Three Phase Induction Motors: Torque Under Running Conditions
TORQUE
UNDER RUNNING CONDITIONS
Let the motor at
standstill,
E2 = Induced emf per phase
X2 = Reactance per phase
R2 = Resistance per phase
Under running
conditions,
Er = rotor emf/phase under running
conditions.
Ir = rotor current/phase under
running conditions.
Zr = rotor impedance/phase.
Now,
Er = sE2
Xr =sX2
Zr = √[ R22
+ (sX2)2 ]
Torque, Tr ∝ ErIr
cos ϕ2
or
Tr ∝ ϕIr cos ϕ2
(Er ∝ ϕ)

Rotor current/phase is
given by,

Where, K is another
constant
It can be proved that K
= 3 / 2πNS
Therefore, the
expression for torque becomes,

If the stator supply
voltage V is constant, then stator flux and hence E2 will be
constant.

………….. (5.17)
Where, K2 is
another constant.
It may be shown that,
the running torque is,
(i) directly proportional
to slip i.e., if slip increases, the torque will increase and vice versa.
(ii) directly
proportional to square of supply voltage (`.` E2 ∝ V)
Basic Electronics and Electrical Engineering: Chapter 5: Induction Motors : Tag: Basic Engineering : Three Phase Induction Motors - Torque Under Running Conditions
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