Current State of Surge Testing Induction Machines

Surge testing of motor coils has been an industry practice since J. L. Rylander published “A High Frequency Voltage Test for Insulation of Rotating Electrical Apparatus” in Transactions of the AIEE, in February of 1926. Since that time the use of surge testing to diagnose failing or damaged insulation in motors has advanced greatly. In 1926, the indication of a turn-turn insulation failure was the drop of the coil voltage amplitude as determined by a vacuum tube rectifier circuit using an apparatus the size of a large work bench with rotating spark gaps for switches and large step up transformers to charge large high voltage capacitors. Today’s instrumentation uses high-speed digitizers to observe the entire waveform. Instead of a work bench, a 50lb portable enclosure contains the entire instrument which includes small high voltage power supplies, power semiconductors for switching elements, small efficient high voltage capacitors and a complete Pentium class computer with color display. The principles of surge testing, the strength of the turn-turn insulation being tested, the motivation to perform the surge test, the required test voltages necessary to succeed with the test, the industry standards surrounding the technology and the modern diagnostic algorithms will be presented. PRINCIPLE OF THE SURGE TEST If a rapidly increasing current is applied to a coil, a voltage will be generated across the coil by the principle of induction. The voltage across the coil is given by V=L*di/dt Where V is the terminal voltage across the coil, L is the coil’s inductance, and The time rate of change of current pulse is di/dt. The terminal voltage V at the leads of the coil is actually a summation of the induced voltage created between individual loops in the coil. If the insulation separating adjacent coils is weak and if the induced voltage is higher than the dielectric strength of the weak insulation, an arc will form between the coils. Surge testing equipment is designed to create the induced voltage between adjacent coils and detect the arcing indicative of weak or failing insulation. Figure 1 below shows a block diagram typical of today’s instrumentation. The internal capacitor is charged to a known voltage by the power supply. At a specific time, a high voltage switch closes which transfers the charge from the capacitor through the windings of the coil. If the resistances and loss of the entire circuit are such that the system is under damped, charge will be able to flow through the inductor and on to the other side of the capacitor resulting in an oscillation This process of ringing will repeat until the resistances and losses in the circuit completely absorb all of the energy that was originally on the capacitor. The measurement of the terminal voltage of the coil vs time gives the surge waveform, which shows the damped oscillation.