GATE 2015 - Electrical Engineering

Electric Drives - AC Motors

Electric Drives - AC Motors

(Description and Applications)

AC Motors

Polyphase Induction Motors

One third of the world's electricity consumption is used for running induction motors driving pumps, fans, compressors, elevators and machinery of various types. The AC induction motor is a common form of asynchronous motor whose operation depends on three electromagnetic phenomena:

• Motor Action - When an iron rod (or other magnetic material) is suspended in a magnetic field so that it is free to rotate, it will align itself with the field. If the magnetic field is moving or rotating, the iron rod will move with the moving field so as to maintain alignment.
• Rotating Field - A rotating magnetic field can be created from fixed stator poles by driving each pole-pair from a different phase of the alternating current supply.
• Transformer Action - The current in the rotor windings is induced from the current in the stator windings, avoiding the need for a direct connection from the power source to the rotating windings.

The induction motor can be considered as an AC transformer with a rotating secondary winding.

• Rotating Fields
Rotating magnetic fields are created by polyphase excitation of the stator windings. In the example below of a 3 phase motor, as the current applied to the winding of pole pair A (phase 1) passes its peak and begins to fall, the flux associated with the winding also begins to weaken, but at the same time the current in the winding of the next pole pair B (phase 2) and its associated flux is rising. Simultaneously the current through the winding of the previous pole pair C ( phase 3) and its associated flux will be negative and rising (towards positive). The net effect is that a magnetic flux wave is set up as the flux created by the stator poles rotates from one pole to the next, about the axis of the machine, at the frequency of the applied voltage. In other words, the rotating flux field appears to the stator as the north and south poles of a magnet rotating about the stator.





The magnitude of the rotating flux wave is proportional applied MMF. Ignoring the effect of the back EMF set up by the induced currents in the rotor windings, the flux density B will be proportional to the applied voltage.

•  Transformer Action
The stator carries the motor primary windings and is connected to the power source. There are normally no external connections to the rotor which carries the secondary windings. Instead the rotor windings are shorted.
When a current flows in the stator windings a current is induced in the shorted secondary windings by transformer action. The magnitude of the rotor current will be proportional to the flux density B in the air gap (and the relative motion, called the slip, of the rotor with respect to the rotating field as we shall see below).
Torque is produced by the reaction between the induced rotor currents and the air-gap flux created by the stator currents.
Many rotor types are used. The most popular AC motors use "squirrel cage" rotors which are constructed from copper or aluminium bars fixed between conducting end rings which provide the short circuit path for the currents induced in the bars.





Since there are no connections to the rotating windings, the costly commutator can be eliminated and with it, a potential source of unreliability.






• Torque Generation (Motor Action)
When the motor is first switched on and the rotor is at rest, a current is induced in the rotor windings (conductors) by transformer action. Another way of seeing this is that the relative motion of the rotating flux passing over the slower moving (initially static) rotor windings causes a current to flow in the windings by generator action.
Once current is flowing in the rotor windings, the motor action due to the Lorentz force on the conductors comes into effect. The reaction between the current flowing in the rotor conductors and the magnetic flux in the air gap causes the rotor to rotate in the same direction as the rotating flux as if it was being dragged along by the flux wave.


Similar to the DC machine, the torque in an induction motor T is proportional to the flux density B and the induced rotor current I. Thus
T = k₁ BI
Where k₁ is a constant depending on the number of stator turns, the number of phases and the configuration of the magnetic circuit.


The rotor speed builds up due to the motor action described above, but as it does so, the relative motion between the rotating stator field and the rotating rotor conductors is reduced. This in turn reduces the generator action and thus the current in the rotor conductors and the torque on the rotor. As the speed of the rotor approaches the speed of the rotating field, known as the synchronous speed, the torque on the rotor drops to zero. Thus the speed of an induction motor can never reach the synchronous speed.


• Slip
The relative motion between the rotating field and the rotating rotor is called the slip and is given by:
S = N_s- N
         N_s
Where S is the slip, N_s is the synchronous speed in RPM, and N is the rotor speed.


Since the rotor current is proportional to the relative motion between the rotating field and the rotor speed, the rotor current and hence the torque are both directly proportional to the slip.


