Download L-27 Concept of slip

SUBJECT- Energy Conversion Technique (BEEE2215)
LECTURE-27
Slip
The difference between the synchronous speed Ns of the rotating stator field and the actual rotor
speed N is called slip. It is usually expressed as a percentage of synchronous speed i.e.,
% age slip, s =
x 100
(i) The quantity Ns - N is sometimes called slip speed.
(ii) When the rotor is stationary (i.e., N = 0), slip, s = 1 or 100 %.
(iii) In an induction motor, the change in slip from no-load to full-load is hardly 0.1% to 3% so
that it is essentially a constant-speed motor
Rotor Torque
The torque T developed by the rotor is directly proportional to:
(i) rotor current
(ii) rotor e.m.f.
(iii) power factor of the rotor circuit
T ∝ ɸ I2 cosɸ2
OR
T = k ɸ I2 cosɸ2
.
where, ɸ = flux per stator pole,
I2 = rotor current at standstill,
BY MANMOHAN PANDA (LECT.)DEPT. OF ELECTRICAL ENGG.
S.I.E.T DHENKANAL
SUBJECT- Energy Conversion Technique (BEEE2215)
ɸ2 = angle between rotor emf and rotor current,
k = a constant.
Now, let E2 = rotor emf at standstill
we know, rotor emf is directly proportional to flux per stator pole, i.e. E2 ∝ ɸ.
therefore, T ∝ E2 I2 cosɸ2
OR
T =k1 E2 I2 cosɸ2.
Starting torque
The torque developed at the instant of starting of a motor is called as starting torque. Starting
torque may be greater than running torque in some cases, or it may be lesser.
We know, T =k1 E2 I2 cosɸ2.
let, R2 = rotor resistance per phase
X2 = stand still rotor reactance
Rotor impedance/phase
Rotor current/phase
Rotor p.f.,
BY MANMOHAN PANDA (LECT.)DEPT. OF ELECTRICAL ENGG.
S.I.E.T DHENKANAL
SUBJECT- Energy Conversion Technique (BEEE2215)
Therefore, starting torque can be given as,
The constant k1 = 3 / 2πNs
Condition For Maximum Starting Torque
If supply voltage V is kept constant, then flux ɸ and E2 both remains constant. Hence,
Hence, it can be proved that maximum starting torque is obtained when rotor resistance is
equal to standstill rotor reactance. i.e. R22 + X22 =2R22 .
Torque Under Running Condition
T ∝ ɸ Ir cosɸ2 .
where, Er = rotor emf per phase under running condition = sE2. (s=slip)
BY MANMOHAN PANDA (LECT.)DEPT. OF ELECTRICAL ENGG.
S.I.E.T DHENKANAL
SUBJECT- Energy Conversion Technique (BEEE2215)
Ir = rotor current per phase under running condition
reactance per phase under running condition will be = sX2
therefore,
as, ɸ ∝ E2.
BY MANMOHAN PANDA (LECT.)DEPT. OF ELECTRICAL ENGG.
S.I.E.T DHENKANAL