Baker D30R, User Manual

Page: 41 / 82

Principles of Surge Testing
12/3/2009 | 71-022 V6
D30R 41
Motivation for Surge testing
Motors are subjected to high energy, high voltage transients in their everyday operating
environment. These transient pulses can damage the insulation in the motor and, given
enough time, cause a catastrophic failure in the motor. The causes of high energy, high
voltage transients are:
−
Motor start-up inrush current coupled with contact bounce in the MCC.
−
Lightening strikes in the power system.
−
Inverter drive transients.
−
Line surges caused by other motors or transformers tripping in the power system.
One of the primary functions of a Baker Tester is to closely simulate the transient voltages
seen by the motor without the high energy that accompanies the normally occurring
transients. These spikes are a significant aging factor for the end turn insulation of an electric
motor.
Contact bounce
Oddly enough one of the major sources for the high-energy transients is the MCC, a device
that is supposed to protect the motor. When the breaker contacts close in the MCC during
startup, they will often “bounce” or chatter, this means that the high inrush current is being
made and broken several times. As a result of interrupting the current, an inductive “kick
back” voltage spike will be developed. Large inrush currents along with the high inductance of
electric motors are what give these “kick back” voltage spikes their high energy.
Lightening strikes
Lightening strikes can often be present on the power system or grid. Although a great
amount of effort is made to protect the grid from the damage caused by lightening, the high
voltage transients caused by strikes still get through to motors.
Inverter transients
Variable speed drives or pulse width modulated drives are based on switching currents very
quickly in such a manner that the motor runs at a pre-set speed. The switching of the current
combined with the obvious fact that the motor is an inductor result in high-speed transients
being generated by the motor drive electronics. These transients are impressed on the motor
where they can slowly degrade the insulation in the motor windings.
Line surges
The stored energy in a motor or transformer must dissipate when that motor or transformer
trips offline from its power system. The energy is either absorbed by the device or is pushed
out onto the power system where other transformers or motors have to absorb the energy.
Often, large transient voltage spikes manifest this energy impressed on the power system.
These spikes are again, a source of damage to motors, especially if the motor already has
weakened insulation.