Medium Voltage Application Guide

03340.B
Current
Torque
MOTORS
magnetic field and the rotor, otherwise known as slip. Slip is commonly expressed as a percentage of the motor's
synchronous speed.
Motor start performance characteristics can vary greatly depending on rotor design and construction, but in general,
a motor with high locked rotor current will produce low locked rotor torque and vice versa. A high resistance rotor
produces relatively high starting torque but runs at high slip which causes inefficiency. To produce superior starting
and running characteristics, specially shaped rotor bars or double cage rotors are used.
Typical start performance characteristics of an induction motor.
7 x FLC
6 x FLC
5 x FLC
2 x FLT
Full voltage motor current
Full voltage motor torque
Load torque (quadratic load, eg
pump)
4 x FLC
3 x FLC
1 x FLT
2 x FLC
1 x FLC
10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Speed
Useful formulae
Motor synchronous speed
Ns = f × 60
p
Motor slip speed (%)
N
slip
(Ns
N r)
= ×100
N
Motor shaft output power
P N r × T r
o =
9550
Motor electrical input power
Pi = 3× V× I × cos
Motor efficiency
Po
eff = × 100
P
i
s
Where:
N s = synchronous speed (rpm)
f = mains supply frequency (Hz)
p = number of stator pole pairs
Where:
N slip = percentage slip speed (%)
N s = synchronous speed (rpm)
= rotor speed (rpm)
N r
Where:
P o = output power (kW)
N r = rotor shaft speed (rpm)
= rotor torque (Nm)
T r
Where:
P i = input power (kW)
V = motor line voltage (kV)
I = motor line current (A)
cosØ = motor power factor
Where:
eff = motor efficiency (%)
P o = motor output power (kW)
= motor input power (kW)
P i
Page 6 Medium Voltage Application Guide 710-12280-00A