by E. Wiedenbrug SM IEEE, G. Frey M IEEE, J. Wilson, M IEEE
Baker Instrument Company engr@bakerinst.com 

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Abstract: 

Impulse testing is an integral part of predictive maintenance of electrical motors.  Through the following questions the influence that extensive impulse testing has on a motor is investigated.  Can impulse testing damage healthy or deteriorated insulation?  Can DC Resistance, Inductance, Megger or HiPot tests diagnose weak turn-to-turn insulation? After failing an impulse test, are motor with weak insulation able to operate? Are motors with a turn-turn short capable of continued operation?  This was accomplished by putting a low voltage motor through extensive testing rigors, until inducing a failure.  Following the failure, additional testing investigated the possible deteriorating effects on turn-turn insulation due to impulse testing beyond the motor’s dielectric breakdown.  NOTE:  This paper was edited from the original version of the IEEE paper published in 2003.

I.                   Introduction

Unplanned downtime of industrial processes is very expensive, frequently with costs exceeding tens of thousands of dollars per hour. It is crucial to utilize scheduled plant downtime, during planned maintenance periods, for identifying equipment likely to fail. Identification of weak motors, and their replacement or repair during a planned outage, is more cost effective than the associated costs of unscheduled plant downtime and rushed repairs.

A.    Insulation Failure Mechanisms

According to IEEE and EPRI [1-2] studies, insulation faults are associated with 26-36% of motor failures.  D.E. Crawford’s investigations on failure mechanisms of motors concluded: “the great majority of failures seemed to be associated with wire” [3]. “This results in low power intermittent arcing which causes erosion of the conductor until enough power is drawn to weld them. Once welding has occurred, high induced currents in the shorted loops lead to rapid stator failure”. He states: “the ultimate motor failure on a motor returned from the field may not indicate what initiated the failure”. In short, most of the stator failures start as turn-turn failures, rapidly developing into catastrophic copper-ground or phase-phase faults.

Internal currents are induced in the motor as soon as an insulation failure reaches a turn-turn short. “These currents have the order of twice the blocked rotor current”. Tallam, Habetler and Harley [4] state that “If left undetected, turn faults can propagate, leading to phase-ground or phase-phase faults.”  The blocked rotor currents are upwards of 6-10 times rated current.  The heating energy is proportional to I²R, which leads to the expected 14,000% – 40,000% of rated wattage deteriorating the insulation in the faulted turns. A basic rule of thumb is, for every additional 10^oC the winding deteriorates at twice the rate. Excessive heating by turn-turn shorts is the reason why faulty motors will almost always fail in a matter of minutes if not seconds.

B.     Fundamentals of insulation testing:

The most common field tests for ensuring insulation integrity are:

DC coil resistance (IEEE Std 118) [7]

Stator inductance (no standard)

Insulation resistance,  (IEEE Std 43) [8]

HiPot (IEEE Std 95) [9]

Polarization index (PI) ( IEEE Std 43) [8]

Impulse test (IEEE Std 522, IEC 34-15, NEMA MG1) [4, 10, 11]

 

II.                Fundamentals of Impulse Testing

Impulse testing is based on sending a voltage impulse with a steep voltage front (high) to the tested coil in the motor.  This voltage impulse is generated by quickly discharging a capacitor into the windings of the motor.  This steep fronted voltage sets up a nonlinear voltage distribution in the coil similar to the ones observed in PWM drives [12-14]. This nonlinear voltage distribution creates a turn-turn voltage difference. By ramping the amplitude of subsequent voltage pulses sent to the motor, the voltage introduced between turns of the same winding is also ramped. The energy from the capacitor that was discharged onto the windings sets up a dampened oscillation between the motor’s inductance L, the capacitor C, and the system’s resistance, R. This oscillation has an internal frequency f according to [10] of:

                                         (1)

 

Any weak turn-turn insulation within the motor will have a low failure voltage.  While the introduced turn-turn voltage is below failure voltage level, there will be no current flowing through the weak insulation.  Discharge current will flow through the insulation as soon as the introduced turn-turn voltage exceeds the breakdown level.  This discharge represents a current path that is in parallel to the shorted windings of the coil, and has the effect of reducing the winding’s effective inductance.

