Induction Motor Speed Control MCQ Quiz in मल्याळम - Objective Question with Answer for Induction Motor Speed Control - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

(V / f) control or frequency control method: 

It is basically frequency control but to maintain B_max constant

The frequency variations should be done by keeping the (V / f) ratio constant.

Formula:

The synchronous speed of the induction motor in rpm is

 rpm 

NS ∝ f

NS = Synchronous speed

f = Frequency

P = No. of poles

Torque produced in the motor is

E2 = Rotor induced emf

R2 = Rotor resistance

X2 = Rotor reactance

s = slip      

The Torque - Speed characteristics:

From the above characteristics, Let motor runs at speed of N_s0 at a frequency of f_o with a slip of S_m0.

Let at frequency f₁ 0, the speed of the motor reduced with N_s1 and slip increased from s_m0 to s_m1.

Now at frequency f₂ > f0, the speed of the motor increases with the synchronous speed of Ns2, and slip decreases from s_m0 to sm2.

∴ the slip is inversely proportional to the frequency applied to the motor.

If frequency reduced then slip will increase. 

Concept:

• The most common method for controlling the speed of an induction motor is the constant V/F method (or variable voltage, variable frequency method).
• If the supply frequency is reduced while keeping the voltage the same, the air gap flux (the magnetic field in the motor) will tend to become too strong or "saturate".
• This saturation can cause a high current in the stator (the stationary part of the motor) and distort the magnetic field.
• To prevent this, the stator voltage should be reduced in proportion to the frequency to keep the air-gap flux constant.
• The strength of the stator's magnetic field is proportional to the ratio of the stator voltage and the frequency.
• Therefore, if the ratio of voltage to frequency (V/F) is kept constant, the magnetic field remains constant.
• By maintaining a constant V/F ratio, the torque (the turning force) that the motor produces also stays roughly constant.
• The constant V/F method results in higher run-time efficiency for the motor.
• Majority of AC (alternating current) speed drives use this method for speed control because of its benefits.
• Along with a wide range of speed control, the constant V/F method also provides a 'soft start' capability, allowing the motor to start up gradually rather than suddenly.

Given P1 = 4, P2 = 6

f2 = 1 Hz; f = 50 Hz

f2 = s2f1­, f1 = s1f

⇒ f2 = s1s2f ⇒ s1s2 = 1/50 = 0.02

For cumulative cascade

Speed of cascade set 

Stator voltage control method:

Consider the approximate torque equation

T ∝ 

Where,

s = slip ,V1 = supply voltage or stator voltage, R2 = rotor resistance

⇒ sV12 = k

⇒ s = 

And, Nr = Ns(1 - s)

• In this method, by keeping the frequency constant the applied voltage to an induction motor should be reduced below its rated value under load conditions to control the speed.
• If stator voltage is reduced by keeping the frequency constant, the slip of the induction motor increases to maintain the load torque constant.
• As slip increases the speed of the motor falls below its rated speed.
• By using this method speeds below the rated value can be achieved.
• During the stator voltage control method of speed control of the induction motor, the motor acts as a constant torque variable power drive.

Disadvantages:

• As we are reducing the voltage in this method, the induction motor draws high current at low voltages, which may overheat the stator winding.
• That's why this method is not suitable for long-duration speed control.

​Rotor resistance method of speed control:

T ∝ 

Where

Rext  = extenal resistance added

• In this method, some external resistance is added in series with the rotor winding under load conditions.
• Here voltage kept constant. To maintain load torque is constant so that slip increases.
• By inserting resistance, the slip of the induction machine increases for constant torque hence the speed reduces.
• By using this method also speeds below the rated value can be achieved.
• During the rotor resistance method of speed control of the induction motor, the motor acts as a constant torque variable power drive.

Disadvantages:

• Due to the presence of additional resistance, copper loss increases leads to a reduction in efficiency of the machines.
• As copper losses increasing heat rise will happen so this method is not suitable for long-duration speed control.

Torque-slip characteristics of ​a three-phase induction motor

Motoring Mode:

• From the curve, the torque and slip is that torque is directly proportional to slip in the low slip region, which is typical in motoring mode.
• This linear relationship holds until the motor reaches its rated speed, after which the torque starts decreasing at higher slips.
   

