# SSC JE 3-Phase Induction Motor Solved Question (2018-2009)

Ques.11. If the air gap in an induction motor is increased_____ (SSC-2018 Set-5)

1. The speed of the rotor increases
2. The power factor will decrease
3. The windage losses will increase
4. Current drawn by the motor increase

Answer.2. The power factor will decrease

Explanation:-

If the air gap of an induction motor is increased, the following will happen:

• The permeability of the magnetic circuit rotor-to-stator will decrease.
• The magnetizing inductance of the motor thus decreases.
• The magnetizing current will increase. This will cause a poorer power factor at all loads.
• The magnetic flux in the air gap will decrease and leakage fluxes will increase. This will cause a reduction in the maximum available torque.

In summary, the maximum available torque will decrease, the power factor will worsen and the motor will run with increased slip.

So it is always good performance-wise to run with as small an air gap as possible, which will reverse all of these effects. But if the air gap is too small, rotor cooling is compromised, and if the rotor expands through overheating (e.g. by exceeding the recommended maximum number of starts per hour) the rotor can “pole” by rubbing or jamming with the stator.

Ques.12. In three-phase induction motor reversing the supply in two of the phases is called as (SSC-2018 Set-5)

1. Pole changing
2. Plugging
3. increase the torque
4. To control current

Explanation:-

Plugging is also called reverse current braking. It is known that the rotor of a polyphase induction motor develops torque in the same direction as the rotating magnetic ﬁeld set up by the stator winding. Also if any two stator leads are reversed, the rotating magnetic ﬁeld is also reversed. lf therefore, the pair of stator leads ore reversed while a motor is rotating, torque is suddenly-produce opposite to the original direction of rotation. This reverse torque causes rotation in the opposite direction as soon as the motor stops, therefore provision must be made to disconnect stator completely from the supply lines when the motor stops.

On reversing the armature current the supply voltage to the armature and back emf developed in it are additive. So, this condition is worse than the starting condition as the applied voltage to the armature is approximately double (V+E = 2V) to the supply voltage.

Hence Plugging in induction motor braking is applied by just reversing the supply phase sequence by interchanging connections of any two phases of the stator, we can attain plugging braking of an induction motor. Due to the reversal of phase sequence, the direction of the rotating magnetic field gets reversed. This produces a torque in the reverse direction and the motor tries to rotate in opposite direction.

This opposite flux acts as the brake and it slows down the motor. During plugging the slip is (2 – s), if the original slip of the running motor is s.

Slip during motoring = s =(Ns – Nr)/Ns

or
Nr/Ns = (1 – s)

During Plugging

Slip =(-Ns – Nr)/-Ns = (Ns + Nr)/Ns = (2 – s)

Note: The method can be applied to both squirrel cage as well as wound rotor induction motors.

The disadvantages of this method are:

1. The kinetic energy of the motor is dissipated in the external resistance in the form of heat. So, this method of braking is inefficient.

2. The braking in this method fails in case of failure of the supply.

Ques.13. The availability of full -rated torque at starting is obtained from an induction motor is (SSC-2018 Set-6)

1. Rotor resistance control
2. Stator voltage control
3. Slip ring control
4. Line current control

Explanation:-

When the load torque is small, the speed control of an induction motor is obtained by variation of stator voltage. In the high torque range, in the case of a wound rotor motor, rotor resistance control is used. For a good dynamic response, external resistance in rotor circuits can be varied statically and steplessly by a high-frequency thyristor chopper circuit. The figure shows such a speed control method for an induction motor.

In the conventional rotor resistance control method, a three-phase variable resistance is inserted in series with the rotor winding as shown in Fig. This way the value of rotor resistance per phase can be controlled. This method is applicable for slip-ring induction motors because we can add external resistance to slip ring rotor and not to squirrel cage rotor. The characteristics such as starting torque and speed can be controlled by varying the rotor resistance per phase. Let us see the effect of the change in rotor resistance on the torque produced.

The torque develops in the 3−φ induction motor is given as

{T_{\max }} = \dfrac{{E_2^2}}{{2{X_2}}}

$T \propto \frac{{sE_2^2R_2′}}{{R_2^{2} + {{(s{X_2})}^2}}}$

Where

X2 = Rotor reactance per phase at standstill
R2 = Rotor resistance per phase at the standstill
E2 = Rotor induced E.M.F per phase on standstill condition

Now, external resistance in added in each phase of rotor through slip rings as shown in Fig.