The rotor current is proportional to the rotor resistance. Increasing the rotor resistance will reduce the current and increase the slip; hence a form of speed and torque control is possible with wound rotor motors. Increased rotor resistance also has the added benefit of reducing the input surge current and increasing starting torque on switch on, but all of these benefits are at the expense of more complex rotor designs and unreliable slip rings to give access to the rotor windings.


• Speed
Synchronous speed in RPM is given by:
N_s = 120 (f)
           P
Where f is the powerline frequency in Hz and P is the number of poles per phase. P must be an even integer since for every north pole there is a corresponding south pole.
The following table shows motors speeds for motors with different numbers of poles working with different AC supply frequencies.
 

Rotor Speed (rpm)

Number of poles	2	4	6	8	10	12
Frequency 50 Hz	3000	1500	1000	750	600	500
Frequency 60 Hz	3600	1800	1200	900	720	600



The actual speed of the motor depends on the load it must drive. Increasing the load on the motor causes it to slow down increasing the slip. The motor speed will settle at an equilibrium speed when the motor torque equals the load torque. This occurs when the slip provides just enough current to deliver the required torque.


• Speed Control
• Pole Changing
Early machines were designed with multiple poles to facilitate speed control by pole changing. By switching in different numbers or combinations of poles a limited number of fixed speeds could be obtained.


• Variable Rotor Resistance
The speed of induction motors can however be varied over a limited range by varying the rotor resistance as noted in the section on slip but only by using wound rotor designs negating many of the advantages of the induction motor.


• Variable Frequency
Since motor speed depends on the speed of the rotating field, speed control can be effected by changing the frequency of the AC power supplied to the motor.


As in most machines the induction motor is designed to work with the flux density just below the saturation point over most of its operating range to achieve optimum efficiency.
The flux density B is given by:
B = k₂ V
         f
Where V is the applied voltage, f is the supply frequency and k₂ is a constant depending on the shape and configuration of the stator poles.
In other words if the flux density is constant, the Volts per Hertz is also a constant. This is an important relationship and it has the following consequences.
• For speed control, the supply voltage must increase in step with the frequency, otherwise the flux in the machine will deviate from the desired optimum operating point. Practical motor controllers based on frequency control must therefore have a means of simultaneously controlling the motor supply voltage. This is known as Volts/Hertz control.
• Increasing the frequency without increasing the voltage will cause a reduction of the flux in the magnetic circuit thus reducing the motor's output torque. The reduced motor torque will tend to increase the slip with respect to the new supply frequency. This in turn causes a greater current to flow in the stator, increasing the IR volt drop across the windings as well as the I²R copper losses in the windings. The result is a major drop in the motor efficiency. Increasing the frequency still further will ultimately cause the motor to stall.
• Increasing the voltage without increasing the frequency will cause the material in the magnetic circuit to saturate. Excessive current will flow giving rise to high heat dissipation due to I²R losses in the windings and high eddy current losses in the magnetic circuit and ultimately failure of the motor due to overheating. Increasing the voltage will not force the motor to exceed the synchronous speed because as it approaches the synchronous speed the torque drops to zero.
  
  The variable frequency is normally provided by an inverter. See more about Motor Controls


Note also that since the induced current in the rotor is proportional to the flux density and the flux density in turn is proportional to the line voltage, the torque, which depends on the product of the flux density and the rotor current, is proportional to the square of the line voltage V.


• Generator Action
If an induction motor is forced to run at speeds in excess of the synchronous speed, the load torque exceeds the machine torque and the slip is negative, reversing the rotor induced EMF and rotor current. In this situation the machine will act as a generator with energy being returned to the supply.
If the AC supply voltage to the stator excitation is simply removed, no generation is possible because there can be no induced current in the rotor.
• Regenerative braking
Thus in traction applications, regenerative braking is not possible below synchronous speed in a machine fed with a fixed frequency supply. If however the motor is fed by a variable frequency inverter then regenerative braking is possible by reducing the supply frequency so that the synchronous speed becomes less than the motor speed.
AC motors can be microprocessor controlled to a fine degree and can regenerate current down to almost a stop whereas DC regeneration fades quickly at low speeds.
• Dynamic Braking
Induction motors can be brought rapidly to a stop (and / or reversed) by reversing one pair of leads which has the effect of reversing the rotating wave. This is known as "plugging". The motor can also be stopped quickly by cutting the AC supply and feeding the stator windings instead with a DC (zero frequency) supply. With both of these methods, energy is not returned to the supply but is dissipated as heat in the motor. These techniques are known as dynamic braking.


