Solid state relay,definition,features and comparison with mechanical relay?

Solid-state relay

Solid-state relay. ~240v 2.5A.

A solid-state relay (SSR) is an electronic switching device that switches on or off when a small external voltage is applied across its control terminals. SSRs consist of a sensor which responds to an appropriate input (control signal), a solid-state electronic switching device which switches power to the load circuitry, and a coupling mechanism to enable the control signal to activate this switch without mechanical parts. The relay may be designed to switch either AC or DC to the load. It serves the same function as an electromechanical relay, but has no moving parts.

solid state relay.....

Packaged solid-state relays use power semiconductor devices such as thyristors and transistors, to switch currents up to around a hundred amperes. Solid-state relays have fast switching speeds compared with electromechanical relays, and have no physical contacts to wear out. Application of solid-state relays must consider their lower ability to withstand momentary overload, compared with electromechanical contacts, and their higher "on" state resistance. Unlike an electromechanical relay, a solid-state relay provides only limited switching arrangements (SPST switching).

Coupling

The control signal must be coupled to the controlled circuit in a way which provides galvanic isolation between the two circuits.

Many SSRs use optical coupling. The control voltage energizes an internal LED which illuminates and switches on a photo-sensitive diode (photo-voltaic); the diode current turns on a back-to-back thyristor, SCR, or MOSFET to switch the load. The optical coupling allows the control circuit to be electrically isolated from the load.

Operation

An SSR based on a single MOSFET, or multiple MOSFETs in a paralleled array, can work well for DC loads. MOSFETs have an inherent substrate diode that conducts in the reverse direction, so a single MOSFET cannot block current in both directions. For AC (bi-directional) operation two MOSFETs are arranged back-to-back with their source pins tied together. Their drain pins are connected to either side of the output. The substrate diodes are alternately reverse biased to block current when the relay is off. When the relay is on, the common source is always riding on the instantaneous signal level and both gates are biased positive relative to the source by the photo-diode.

It is common to provide access to the common source so that multiple MOSFETs can be wired in parallel if switching a DC load. Usually a network is provided to speed the turn-off of the MOSFET when the control input is removed.

In AC circuits, SCR or TRIAC relays inherently switch off at the points of zero load current. The circuit will never be interrupted in the middle of a sine wave peak, preventing the large transient voltages that would otherwise occur due to the sudden collapse of the magnetic field around the inductance. This feature is called zero-crossover switching.

Parameters

SSRs are characterised by a number of parameters including the required activating input voltage, current, output voltage and current, whether it is AC or DC, voltage drop or resistance affecting output current, thermal resistance, and thermal and electrical parameters for safe operating area (e.g., derating according to thermal resistance when repeatedly switching large currents).

Advantages over mechanical relays

Most of the relative advantages of solid state and electromechanical relays are common to all solid-state as against electromechanical devices.

Slimmer profile, allowing tighter packing.

Totally silent operation.

SSRs switch faster than electromechanical relays; the switching time of a typical optically coupled SSR is dependent on the time needed to power the LED on and off - of the order of microseconds to milliseconds.

Increased lifetime, even if it is activated many times, as there are no moving parts to wear and no contacts to pit or build up carbon.

Output resistance remains constant regardless of amount of use.

Clean, bounceless operation.

No sparking, allows it to be used in explosive environments, where it is critical that no spark is generated during switching.

Inherently smaller than a mechanical relay of similar specification (if desired may have the same "casing" form factor for interchangeability).

Much less sensitive to storage and operating environment factors such as mechanical shock, vibration, humidity, and external magnetic fields.

Disadvantages

Voltage/current characteristic of semiconductor rather than mechanical contacts:

When closed, higher resistance (generating heat), and increased electrical noise

When open, lower resistance, and reverse leakage current (typically µA range)

Voltage/current characteristic is not linear (not purely resistive), distorting switched waveforms to some extent. An electromechanical relay has the low ohmic (linear) resistance of the associated mechanical switch when activated, and the exceedingly high resistance of the air gap and insulating materials when open.

Some types have polarity-sensitive output circuits. Electromechanical relays are not affected by polarity.

Possibility of spurious switching due to voltage transients (due to much faster switching than mechanical relay)

Isolated bias supply required for gate charge circuit

Higher transient reverse recovery time (Trr) due to the presence of the body diode

Tendency to fail "shorted" on their outputs, while electromechanical relay contacts tend to fail "open".

Signal in the context of electrical engineering?

Signal 

A signal as referred to in communication systems, signal processing, and electrical engineering is a function that "conveys information about the behavior or attributes of some phenomenon". In the physical world, any quantity exhibiting variation in time or variation in space (such as an image) is potentially a signal that might provide information on the status of a physical system, or convey a message between observers, among other possibilities. The IEEE Transactions on Signal Processing states that the term "signal" includes audio, video, speech, image, communication, geophysical, sonar, radar, medical and musical signals.

Typically, signals are provided by a sensor, and often the original form of a signal is converted to another form of energy using a transducer. For example, a microphone converts an acoustic signal to a voltage waveform, and a speaker does the reverse.

The formal study of the information content of signals is the field of information theory. The information in a signal is usually accompanied by noise. The term noise usually means an undesirable random disturbance, but is often extended to include unwanted signals conflicting with the desired signal (such as crosstalk). The prevention of noise is covered in part under the heading of signal integrity. The separation of desired signals from a background is the field of signal recovery, one branch of which is estimation theory, a probabilistic approach to suppressing random disturbances.

