Issue 2 - 201988

TECHNICAL

Predictive condition monitoring for motors requires relatively simple, straightforward testing procedures that don’t compromise the health of motors in good condition. For example, hipot testing of windings can lead to early breakdown of winding insulation. The test procedure proposed here, involving an impulse test as a part of multimeter – capable also of measuring insulation resistance as well as a suite of other parameters – provides a sensitive test of winding integrity with the capability of indication of incipient failure, therefore providing an excellent basis for predictive maintenance.

BY PAUL ANTONY

Power Parameters Pty Ltd

Motor Testing

Issue 2 - 2019 89

TECHNICAL

To a large extent, moderately rated motors (under 100 kVA) are subject to replacement rather than repair; however, that of itself doesn’t obviate the advantages associated with condition monitoring, and in particular the testing of stator windings. In this article, a reliable stator winding testing procedure is discussed. It’s one thing to test for conditions likely to lead to failure, but if replacement rather than repair is the end result then the only economic advantage is the avoidance of an unexpected breakdown with its associated costs of process interruption. There are often neglected problems associated with the drive cable and motor impedance mismatch in variable speed, inverter drives. This can result in dangerous levels of voltage build-up at the motor terminals. Of course, the use of an inappropriate drive cable is an installation matter, but in due course, problems developing in an inverter can cause identical mismatch problems leading to voltage build-up. In short, rather than taking a laid-back approach, sometimes summed up by the ‘if it ain’t broke, don’t fix it’ expression, there’s great benefit in subjecting motors to simple stator tests that don’t require removal of the rotor, in other words: readily performed, non-invasive tests. Insulation tests are an obvious measure, but don’t necessarily reveal interturn shorts. A more subtle approach is that of stator winding signature testing, as discussed in this article.

A traditional method for testing shorts in field windings, laborious and not recommended but listed for completeness sake, is the ‘growler’. It comprises of a of a coil of wire on an iron core connected to an alternating current. Essentially, the primary of a

transformer, the crude primary winding induces voltage in the field windings. Current can only flow in shorted windings. A thin strip of metal, used as a probe, will detect the circulating current because of induced eddy current, causing it to vibrate and to emit an unpleasant audible tone and overtones. As is evident from the description, the test requires the removal of the rotor. It is therefore suitable for use in rewinding shops, but not practical in the field.

The use of signature style testing on the other hand is well suited to routine testing, and if performed at regular intervals, can reliably indicate deterioration of stator windings. The general expression for the current after connection of voltage to an inductive circuit is of the form:

e=ri+L di

dt

Where r is resistance of a winding, and L the inductance. If the voltage applied is in fact via a capacitor, we have the basis for an oscillatory circuit. The expression for the circuit becomes:

q=ri+L di

C dt

Where q is charge on the capacitor, C.The effect of the capacitor in series with the field coil is to set up a resonance (ringing) mode of frequency as per the following expression.

Where f is frequency in Hz, the effect of an interturn short is to reduce both L and R and therefore to increase the ringing frequency. In order to make the test a practical proposition, the

comparison between windings on a poly-phase motor should be based on the period rather than frequency. The motor tester discussed here (the Metrahit COIL) is a combination digital multimeter with insulation tester and coil test adaptor that utilises the adaptor by charging its interior capacitor to 1000 volts DC. The discharge into the stator phase winding to be tested causes resonant oscillation, and the period of a specified oscillation is shown digitally. The oscillation dies off at an exponential rate as indicated by the expression below.

In order to determine a faulty winding, it’s necessary to perform a comparison with remaining windings. The diagram in Figure 1 indicates the aforementioned waveform for two windings, indicating a slight left shift for one of the windings (i.e. a higher resonant frequency).

One way of overcoming slight statistical shifts that might hide an otherwise indicative result, is to use the method ‘error to area ratio’ concept. In this method, two waveforms resulting from two windings have one subtracted from the other, and this is than divided by the sum of the waveforms. In other words, in the case of two identical resonant waveforms, the result for this ratio would 0%.

Figure 1: The red trace indicates a winding with inter-turn short.

The resonant frequency comparison test for motor windings is a powerful testing method, particularly because an insulation test of the windings often does not reveal an incipient fault. A motor with a winding with partial breakdown of the insulation in a certain section, will often withstand a large number of starts without a breakdown. It has also been shown that the resonance test doesn’t compromise healthy windings. This is an important feature in implementing a predictive condition monitoring regime for motors.Measurement scheme for motor testing with Metrahit COIL.