In the induction motor rotor always rotates speed less than synchronous speed. The difference between the rotor speed (N) and the rotating magnetic flux speed (Ns) is called slip. The induction motor slip is usually expressed as a percentage of synchronous speed (Ns) and is represented by symbol s.
Mathematically, Percentage slip, % s = [(Ns – N)/Ns] x 100
or Fractional slip, s = (Ns – N)/Ns
The difference between synchronous speed and rotor speed is called slip speed
i.e. Slip speed = Ns – N
The value of induction motor slip at full load varies from about 6% for small motors to about 2% for large motors.
Importance of Slip in Induction Motor
Slip plays an important role in the operation of the induction motor. The torque produced by the induction motor is directly proportional to induction motor slip. At no-load induction motor requires small torque to meet with the frictional, iron and other losses, therefore slip is small.
When the motor is loaded, greater torque is required to drive the load, therefore, slip increases and rotor speed decreases slightly. Thus induction motor slip adjusts itself to such a value so as to meet the required driving torque.
At standstill N = 0 hence s = 1 whereas at N = Ns, s = 0 (imaginary condition).
Induced EMF in Rotor
E2r = sE2
Where E2 = Induced EMF per phase at standstill.
E2r = Rotor induced EMF per phase in running condition.
At standstill s = 1 hence E2r = E2 whereas at N = Ns, s = 0, E2r = 0.
Thus rotor induced EMF fluctuates between 0 and E2 for the rotor speeds between N = Ns and N = 0.
Frequency of Induced EMF in Rotor
The expression for frequency of induced EMF in the rotor is:
fr = sf1
i.e. rotor EMF frequency = fractional slip x supply frequency
At standstill, induction motor slip s is 1, hence the frequency of induced EMF in the rotor of the induction motor is same as that of supply frequency and reduces with increase in speed (due to the reduction in slip).
There is no effect of rotor induced EMF frequency on its resistance. Hence the rotor resistance remains constant irrespective of the speed of the induction motor.
Let X2 be the rotor reactance per phase at standstill. The rotor frequency at standstill is fr = f1.
Therefore, X2 = 2πf1L2 ohm/phase
In the running condition the frequency of rotor voltage is fr = sf1. Hence rotor reactance in the running condition X2r is given by,
X2r = 2πfrL2 = 2πsf1L2 = s (2πf1L2) = sX2
X2r = sX2.
At standstill, induction motor slip s is 1, hence reactance of rotor of the induction motor is same as reactance of rotor at standstill and reduces with increase in speed (due to the reduction in slip).
The rotor impedance per phase at standstill is given by,
Z2 = (R22 + X22)1/2 ohm/phase
The rotor impedance per phase at running condition is given by,
Z2r = (R22 + sX22)1/2 ohm/phase
Where R2 = Rotor resistance per phase
X2 = Reactance of the rotor winding per phase at standstill.
Rotor Power Factor
At standstill, the rotor power factor is given by,
cos φr = R2/Z2 = R2/(R22 + X22)1/2
At running condition, the rotor power factor is given by,
cos φ2r = R2/Z2r = R2/[R22 + (sX2)2]1/2
Thanks for reading about importance of slip in induction motor.
Induction Motors — 2 | Objective Type Question Answers
#1 A pump induction motor is switched on to a supply 30% lower than its rated voltage. The pump runs. What will eventually happen ? It will
get heated and subsequently get damaged.
#2 5 H.P., 50 Hz, 3-phase, 440 V, induction motors are available for the following R.P.M. Which motor will be the costliest ?
#3 The torque developed in an induction motor is nearly proportional to
#4 Short-circuit test on an induction motor cannot be used to determine
#5 In a three-phase induction motor
iron losses in stator will be more than that in rotor
#6 In case of 3-phase induction motors, plugging means
interchanging two supply phases for quick stopping
#7 Which is of the following data is required to draw the circle diagram for an induction motor ?
Block rotor test, no-load test and stator resistance test.
#8 In three-phase induction motors sometimes copper bars are placed deep in the rotor to
improve starting torque
#9 In a three-phase induction motor
power factor at starting is low as compared to that while running
#10 The transformation ratio of an induction motor can be found by
short-circuit test only
#11 The power scale of circle diagram of an induction motor can be found from
short-circuit test only
#12 The shape of the torque-slip curve of induction motor is
#13 A change of 4% of supply voltage to an induction motor will produce a change of approximately
16% In the rotor torque
#14 The complete circle diagram of induction motor can be drawn with the help of data found from
of the above.
#15 In the squirrel-cage induction motor the rotor slots are usually given slight skew
to reduce the magnetic hum and locking tendency of the rotor
#16 The torque of a rotor in an induction motor under running condition is maximum
at the value of the slip which makes rotor reactance per phase equal to the resistance per phase
#17 What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero?
the rotor will not run
#18 The circle diagram for an induction motor cannot be used to determine
#19 Blocked rotor test on induction motor is used to find out
all of the above.
#20 An Induction motor can run at synchronous speed when
E.M.F. is injected in the rotor circuit.
Do not forget to click the “finish” button to see the correct answers and result.
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