Engineering disciplines such as electrical engineering have led the way in the design, study, and implementation of systems involving transmission, storage, and manipulation of information. In the latter half of the 20th century, electrical engineering itself separated into several disciplines, specialising in the design and analysis of systems that manipulate physical signals; electronic engineering and computer engineering as examples; while design engineering developed to deal with functional design of man–machine interfaces.

Definitions

Definitions specific to sub-fields are common. For example, in information theory, a signal is a codified message, that is, the sequence of states in a communication channel that encodes a message.

In the context of signal processing, arbitrary binary data streams are not considered as signals, but only analog and digital signals that are representations of analog physical quantities.

In a communication system, a transmitter encodes a message to a signal, which is carried to a receiver by the communications channel. For example, the words "Mary had a little lamb" might be the message spoken into a telephone. The telephone transmitter converts the sounds into an electrical voltage signal. The signal is transmitted to the receiving telephone by wires; at the receiver it is reconverted into sounds.

In telephone networks, signalling, for example common-channel signaling, refers to phone number and other digital control information rather than the actual voice signal.

Signals can be categorized in various ways. The most common distinction is between discrete and continuous spaces that the functions are defined over, for example discrete and continuous time domains. Discrete-time signals are often referred to as time series in other fields. Continuous-time signals are often referred to as continuous signals even when the signal functions are not continuous; an example is a square-wave signal.

A second important distinction is between discrete-valued and continuous-valued. Particularly in digital signal processing a digital signal is sometimes defined as a sequence of discrete values, that may or may not be derived from an underlying continuous-valued physical process. In other contexts, digital signals are defined as the continuous-time waveform signals in a digital system, representing a bit-stream. In the first case, a signal that is generated by means of a digital modulation method is considered as converted to an analog signal, while it is considered as a digital signal in the second case.

Another important property of a signal (actually, of a statistically defined class of signals) is its entropy or information content.

Analog and digital signals

A digital signal has two or more distinguishable waveforms, in this example, high voltage and low voltages, each of which can be mapped onto a digit. Characteristically, noise can be removed from digital signals provided it is not too large.

Two main types of signals encountered in practice are analog and digital. The figure shows a digital signal that results from approximating an analog signal by its values at particular time instants. Digital signals are quantized, while analog signals are continuous.

analog signal

digital signal

Digital signals often arise via sampling of analog signals, for example, a continually fluctuating voltage on a line that can be digitized by an analog-to-digital converter circuit, wherein the circuit will read the voltage level on the line, say, every 50 microseconds and represent each reading with a fixed number of bits. The resulting stream of numbers is stored as digital data on a discrete-time and quantized-amplitude signal. Computers and other digital devices are restricted to discrete time.

Time discretization

Discrete-time signal created from a continuous signal by sampling

One of the fundamental distinctions between different types of signals is between continuous and discrete time. In the mathematical abstraction, the domain of a continuous-time (CT) signal is the set of real numbers (or some interval thereof), whereas the domain of a discrete-time (DT) signal is the set of integers (or some interval). What these integers represent depends on the nature of the signal; most often it is time.

If for a signal, the quantities are defined only on a discrete set of times, we call it a discrete-time signal. A simple source for a discrete time signal is the sampling of a continuous signal, approximating the signal by a sequence of its values at particular time instants.

A discrete-time real (or complex) signal can be seen as a function from (a subset of) the set of integers (the index labeling time instants) to the set of real (or complex) numbers (the function values at those instants).

A continuous-time real (or complex) signal is any real-valued (or complex-valued) function which is defined at every time t in an interval, most commonly an infinite interval.

Amplitude quantization

Digital signal resulting from approximation to an analog signal, which is a continuous function of time

If a signal is to be represented as a sequence of numbers, it is impossible to maintain exact precision - each number in the sequence must have a finite number of digits. As a result, the values of such a signal belong to a finite set; in other words, it is quantized. Quantization is the process of converting a continuous analog audio signal to a digital signal with discrete numerical values.

Signal processing

Main article: Signal processing

Signal transmission using electronic signals

A typical role for signals is in signal processing. A common example is signal transmission between different locations. The embodiment of a signal in electrical form is made by a transducer that converts the signal from its original form to a waveform expressed as a current (I) or a voltage (V), or an electromagnetic waveform, for example, an optical signal or radio transmission. Once expressed as an electronic signal, the signal is available for further processing by electrical devices such as electronic amplifiers and electronic filters, and can be transmitted to a remote location by electronic transmitters and received using electronic receivers.

Signals and systems

In Electrical engineering programs, a class and field of study known as "signals and systems" (S and S) is often seen as the "cut class" for EE careers, and is dreaded by some students as such. Depending on the school, undergraduate EE students generally take the class as juniors or seniors, normally depending on the number and level of previous linear algebra and differential equation classes they have taken.

The field studies input and output signals, and the mathematical representations between them known as systems, in four domains: Time, Frequency, s and z. Since signals and systems are both studied in these four domains, there are 8 major divisions of study. As an example, when working with continuous time signals (t), one might transform from the time domain to a frequency or s domain; or from discrete time (n) to frequency or z domains. Systems also can be transformed between these domains like signals, with continuous to s and discrete to z.

Although S and S falls under and includes all the topics covered in this article, as well as Analog signal processing and Digital signal processing, it actually is a subset of the field of Mathematical modeling. The field goes back to RF over a century ago, when it was all analog, and generally continuous. Today, software has taken the place of much of the analog circuitry design and analysis, and even continuous signals are now generally processed digitally. Ironically, digital signals also are processed continuously in a sense, with the software doing calculations between discrete signal "rests" to prepare for the next input/transform/output event.

