Measurement of Slip

Measurement of Slip


The measurement of slip is important for the range of loads for which efficiency is to be determined. The slip of a three-phase induction motor is defined as;

(1)   \begin{equation*} % slip = \frac{N_s-N}{N_s} \times 100 \end{equation*}

Where Ns is a synchronous speed.

The synchronous speed depends on the number of poles P and supply frequency f. The equation of synchronous speed is;

(2)   \begin{equation*} N_s = \frac{120f}{P} \end{equation*}

When the load is connected to an induction motor, the speed of a rotor is slightly less than the synchronous speed. The speed of a rotor is N.

The rotor speed changes with changes in load. And hence, the slip changes with changes in load.

Therefore, it is most important to measure the accurate slip of the motor. Generally, the variation of slip from no-load to full-load is between (0.5-1) % to (5-12) %.

The following methods are used to measure the slip.

  • Direct measurement of speed by tachometer
  • Stroboscopic method
  • Galvanometer method

Now, in this article, we will discuss these methods in brief for the measurement of slip.

Direct Measurement of Speed by Tachometer

In this method, the speed of a rotor is measured using a tachometer. The frequency is monitored on the stator side (supply frequency).

With the help of frequnecy and number of poles, we can calculate the synchronous speed by equation-2.

For, number of poles is equal to 4 and frequency is 50 Hz.

    \[ N_s = \frac{120f}{P} \]

    \[ N_s = \frac{120\times50}{4} \]

    \[ N_s = 1500 RPM \]

Now, find the speed of a rotor with the tachometer and put the above values in the below table.

f Ns N % Slip
50 1500 1470
50 1500 1455

    \[ % slip = \frac{N_s-N}{N_s} \]

    \[ % slip = \frac{1500-1470}{1500} \times 100 = 2% \]

Stroboscopic Method

In this method of slip measurement, there is no physical contact with the shaft. If precise time measurement is made, the stroboscopic method is quite accurate.

A disc having an equal number of black and white sectors is mounted on the shaft. The sectors are equal to the number of poles in the motor and all sectors are identical in shape.

A neon lamp illuminates the disc and emits light pluses twice in a cycle of the stator supply.

Therefore, for 50 Hz frequency, it becomes bright 100 times in a second.

If the disc rotates at synchronous speed, the time of ¼ revolution will be the same as the interval between two light pulses. And the disc appears stationary.

If the disc rotates at less than synchronous speed, the time for ¼ revolution of a disc becomes greater than an interval of light flickers.

Therefore, the sectors appear to move in the opposite direction and the rate of their rotation is the slip speed in RPM.

The time taken for a fixed number of revolutions of the disc is measured from which slip-speed is found and slip is calculated.

Stroboscopic Method
Stroboscopic Method

For example, the disc takes 10 second for 5 number of revolutions and machine has 4 number of poles.

In this condition, the synchronous speed is 1500 RPM.

In 10 seconds, disc rotates 5 number of revolutions. Hence in 60 seconds, it takes 30 number of revolutions.

Therefore, the slip speed in 60 second is 30 revolutions.

Slip speed = 30 RPM

Synchronous speed Ns = 1500 RPM

    \[ % slip = \frac{slip speed}{N_s} \times 100 = \frac{30}{1500} \times 100 \]

    \[ % slip = 2% \]

Galvanometer Test

The frequency of rotor EMF is slip times the stator frequency. In the case of a slip ring induction motor, a galvanometer connected between slip rings gives oscillations per second corresponding to rotor EMF frequency.

If the motor has a P number of poles, the rotor EMF undergoes P/2 electrical cycles for every revolution.

Galvanometer Test
Galvanometer Test

For a 4 poles machine, there are two cycles completed in one revolution. The rotor frequency f can be seen by the oscillation of the galvanometer connected to the rotor circuit.

For example, the motor takes 1 second for one revolution.

The frequency of rotor EMF f’ = 1 cycle/second

    \[ % slip 's' = \frac{f'}{f} \times 100 \]

    \[ % slip 's' = \frac{1}{50} \times 100 = 2% \]

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