Electrical Motors

An electrical machine is a device that can convert either mechanical energy to electrical energy or electrical energy to mechanical energy

When it converts electrical energy to mechanical energy through the action of a magnet, it is called a motor.

electric machines rotate about an axis, called the shaft of the machine. Because of the rotational nature of machinery, it is important to have a basic understanding of rotational motion

Principles and design : with a simple loop of wire rotating within a uniform magnetic field ,a loop of wire in a uniform magnetic field is the simples possible machine that produces a sinusoidal ae voltage.

This case is not representative of real ac machines, since the flux in real ae machines is not constant in either magnitude or direction.

machine consisting of a large stationary magnet producing an essentially constant and uniform magnetic field and a rotating loop of wire within that field. The rotating part of the machine is called the rotor, and the stationary part of the machine is called the stator.

the induced voltage eiad = 2vBl sin Beta that main reason to generate electric field lead to current and through wires of stator windings to generate magnetic field affecting on rotor windings to repeat same operation causing to generate magnetic fields

Thus, the voltage generated in the loop is a sinusoid whose magnitude is equal LO the product of the flux inside the machine and the speed of rotation of the machine. This is also true of real ac machines. In general, the voltage in any real machine will depend on three factors:

The flux in the machine

The speed of rotation

A constant representing the construction of the machine (the number of loops, etc.)

The Torque Induced in a Current-Carrying Loop :the rotor loop is at some arbitrary angle 8 with respect to the magnetic field, and that a current i is flowing in the loop, as shown in Figure 3-4. If a current flows in the loop, then a torque will be induced on the wire loop. To determine the magnitude and direction of the torque Tind = 2rilB sin (theta)

Thus, the torque induced in the loop is proportional to the strength of the loop's magnetic field, the strength at the external magnetic field, and the sine of the angle between them. This is also true of real ac machines. In general, the torque in any real machine will depend on four factors: .

The strength of the rotor magnetic field

The strength of the external magnetic field

The sine of the angle between them

A constant representing the construction of the machine (geometry, etc.)

The fundamental principle of ac machine operation is that if a three-phase set of currents, each of equal magnitude and differing in phase by 120°, flows in a three-phase winding, then it will produce a rotating magnetic field of constant magnitude. The three-phase winding consists of three separate windings spaced 120 electrical degrees apart around the surface of the machine. The rotating magnetic field concept is illustrated in the simplest case by an empty stator containing just three coils, each 120 apart . Since such a winding produces only one north and one south magnetic pole, it is a two pole winding. To understand the concept of the rotating magnetic field, we will apply a set of currents to the stator of

The Relationship between Electrical Frequency and the Speed of Magnetic Field Rotation

Frequency = Ns*P/120

P:number of poles of rotor

Ns: speed of rotation

Importance of winding insulation

Another interesting fact can be observed about the resulting magnetic field. If the current in any two of the three coils is swapped, the direction of the magnetic field 's rotation will be reversed. This means that it is possible to reverse the direction of rotation of an ac motor just by switching the connections on any two of the three coils.

One of the most critical parts of an ac machine design is the insulation of its windings. If the insulation of a motor breaks down, the machine shorts out. The repair of a machine with shorts out insulation is quite expensive, if it is even possible. To prevent the winding insulation from breaking down as a result of overheating, it is necessary to limit the temperature of the windings. This can be partially done by providing a cooling air circulation over them, but ultimately the maximum winding temperature limits the maximum power that can be supplied continuously by the machine.

Insulation rarely fails from immediate breakdown at some critical temperature. Instead, the increase in temperature produces a gradual degradation of the insulation, making it subject to failure from another cause such as shock, vibration, or electrical stress. There was an old rule of thumb that said that the life expectancy of a motor with a given type of insulation is halved for each 10 percent ( rise in temperature above the rated temperature of the winding. This rule still applies to some extent today.

To standardize the temperature limits of machine insulation, the National Electrical Manufacturers Association (NEMA) in the United States has defined a series of insulation system classes. Each insulation system class specifies the maximum temperature rise permissible for that class of insulation. There are three common NEMA insulation classes for integral-horsepower ac motors: E, F, and H. Each class represents a higher permissible winding temperature than the one before it. For example, the armature winding temperature rise above ambient temperature in one type of continuously operating ac induction motor must be limited to 80'e for class B, 105 for class F, and 125 for class H insulation. The effect of operating temperature on insulation life for a typical machine can be quite dramatic. . The specific temperature specifications for each type of ac motor and generator are set out in great detail in NEMA Standard MG 1-1993, Motors and Generators. Similar standards have been defined by the International Electrotechnical Commission (lEC) and by various national standards organizations in other countries.