• Starting
Three phase induction motors and some synchronous motors are not self starting but design modifications such as auxiliary or "damper" windings on the rotor are incorporated to overcome this problem.


Usually an induction motor draws 5 to 7 times its rated current during starting before the speed builds up and the current is modified by the back EMF. In wound rotor motors the starting current can be limited by increasing the resistance in series with the rotor windings.
In squirrel cage designs, electronic control systems are used to control the current to prevent damage to the motor or to its power supply.
Even with current control the motor can still overheat because, although the current can be limited, the speed build up is slower and the inrush current, though reduced, is maintained for a longer period.


• Power Factor
The current drawn by an induction motor has two components, the current in phase with the voltage which governs the power transfer to the load and the inductive component, representing the magnetising current in the magnetic circuit, which lags 90° behind the load current.
The power factor is defined as cosΦ where Φ is the net lag of the current behind the applied voltage due to the in phase and out of phase current components. The net power delivered to the load is VAcosΦ where V is the applied voltage, A is the current which flows.


Various methods of power factor correction are used to reduce the current lag in order to avoid losses due to poor power factor. The simplest is to connect a capacitor of suitable size across the motor terminals. Since the current through a capacitor leads the voltage, the effect of the capacitor is to counter-balance the inductive element in the motor canceling out the current lag.
Power factor correction can also be accomplished in the motor controller.

• Characteristics
One of the major advantages of the induction motor is that it does not need a commutator. Induction motors are therefore simple, robust, reliable, maintenance free and relatively low cost.
They are normally constant speed devices whose speed is proportional to the mains frequency.
Variable speed motors are also possible by using motor controllers which provide a variable frequency output.


• Applications
Three phase induction motors are used wherever the application depends on AC power from the national grid. Because they don't need commutators they are particularly suitable for high power applications.
They are available with power handling capacities ranging from a few Watts to more than 10 MegaWatts.
They are mainly used for heavy industrial applications and for machine tools.
The availability of solid state inverters in recent years means that induction motors can now be run from a DC source. They are now finding use in automotive applications for electric and hybrid electric vehicles. Induction motors are seen as more rugged for these applications than permanent magnet motors which are vulnerable to possible degradation or demagnetization of the magnets due to over-temperature or accidental over-current at power levels over about 5 kW. Nevertheless, the induction motor can be ill-suited for some automotive applications because of the difficulties associated with extracting heat from the rotor, efficiency problems over wide speed and power ranges, and a more expensive manufacturing process due to distributed windings. Permanent magnet and reluctance motors may offer better solutions for these applications.

Wound Rotor Induction Motor

Now of historic interest only, these motors were designed to permit control of the speed - torque characteristics of the machine. They used conventional windings on the rotor which were accessible through slip rings. The rotor windings were not connected to the supply line but current through the windings could be controlled by external rheostats connected in series with the windings. Modern electronic controls have made these designs obsolete.

Single Phase Induction Motors

At first sight it might be assumed that it would be impossible to create a rotating field using only a single phase supply. With the aid of an auxiliary stator winding displaced from the main winding it is however possible to create a second MagnetoMotive Force (MMF) in the auxiliary winding, out of phase with the MMF in the main winding, and this is sufficient to generate the rotating field.

• Capacitor-Run Motors
The necessary phase difference between the main and auxiliary windings can be provided by connecting a high value capacitor in series with the auxiliary winding. These motors are commonly used in household washing machines, refrigerators and shower pumps and can easily be identified by the large electrolytic capacitor strapped to the motor body.
As an alternative to using an external capacitor, the split-phase method uses a high resistance auxiliary winding. The difference in the impedance of the two windings is sufficient to create the necessary phase difference between the currents in the two windings.
• Shaded Pole Motors
The shaded pole motor uses another, rather crude, method of inducing a second stator MMF, out of phase with the main MMF in order to create the desired rotating field from a single phase AC supply. A short circuited turn of thick copper, known as the shading ring, is mounted in a slot in the pole piece. Some of the magnetic flux produced by the main winding induces a current in the shading ring which produces its own weak flux which opposes and retards the main flux through the ring so that the resulting flux through the ring is out of phase with the main flux. Thus there is a phase difference between one side of the pole and the other. Though inefficient, this method is once more sufficient to set up a rotating field.

