Baker D30R User Manual - Testing

12/3/2009 | 71-022 V6 

D30R 

39 

6  

Principles and theory of Surge 

testing 

Principles of Surge testing  

Surge testing is performed to detect insulation damage between turns within a motor’s 

winding. This type of insulation problem cannot be found any other way than by surge testing. 

The surge test consists of applying a short, fast rise time, high current impulse to a winding. 

This high rise time impulse will induce, via Lenz’s Law, a voltage difference between adjacent 

loops of wire within the winding. If the insulation between the two loops of wire is damaged 

or somehow weakened, and if the voltage difference between the wires is high enough there 

will be an arc between the wires. The arc is detected by observing a shift in the surge 

waveform. 

The Surge test is performed with an impulse generator and an oscilloscope type display to 

observe the “surge waveform” in progress. The surge waveform is a representation of the 

voltage present across the test leads of the Baker tester during a test. The indication of a 

turn-to-turn fault is a shift to the left, and/or a decrease in amplitude of the surge test 

waveform as the test voltage is increased.  

 Surge testing theory 

As mentioned above very short high current pulses are applied to the coil during a Surge test 

to create a voltage gradient (or potential) across the length of the wire in the winding. This 

gradient produces a momentary voltage stress between turns. 

  The coil will respond to the surge pulse with a ringing or damped sinusoidal waveform 

pattern. Each coil has its own unique signature ringing or wave pattern, which can be 

displayed on a CRT display screen as shown below. 

     The wave pattern observed during a Surge Test is directly related to the coil’s inductance. 

(There are other factors influencing the wave pattern but inductance is the primary one.) The 

coil becomes one of two elements in what is known as a tank circuit – a 

LC-type

 circuit made 

up of the coil’s inductance 

(L)

 and the surge tester’s internal capacitance 

(C). 

    Inductance (L) 

of a coil is basically set by the number of turns in a winding and the type of 

iron core it rests in. The frequency of the wave pattern is determined by the formula: 

LC

Frequency

π

2

1

=