Page 65 - Energize July 2022

TECHNICAL

Understanding motor inrush

current

Motor inrush current – the large spike of current drawn by a motor as it starts up – can have an adverse
effect on supply networks and protection systems.

by Andy Sagl, Megger

hen an AC motor is energised using a conventional The inrush current can cause motor protective devices
contactor starter, a large spike of current flows (overloads and fuses) to operate if these have not been correctly
Wthrough the motor and the conductors feeding it. selected, but more typically the voltage dip caused by the large
This current, which is well in excess of the rated current shown current flow (see Figure 2) causes malfunctions in other devices
on the motor’s nameplate, is needed to overcome the combined connected to the same circuit as the motor. The voltage dips can
inertia of the stationary motor shaft and the load the motor is trip controllers and loads off line. Constant power devices will
driving. increase their current draw to compensate for the lower voltage,
When three-phase power is applied to a motor, the stator which can lead to the tripping of over-current protection devices.
windings, which are the stationary windings in the motor And, in severe cases, the voltage dip may be so great that the
frame, are energised. The current in these windings generates motor cannot develop enough torque to start.
a rotating magnetic field which induces current in the rotor Motor load, as well as the characteristics of the supply system,
winding - the winding on the rotating part of the motor. The affect the motor inrush current. A heavily loaded motor will draw
rotor current also produces a magnetic field and the fields
produced by the stator and rotor interact in a way that causes
the rotor to rotate.
The rotor speeds up until it reaches close to synchronous
speed, which is the speed of the rotating field produced by the
stator. The rotor never quite reaches synchronous speed however,
because if it did, there would be no induced rotor current, and
the motor would produce no torque. The difference between the
actual speed of the rotor and the synchronous speed is usually
expressed in terms of slip, where:

slip = (synchronous speed - speed of rotation) / synchronous speed (1).
Figure 1: Inrush current falls and slip decreases as rotor speed increases.
When the motor is stationary, the slip is 1. When the motor is
running normally, the value of slip depends on the load, but
typically ranges from around 0,05 for small motors to as little as
0,01 for large motors.
At start up, the slip = 1 and this large value of slip is the biggest
contributor to the inrush current. As the rotor speeds up, the
slip decreases and the inrush current falls to the normal running
current of the motor, as shown in Figure 1. The magnitude of the
inrush current depends on the type of motor and the starting
method. For standard industrial motors started directly on line,
inrush currents between eight and ten times the normal running
current are typical. For high efficiency motors, the inrush current
can be even higher. Figure 2: Large current flow causes voltage dips affecting other devices

energize | July 2022 | 63