In past EE curricula S and S, as it is often called, involved circuit analysis and design via mathematical modeling and some numerical methods, and was updated several decades ago with Dynamical systems tools including differential equations, and recently, Lagrangians. The difficulty of the field at that time included the fact that not only mathematical modeling, circuits, signals and complex systems were being modeled, but physics as well, and a deep knowledge of electrical (and now electronic) topics also was involved and required.

Today, the field has become even more daunting and complex with the addition of circuit, systems and signal analysis and design languages and software, from MATLAB and Simulink to NumPy, VHDL, PSpice, Verilog and even Assembly language. Students are expected to understand the tools as well as the mathematics, physics, circuit analysis, and transformations between the 8 domains.

Because mechanical engineering topics like friction, dampening etc. have very close analogies in signal science (inductance, resistance, voltage, etc.), many of the tools originally used in ME transformations (Laplace and Fourier transforms, Lagrangians, sampling theory, probability, difference equations, etc.) have now been applied to signals, circuits, systems and their components, analysis and design in EE. Dynamical systems that involve noise, filtering and other random or chaotic attractors and repellors have now placed stochastic sciences and statistics between the more deterministic discrete and continuous functions in the field. (Deterministic as used here means signals that are completely determined as functions of time).

EE taxonomists are still not decided where S&S falls within the whole field of signal processing vs. circuit analysis and mathematical modeling, but the common link of the topics that are covered in the course of study has brightened boundaries with dozens of books, journals, etc. called Signals and Systems, and used as text and test prep for the EE, as well as, recently, computer engineering exams. 

Short circuit concepts in a comprehensive way?

Short circuit

Definition

A short circuit is an abnormal connection between two nodes of an electric circuit intended to be at different voltages. This results in an excessive electric current limited only by the Thévenin equivalent resistance of the rest of the network and potentially causes circuit damage, overheating, fire or explosion. Although usually the result of a fault, there are cases where short circuits are caused intentionally, for example, for the purpose of voltage-sensing crowbar circuit protectors.

In circuit analysis, a short circuit is a connection between two nodes that forces them to be at the same voltage. In an ideal short circuit, this means there is no resistance and no voltage drop across the short. In real circuits, the result is a connection with almost no resistance. In such a case, the current is limited by the rest of the circuit.

Examples

A common type of short circuit occurs when the positive and negative terminals of a battery are connected with a low-resistance conductor, like a wire. With low resistance in the connection, a high current exists, causing the cell to deliver a large amount of energy in a short time.

A large current through a battery can cause the rapid buildup of heat, potentially resulting in an explosion or the release of hydrogen gas and electrolyte (an acid or a base), which can burn tissue, cause blindness or even death. Overloaded wires can also overheat, sometimes causing damage to the wire's insulation, or a fire. High current conditions may also occur with electric motor loads under stalled conditions, such as when the impeller of an electrically driven pump is jammed by debris; this is not a short, though it may have some similar effects.

In electrical devices unintentional short circuits are usually caused when a wire's insulation breaks down, or when another conducting material is introduced, allowing charge to flow along a different path than the one intended.

In mains circuits, short circuits may occur between two phases, between a phase and neutral or between a phase and earth (ground). Such short circuits are likely to result in a very high current and therefore quickly trigger an overcurrent protection device. However, it is possible for short circuits to arise between neutral and earth conductors, and between two conductors of the same phase. Such short circuits can be dangerous, particularly as they may not immediately result in a large current and are therefore less likely to be detected. Possible effects include unexpected energisation of a circuit presumed to be isolated. To help reduce the negative effects of short circuits, power distribution transformers are deliberately designed to have a certain amount of leakage reactance. The leakage reactance (usually about 5 to 10% of the full load impedance) helps limit both the magnitude and rate of rise of the fault current.

A short circuit may lead to formation of an electric arc. The arc, a channel of hot ionized plasma, is highly conductive and can persist even after significant amount of original material of the conductors was evaporated. Surface erosion is a typical sign of electric arc damage. Even short arcs can remove significant amount of materials from the electrodes. The temperature of the resulting electrical arc is very high (tens of thousands of degrees Fahrenheit), causing the metal on the contact surfaces to melt, pool and migrate with the current, as well as to escape into the air as fine particulate matter.

Damage

A short circuit fault current can, within milliseconds, be thousands of times larger than the normal operating current of the system. Damage from short circuits can be reduced or prevented by employing fuses, circuit breakers, or other overload protection, which disconnect the power in reaction to excessive current. Overload protection must be chosen according to the current rating of the circuit. Circuits for large home appliances require protective devices set or rated for higher currents than lighting circuits. Wire gauges specified in building and electrical codes are chosen to ensure safe operation in conjunction with the overload protection. An overcurrent protection device must be rated to safely interrupt the maximum prospective short circuit current.

In an improper installation, the overcurrent from a short circuit may cause ohmic heating of the circuit parts with poor conductivity (faulty joints in wiring, faulty contacts in power sockets, or even the site of the short circuit itself). Such overheating is a common cause of fires. An electric arc, if it forms during the short circuit, produces high amount of heat and can cause ignition of combustible substances as well.

In industrial and utility distribution systems, dynamic forces generated by high short circuit currents cause conductors to spread apart. Busbars, cables, and apparatus can be damaged by the forces generated in a short circuit.