Efficiency and voltage regulation

The efficiency of a machine is defined by the equation : Eff= (Pout/Pin )*100%

The Losses in machines that occur can be divided into four basic categories: .

Electrical or copper losses: are the resistive heating losses that occur in the stator (armature) and rotor (field) windings of the machine.

Core losses :are the hysteresis losses and eddy current losses occurring in the metal of the motor

Mechanical losses :are the losses associated with mechanical effects. There are two basic types of mechanical losses: friction and windage. Friction losses are losses caused by the friction of the bearings in the machine, while windage losses are caused by the friction between the moving parts of the machine and the air inside the motor's casing. These losses vary as the cube of the speed of rotation of the machine. The mechanical and core losses of a machine are often lumped together and called the no-load rotational loss of the machine. At no load, all the input power must be used to overcome these losses. Therefore, measuring the input power to the stator of an ac machine acting as a motor at no load will give an approximate value for these losses.

Stray load losses: are losses that cannot be placed in one of the previous categories. No matter how carefully losses are accounted for, some always escape inclusion in one of the above categories. All such losses are lumped into stray losses. For most machines, stray losses are taken by convention to be I percent of full load.

One of the most convenient techniques for accounting for power losses in a machine is the power-flow diagram.

mechanical power is output of the motor , and then the stray losses, mechanical losses, and core losses are subtracted. After they have been subtracted, the remaining power is ideally converted from mechanical to electrical form at the point labelled Pcon" The mechanical power that is converted is given by Pcon=(Torque)*(speed)

and the same amount of Mechanical power is produced. However, this is not the power that appears at the machine's terminals. Before the terminals are reached, the electrical copper losses must be subtracted.

Speed regulation

Motors are often compared to each other by using a figure of merit called speed regulation. Speed regulation (SR) is a measure of the ability of a motor to keep a constant shaft speed as load varies. It is defined by the equation

SR=((Nn,l-Nf,l)/Nf,l)*100%

It is a rough measure of the shape of a motor's torque-speed characteristic-a positive speed regulation means that a motor's speed drops with increasing load, and a negative speed regulation means a motor's speed increases with increasing load. The magnitude of the speed regulation tells approximately how steep the slope of the torque-speed curve is.

Classification

There are two major types of ac machines: synchronous machines and induction machines. The principal difference between the two types is that synchronous machines require a dc field current to be supplied to their rotors, while induction machines have the field current induced in their rotors by by transformer action

A synchronous motor is the same physical machine as a synchronous generator, except that the direction of real power flow is reversed

A synchronous motor has no net starting torque and so cannot start by itself. There are three main ways to start a synchronous motor:

Reduce the stator frequency to a safe starting level.

Use an external prime mover

Put amortisseur or damper windings on the motor to accelerate it to near synchronous speed before a direct current is applied to the field windings.

If damper windings are present on a motor, they will also increase the stability of the motor during load transients the behaviour of synchronous motors under varying conditions of load and field current as well as the question of power-factor correction with synchronous motors.

Synchronous motors supply power to loads that are basically constant-speed devices. They are usually connected to power systems very much larger than the individual motors, so the power systems appear as infinite buses to the motors. This means that the terminal voltage and the system frequency will be constant regardless of the amount of power drawn by the motor. The speed of rotation of the motor is locked to the rate of rotation of the magnetic fields. and the rate of rotation of the applied mechanical fields is locked to the applied electrical frequency. so the speed of the synchronous motor will be constant regardless of the load This fixed rate of rotation is given Nm=120Fe/P where Nm is the mechanical rate of rotation, Fse is the stator electrical frequency, and P is the number of poles in the motor.

The steady-state speed of the motor is constant from no load all the way up to the maximum torque that the motor can supply (called the pull-out torque)

The maximum or pull-out torque occurs when angle = 90°. Normal full-load torques are much less than that. however. In fact. the pull-out torque may typically be three times the full-load torque of the machine.

