Induction Motor Startup And Losses Calculation

Main Objectives

The main objectives while starting an induction motor are:

1. To handle high-starting current
2. To achieve high-starting torque.

As we know, rotor resistance determines starting torque. Usually, this rotor resistance is small, giving small starting torque, but good running conditions. So, the squirrel-cage motor can run only with low-starting loads.

If the rotor resistance is increased by some means, then the slip and speed at which maximum torque occurs can be shifted. For that purpose, external resistance can be introduced in the rotor circuit, which is done inthe case of slip ring or wound rotor type motors.

This happens due to the fact that there is a transformer action between the stator winding and the rotor winding, and the rotor conductors are short-circuited. This causes heavy current flow through the rotor. If, for reducing this heavy starting current, starting voltage applied is reduced then it affects the starting torque as well.

Losses Calculation

The following are the losses in an induction motor:

1. Core loss in the stator and the rotor
2. Stator and rotor copper losses
3. Friction and windage loss.

Core loss is due to the main and leakage fluxes. As the voltage is assumed constant, the core loss can also be approximated as a constant. DC can measure the stator resistance. The hysteresis and eddy current loss in the conductors increase the resistance, and the effective resistance is taken at 1.2 times the DC resistance.

The rotor copper loss is calculated by subtracting the stator copper loss from the total measured loss or the rotor I²R loss. The friction and windage loss may be assumed constant, irrespective of the load.

Example With Calculations

Consider a three-phase 440 V, 50 Hz, six-pole induction motor. The motor takes 50 kW at 960 rpm for a certain load. Assume stator losses of 1 kW and friction and windage loss of 1.5 kW.

To determine the percentage slip, rotor copper loss, rotor output, and efficiency of the motor, perform the following function:

Percentage slip

The synchronous speed of the motor = (50 ×120) / 6 = 6000 / 6 = 1000 rpm
Slip = (Synchronous speed – Actual speed) = 1000 – 960 = 40 rpm
Percentage slip = [(40 / 1000) × 100] = 4% = 0.04

Rotor copper loss

Rotor input = 50 1 = 49 kW
Rotor copper loss = Rotor input × Slip = 49 × 0.04 = 1.96 kW

Rotor output

Rotor output = Rotor input – Rotor copper loss – Friction and Windage loss
= 49 – 1.96 + 1.5
= 49 – 3.46
= 45.54 kW

Motor efficiency

Motor efficiency = Rotor output/Motor input
= 45.54 / 50 = 0.9108
= 91.08%

Century Electric Repulsion Start Induction Motor (VIDEO)

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Resource: Practical Troubleshooting of Electrical Equipment and Control Circuits – Mark Brown, Jawahar Rawtani and Dinesh Patil (Get it from Amazon)