Measurement of slip of induction motor using Galvanometer

Hello there, friends. In this section, I will explain how to measure a slip using rotor and stator frequencies(50 Hz or 60 Hz). Here the proposed method is the Galvanometer method. Explained with an example problem.

Rotor- Rotating part of the electrical motor.

Stator- static part of the electrical motor.

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.Measuring the slip of an induction motor by differentiating the supply frequencies of the rotor and frequency of stator can be done by the method of the galvanometer.

The formula for the slip

As I have stated that this method can be attained using the concept of stator and rotor's supply frequency differentiation. 

S= fr/f

fr  =  Rotor Frequency

f   =  Frequency

Definition of Slip

The slip of the motor can be defined as the ratio of the stator to the rotor supply frequency.

Required things

For measuring the slip of motor using this method, we require the following things

• Galvanometer
• Connecting wires

What is a galvanometer?

A galvanometer is a measuring device connected suitably to the circuit to measure the magnitude and direction of the current. By using a galvanometer, we can measure a very small amount of current up to microamps (uA).

Working principle of Galvanometer

The working principle of moving coil galvanometer is due to Magnetism and the Magnetic effect of electric current.

How does galvanometer help in slip Measurement?

Slip measurement with a galvanometer is very simple. Slip can be measured by following some of the constant rules.

Consider an electrical motor, for instance, an induction motor. Now follow these steps to measure slip.

Steps to measure rotor frequency & stator frequency

1. Connect the Null type Galvanometer to the brushes of the induction motor,  
2. Due to relative speed, some EMF is induced across the shaft of the induction motor.
3. This induced EMF is applied to the galvanometer through brushes.
4. Count the galvanometer vibration cycles (Movement of a needle) for one second time.
5. This count gives the value of Rotor EMF frequency fr and Since fs is generally known,

∵ fr= S*fs

fr  =  Rotor Frequency (obtained by galvanometer)

f   =  Supply Frequency (Mentioned in the nameplate detail)

After taking these two reading, we need to substitute these values in the formula

S= fr/f

Problem-1

The Induction motor is supplied with 3 phase, 50 Hz in industry. Calculate the value of rotor frequency, when slip is 6.

Given Data

Slip= 6 

Stator frequency = 50 Hz

Voltage for induction motor= 440 V

Formula

fr= S* f

Find the rotor frequency

Rotor frequency =?

Solution

To calculate the fr rotor frequency substitute the Slip and stator frequency in a formula.

fr=6*50

fr=30 Hz\

The rotor frequency is 50 Hz.

The stator frequency is 30 Hz.

The value of the slip is 6.

Conclusion

Slip calculation is more important while designing an induction motor. Slip is directly involved with the torque of the induction motor. Low slip produces low torque.

Thank you... If you have any doubt please post a comment.