Related concepts

In electronics, the ideal model (infinite gain) of an operational amplifier is said to produce a virtual short circuit between its input terminals because no matter what the output voltage is, the difference of potential between its input terminals is zero. If one of the input terminals is connected to the ground, then the other one is said to provide a virtual ground because its potential is (ideally) identical to that of the ground. An ideal operational amplifier also has infinite input impedance, so unlike a real short circuit, no current flows between the terminals of the virtual short. Due to these differences, the terminology can be confusing; one textbook parenthetically suggests that "virtual open circuit" may be equally suitable because no current flows.

Flyback Transformer definition,features and various aspects.

Flyback transformer

A flyback transformer (FBT), also called a line output transformer (LOPT), is a special type of electrical transformer. It was initially designed to generate high voltage sawtooth signals at a relatively high frequency. In modern applications, it is used extensively in switched-mode power supplies for both low (3 V) and high voltage (over 10 kV) supplies.

Operation and usage

The primary winding of the flyback transformer is driven by a switch from a DC supply (usually a transistor). When the switch is switched on, the primary inductance causes the current to build up in a ramp. An integral diode connected in series with the secondary winding prevents the formation of secondary current that would eventually oppose the primary current ramp.

When the switch is turned off, the current in the primary falls to zero. The energy stored in the magnetic core is released to the secondary as the magnetic field in the core collapses. The voltage in the output winding rises very quickly (usually less than a microsecond) until it is limited by the load conditions. Once the voltage reaches such level as to allow the secondary current to flow, then the current in the secondary winding begins to flow in the form of a descending ramp.

The cycle can then be repeated. If the secondary current is allowed to discharge completely to zero (no energy stored in the core), then it is said that the transformer works in discontinuous mode (DCM). When some energy is always stored in the core (and the current waveforms look trapezoidal rather than triangular), then this is continuous mode (CCM). This terminology is used especially in power supply transformers.

The low voltage output winding mirrors the sawtooth of the primary current and, e.g. for television purposes, has fewer turns than the primary, thus providing a higher current. This is a ramped and pulsed waveform that repeats at the horizontal (line) frequency of the display. The flyback (vertical portion of the sawtooth wave) can be a potential problem to the flyback transformer if the energy has nowhere to go: the faster a magnetic field collapses, the greater the induced voltage, which, if not controlled, can flash over the transformer terminals. The high frequency used permits the use of a much smaller transformer. In television sets, this high frequency is about 15 kilohertz (15.625 kHz for PAL, 15.734 kHz for NTSC), and vibrations from the transformer core caused by magnetostriction can often be heard as a high-pitched whine. In modern computer displays, the frequency can vary over a wide range, from about 30 kHz to 150 kHz.

The transformer can be equipped with extra windings whose sole purpose is to have a relatively large voltage pulse induced in them when the magnetic field collapses as the input switch is turned off. There is considerable energy stored in the magnetic field and coupling it out via extra windings helps it to collapse quickly, and avoids the voltage flash over that might otherwise occur. The pulse train coming from the flyback transformer windings is converted to direct current by a simple half wave rectifier. There is no point in using a full wave design as there are no corresponding pulses of opposite polarity. One turn of a winding often produces pulses of several volts. In older television designs, the transformer produced the required high voltage for the CRT accelerating voltage directly with the output rectified by a simple rectifier. In more modern designs, the rectifier is replaced by a voltage multiplier. Color television sets also have to use a regulator to control the high voltage. The earliest sets used a shunt vacuum tube regulator, but the introduction of solid state sets employed a simpler voltage dependant resistor. The rectified voltage is then used to supply the final anode of the cathode ray tube.

There are often auxiliary windings that produce lower voltages for driving other parts of the television circuitry. The voltage used to bias the varactor diodes in modern tuners is often derived from the flyback transformer ("Line OutPut Transformer" LOPT). In tube sets, a one or two-turn filament winding is located on the opposite side of the core as the HV secondary, used to drive the HV rectifier tube's heater.

Practical considerations

In modern displays, the LOPT, voltage multiplier and rectifier are often integrated into a single package on the main circuit board. There is usually a thickly insulated wire from the LOPT to the anode terminal (covered by a rubber cap) on the side of the picture tube.

One advantage of operating the transformer at the flyback frequency is that it can be much smaller and lighter than a comparable transformer operating at mains (line) frequency. Another advantage is that it provides a failsafe mechanism — should the horizontal deflection circuitry fail, the flyback transformer will cease operating and shut down the rest of the display, preventing the screen burn that would otherwise result from a stationary electron beam.

Construction

The primary is wound first around a ferrite rod, and then the secondary is wound around the primary. This arrangement minimizes the leakage inductance of the primary. Finally, a ferrite frame is wrapped around the primary/secondary assembly, closing the magnetic field lines. Between the rod and the frame is an air gap, which increases the reluctance. The secondary is wound layer by layer with enameled wire, and Mylar film between the layers. In this way parts of the wire with higher voltage between them have more dielectric material between them.

Applications

The flyback transformer is used in the operation of CRT-display devices such as television sets and CRT computer monitors. The voltage and frequency can each range over a wide scale depending on the device. For example, a large color TV CRT may require 20 to 50 kV with a horizontal scan rate of 15.734 kHz for NTSC devices. Unlike a power (or "mains") transformer which uses an alternating current of 50 or 60 hertz, a flyback transformer typically operates with switched currents at much higher frequencies in the range of 15 kHz to 50 kHz.