When the torque on the shaft of a synchronous motor exceeds the pull-out torque, the rotor can no longer remain locked to the stator and net magnetic fields. Instead. the rotor starts to slip behind them. As the rotor slows down. the stator magnetic field "laps" it repeatedly, and the direction of the induced torque in the rotor reverses with each pass. The resulting huge torque surges, first one way and then the other way, cause the whole motor to vibrate severely. The loss of synchronization after the pull-out torque is exceeded is known as slipping pole

The Effect of Load Changes on a Synchronous Motor

If a load is attached to the shaft of a synchronous motor, the motor will develop enough torque to keep the motor and its load turning at a synchronous speed and a synchronous motor operating initially with a leading power factor and if the load on the shaft of the motor is increased, the rotor will initially slow down. As it does, the torque angle becomes larger, and the induced torque increases. The increase in induced torque eventually speeds the rotor back up, and the motor again turns at synchronous speed but with a larger torque angle and effect directly on energy consumption by KWH.

A phasor diagram for a synchronous motor shows the relationships between various electrical quantities like voltage and current

The Effect of Field Current Changes on a Synchronous Motor

There is one other quantity on a synchronous motor that can be readily adjusted-its field current.

a synchronous motor initially operating at a lagging power factor. Now, increase its field current and see what happens to the motor. Note that an increase in field current increases the magnitude of EA but does not affect the real power supplied by the motor. The power supplied by the motor changes only when the shaft load torque changes. Since a change in IF does not affect the shaft speed N,JI' and since the load attached ( to the shaft is unchanged, the real power supplied is unchanged. Of course, VT is also constant, since it is kept constant by the power source supplying the motor. The distances proportional to power on the phasor diagram (EA sin (beta) and fA cos fJ) ( must therefore be constant. When the field current is increased, EA must increase, but it can only do so by sliding out along the line of constant power.

Notice that as the value of EA increases, the magnitude of the armature current IA first decreases and then increases again. At low EA' the armature current is lagging, and the motor is an inductive load. It is acting like an inductor-resistor combination, consuming reactive power Q. As the fie ld current is increased, the armature current eventually lines up with V￠. and the motor looks purely resistive. As the field current is increased further, the armature current becomes leading, and the motor becomes a capacitive load. It is now acting like a capacitor-resistor combination, consuming negative reactive power -Q or, alternatively, supplying reactive power Q to the system.

for the obvious reason that it is shaped like the letter V. There are several V curves drawn, corresponding to different real power levels. For each curve, the minimum armature current occurs at unity power factor, when only real power is being supplied to the motor. At any other point on the curve, some reactive power is being supplied to or by the motor as well. For field current less than the value giving minimum lA, the armature current is lagging, consuming Q. For field currents greater than the value giving the minimum lA' the armature current is leading, supplying Q to the power system as a capacitor would. Therefore, by controlling the field current of a synchronous motor, the reactive power supplied to or consumed by the power system can be controlled.

The Synchronous Motor and Power-Factor Correction

an infinite bus whose output is connected through a transmission line to an industrial plant at a distant point. The industrial plant shown consist of three loads. Two of the loads are induction motors with lagging power factors, and the third load is a synchronous motor with a variable power factor.

the ability to adjust the power factor of one or more loads in a power system can significantly affect the operating efficiency of the power system. The lower the power factor of a system, the greater the losses( in the power lines feeding it. Most loads on a typical power system are induction motors, so power systems are almost invariably lagging in power factor. Having one or more leading loads (overexcited synchronous motors) on the system can be useful for the following reasons:

A leading load can supply some reactive power Q for nearby lagging loads, instead of it coming from the generator. Since the reactive power does not have to travel over the long and fairly high-resistance transmission lines, the transmission line current is reduced and the power system losses are much lower.

Since the transmission lines carry less current, they can be smaller for a given rated power flow. A lower equipment current rating reduces the cost of a power system significantly.

In addition, requiring a synchronous motor to operate with a leading power factor means that the motor must be run overexcited. This mode of operation increases the motor's maximum torque and reduces the chance of accidentally exceeding the pull-out torque.

