Surge testing is a technique to test the dielectric strength of the insulation in windings. These windings can be solenoids, toroids, transformers, or some other type of winding like those found in electric motors. The technique involves applying a fast rise time current pulse to the winding which induces a turn to turn voltage between adjacent loops of wire. If that induced voltage is high enough to overcome weak insulation, an arc will be formed between the wires. This arc is picked up by the surge test equipment and displayed for the operator.
Before moving on with more applications, the technique will be explained in some detail. Again, surge involves applying a fast rise time current pulse to the winding or coil. Looking at the basic equation for voltage across a coil (below), the voltage across the terminals of the coil is proportional to the time rate of change of the current. The inductance of the coil is the proportionality constant.
(where L = inductance, i is the current and t is time. The term di/dt is the time rate of change of current).
Simply said, apply a pulse to a winding, and there will be a voltage across the terminals. In surge testing, we always measure the voltage instead of the current – the reason being to not exceed some breakdown voltage of some other insulation in the winding.
The induced voltage from a current pulse (which is observed at the terminals of the winding) is actually distributed across all the turns in the winding. For example, if the winding is 100 turns and the observed terminal voltage is 1000Volts, the voltage between turns is 10 volts….in a simple world.
In reality, the voltage distribution will not be even distributed as shown above. More on this later.
If the voltage is high enough across damaged turn insulation in the coil, then an arc will form. From Paschen’s minimum breakdown principle, the voltage across the turns must be above 375V and the coils close enough to each other for an arc to form. When this arc occurs, the shape of the surge waveform changes as will be discussed later on.