Failure and repair

Flyback transformers are a frequent source of failure in CRT displays. Often, the CRT itself is blamed when the display has actually experienced a flyback transformer failure. The high voltage present in the many turns of wire, with the thin insulation required for the transformer to be of reasonable size, can result in leakage between the windings. As the leakage heats the insulation, it carbonizes and increases conduction. In turn, heat and carbonization continue degrading the insulation until the leaked current is high enough for the high voltage to arc between the windings, and destroy the transformer (and sometimes other components in the display). As a result, replacement flyback transformers for almost every set on the market are available through dealers in electronic parts, typically for under $50. The problem is exacerbated by the tendency of the flyback to accumulate a coating of dust due to electrostatic attraction, which serves as a path to ground for leaks which might otherwise not be of sufficient magnitude to initiate the chain of events leading to destructive failure, as described.

As a result, occasional cleaning of the accumulated dust from the high voltage circuitry inside a television can be beneficial if proper precautions are taken — however the small amount of additional life that is gained for the flyback transformer rarely justifies the time and effort necessary. It is debated among technicians if displays installed in dirty, dusty locations experience more failures than those in cleaner locations, but many do say that dirty conditions contribute to malfunctions.

Another common failure mode is for one of the windings to develop a short circuit turn. This is often the high voltage winding with its small wire and thin enamel insulation. Once a short circuit turn occurs, the transformer is prevented from ringing (magnetically speaking) as the shorted turn "damps" the inductance. It is not possible to detect the shorted turn using conventional resistance measuring equipment.

In many recent televisions, after replacing the flyback transformer, the control firmware must be recalibrated to account for slight differences in performance between transformers in order to maintain accurate picture reproduction. In older televisions and monitors, these needed adjustments were performed by turning potentiometers inside, or on the back of the set to achieve optimal picture quality. Also, when flyback transformers fail, they frequently will also damage the horizontal output transistor that drives the flyback transformer, and sometimes even blow fuses in the low voltage power supply circuits.

Unless the owner of the display device has enough experience and knowledge to repair it themselves, the failure of a flyback transformer frequently condemns the device as unrepairable, because the cost of repair can be higher than the replacement cost. Although the cost of the flyback transformer — and other damaged parts — is relatively inexpensive, the labor time needed to disassemble, replace the parts, and then re-adjust the display can make the repair prohibitively expensive.

Safety considerations

A flyback transformer and its associated circuitry operate at very high voltages at low currents (<1mA-15mA), far beyond mains voltage. While most flybacks do not supply enough power to kill directly, the voltage they employ can cause violent muscle spasms if touched; and such spasms usually cause injury. A common injury that occurs when one is shocked is actually to be injured not as much by the shock itself, but when the victim's hand or arm is thrown back against other internal components in the display device. Therefore, only trained persons should touch or modify these devices, after first ensuring that the transformer is switched off and any stored energy has been safely discharged. The CRT attached to the flyback has an inherent capacitance which can hold a high voltage charge for more than a week after the power is switched off. Often, a high-resistance bleeder resistor is connected internally within the flyback transformer to ensure the charge is safely grounded when not in use, but many sets lack this, especially older models

DC motor,electromagnetic motor and different stators?

DC motor

DC motors were the first type widely used, since they could be powered from existing direct-current lighting power distribution systems. A DC motor's speed can be controlled over a wide range, using either a variable supply voltage or by changing the strength of current in its field windings. Small DC motors are used in tools, toys, and appliances. The universal motor can operate on direct current but is a lightweight motor used for portable power tools and appliances. Larger DC motors are used in propulsion of electric vehicles, elevator and hoists, or in drives for steel rolling mills. The advent of power electronics has made replacement of DC motors with AC motors possible in many applications.

Electromagnetic motors

A coil of wire with a current running through it generates an electromagnetic field aligned with the center of the coil. The direction and magnitude of the magnetic field produced by the coil can be changed with the direction and magnitude of the current flowing through it.

A simple DC motor has a stationary set of magnets in the stator and an armature with one or more windings of insulated wire wrapped around a soft iron core that concentrates the magnetic field. The windings usually have multiple turns around the core, and in large motors there can be several parallel current paths. The ends of the wire winding are connected to a commutator. The commutator allows each armature coil to be energized in turn and connects the rotating coils with the external power supply through brushes. (Brushless DC motors have electronics that switch the DC current to each coil on and off and have no brushes.)

The total amount of current sent to the coil, the coil's size and what it's wrapped around dictate the strength of the electromagnetic field created.

The sequence of turning a particular coil on or off dictates what direction the effective electromagnetic fields are pointed. By turning on and off coils in sequence a rotating magnetic field can be created. These rotating magnetic fields interact with the magnetic fields of the magnets (permanent or electromagnets) in the stationary part of the motor (stator) to create a force on the armature which causes it to rotate. In some DC motor designs the stator fields use electromagnets to create their magnetic fields which allow greater control over the motor.

At high power levels, DC motors are almost always cooled using forced air.

Different number of stator and armature fields as well as how they are connected provide different inherent speed/torque regulation characteristics. The speed of a DC motor can be controlled by changing the voltage applied to the armature. The introduction of variable resistance in the armature circuit or field circuit allowed speed control. Modern DC motors are often controlled by power electronics systems which adjust the voltage by "chopping" the DC current into on and off cycles which have an effective lower voltage.

