MANUAL MOTOR STARTER GUIDE
APRIL 19
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Amplitude of inrush current
The amplitude of the inrush current from IE1 to IE2 and IE3 / IE4 depends on the following factors in the respective
application:
−
The structure of the motor
−
Network conditions (in particular the size of the short-circuit power of the transformer and thus the voltage
stability)
−
The length and routing of the motor cables
−
The switch-on phase position in the respective phase
3.2.7 Power factor
A motor always consumes active power, which it converts into mechanical action. Reactive power is also required for
the magnetization of the motor, but it does not perform any action. In the diagram below the active and reactive
power is represented by P and Q, which together give the apparent power S.
The ratio between the active power P (kW) and the apparent power S (kVA) is known as the power factor, and is often
designated as the cos
ϕ
. A normal value is between 0.7 and 0.9. When running, where the lower value is for small or
low loaded motors and the higher for large ones.
Figure 21: Diagram indicating P, Q, S and cos
ϕ
.
3.2.8 Torque
The starting torque for a motor differs significantly depending on the size of the motor. A small motor, e.g.,
≤
30 kW,
normally has a value of between 1.5 and 2.5 times the rated torque, and for a medium size motor, say up to 250 kW, a
typical value is between 2 to 3 times the rated torque. Very large motors tend to have a low starting torque, some-
times even lower than the rated torque. It is not possible to start such a motor fully loaded, not even a direct online
start.
𝑇𝑇
𝑛𝑛
= Rated torque (Nm)
𝑃𝑃
𝑟𝑟
= Rated motor power (kW)
𝑛𝑛
𝑟𝑟
= Rated motor speed (rpm)
Figure 22: Diagram of the torque vs. speed.