• Characteristics
Single phase induction motors are less efficient than polyphase machines and were developed mainly for domestic use since most dwellings are only supplied with single phase power.
No control of speed.
• Applications
All kinds of household appliances and light industrial applications.

Synchronous AC Motors

The synchronous motor is similar to the induction motor in that it is a polyphase machine in which the stator produces a rotating field, however the rotor is constructed from either permanent magnets or electromagnets energised by direct current supplied through slip rings. 

• Torque
The torque depends on the attraction of the rotor magnets to the rotating magnetic poles and not on relative motion between the windings in the rotor and the rotating magnetic field. It can therefore lock on to the rotating field. See Alternative Motor Action. Unlike squirrel cage induction motors, synchronous motors can run and produce torque at synchronous speed.
They are difficult to start on mains frequency because the rotating field is too fast so they need to start at lower frequency or they need unexcited auxiliary windings or a rudimentary squirrel cage to bring the rotor up to synchronous speed. As the motor approaches synchronous speed it will suddenly snap in to synchronisation.
• Pull In Torque
To achieve synchronisation the motor torque must be greater then the load torque. The torque developed when the motor locks on to synchronous speed is called the pull in torque. If the load is greater than the pull in torque the motor will not reach synchronous speed.
• Pull Out Torque
As the load on the motor is increased, the motor torque and the torque angle will also increase. However if the torque angle exceeds 90 degrees the torque will begin to fall and the motor will lose synchronisation and eventually stop. The pull out torque is typically 1.5 times the continuously rated torque.


• Characteristics
Synchronous operation.
• Applications
Fixed speed applications such as clocks and timers

Synchronous Reluctance Motors

The operating principle of the basic reluctance motors is described in the section about switched reluctance motors.

The so called "synchronous" reluctance motor is designed to run on mains frequency alternating current and it uses distributed stator windings similar to those used in squirrel cage induction motors. The rotor however needs salient poles to create a variable reluctance in the motor's magnetic circuit which depends on the angular position of the rotor. These salient poles can be created by milling axial slots along the length of a squirrel cage rotor. See diagram below.

• Characteristics
The synchronous reluctance motor is not self starting without the squirrel cage. During run up it behaves as an induction motor but as it approaches synchronous speed, the reluctance torque takes over and the motor locks into synchronous speed.
• Applications
Used where regulated speed control is required in applications suc as metering pumps and industrial process equipment.

Hysteresis Motor

The Hysteresis Synchronous motor consists of a wound stator producing a rotating field and a rotor in the form of a cylindrical shell with crossbars all made from hard steel with relatively high magnetic hysteresis.

At start up, the combined effects of eddy currents in the steel causing induction motor action and remanent magnetism in the steel causing the magnetic poles to follow the rotating field, together cause the motor speed to build up. As the motor approaches synchronous speed the magnetic effect of the crossbars behaving like a permanent magnet causes the motor to lock on to synchronous speed. The net result is that the torque is roughly constant at all speeds.

• Characteristics
Simple design
Starts as an induction motor and locks in as a synchronous motor.
Having a smooth rotor of homogenous material, the noise and vibration produced is inherently low. Since there are no pole faces or saliencies, the magnetic path is of constant permeability, thus eliminating the magnetic pulsations which are the major cause of noise in the salient pole type.
• Applications
Their efficiency is low, and applications are restricted to small power ratings.
Used extensively in tape recorders and clocks.
Now mostly replaced by permanent magnet motors.

Universal Motors

An AC motor which uses separately excited rotor windings using a commutator to feed current to rotor coils behaves in much the same way as a brushed DC motor and can in fact be used as a universal motor taking its supply either from an AC or a DC source.