Corresponding torque:-

$T \propto \frac{{sE_2^2R_2′}}{{R_2^{‘2} + {{(s{X_2})}^2}}}$

Similarly, the starting torque at slip = 1 for R2 and R’2 can be written as

${T_{st}} \propto \frac{{sE_2^2R{_2}}}{{R_2^{2} + {{(s{X_2})}^2}}}$

and

${T_{st}} \propto \frac{{sE_2^2R{‘_2}}}{{R_2^{‘2} + {{(s{X_2})}^2}}}$

Maximum starting torque is obtained when the slip is equal to the ratio between the rotor resistance (R2) and the rotor inductive reactance (X2).This slip is also known as slip at maximum torque, labeled as Sm.

${T_{\max }} = \dfrac{{E_2^2}}{{2{X_2}}}$

It can be observed that Tmax is independent of R2; hence whatever may be the rotor resistance, the maximum torque produced never changes, but the slip and speed at which it occurs depend on R2.

For R2

Sm = R2 ⁄ X2

For R’2

Sm = R’2 ⁄ X2

As R’2 > R2. then S’m > Sm . Due to this, we get a new torque-slip characteristic for rotor resistance R’2.

From the slip-torque characteristics of the induction motor shown in Fig.  it is clear that the starting torque T‘st with rotor resistance R’2 is greater than staring torque Ts, with R2. Therefore, staring torque can be improved by adding external rotor resistance. If now resistance in further added to the rotor to get resistance R”2 and so on, it can be seen that Tmax remains the same but the slip at which it occurs increases to S”m and so on. Similarly, starting torque also increases to T”st and so on as shown in Fig. If maximum torque Tmax is required at the start, then Sm = 1 as at start slip is always unity.

Sm = R2 ⁄ X= 1

R2 = X2 which is the condition for getting Tst = Tmax

Thus, by adding external resistance to the rotor until it becomes equal to X2, the maximum torque can be achieved at the start. It is represented by point A as shown in fig. During running conditions, the external resistance is removed in order to increase efficiency by reducing I2R losses. Hence such added resistance is cut off gradually and finally removed from the rotor circuit in the normal running condition of the motor. This method is specially designed to improve the starting torque at the time of staring itself.

• Reduce the starting current
• Increase the starting torque
• The power factor of the line is improved
• There are no harmonics in the line current
• Speed control is smooth and of the wide range.

1. Efficiency is reduced due to the wastage of slip energy in the rotor circuit.
2. If rotor resistances are not equal, unbalance in currents and voltages.

Ques.14. Why is the rotor skewed? (SSC-2017)

1. To prevent starting current
2. To provide stability
3. To reduce magnetic turn
4. To reduce rotor locking tendency

Answer.4. To reduce rotor locking tendency

Explanation:

Rotor conductors are skewed because of these two main reasons-

1. Primarily to prevent the cogging phenomenon. It is a phenomenon in which, if the rotor conductors are straight, there are chances of magnetic locking or strong coupling between rotor & stator.

2. To avoid crawling. Crawling is a phenomenon where harmonic components introduce oscillations in torque.

Ques.15. The speed of the motor for the frequency of 60 Hz and 4 poles motor is? (SSC-2017)

1. 3600 rpm
2. 1800 rpm
3. 1500 rpm
4. None of these

Explanation:

The speed of the motor is given as

Ns =120f/P

=120 x 60/4

= 1800 rpm

Ques.16. A 3 phase, 400-V, 50 Hz 4 pole induction motor is fed from a 3-phase supply and runs of 1425 rpm. The frequency of the rotor emf is (SSC-2017)

1. 2.5 Hz
2. 50 Hz
3. 48 Hz
4. Zero

Explanation:

Synchronous speed

Ns = 120f/P

= 120 x 50/ 4

= 1500 rpm

Rotor speed = 1425 rpm

Slip = (Ns – Nr)/Ns

= (1500 – 1425)/1500

= 0.05

∴ Frequency of rotor emf

= 0.05 x 50

= 2.5 Hz

Ques.17. In the case of traveling cranes, the motor preferred for boom hoist is ______  (SSC-2016)

1. Slip Ring Induction Motor
2. Squirrel cage induction motor
3. Synchronous Motor
4. Single-phase motor

Explanation:-

A hoist having a spar projecting from the mast to support and guide the load is called Boom Hoist.

For crane travel, trolley travel and boom hoist of traveling crane require high starting torque and constant speed in operation. Hence slip ring induction motor is preferred.