Since the series-wound DC motor develops its highest torque at low speed, it is often used in traction applications such as electric locomotives, and trams. The DC motor was the mainstay of electric traction drives on both electric and diesel-electric locomotives, street-cars/trams and diesel electric drilling rigs for many years. The introduction of DC motors and an electrical grid system to run machinery starting in the 1870s started a new second Industrial Revolution. DC motors can operate directly from rechargeable batteries, providing the motive power for the first electric vehicles and today's hybrid cars and electric cars as well as driving a host of cordless tools. Today DC motors are still found in applications as small as toys and disk drives, or in large sizes to operate steel rolling mills and paper machines. Large DC motors with separately excited fields were generally used with winder drives for mine hoists, for high torque as well as smooth speed control using thyristor drives. These are now replaced with large AC motors with variable frequency drives.

If external power is applied to a DC motor it acts as a DC generator, a dynamo. This feature is used to slow down and recharge batteries on hybrid car and electric cars or to return electricity back to the electric grid used on a street car or electric powered train line when they slow down. This process is called regenerative braking on hybrid and electric cars. In diesel electric locomotives they also use their DC motors as generators to slow down but dissipate the energy in resistor stacks. Newer designs are adding large battery packs to recapture some of this energy.

Brushed

A brushed DC electric motor generating torque from DC power supply by using an internal mechanical commutation. Stationary permanent magnets form the stator field. Torque is produced by the principle that any current-carrying conductor placed within an external magnetic field experiences a force, known as Lorentz force. In a motor, the magnitude of this Lorentz force (a vector represented by the green arrow), and thus the output torque,is a function for rotor angle, leading to a phenomenon known as torque ripple) Since this is a single phase two-pole motor, the commutator consists of a split ring, so that the current reverses each half turn ( 180 degrees).

The brushed DC electric motor generates torque directly from DC power supplied to the motor by using internal commutation, stationary magnets (permanent or electromagnets), and rotating electrical magnets.

Advantages of a brushed DC motor include low initial cost, high reliability, and simple control of motor speed. Disadvantages are high maintenance and low life-span for high intensity uses. Maintenance involves regularly replacing the carbon brushes and springs which carry the electric current, as well as cleaning or replacing the commutator. These components are necessary for transferring electrical power from outside the motor to the spinning wire windings of the rotor inside the motor. Brushes consist of conductors.

Brushless

Typical brushless DC motors use one or more permanent magnets in the rotor and electromagnets on the motor housing for the stator. A motor controller converts DC to AC. This design is mechanically simpler than that of brushed motors because it eliminates the complication of transferring power from outside the motor to the spinning rotor. The motor controller can sense the rotor's position via Hall effect sensors or similar devices and can precisely control the timing, phase, etc., of the current in the rotor coils to optimize torque, conserve power, regulate speed, and even apply some braking. Advantages of brushless motors include long life span, little or no maintenance, and high efficiency. Disadvantages include high initial cost, and more complicated motor speed controllers. Some such brushless motors are sometimes referred to as "synchronous motors" although they have no external power supply to be synchronized with, as would be the case with normal AC synchronous motors.

Uncommutated

Other types of DC motors require no commutation.

Homopolar motor – A homopolar motor has a magnetic field along the axis of rotation and an electric current that at some point is not parallel to the magnetic field. The name homopolar refers to the absence of polarity change.

Homopolar motors necessarily have a single-turn coil, which limits them to very low voltages. This has restricted the practical application of this type of motor.

Ball bearing motor – A ball bearing motor is an unusual electric motor that consists of two ball bearing-type bearings, with the inner races mounted on a common conductive shaft, and the outer races connected to a high current, low voltage power supply. An alternative construction fits the outer races inside a metal tube, while the inner races are mounted on a shaft with a non-conductive section (e.g. two sleeves on an insulating rod). This method has the advantage that the tube will act as a flywheel. The direction of rotation is determined by the initial spin which is usually required to get it going.

Permanent magnet stators

A PM motor does not have a field winding on the stator frame, instead relying on PMs to provide the magnetic field against which the rotor field interacts to produce torque. Compensating windings in series with the armature may be used on large motors to improve commutation under load. Because this field is fixed, it cannot be adjusted for speed control. PM fields (stators) are convenient in miniature motors to eliminate the power consumption of the field winding. Most larger DC motors are of the "dynamo" type, which have stator windings. Historically, PMs could not be made to retain high flux if they were disassembled; field windings were more practical to obtain the needed amount of flux. However, large PMs are costly, as well as dangerous and difficult to assemble; this favors wound fields for large machines.

To minimize overall weight and size, miniature PM motors may use high energy magnets made with neodymium or other strategic elements; most such are neodymium-iron-boron alloy. With their higher flux density, electric machines with high-energy PMs are at least competitive with all optimally designed singly fed synchronous and induction electric machines. Miniature motors resemble the structure in the illustration, except that they have at least three rotor poles (to ensure starting, regardless of rotor position) and their outer housing is a steel tube that magnetically links the exteriors of the curved field magnets.

Wound stators

A field coil may be connected in shunt, in series, or in compound with the armature of a DC machine (motor or generator)

There are three types of electrical connections between the stator and rotor possible for DC electric motors: series, shunt/parallel and compound (various blends of series and shunt/parallel) and each has unique speed/torque characteristics appropriate for different loading torque profiles/signatures.

Servo motor verses Stepper motor?

Industrial servomotor

The grey/green cylinder is the brush-type DC motor. The black section at the bottom contains the planetary reduction gear, and the black object on top of the motor is the optical rotary encoder for position feedback. This is the steering actuator of a large robot vehicle.

