Motor insulation test voltages: how high is too high?
Author - Stephen Drennan
A recent article in Electrical Tester discussed insulation test voltages for cables and looked at the confusion that sometimes exists in relation to choosing the most appropriate test voltage and test method for a particular application. This article looks at similar issues, but deals with the testing of motors rather than cables.
As mentioned in the previous article, there are many options for testing insulation, and Megger supplies an extensive range of testers for such applications, from 50 V to 15 kV insulation testers, through VLF and AC Tan Delta test sets to diagnostic Dielectric Frequency Response instruments and, under the Baker brand, specialist motor testing equipment up to 40 kV.
When testing motors, it’s important to keep in mind that they are complex electro-mechanical devices with a complex set of failure modes and related diagnostic options. We will look at the failures of motor electrical/insulation systems, how to manage the life of motors by adopting an appropriate test regime and the concerns that are sometimes voiced about the ‘high’ voltages used for surge testing.
A test regime designed for a particular facility must first consider how maintenance efforts can be optimized for the various motors in use. Permanent continuous monitoring using, for example, the Baker NetEP system may be selected for the most critical motors. By monitoring a wide range of parameters, including the current and current spectra, the NetEP can pick up many issues that may be developing – from broken rotor bars to saturating supply transformers. The system generates a series of ‘Watch’, ‘Caution’ and ‘Warning’ flags to facilitate planning of maintenance actions.
On-Line testing – that is, testing while the motor is running – can also be carried out on an as-needed basis with a portable on-line monitor like the Baker EXP4000.
These on-line techniques, of course, cannot tell you about all types of incipient faults. Indeed, when warnings are given, a full diagnosis may require complementary off-line testing. In addition, off-line tests are often used on their own in a planned maintenance regime that takes motors off-line for testing and other maintenance activities at appropriate intervals. Compared with on-line tests, off-line tests provide different insights into the motor’s condition; the two approaches to testing are, therefore, complementary.
Off-line testing operates by applying a stimulus of some kind to the motor and measuring the response. This is similar to a doctor asking you to cross your legs and then tapping you on the knee to see how your leg kicks up. This quick check enables a doctor to evaluate your spinal neurological response – it’s not about checking your particular skill in literal knee-jerk reactions!
Go/no-go tests are an invaluable part of every maintenance regimen. Options include insulation resistance, leakage current, polarisation index and step voltage testing, and there are few who would argue against the benefits of these core electrical tests. However, these tests will not reveal one of the most common early initiators omotor faults: inter-turn insulation breakdown. To detect such a breakdown, it is necessary to use a surge test. Unfortunately, this is the area where some confusion has been generated in relation to the ‘high’ voltages involved.
In a surge test, a short-duration test current with a fast rise time – typically around 100 ns – is generated and applied to the motor coil. The test equipment captures the coil’s response, which takes the form of a decaying oscillation – or ‘ringing transient’. If the motor coil were in air, the pulse would be travelling at almost the speed of light and the voltage would be evenly distributed across the coils. But in a motor, the coil is not in air, it is wrapped around a steel core, and so the pulse travels much more slowly.
In fact, it will typically take the pulse 100 ns to travel from one coil-turn to the next, which is equal to the rise time of the pulse. The result is that the pulse produces a significant voltage difference between adjacent turns of the coil, something that it is impossible to achieve with any other test technique. The surge test will, therefore, reveal turn-to-turn insulation weaknesses.
A bad turn will short and this will be shown by a jump in the ringing frequency of the coil (it becomes in effect a ‘different’ coil at this point in the test). Large frequency jumps can be clearly seen on the tester display, but the instrument’s software also uses mathematical analysis to reveal anomalies that are less easy to spot by eye.
FIGURE 1: Surge test detection of inter-turn insulation problems
Figure 1 provides examples of surge traces for a good winding and a winding that has weak insulation or a turn-to-turn short. The three traces – corresponding to the three phases of the motor’s windings – should overlay as one on the graph but, as can be seen, the trace for a winding with a turn-to-turn short is different from the traces for the other two phases.
FIGURE 2: Error Area Ratio (EAR) analysis
To make the identification of these issues easier, Megger’s Baker test equipment uses EAR (Error Area Ratio) analysis. This shows clearly the difference between the pulses (Figure 2) and, when the limits are set, weak turn-to-turn insulation can be identified quickly.
There are other effects due to filtering of the higher frequency components of the pulse-wave as a result of attenuation by the steel and the coils own inductance. The speed of propagation of the pulse wave is, however, the key aspect that allows the surge test to detect bad turns.
But what overall voltage is necessary to show up these turn-to-turn faults, and will this voltage be harmful if applied to a fault-free motor? How do the test voltages relate to the dielectric strength of the windings?
To answer these questions, let’s look at an example. When a 415 V motor is assembled, the insulation applied to the wire used to wind the stator has a dielectric strength of approximately 8000 V. During its lifetime, this insulation will degrade primarily due to heat in the motor, but also as a result of environmental conditions and the coil movements which arise from starting, stopping and load changes.
FIGURE 3: Thinning of winding insulation over time
In Figure 3, it can be seen that the insulation becomes thinner over the life of the motor.
FIGURE 4: Surge test voltage vs the dielectric strength of a 460 V motor and typical voltage spikes it sees in service
The surge test can detect the onset of this thinning without detrimental effect on the motor’s performance. The graph in Figure 4 illustrates the voltages involved relative to the dielectric strength of the motor winding insulation. Also illustrated are the ‘normal’ voltage spikes; motors in operation in an industrial or commercial power system see such voltage spikes all the time, whether from a circuit breaker being opened and closed or through the operation of variable speed drives
For a motor in normal operation, which has not endured significant electro-mechanical thinning of the insulation, the surge-test voltage is far below the dielectric strength of the insulation.
To come back to the analogy of the doctor hitting your knee with a little rubber hammer: If that test was being carried out with a proper steel-headed claw-hammer, and the doctor was built like Arnold Schwarzenegger, and Arnie wasn’t paying attention and just took a full swing at your knee, maybe you would be right to be worried about having your knee hit with a hammer. But you know that, in real life, that’s not how things are.
And it’s the same with a surge test generated by a Baker DX tester. It applies a voltage and rise-time to enable you to see the inter-coil response, but with a signal controlled in voltage, time and energy, so that the impact on the motor is similar to the spikes that the motor receives as a result of typical power-system variations during everyday operation.
In conclusion, a surge test should form part of the diagnostic toolkit of every motor maintenance and facilities management professional, who can be confident that a fault found by doing this test has avoided a probable unplanned failure and plant shut-down. What’s more, surge testing is actually recommended by many motor test standards.