In SLIP RING induction motor the ends of the rotor windings are externally connected by a variable rheostat (resistance is varied in order to give it proper starting and running current). So more the resistance, more the torque. When we add resistance to the rotor the torque is high, the slip is high and the current is reduced.

With the correct value of (usually) resistance inserted in the rotor circuit, a near-unity relationship between torque and supply current at the start can be achieved, such as 100%full load torque (FLT), with 100% full load current (FLC) and 200% FLT with 200 percent FLC. This is comparable with the starting capability of the dc machine.

Not only high starting efficiency but also smoothly controlled acceleration historically gave the slip ring motor a great popularity for lift, hoist and crane applications.

Ques.18. The purpose of skewing of rotor slots in an induction motor is to________  (SSC-2016)

1. Reduce magnetic hum
2. Increase the distribution factor
3. Reduce the locking tendency of the rotor
4. Increase the air gap

Answer.3. Reduce the locking tendency of the rotor

Explanation:-

There are two primary sources of noise in polyphase induction motors: magnetically induced noise, called Magnetic noised and noise Induced by the flow of air, called Windage noised.

The level of noise of magnetic origin is variable because it depends on the design, the load, the speed, and the power supply. For low-speed machines, magnetic noise almost always prevails. It is generated by electromagnetic forces, which occur between the stator and the rotor. They produce vibrations of the machine, mainly the stator.

When the frequencies of the electromagnetic forces are close to the resonance frequencies of the stator, the vibrations and the noise are amplified. The magnetic noise of rotating machines can easily be distinguished from other noises by cutting off the electric supply: the magnetic noise is immediately stopped while aerodynamic and mechanical noises decrease slowly with the speed.

Rotor conductors are skewed because of these following main reasons

• Cogging:- Cogging is magnetic locking. When an induction motor refuses to start even if the full voltage is applied to it, this is called as cogging. This happens when the rotor slots and stator slots are the same in number or they are integer multiples of each other. due to this the opposite poles of the stator and rotor come in front of each other and get locked. Skewing the rotor bars prevents locking thus preventing Cogging.
• Skewing help the motor run more quietly because the magnetic fields are slightly skewed to offset alignment with the rotor field coils. This feature tends to reduce vibration or magnetic hum as the rotor speed changes slightly every time the conductor bars align with the rotor magnetic field. As the speed is the function of frequency so the induction motor is unable to attend the resonance frequency, therefore, the magnetic vibration is reduced.
• Skewing makes the rotor conductor longer with the reduced cross area. This increases the rotor conductor resistance hence starting performance and the torque of an induction motor is improved.
• Crawling:– Crawling is a phenomenon where harmonic components introduce oscillations in torque. With the bar skewed, the amount of the bar cutting the field line grows continuously and the next bar starts cutting the field lines as the first finishes. Due to this, we get Uniform Torque.

Ques.19. In motor circuit static frequency changers are used for __________  (SSC-2016)

1. Power factor improvement
2. Speed regulation
3. Improve cooling
4. Reversal of direction

Explanation:-

## Static Frequency Converter

An ac motor can also be used as a drive motor. Among the motors used are induction motor and synchronous motors. Both slip ring and squirrel cage induction motors are used. Squirrel cage motor has advantages over slip ring one in that it is robust and has the simple construction. When compared to a dc motor it has a large power/weight ratio. An ac motor is normally a constant speed machine and its speed control over a wide range in a smooth and step-less manner had been a problem.

It is well known that the speed of an ac motor can be varied if the supply frequency can be varied. The speed control can be smooth if a continuously variable frequency supply is available. Rotating machines have been used to provide a variable frequency supply. Similar to the conventional Ward-Leonard system for dc motor, an MG (Motor- Generator) set is used here to provide variable frequency supply. The system has the disadvantage of high initial cost, poor efficiency and it occupies plenty of space.

But the advent of thyristors and thyristor power converters has paved the way for static frequency conversion. The ability to achieve a voltage or current of controllable frequency using thyristors presents the possibility of a variable speed drive using an ac motor. The static frequency conversion has several advantages over rotating frequency conversion equipment. The ac motor can have precise speed control. However, when the supply frequency is varied, the voltage applied to the motor should also be simultaneously varied to avoid saturation. Static frequency conversion equipment can be controlled to, provide a variable voltage supply and the advantage being that they can be independently varied. The ac motor is operated at constant flux at every operating frequency.

The static frequency converter is a device that alters the frequency of the input signal according to the input set point. As the name specifies, it consists of solid-state switching devices which are either on or off according to the input control signal.

