Question

In adjustable frequency 3-phase induction motor drives, for constant power application the slip speed is kept
1. Constant
2. Proportional to synchronous speed
3. Inversely proportional to synchronous speed
4. Proportional to square of synchronous speed

Answer 1

Correct Answer - Option 1 : Constant

Concept:

In a 3-phase induction motor, the synchronous speed can be calculated as

\({N_s} = \;\frac{{120f}}{P}\)

\(s = \;\frac{{{N_s} - {N_r}}}{{{N_s}}}\)

Where, Ns = Synchronous speed in rpm,

Nr = Rotor speed in rpm,

s = Slip of the machine,

f = Supply frequency in Hz,

P = Number of poles.

Slip speed: The difference between the synchronous speed and the actual speed of the rotor is known as the slip speed.

Slip speed sN_s = N_s - N_r

Explanation:

In adjustable frequency 3-phase induction motor drives, the induction motor is supplied by a dedicated generator so that frequency can be easily varied by changing the speed of prime mover and for constant power application, the slip speed is kept constant.

At a frequency f₁ we got a field speed of N_s1 then by performing the experiment we find out the rotor speed N_r1 of the motor then we will calculate the slip s₁.

Slip speed = s₁N_s1

Now when the frequency is changed to f2 then the field speed changes to N_s2 then by keeping the slip speed constant we can calculate theoretically the rotor speed N_r2 at which the motor can rotate for that particular frequency f₂ as shown

s1Ns1 = N_s2 - N_r2

Nr2 = Ns2 - s1Ns1

Also, s1Ns1 = s₂N_s2 = constant