Induction Machine Speed Control

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Questions and Answers

When the rotor resistance increases, what happens to the motor speed?

The speed increases

The speed decreases (correct)

The motor stops

The speed remains constant

What is the condition for maximum torque at starting in a three-phase WRIM?

Slip equals zero

Slip equals one (correct)

Slip equals three

Slip equals two

What is the value of the resistance to be inserted in the rotor circuit per phase to provide maximum torque at starting?

0.07 ohm (correct)

0.08 ohm

0.06 ohm

0.05 ohm

What is the purpose of adding rotor resistance per phase externally?

To achieve 80% of the maximum torque at starting Signup and view all the answers

What is the type of cascading where the induced torque produced by the main motor acts in the same direction as the auxiliary motor?

Cumulative Cascading Signup and view all the answers

What is the effect of cumulative cascading on the number of poles?

The number of poles is the sum of the poles of the two motors Signup and view all the answers

What is the type of cascading where the induced torque produced by the main motor acts in the opposite direction to the auxiliary motor?

Differential Cascading Signup and view all the answers

What is the effect of differential cascading on the number of poles?

The number of poles is the difference of the poles of the two motors Signup and view all the answers

What is the speed of the main induction motor?

Ns1 = 120f/p1 Signup and view all the answers

What is the speed of the auxiliary induction motor?

Ns2 = 120f/p2 Signup and view all the answers

What is the expression for the actual speed of a 3-phase I.M?

$$N_r = (1 - s)N_s$$ Signup and view all the answers

What is the speed of the cascade connection of three-phase induction motors when cumulative cascading is done?

Ns = 120f/(p1 + p2) Signup and view all the answers

What is the formula for the synchronous speed of a 3-phase I.M?

$$N_s = 120 f / (2P)$$ Signup and view all the answers

What is the speed of the cascade connection of three-phase induction motors when differential cascading is done?

Ns = 120f/(p1 - p2) Signup and view all the answers

What happens to the motor speed if the induced torque is less than the load torque?

The motor speed will decrease. Signup and view all the answers

What is the method of speed control that involves changing the stator voltage?

Stator Voltage Control Signup and view all the answers

What is the rated voltage applied to the stator in the slip power recovery scheme?

Rated voltage is applied to the stator Signup and view all the answers

What is the purpose of the diode bridge rectifier in the slip power recovery scheme?

To rectify the AC voltage Signup and view all the answers

What is the expression for the induced torque in a 3-phase I.M?

$$T_{ind} = \frac{3V_{TH}^2}{R_{TH}^2 + (X_{TH} + X_2)^2}$$ Signup and view all the answers

What is the air-gap power that is not converted into mechanical power?

Slip power Signup and view all the answers

What are the two main methods of controlling the speed of an I.M?

Stator and rotor parameters Signup and view all the answers

What is the purpose of the slip power recovery scheme?

To recover the slip power from the motor Signup and view all the answers

What is the effect of increasing the stator voltage in a 3-phase I.M?

The motor speed will increase. Signup and view all the answers

What is the term for the difference between the synchronous speed and the actual speed of a 3-phase I.M?

Slip Signup and view all the answers

What is the purpose of connecting external variable resistors in speed control using external rotor resistance?

To vary the rotor resistance Signup and view all the answers

What is the effect of increasing the rotor resistance on the maximum torque occurrence?

It shifts to higher slip values Signup and view all the answers

What type of machine is required for rotor resistance control?

Slip-ring machine Signup and view all the answers

Why is the method of rotor resistance control not very efficient?

Due to power loss in the resistor Signup and view all the answers

What is the effect of increasing the external rotor resistance on the torque-slip characteristics?

The critical slip Sm increases Signup and view all the answers

What happens to the operating point of the induction machine when the rotor resistance is increased?

It shifts from point A to point B Signup and view all the answers

What type of load torque benefits from the speed control range of this method?

Both constant torque type and fan-type Signup and view all the answers

What is an advantage of using rotor resistance control for speed control?

Wide speed control range Signup and view all the answers

Study Notes

Speed Control Techniques of Induction Motors

There are several speed control techniques of induction motors, including:
- Stator Voltage Control
- Supply Frequency Control
- Volt/Hertz Control
- Stator Current Control
- Pole Changing Methods

Speed Control Using External Rotor Resistance

Speed control can be achieved by changing the rotor resistance of an induction motor
The maximum torque point shifts to higher slip values with an increase in rotor resistance, while retaining a constant torque
The starting torque increases with an increase in rotor resistance
The speed control range is more with this method

Drawbacks of Speed Control Using External Rotor Resistance

The method requires a slip-ring machine
The additional resistance and operation at high slips entail power loss in the resistor

Speed Control of WRIM using Cascade Connection

Cascade connection of two motors allows for speed control
The torque produced by the main and auxiliary motors can act in the same direction (Cumulative Cascading) or opposite directions (Differential Cascading)
Four different speeds can be extracted using cascade connection

Slip Power Recovery Scheme

The scheme is applied to WRIM only
Rated voltage is applied to the stator, and the rotor voltage is rectified and inverted to feed back to the supply
Slip power is recovered from the motor, and the speed is reduced

Actual Speed of a 3-Phase I.M

The actual speed of a 3-phase induction motor depends on two factors: synchronous speed (Ns) and slip (s)
Changing either of these factors can vary the speed of the motor

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Description

Learn about the relationship between rotor resistance and torque in induction machines, including how it affects maximum torque and slip values.