The following figure shows the essential elements of a static frequency converter. The 3-phase supply is rectified and filtered to produce a DC bus, which powers the inverter section of the static frequency converter. The inverter consists of three pairs of semiconductor switches (MOSFET, GTO, power transistor, IGBT, etc.) with associated diodes. Each pair of switches provides the power output for one phase of the motor. Each pair of semiconductor switches is driven by the control electronics to generate a high-frequency square wave carrier pulse waveform at each of the phase outputs.

Since the carrier is identical on all three phases, the net voltage appearing across any phase of the motor windings due to the carrier alone is zero. In order to drive the motor, the control electronics generate three low-frequency sine waves, 120 degrees apart, which modulate the carrier pulses to each pair of switches. The width of the positive and negative pulse within each carrier cycle is modulated according to the amplitude of the low-frequency sine waveform of that phase. As a result, the average voltage presented to the motor winding is approximately sinusoidal. The two other phases of the motor winding have similar average voltages spaced 120 degrees apart.

As static frequency converters operate with an output frequency from a few Hz up to about 100 Hz, they use a carrier frequency in the range of 2 kHz up to about 10 kHz. As power semiconductors improve, the trend is to increase carrier frequencies up to ultrasonic frequencies (> 18 kHz), which can lower losses in the motor since the current is more sinusoidal. The downside is higher switching losses in the inverter and potential for more radiated frequency noise.

### Types of static Frequency converters

There are three basic types of Static converters as follows:

• Cyclone converter

The static frequency conversion is achieved using single-stage conversion devices like cyclo- converters or two-stage conversion devices like dc-link converters. Using a cyclo-converter, a very good quality low-speed drive can be obtained having inherent regeneration and reverse rotation possibilities.

A dc link inverter uses two stages for frequency conversion. The ac is first rectified to dc which is inverted back to ac of desired frequency. The voltage control of the inverter is obtained externally by the control of the firing angle of the rectifier. Sometimes certain advantages are achieved by controlling the voltage in the inverter itself. In these cases, the inverters are voltage source inverters (VSI).

The advantages of static frequency conversion devices are as follows:

1. The installation costs are lowered. Space occupied is smaller compared to the conventional rotating devices. The alignment problems are not there.
2. The efficiency of conversion is improved which in turn reduces the running costs also. Periodic maintenance of rotating parts is not required.
3. The voltage and frequency can be independently varied so that the motor operates at constant flux. So it is completely independent of fluctuations in the ac supply voltage and frequency.
4. Static converters occupy less space and offer low noise compared to mechanical converters with heavy machinery.
5. It offers a wide variety of voltage and frequency control with great control due to its independence of supply voltage variations.
6. The output volts/hertz can be adjusted to suit the adjustable speed motors and a large starting torque can be provided if it is needed.

Ques.20. NEMA standards rate motors according to________  (SSC-2016)

1. Weight
2. Horsepower
3. Voltage
4. Frame

Explanation:-

• Standards agencies such as the National Electrical Manufacturers Association (NEMA) and International Electrotechnical Commission (IEC) specify certain information that describes the physical and operating characteristics of each type of motor design, which is listed on the motor’s nameplate.
• The purpose of standardizing this information is to ensure the correct interchangeability of motors between different motor manufacturers.
• Both NEMA and IEC motors and motor control components are common in the electrical industry today because the electrical equipment is manufactured in the global economy under both standards.
• NEMA rated motor power in horsepower, and IEC rated motor power in kilowatts.
• The rated voltage of a motor listed on the nameplate is called the terminal voltage because it is the actual voltage on the motor’s terminals at which the manufacturer designed the motor to operate.
• Nominal voltage is the design or configuration voltage of the electrical distribution system.
• Motor terminal voltages are rated slightly less than nominal system voltages to compensate for electrical distribution system voltage drops
• NEMA standard motors are designed to operate within a 10% voltage deviation tolerance from the nameplate rated voltage.
• The nameplate current rating of a motor is measured when the motor is loaded to its full rated horsepower.
• The nameplate current rating of a motor is measured when the motor is loaded to its full rated horsepower.
• The nameplate RPM rating of a motor is measured when the motor is loaded to its full rated horsepower.
• Motors below 1 horsepower are referred to as fractional horsepower motors, and motors 1 horsepower or more are called integral horsepower motors.
• The locked rotor kVA per horsepower multiplier is used to determine the maximum locked-rotor amperes a motor design can draw.
• The NEMA duty ratings for motors are intermittent, continuous duty, and special duty.

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