Generating Mode:

• In generating mode, a three-phase induction motor operates as an induction generator.
• This happens when the rotor of the induction motor is driven faster than the synchronous speed of the rotating magnetic field generated by the stator.
• When in generating mode, the motor converts mechanical energy (from an external prime mover, such as a turbine) into electrical energy, supplying power back to the grid or load.

Braking Mode:

• In braking mode, an induction motor decelerates and absorbs energy from the system, acting as a brake.
• This mode is used to quickly reduce the speed of the motor and is particularly useful in applications where controlled stopping is required.
• There are three common methods of braking in induction motors: regenerative braking, dynamic braking, and plugging.​

Variable Frequency Drives:

• Induction motors are designed for a specific voltage per frequency ratio (V/f).
• Voltage is the supply voltage to the motor, and frequency is the supply frequency.
• The V/f ratio is directly proportional to the amount of magnetic flux in the motor magnetic material (stator and rotor core laminations).
• The torque developed on the motor shaft is proportional to the strength of the rotating flux.
• The type and the amount of magnetic material used in motor construction are factors to define motor power rating.

Torque speed characteristics:

From the characteristics,

• The pullout torque is constant at all points below the rated speed, except at low frequencies.
• At low frequencies, the pullout torque is reduced because of the effect of stator resistance.
• As the frequency approaches zero, the voltage drop due to stator resistance becomes important.
• Flux reduction causes the torque reduction to become prominent.
• This effect is known and easily mitigated by low-speed voltage boosting: increasing the V/f ratio at low frequencies to restore the flux.

The typical set of torque-speed curves for a drive with low-speed voltage boosting:

Therefore, In V/f control of induction motors, the ratio of V/f is boosted during below 5 Hertz

The correct answer is option no "1".

Explanation:-

(V / f) control or frequency control method: 

It is frequency control but to maintain Bmax constant

The frequency variations should be done by keeping the (V / f) ratio constant.

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The Torque - Speed characteristics:

• From the above characteristics, Let motor runs at speed of Ns0 at a frequency of fo with a slip of Sm0.

Let at frequency f1 0, the speed of the motor reduced with Ns1 and slip increased from sm0 to sm1.

• Now at frequency f2 > f0, the speed of the motor increases with the synchronous speed of Ns2, and slip decreases from sm0 to sm2.

∴ the slip is inversely proportional to the frequency applied to the motor.

If frequency reduced then slip will increase., But the T_max (Maximum Torque) will remain constant.

The speed of the induction motor can be controlled by the following methods:

1) V/f control (or) frequency control

2) Changing the number of stator poles

3) Controlling supply voltage

4) Changing winding resistance by adding rheostat in the stator circuit

In AC locomotives, pole changing method of speed control is used for squirrel cage induction motors.

Important Points:

Rheostatic control is the simplest but the least efficient method of speed control of 3-phase induction motors. This method of speed control is employed in light locomotives and motor coaches where a single economical speed is sufficient and energy consump­tion is of no importance.

(V / f) control or frequency control method: 

It is basically frequency control but to maintain B_max constant

The frequency variations should be done by keeping the (V / f) ratio constant.

Formula:

The synchronous speed of the induction motor in rpm is

 rpm 

NS ∝ f

NS = Synchronous speed

f = Frequency

P = No. of poles

Torque produced in the motor is

E2 = Rotor induced emf

R2 = Rotor resistance

X2 = Rotor reactance

s = slip      

The Torque - Speed characteristics:

From the above characteristics, Let motor runs at speed of N_s0 at a frequency of f_o with a slip of S_m0.

Let at frequency f₁ 0, the speed of the motor reduced with N_s1 and slip increased from s_m0 to s_m1.

Now at frequency f₂ > f0, the speed of the motor increases with the synchronous speed of Ns2, and slip decreases from s_m0 to sm2.

∴ the slip is inversely proportional to the frequency applied to the motor.

If frequency reduced then slip will increase. 

Concept:

The slip of the induction motor is given by:

where, s = Slip

N_s = Speed of stator field

N_r = Rotor speed

Calculation:

The stator field is N_s and the rotor is rotating at the speed 'N_r' in the direction of the stator field.

Then stator field cuts the rotor conductors at the speed of 'N_s - N_r'.

Due to this a rotor field is induced which rotates at the speed of 'sN_s'

with respect to the rotor in its direction

Then, the speed of the rotor field with respect to the stator body is given by

= sN_s + N_r

= N_s - N_r + N_r

= Ns