Unlike induction and synchronous motors, the speed of universal motors is not limited by the electric mains supply frequency and can easily exceed one revolution per cycle. This makes them useful for household appliances such as blenders, vacuum cleaners and hair dryers which need high-speed operation. Speeds of up to 30,000 RPM are possible but the current carrying capacity is limited by the commutator and brushes which restricts their use to low power applications of about 1 kilowatt or less. 

SCR Control circuits

SCR Control circuits

90° Phase Control of SCR.

Phase control of SCR

In ac circuits, the SCR can be turned on by the gate at any angle a with respect to the applied voltage. This angle α is called the firing angle. Power control is obtained by varying the firing angle and this is known as phase control. In the phase-control circuit given in fig. 1, the gate triggering voltage is derived from the ac supply through resistors R₁, R₂ and R₃. The variable resistance R₂ limits the gate current during positive half cycles of the supply. If the moving contact is set to the top of resistor R₂, resistance in the circuit is the lowest and the SCR may trigger almost immediately at the commencement of the positive half cycle of the input. If, on the other hand, the moving contact is set to the bottom of resistor R₂, resistance in the circuit is maximum, the SCR may not switch on until the peak of the positive half-cycle. By adjust­ing R₂ between these two extremes, SCR can be switched on somewhere between the commencement and peak of the positive half-cycle, that is between 0° and 90°. If the trigger­ing voltage V_T is not large enough to trigger SCR at 90°, the device will not trigger on at all, because V_T has the maximum value at the peak of the input and decreases with the fall in voltage. This operation is sometimes referred to as half-wave variable-resistance phase control. It is an effective method of controlling the load power.

Diode D is provided to protect the SCR gate from the negative voltage that would otherwise be applied during the negative half cycle of the input. It can be seen from the circuit diagram shown in fig.a, that at the instant of turning on of the SCR gate current flows through R_L and diode. So

V_T=V_D + V_G + I_GR_L

180 degree Phase Control.

Phase Control-SCR

The circuit shown in figure, can trigger the SCR from 0° to 180° of the input waveform. In the circuit shown here, the resistor R and capacitor C determine the point in the input cycle at which the SCR triggers. During the negative half cycle of the input, capacitor C is charged negatively (with the polarity shown in the figure) through diode D₂ to the peak of the input voltage because diode D₂ is forward-biased. When the peak of the input negative half cycle is passed, diode D₂ gets reverse-biased and capacitor C commences to discharge through resistor R. Depending upon the time constant, that is CR, the capacitor C may be almost completely discharged at the commencement of the positive half cycle of the input, or it may retain a partially negative charge until almost 180° of positive half cycle has passed. So long as the capacitor C remains negatively charged, diode D₁, is reverse-biased and the gate cannot go positive to trigger the SCR into conduction. Thus R and /or C can be adjusted to affect SCR triggering anywhere from 0° to 180° of the input ac cycle.

Pulse Control of an SCR.

Pulse Control Circuit

The simplest of SCR control circuits is shown in figure. If SCR were an ordinary rectifier, it would develop half-wave rectified ac voltage across the load R_L. The same would be true if the gate of the SCR had a continuous bias voltage to keep it on when the anode-cathode voltage V_AK goes positive. A trigger pulse applied to the gate can switch the device at any time during the positive half-cycle of the input. The resultant load waveform is a portion of positive half cycle commencing at the instant at which the SCR is triggered. Resistor R_G holds the gate-cathode voltage, V_G at zero when no trigger input is present. The instantaneous level of load current can/be determined from the following relation

SCR Applications

SCR Applications

The ability of an SCR to control large currents to a load by means of small gate current makes the device very useful in switching and control applications. A few of the possible applications for the SCR are listed in theintroduction to SCR blog post.

Here we will consider six applications of SCR like power control, switching, zero-voltage switching, over-voltage protection, pulse circuits and battery charging regulator.

1. Power Control.

SCR Power Control Circuit

Because of the bistable characteristics of semiconductor devices, whereby they can be switched on and off, and the efficiency of gate control to trigger such devices, the SCRs are ideally suited for many industrial applications. SCRs have got specific advantages over saturable core reactors and gas tubes owing to their compactness, reliability, low losses, and speedy turn-on and turn-off.

The bistable states (conducting and non-conducting) of the SCR and the property that enables fast transition from one state to the other are made use of in the control of power in both ac and dc circuits.