The use of synchronous motors or other equipment to increase the overall power factor of a power system is called power-factor correction. Since a synchronous motor can provide power-factor correction and lower power system costs, many loads that can accept a constant-speed motor (even though they do not necessarily need one) are driven by synchronous motors. Even though a synchronous motor may cost more than an induction motor on an individual basis, the ability to operate a synchronous motor at leading power factors for power-factor correction saves money for industrial plants. This results in the purchase and user of synchronous motors. Any synchronous motor that exists in a plant is run overexcited as a matter of course to achieve power-factor correction and to increase its pull-out torque.

However, mnning a synchronous motor overexcited requires a high field current and flux, which causes significant rotor heating. An operator must be careful not to overheat the field windings by exceeding the rated field current.

The Synchronous Capacitor or Synchronous Condenser

A synchronous motor purchased to drive a load can be operated overexcited to supply reactive power Q for a power system. In fact, at some times in the past a synchronous motor was purchased and run without a load, simply for power factor correction,

Today, conventional static capacitors are more economical to buy and use than synchronous capacitors. However, some synchronous capacitors may still be in use in older industrial plants.

Induction motor

An induction motor has the same physical stator as a synchronous machine, with a different rotor construction. A typical two-pole stator It looks (and is) the same as a synchronous machine stator. There are two different types of induction motor rotors which can be placed inside the stator. One is caned a cage rotor, while the other is called a wound rotor.

induction motors are singly excited machines, their power and torque relationships are considerably different from the relationships in the synchronous machines previously studied

The input power to an induction motor An is in the form of three-phase electric voltages and current . The first losses encountered in the machine are copper losses in the stator windings (the stator copper loss PSCL ). Then some amount of power is lost as hysteresis and eddy currents in the stator (Pcore ). The power remaining at this point is transferred to the rotor of the machine across the air gap between the stator and rotor. This power is called the air-gap power PAG of the ( machine. After the power is transferred to the rotor, some of it is lost as [ 2R losses (the rotor copper loss PRCL), and the rest is converted from electrical to mechanical form (PconJ. Finally, friction and windage losses PF&W and stray losses Pmisc are subtracted. The remaining power is the output of the motor Pout>

Because of the nature of core losses, where they are accounted for in the machine is somewhat arbitrary. The core losses of an induction motor come partially from the stator circuit and partially from the rotor circuit. Since an induction motor normally operates at a speed near synchronous speed, the relative motion of the magnetic fields over the rotor surface is quite slow, and the rotor core losses are very tiny compared to the stator core losses. Since the largest fraction of the core losses comes from the stator circuit, all the core losses are lumped together at that point on the diagram.

The higher the speed of an induction motor, the higher its friction, windage, and stray losses. On the other hand, the higher the speed of the motor (up to Nsync , the lower its core losses. Therefore, these three categories of losses are sometimes lumped together and called rotational losses. The total rotational losses of a motor are often considered to be constant with changing speed, since the component losses change in opposite directions with a change in speed.

the stator copper losses, the core losses, and the rotor copper losses can be found. The stator copper losses in the three phases are given by Pscl = (3I^2 R)

The core losses are given by Pcore losses = (3E^2*Gc1)

so the air-gap power can be found as

Pair-gap=Pin-Pscl-Pcore

The air-gap power can be consumed is in the resistor.

Therefore, the air-gap power can also be given by

Pair-gap=3I^2*R/s

The actual resistive losses in the rotor circuit are rotor copper losses given by the equation Prcl = 3I^2*R

After stator copper losses, core losses, and rotor copper losses are subtracted from the input power to the motor, the remaining power is converted from electrical to mechanical for111. This converted power, which is sometimes called developed mechanical power, is given by

Pconv=Pair-gap - Prcl

Pconv=3I^2*R((1-s)/s)

Therefore, the lower the slip of the motor, the lower the rotor losses in the machine.

Note also that if the rotor is not turning, the slip s = 1 and the air-gap power is entirely consumed in the rotor. This is logical, since if the rotor is not turning, the output power Pout=Torque*Speed must be zero.

The induced torque Tind in a machine was defined as the torque generated by the internal electric-ta-mechanical power conversion. This torque differs from the torque actually available at the terminals of the motor by an amount equal to the friction and windage torques in the machine. The induced torque is given by the equation

Tind=Pconv/N This torque is also caned the developed torque of the machine.