Changes in the effective inductance of the motor that occurs during the internal discharge increases the frequency of this oscillation.  Impulse testing equipment monitors the voltage trace at the coil terminals or the terminal leads to the motor. This determines the increase in frequency. This frequency increase is easiest viewed by a ‘shift to the left’ of the impulse waveform as shown in Figure 1. 

Figure  1:  Passed and Failed Impulse Waveforms

This shift to the left can be very slight.  For that reason it is advisable to utilize a numeric method of observation which allows an accurate, repeatable and unbiased assessment. One such method is the Error to Area Ratio (EAR). The EAR is defined as:

                                   (2)

where F^(1,2) are the two waveforms; and i, j are summation of the samples of the digitized waveforms.  Identical waveforms will have an EAR of 0%, while very similar waveforms will have EAR of few percent. In a production environment, phase-phase comparison of impulse waveforms is commonly performed before the motor is assembled. This allows for identifying weak turn-turn insulation, as well as reversed coils or different numbers of turns per phase. In PM applications, subsequent pulses of the same winding are compared during the ramping of the pulse voltage. This has the advantage that the rotor position does not influence the test, while still investigating turn-turn insulation. An EAR of 5%, for example, is very difficult to discern on the display, yet can be the symptom of a failed winding.

Impulse testing and some of the insulation deterioration mechanisms are similar. Therefore it is of interest to investigate whether impulse testing, as commonly performed in Predictive Maintenance (PM), could deteriorate insulation condition.

III.             Test Protocol

The study test protocol was designed to answer the questions asked by exposing one motor to the rigors of testing.  The first test motor is a new factory 5hp 6 pole 460V connected dual voltage Hemco Global HE EPACT Compliant motor with a rated current of 7.7A, rated speed of 1170rpm and a 215T frame.  This motor was chosen because it was the least expensive new motor with these ratings. The hope was that the least expensive motor gave a lower average in insulation quality. In order to have a baseline reading of all tests, the motor was tested prior to starting the study.

Table 1: Testing New Motor

DC Resistance	Phase 1-2:  1.88 Phase 2-3:  1.78 Phase 3-1:  1.88
Insulation Resistance	20 GΩ
HiPot	Pass
Impulse	1-2% >=2kV

The first question, whether impulse testing damages healthy windings, was investigated by exposing the motor to 3 million impulses at the suggested 2kV voltage level.  To perform a normal test, modern impulse testers ramp each phase of a 460V winding with approximately  80 impulses of voltage levels increasing from 0V to 2kV. A very intensive PM program will test each motor up to twice a year for turn-turn insulation quality. This frequency of testing would expose a motor to a total of 3 million pulses (one million per phase) only after the motor was in operation for over 6,200 years. 

DC resistance per phase, insulation resistance, HiPot and Impulse test including EAR were recorded prior to each set of 3 million impulses. The PI test was not performed because IEEE 43 suggests no useful PI information will be obtained by testing such small motors.

Table 2:  Results from Suggested Test Voltage Level

	Results
DC Resistance	Phase1-2:  1.88  Phase2-3:  1.78  Phase 3-1: 1.88
Insulation Resistance	20.4 G Ω
HiPot	Pass
Impulse	Pulse-Pulse EAR:  1-2%  <= 2kV

To allow for possible misuse of the impulse tester when checking a motor’s turn-turn insulation, we tested the motor, again for 3 million impulses at both, 3kV and 4kV. This represent 150% and 200% over suggested test voltage. The same results were seen after this testing was done.