Industrial servomotors and gearboxes, with standardized flange mountings for interchangeability

A servomotor is a rotary actuator or linear actuator that allows for precise control of angular or linear position, velocity and acceleration. It consists of a suitable motor coupled to a sensor for position feedback. It also requires a relatively sophisticated controller, often a dedicated module designed specifically for use with servomotors.

Servomotors are not a specific class of motor although the term servomotor is often used to refer to a motor suitable for use in a closed-loop control system.

Servomotors are used in applications such as robotics, CNC machinery or automated manufacturing.

Mechanism

A servomotor is a closed-loop servomechanism that uses position feedback to control its motion and final position. The input to its control is a signal (either analogue or digital) representing the position commanded for the output shaft.

The motor is paired with some type of encoder to provide position and speed feedback. In the simplest case, only the position is measured. The measured position of the output is compared to the command position, the external input to the controller. If the output position differs from that required, an error signal is generated which then causes the motor to rotate in either direction, as needed to bring the output shaft to the appropriate position. As the positions approach, the error signal reduces to zero and the motor stops.

The very simplest servomotors use position-only sensing via a potentiometer and bang-bang control of their motor; the motor always rotates at full speed (or is stopped). This type of servomotor is not widely used in industrial motion control, but it forms the basis of the simple and cheap servos used for radio-controlled models.

More sophisticated servomotors use optical rotary encoders to measure the speed of the output shaft and a variable-speed drive to control the motor speed.Both of these enhancements, usually in combination with a PID control algorithm, allow the servomotor to be brought to its commanded position more quickly and more precisely, with less overshooting.

Servomotors vs. stepper motors

A servomotor consumes power as it rotates to the commanded position but then the servomotor rests. Stepper motors continue to consume power to lock in and hold the commanded position.

Servomotors are generally used as a high-performance alternative to the stepper motor. Stepper motors have some inherent ability to control position, as they have built-in output steps. This often allows them to be used as an open-loop position control, without any feedback encoder, as their drive signal specifies the number of steps of movement to rotate, but for this the controller needs to 'know' the position of the stepper motor on power up. Therefore, on first power up, the controller will have to activate the stepper motor and turn it to a known position, e.g. until it activates an end limit switch. This can be observed when switching on an inkjet printer; the controller will move the ink jet carrier to the extreme left and right to establish the end positions. A servomotor will immediately turn to whatever angle the controller instructs it to, regardless of the initial position at power up.

The lack of feedback of a stepper motor limits its performance, as the stepper motor can only drive a load that is well within its capacity, otherwise missed steps under load may lead to positioning errors and the system may have to be restarted or recalibrated. The encoder and controller of a servomotor are an additional cost, but they optimise the performance of the overall system (for all of speed, power and accuracy) relative to the capacity of the basic motor. With larger systems, where a powerful motor represents an increasing proportion of the system cost, servomotors have the advantage.

There has been increasing popularity in closed loop stepper motors in recent years. They act like servomotors but have some differences in their software control to get smooth motion. The top 3 manufacturers of closed loop stepper motor systems employ magnetic encoders as their feedback device of choice due to low cost and resistance to vibration. The main benefit of a closed loop stepper motor is the cost to performance ratio. There is also no need to tune the PID controller on a closed loop stepper system.

Many applications, such as laser cutting machines, may be offered in two ranges, the low-priced range using stepper motors and the high-performance range using servomotors.

Encoders

The first servomotors were developed with synchros as their encoders. Much work was done with these systems in the development of radar and anti-aircraft artillery during World War II. 

Simple servomotors may use resistive potentiometers as their position encoder. These are only used at the very simplest and cheapest level, and are in close competition with stepper motors. They suffer from wear and electrical noise in the potentiometer track. Although it would be possible to electrically differentiate their position signal to obtain a speed signal, PID controllers that can make use of such a speed signal generally warrant a more precise encoder.

Modern servomotors use rotary encoders, either absolute or incremental. Absolute encoders can determine their position at power-on, but are more complicated and expensive. Incremental encoders are simpler, cheaper and work at faster speeds. Incremental systems, like stepper motors, often combine their inherent ability to measure intervals of rotation with a simple zero-position sensor to set their position at start-up.

Instead of servomotors, sometimes a motor with a separate, external linear encoder is used. These motor + linear encoder systems avoid inaccuracies in the drivetrain between the motor and linear carriage, but their design is made more complicated as they are no longer a pre-packaged factory-made system.

Motors

The type of motor is not critical to a servomotor and different types may be used. At the simplest, brushed permanent magnet DC motors are used, owing to their simplicity and low cost. Small industrial servomotors are typically electronically commutated brushless motors. For large industrial servomotors, AC induction motors are typically used, often with variable frequency drives to allow control of their speed. For ultimate performance in a compact package, brushless AC motors with permanent magnet fields are used, effectively large versions of Brushless DC electric motors.

Drive modules for servomotors are a standard industrial component. Their design is a branch of power electronics, usually based on a three-phase MOSFET or IGBT H bridge. These standard modules accept a single direction and pulse count (rotation distance) as input. They may also include over-temperature monitoring, over-torque and stall detection features.As the encoder type, gearhead ratio and overall system dynamics are application specific, it is more difficult to produce the overall controller as an off-the-shelf module and so these are often implemented as part of the main controller.

