What happened to maximum torque and starting torque of slip ring induction motor on adding the rotor resistance?

This value of resistance is called Critical Resistance. When slip at starting becomes equal to slip corresponding to maximum torque. Thus by adding external resistance to rotor till it becomes equal to, the maximum torque can be achieved at start.

What is the starting torque of slip ring induction motor?

High starting torque compared to squirrel cage induction motors about 2.5 times full load torque. Pull out torque can be achieved at even zero speed. Squirrel cage induction motors take 6 to 7 times full load current but slip-ring induction motors take low starting current approx 2.5 to 3.5 times full load current.

How can we increase the starting torque of an induction motor?

There are three ways to improve the starting torque of Single phase induction Motor

1. Internal Rotor Resistance. The across the line starting torque at low speed can be increased by increasing the rotor resistance.
2. External Rotor Resistance.
3. Variable Voltage/Frequency.

What is the effect on the starting and maximum torque of an induction motor if a resistance is inserted in its rotor circuit?

The starting torque of an induction motor increases with increasing rotor resistance. However, we know that, with increasing rotor resistance, rotor current will reduce, and hence rotor flux will reduce. Starting torque is the torque produced by induction motor when it is started.

What effect does the slip ring starting resistance have on the starting torque and the starting current?

By increasing the resistance in the rotor via the slip rings, the pullout torque can be achieved at much lower speeds, allowing higher initial torque and lower starting current.

Why is high starting torque required?

Locked Rotor or Starting Torque The Locked Rotor Torque or Starting Torque is the torque an electrical motor develops when starting at zero speed. A high Starting Torque is more important for application or machines hard to start – like positive displacement pumps, cranes etc.

How do you increase the starting torque of a slip ring induction motor?

The starting torque of a slip ring induction motor can be…

1. Adding external resistance to rotor.
2. Adding external inductance to rotor.
3. Adding external capacitance to rotor.
4. Adding external RLC circuit to rotor.

What is the starting torque of squirrel cage induction motor?

The starting torque of a squirrel cage induction motor is (d) zero.

What is the effect of rotor resistance on maximum torque and slip corresponding to maximum torque?

The maximum torque is inversely proportional to rotor reactance. The maximum torque is independent of rotor resistance. The slip at which maximum torque occur depends upon rotor resistance, R2. So, by varying the rotor resistance, maximum torque can be obtained at any required slip.

What is the effect of rotor resistance on maximum torque and slip corresponding to maximum torque explain with neat diagram?

The maximum torque is independent of the rotor resistance. But the exact location of the maximum torque Ʈmax is dependent on it. The greater, the value of the R2, the greater is the value of the slip at which maximum torque occurs. As the rotor resistance increases, the pullout speed of the motor decreases.

Why the starting torque of induction motor is low?

It suffers from the disadvantage of a low starting torque. It is because the rotor bars are permanently short-circuited and it is not possible to add any external resistance to the rotor circuit to have a large starting torque.

How does rotor resistance affect the slip in induction motor?

In the most direct answer to the question, adding resistance increases the slip for maximum torque (more torque at lower speed) by driving more current into the rotor circuit at lower speed. If rotor resistance were zero (superconductor), the induction motor would operate synchronously, but would be very hard to start!

What is the function of three phase slip ring induction motor?

These are used in increasing the starting torque of the motor. Slips rings are connected to the external high resistance circuit. The addition of these rings along with the external high resistance increases the resistance and decreases the inductive reactance. The function of a three-phase slip ring induction motor is shown in the figure below.

How to increase the starting torque at low speed?

Internal Rotor Resistance – The across the line starting torque at low speed can be increased by increasing the rotor resistance. This will decrease the efficiency of the motor, and the heat on the rotor.

Can You Bridge the starting torque of a slip ring motor?

If money is tight and you don’t have funds to change the motor, you could always use the starting torque of the slip ring motor and bridge it out once moving. Complex and expensive to maintain but it is a solution. Also, if you require an induction motor to produce the same starting torque you will require a larger motor.