Separating the Rotor Copper Losses and the Power Converted in an Induction Motor's Equivalent Circuit Part of the power coming across the air gap in an induction motor is consumed in the rotor copper losses, and part of it is converted to mechanical power to drive ( the motor's shaft. It is possible to separate the two uses of the air-gap power and to indicate them separately on the motor

Induction Motor Design Classes

. Design class A motors are the standard motor design, with a normal starting torque, a normal starting current, and low slip

. Design class B motors have a normal starting torque, a lower starting current, and low slip. This motor produces about the same starting torque as the class A motor with about 25 percent less current.

DESIGN CLASS C. Design class C motors have a high starting torque with low starting current and low slip (less than 5 percent) at full load.

. Design class D motors have a very high starting torque (275 percent or more of the rated torque) and a low starting current, but they also have a high slip at full load. T

In addition to these four design classes, NEMA used to recognize design classes E and F, which were called soft-start induction motors

These designs were distinguished by having very low starting currents and were used for low-starting-torque loads in situations where starting currents were a problem. These designs are now obsolete.

A magnetic motor starter circuit of this sort has several built-in protective features:

Under voltage protection

Short-circuit protection

Overload protection

Drivers and speed control methods

An induction motor starting circuit with resistors to reduce the starting current flow there are additional components present to control removal of the starting resistor. Relays1TD. 2TD. and 3TD are so-called time-delay relays, meaning that when they are energized there is a set time

When the start button is pushed in this circuit, the M relay energizes and power is applied to the motor as before. Since the 1TD. 2TD. and 3TD contacts are all open, the full starting resistor is in series with the motor, reducing the starting current.

SPEED CONTROL OF INDUCTION MOTORS

Induction Motor Speed Control by Pole Changing

There are two major approaches to changing the number of poles in an induction motor:

1. The method of consequent poles

2. Multiple stator windings

Speed Control by Changing the Line Frequency

Speed Control by Changing the Line Voltage

Speed Control by Changing the Rotor Resistance

SOLID-STATE INDUCTION MOTOR DRIVES

the method of choice today for induction motor speed control is the solid-state variable-frequency induction motor drive

The drive is very flexible: its input power can be either single-phase or three-phase, either 50 or 60 Hz, and any where from 208 to 230 V. The output from this drive is a three-phase set of voltages whose frequency can be varied from 0 up to 120 Hz and whose voltage can be varied from 0 V up to the rated voltage of the motor. The output voltage and frequency control are achieved by using the pulse width modulation (PWM) techniques.' Both output frequency and output voltage can be controlled independently by pulse-width modulation. illustrates the manner in which the PWM drive can control the output frequency while maintaining a constant rms voltage level,.it is often desirable to vary the output frequency and output rms voltage together in a linear fashion

Frequency (Speed) Adjustment The output frequency of the drive can be controlled manually from a control mounted on the drive cabinet, or it can be controlled remotely by an external voltage or current signa1. The ability to adjust the frequency of the drive in response to some external signal is very important, since it permits an external computer or process controller to control the speed of the motor in accordance with the overall needs of the plant

A Choice of Voltage and Frequency Patterns The types of mechanical loads which might be attached to an induction motor vary greatly. Some loads such as fans require very little torque when starting (or running at low speeds) and have torques which increase as the square of the speed, Other loads might be harder to start, requiring more than the rated full-load torque of the motor just to get the load moving. This drive provides a variety of voltage versus-frequency patterns which can be selected to match the torque from the induction motor to the torque required by its load.

Independently Adjustable Acceleration and Deceleration Ramps

When the desired operating speed of the motor is changed, the drive controlling it will change frequency to bring the motor to the new operating speed. If the speed change is sudden (e.g., an instantaneous jump from 900 to 1200 r/min), the drive does not try to make the motor instantaneously jump from the old desired speed to the new desi red speed. Instead, the rate of motor acceleration or deceleration is limited to a safe level by special circuits built into the electronics of the drive. These rates can be adjusted independently for accelerations and decelerations.

DETERMINING CIRCUIT MODEL PARAMETERS

The equivalent circuit of an induction motor is a very useful tool for determining the motor's response to changes in load. However, if a model is to be used for a real machine, it is necessary to determine what the element values are that go into the model

the tests are:

The no-load test of an induction motor measures the rotational losses of the motor and provides information about its magnetization current.

The DC Test for Stator Resistance

The Locked-Rotor Test