Control

Most modern servomotors are designed and supplied around a dedicated controller module from the same manufacturer. Controllers may also be developed around microcontrollers in order to reduce cost for large-volume applications.

Integrated servomotors

Integrated servomotors are designed so as to include the motor, driver, encoder and associated electronics into a single package.

What is Flyback diode?

Flyback diode

A flyback diode (sometimes called a snubber diode, commutating diode, freewheeling diode, suppressor diode, suppression diode, clamp diode, or catch diode) is a diode used to eliminate flyback (back EMF), which is the sudden voltage spike seen across an inductive load when its supply current is suddenly reduced or interrupted.

schematic diagram of flyback diagram

Working principle

In its most simplified form with a voltage source connected to an inductor with a switch, we have 2 states available. In the first steady-state, the switch has been closed for a long time such that the inductor has become fully energized and is behaving as though it were a short. Current is flowing "down" from the positive terminal of the voltage source to its negative terminal, through the inductor. When the switch is opened , the inductor will attempt to resist the sudden drop of current (dI/dt is large therefore V is large) by using its stored magnetic field energy to create its own voltage. An extremely large negative potential is created where there once was positive potential, and a positive potential is created where there was once negative potential. The switch, however, remains at the voltage of the power supply, but it is still in contact with the inductor pulling down a negative voltage. Since no connection is physically made to allow current to continue to flow (due to the switch being open), the large potential difference can cause electrons to "arc" across the air-gap of the open switch (or junction of a transistor). This is undesirable for the reasons mentioned above and must be prevented.

A flyback diode solves this starvation-arc problem by allowing the inductor to draw current from itself (thus, "flyback") in a continuous loop until the energy is dissipated through losses in the wire, the diode and the resistor . When the switch is closed the diode is reverse-biased against the power supply and doesn't exist in the circuit for practical purposes. However, when the switch is opened, the diode becomes forward-biased relative to the inductor (instead of the power supply as before), allowing it to conduct current in a circular loop from the positive potential at the bottom of the inductor to the negative potential at the top (assuming the power supply was supplying positive voltage at the top of the inductor prior to the switch being opened). The voltage across the inductor will merely be a function of the forward voltage drop of the flyback diode. Total time for dissipation can vary, but it will usually last for a few milliseconds.

In these images we observe classic signs of back EMF and its elimination through the use of a flyback diode (1N4007). The inductor in this case is a solenoid connected to a 24V DC power supply using 20 awg wire. Each waveform was taken using a digital oscilloscope set to trigger when the voltage across the inductor dipped below zero. In Figure 1 the voltage as measured across the switch bounces/spikes to around -300 V. a flyback diode was added in antiparallel with the solenoid. Instead of spiking to -300 V, the flyback diode only allows approximately -1.4 V of potential to be built up (-1.4 V is a combination of the forward bias of the 1N4007 diode (1.1 V) and the foot of wiring separating the diode and the solenoid). In both cases, the total time for the solenoid to discharge is a few milliseconds.

Design

In an ideal flyback diode selection, one would seek a diode which has very large peak forward current capacity (to handle voltage transients without burning out the diode), low forward voltage drop, and a reverse breakdown voltage suited to the inductor's power supply. Depending on the application and equipment involved, some voltage surges can be upwards of 10 times the voltage of the power source, so it is critical not to underestimate the energy contained within an energized inductor.

When used with a DC coil relay, a flyback diode can cause delayed drop-out of the contacts when power is removed, due to the continued circulation of current in the relay coil and diode. When rapid opening of the contacts is important, a low-value resistor can be placed in series with the diode to help dissipate the coil energy faster, at the expense of higher voltage at the switch.

Schottky diodes are preferred in flyback diode applications for switching power converters, because they have the lowest forward drop (~0.2 V rather than >0.7 V for low currents) and are able to quickly respond to reverse bias (when the inductor is being re-energized). They therefore dissipate less energy while transferring energy from the inductor to a capacitor.

When the flyback diode is used to simply dissipate the inductive energy, as with a solenoid or motor, cheap 1N540x and 1N400x general-purpose diodes are used instead.

Induction at the opening of a contact

According to Lenz's law, if the current through an inductance changes, this inductance induces a voltage so the current will go on flowing as long as there is energy in the magnetic field. If the current can only flow through the air, the voltage is therefore so high that the air conducts. That is why in mechanically-switched circuits, the near-instantaneous dissipation which occurs without a flyback diode is often observed as an arc across the opening mechanical contacts. Energy is dissipated in this arc primarily as intense heat which causes undesirable premature erosion of the contacts. Another way to dissipate energy is through electromagnetic radiation.

Similarly, for non-mechanical solid state switching (i.e., a transistor), large voltage drops across an unactivated solid state switch can destroy the component in question (either instantaneously or through accelerated wear and tear).

Some energy is also lost from the system as a whole and from the arc as a broad spectrum of electromagnetic radiation, in the form of radio waves and light. These radio waves can cause undesirable clicks and pops on nearby radio receivers.

To minimise the antenna-like radiation of this electromagnetic energy from wires connected to the inductor, the flyback diode should be connected as physically close to the inductor as practicable. This approach also minimises those parts of the circuit that are subject to an unwanted high-voltage a good engineering practice.

Applications

Flyback diodes are commonly used when inductive loads are switched off by semiconductor devices: in relay drivers, H-bridge motor drivers, and so on. A switched-mode power supply also exploits this effect, but the energy is not dissipated to heat and instead used to pump a packet of additional charge into a capacitor, in order to supply power to a load.