SCR Phase Control

In ac circuits the SCR can be turned-on by the gate at any angle α with respect to applied voltage. This angle α is called the firing angle and power control is obtained by varying the firing angle. This is known asphase control. A simple half-wave circuit is shown in figure a. for illustrating the principle of phase control for an inductive load. The load current, load voltage and supply voltage waveforms are shown in figure b. TheSCR will turn-off by natural commutation when the current becomes zero. Angle β is known as the conduction angle. By varying the firing angle a, the rms value of the load voltage can be varied. The power consumed by the load decreases with the increase in firing angle a. The reactive power input from the supply increases with the increase in firing angle. The load current wave-form can be improved by connecting a free-wheeling diode D₁, as shown by the dotted line in fig-a. With this diode, SCR will be turned-off as soon as the input voltage polarity reverses. After that, the load current will free wheel through the diode and a reverse voltage will appear across the SCR. The main advantage of phase control is that the load current passes through a natural zero point during every half cycle. So, the device turns-off by itself at the end of every conducting period and no other commutating circuit is required.

Power control in dc circuits is obtained by varying the duration of on-time and off-time of the device and such a mode of operation is called on-off control or chopper control. Another important application of SCRs is ininverters, used for converting dc into ac. The input frequency is related to the triggering frequency of SCRs in the inverters. Thus, variable frequency supply can be easily obtained and used for speed control of ac motors, induction heating, electrolytic cleaning, fluorescent lighting and several other applications. Because of the large power-handling capacity of the SCRs, the SCR controlled inverter has more or less replaced motor-generator sets and magnetic frequency multipliers for gener­ating high frequency at large power ratings.

Operation of Power Control in SCR

A commonly used circuit for controlling power in load R_L using two SCRs is shown in figure. Potentiometer R controls the angle of conduction of the two SCRs. The greater the resistance of the pot, lesser will be the voltage across capacitors C₁ and C₂ and hence smaller will be the time duration of conduction of SCR₁ and SCR₂ during a cycle.

During positive half cycle capacitor C₂ gets charged through diode D₁, pot R, and diode D₄. When the capacitor gets fully charged, (charge on the capacitor depending upon the value of R) it discharges through Zener diode Z. This gives a pulse to the primary and thereby secondary of the transformer T₂. Thus SCR₂, which is forward biased, is turned on and conducts through load R_L. During negative half cycle similar action takes place due to charging of capacitor C₁ and SCR₁ is triggered. Thus power to a load is controlled by using SCRs.

2. Switching.

Thyristor, being bistable device is widely used for switching of power signals owing to their long life, high operation speed and freedom from other defects associated with mechanical and electro-mechanical switches.

AC Circuit Breaker using SCR

Figure shows a circuit in which two SCRs are used for making and breaking an ac circuit. The input voltage is alternating and the trigger pulses are applied to the gates of SCRs through the control switch S. Resistance R is provided in the gate circuit to limit the gate current while resistors R₁ and R₂ are to protect the diodes D₁and D₂ respectively.

For starting the circuit, when switch S is closed, SCR₁ will fire at the beginning of the positive half-cycle (the gate trigger current is assumed to be very small) because during positive half cycle SCR₁ is forward biased. It will turn-off when the current goes through the zero value. As soon as SCR₁ is turned-off, SCR₂ will fire since the voltage polarity is already reversed and it gets the proper gate current. The circuit can be broken by opening the switch S. Opening of gate circuit poses no problem, as current through this switch is small. As no further gate signal will be applied to the SCRs when switch S is open, the SCRs will not be triggered and the load current will be zero. The maximum time delay for breaking the circuit is one half-cycle.

Thus several hundred amperes of load current can be switched on/off simply by han­dling gate current of few mA by an ordinary switch. The above circuit is also called the static contactor because it does not have any moving part.

DC circuit breaker

SCR Application-DC Circuit Breaker

As shown in figure, Capacitor C pro­vides the required commutation of the main SCR since the cur­rent does not have a natural zero value in a dc circuit. When the SCR₁ is in conducting state, the load voltage will be equal to the supply voltage and the capacitor C will be charged through resis­tor R. The circuit is broken by turning-off SCR₁. This is done by firing SCR₂, called the auxiliary SCR. Capacitor C discharges through SCR₂and SCR₁. This discharge current is in opposite direction to that flowing through SCR₁ and when the two become equal SCR₂ turns-off. Now capacitor C gets charged through the load and when the capacitor C gets fully charged, the SCR₂ tums-off. Thus the circuit acts as a dc circuit breaker. The resistor R is taken of such a value that current through R is lower than that of holding current.