Table 3: 150% - 200% Testing Voltage

DC Resistance	Phase 1-2:  1.88 Phase 2-3:  1.78 Phase 3-1:  1.88
Insulation Resistance	20 GΩ
HiPot	Pass
Impulse	1-2% >=4kV

 

Since the question whether impulse testing deteriorates already damaged insulation needed evaluation too, we manufactured a weakness within the insulation. This was achieved by repeating the 3 million impulses at successive 1kV increased levels, until failure occurred. This failure took place at a 7kV level, or 350% over the recommended test voltage. After failure, subsequent impulse tests showed the punctured insulation to have a breakdown voltage of 1700V.  Again, we recorded DC resistance, Insulation Resistance, HiPot values and EAR.

Table 4:  Results of Testing After Applied Weakness

	Results
DC Resistance	Phase 1-2:  1.88 Phase 2-3:  1.78 Phase 3-1:  1.88
Insulation Resistance	20.4 GΩ
HiPot	Pass
Impulse	20% > 1.7kV

In order to investigate whether impulse testing further damages already damaged insulation, an additional 20 million impulses were performed on the failed phase at voltage levels above failing voltage level. The attempt here was to further deteriorate the motor and try to induce a lower failure voltage level. After concluding this additional testing, DC resistance, insulation resistance, HiPot and EAR were recorded.

To assess whether a motor that has failed the impulse test is still capable of running, we performed repeated starts on this motor.  During this process we monitored voltage spikes on phase A (the failed phase) which happened during the closing and the opening of the breaker process. A Tektronix TDS360 Oscilloscope captured the traces utilizing a Tektronix P6015A High Voltage Probe. Steady state operation was recorded with Fluke 77 multimeters and clamp-on probes.

The motor withstood 42 starts and stops, running under normal operation after each one of these starts. On the 43^rd start the motor started to sound louder, like an overloaded transformer. Within 40 seconds data was taken on the stator currents.  Motor operation was stopped due to the amount of smoke released by the motor, and the fire hazard. The currents prior to the turn-turn short were balanced, 7.2A per phase. The stator currents during the 40 seconds of operation with the turn-turn short were: Phase A = 11.9A; Phase B =  8.3A; Phase C = 7.2A.

After finishing these tests, we measured once more the DC resistance, insulation resistance, HiPot and EARs.

Table 5:  After Short Turn

DC Resistance	Phase 1-2:  1.85 Phase 2-3:  1.78 Phase 3-1:  1.78
Insulation Resistance	20 GΩ
HiPot	Pass
Impulse	25% any voltage

 

IV.             Results

The testing is divided into four segments:

Prior to test: The motor is in mint condition

Prior to impulse failure: The motor has withstood over 20 million impulses ranging from 100% rated testing voltage to 300% rated testing voltage without breaking down.

Prior to shorted failure:  The motor has been exposed to additional 20 million impulses above the 1700V breakdown voltage.

After shorted turn-turn:  The motor has started, ran and stopped 42 times without noticeable deterioration, and additionally started a 43^rd time with a shorted turn-turn fault. It ran less than 40 seconds in this condition, during which it released smoke, and was stopped due to fire hazard. 

 

Table 6:  Complete Results

	Baseline	2kV Test Voltage	3-4kV Test Voltage	Introduced Weakness	Post 43 Starts & Stops
DC Resistance Phase 1-2 Phase 2-3 Phase 3-1	1.88 1.78 1.88	1.88 1.78 1.88	1.88 1.78 1.88	1.88 1.78 1.88	1.85 1.78 1.78
Insulation Resistance	20.4 GΩ	20.4 GΩ	20.4 GΩ	20.4 GΩ	20 GΩ
HiPot	Pass	Pass	Pass	Pass	Pass
Impulse (Pulse-Pulse EAR)	1-2% <=2kV	1-2% <=4kV	1-2%<=2kV	20% >1.7kV	25% any Voltage