3. Zero Voltage Switching.

SCR Switching Application

In some ac circuits it is necessary to apply the voltage to the load when the instantaneous value of this voltage is going through the zero value. This is to avoid a high rate of increase of current in case of purely resistive loads such as lighting and furnace loads, and thereby reduce the generation of radio noise and hot-spot temperatures in the device carrying the load current. The circuit to achieve this is shown in figure. Only half-wave control is used here. The portion of the circuit shown by the dotted lines relates to the negative half cycle. Whatever may be the instant of time when switch S is opened (either during the positive or the nega­tive half cycle), only at the begin­ning of the following positive half-cycle of the applied voltage SCR₁ will be triggered. Similarly, when switch S is closed, SCR₁ will stop conducting at the end of the present or previous positive half-cycle and will not get triggered again. Resis­tors R₃ and R₄ are designed on the basis of minimum base and gate currents required for transistor Q₁ and SCR₁. Resistors R_l and R₂ govern rates of the charging and discharging of capacitor C₁ Resis­tor R₅ is used for preventing large discharge currents when switch S is closed.

4. Over-Voltage Protection.

Over Voltage Circuit Protection

SCRs can be employed for protecting other equipment from over-voltages owing to their fast switching action. The SCR employed for protection is connected in parallel with the load. Whenever the voltage exceeds a specified limit, the gate of the SCR will get energized and trigger the SCR. A large current will be drawn from the sup­ply mains and voltage across the load will be reduced. Two SCRs are used—one for the positive half-cycle and the other for negative half-cycle, as shown in figure. Resistor R₁ limits the short-circuit current when the SCRs are fired. Zener diode D₅ in series with resistors R_x and R₂ consti­tutes a voltage-sensing circuit.

5. Pulse Circuits.

SCR-Pulse Circuit

SCRs are used for producing high voltage/current pulses of desired waveform and duration. The capacitor C is charged during the positive half cycle of the input supply and the SCR is triggered during the nega­tive half-cycle. The capacitor will discharge through the output circuit, and when the SCR forward currentbecomes zero, it will turn-off. The output circuit is designed to have discharge current of less than a milli-second duration. The capacitor will again get charged in the following positive half-cycle and the SCR will be triggered again in the negative half-cycle. Thus the frequency of the output pulse will be equal to the frequency of the input supply. For limiting the charging current resistor R is used. High voltage/current pulses can be used in spot welding, electronic ignition in automo­biles, generation of large magnetic fields of short duration, and in testing of insulation.

6. Battery Charging Regulator.

Battery Charging Regulator

The basic components of the circuits are shown in figure. Diodes D₁ and D₂ are to establish a full-wave rectified sig­nal across SCR₁ and the 12 V battery to be charged. When the battery is in dis­charged condition, SCR₂ is in the off-state as will be clear   after discussion. When the full-wave rectified input is large enough to give the required turn-on gate current (controlled by resistor R₁), SCR₁ will turn on and the charging of the battery will commence. At the commencement of charging of battery, voltage V_R determined by the simple volt­age-divider circuit is too small to cause 11.0 V zener conduction. In the off-state Zener diode is effectively an open-circuit maintaining SCR₂ in the off-state because of zero gate current. The capacitor C is included in the circuit to prevent any voltage transients in the circuit from accidentally turning on of the SCR₂. As charging continues, the battery voltage increases to a point when V_R is large enough to both turn on the 11.0 V Zener diode and fire SCR₂. Once SCR₂ has fired, the short circuit representation for SCR₂ will result in a voltage-divider circuit determined by R₁ and R₂ that will maintain V₂ at a level too small to turn SCR₁ on. When this occurs, the battery is fully charged and the open-circuit state of SCR₁ will cut off the charging current. Thus the regulator charges the battery whenever the voltage drops and prevents overcharging when fully charged. There are many more applications of SCRs such as in soft start circuits, logic and digital circuits, but it is not possible to discuss all these here.

Synchronous motors Advantages and Disadvantages

Synchronous motors Advantages Disadvantages

In industries induction motors are employed mostly because of less cost, rugged construction, good starting torques and very less maintenance. Synchronous motors are rarely used in industries for drive applications. They are generally used as power factor correction device. In industries they are employed to improve the power factor of the system. Some of the advantages and disadvantages related to synchronous motors are explained below: 

Advantage or Merits:

• One of the major advantage of using synchronous motor is the ability to control the power factor. An over excited synchronous motor can have leading power factor and can be operated in parallel to induction motors and other lagging power factor loads thereby improving the system power factor. 
• In synchronous motor the speed remains constant irrespective of the loads. This characteristics helps in industrial drives where constant speed is required irrespective of the load it is driving. It also useful when the motor is required to drive another alternator to supply at a different frequency as in frequency changes
• Synchronous motors can be constructed with wider air gaps than induction motors which makes these motors mechanically more stable
• In synchronous motors electro-magnetic power varies linearly with the voltage
• Synchronous motors usually operate with higher efficiencies ( more than 90%) especially in low speed and unity power factor applications compared to induction motors

  Disadvantages or Demerits:

• Synchronous motors requires dc excitation which must be supplied from external sources
• Synchronous motors are inherently not self starting motors and needs some arrangement for its starting and synchronizing
• The cost per kW output is generally higher than that of induction motors
• These motors cannot be used for variable speed applications as there is no possibility of speed adjustment unless the incoming supply frequency is adjusted (Variable Frequency Drives)
• Synchronous motors cannot be started on load. Its starting torque is zero
• These motors have tendency to hunt
• When loading on the synchronous motor increases beyond its capability, the synchronism between rotor and stator rotating magnetic field is lost and motor comes to halt 
• Collector rings and brushes are required resulting in increase in maintenance
• Synchronous motors cannot be useful for applications requiring frequent starting or high starting torques required

Engineering Services (IES) Questions

Engineering Services (IES) Questions and Answers:Machines 2

Indian Engineering Services Objective Questions and Answers

1) Match the Following   ( IES- 2003)

      List I                                        List II 

A. Transformer                           1. Slip Test

B. DC Motor                               2. Blocked Rotor Test

C. Alternator                               3. Sumpner's Test

D. Induction Motor                      4. Swinburne's Test

Codes:

         A          B          C         D

a)      3           4          1         2

b)      4           3          2         1

c)      3           4          2         1

d)      4           3          1         2

2) Possible three to three phase transformer connection for parallel operation is: ( IES- 2002)

a) Delta-Star to Delta-Star

b) Delta-Delta to Delta-Star

c) Star-Star to Delta-Star

d) Delta-Star to Star-Delta

3) Stepper Motors are widely used for  ( IES- 2002)

a) Very high power requirement

b) Very high speed operation

c) Very low speed operation

d) Control system applications

4) Armature torque of a d.c motor is a function of which of the following factors ( IES- 2003)

1. Speed

2. Field Flux

3. Armature Current

4. Residual Magnetism

Select the correct answer

a) 2 and 3

b) 1 and 4

c) 3 and 4

d) 1 and 2

5) The dummy coils in the DC machine are useful to

a) Increase the efficiency of the machine

b) Improve the commutation

c) Reduce the armature reaction

d) Maintain mechanical balance

6) The synchronous reactance is the ( IES- 2003)

a) Reactance due to armature reaction of the machine

b) Reactance due to the leakage flux

c) Combined reactance due to leakage flux and armature reaction

d) Reactance either due to armature reaction or leakage flux

7) The crawling in the induction motor is due to  ( IES- 2003)

a) Improper design of the stator laminations

b) Low voltage supply

c) Improper design of rotor laminations

d) Harmonics developed in the motor

8) In an Induction motor, when the number of stator slots is equal to an integral multiple of rotor slots  ( IES- 2003)

a) There must be a discontinuity in the torque slip charcteristics

b) A high starting torque will be available

c) The maximum torque will be high

d) The machine may fail to start

Answers:

(1) a (2) a (3) d (4) a

(5) d (6) c (7) d (8) d