Traditionally Surge testing has been the dominant method for detecting winding defects in electric motors, but this test is well suited to a workshop environment as the instruments tend to be bench friendly.

An alternative which is Maintenance friendly (Portable) is the use of Current Frequency Response testing.

The Test is listed in the IEEE 1415:2006 as an effective method of determining winding condition.

The Current Frequency Response test injects a tone at a known voltage (Approximately 7 volts) into the winding and because this test is current determined the distance from the motor is relatively unlimited unlike surge testing. Therefore motor health can be measured from the Motor Control Cabinet many hundred of meters from the motor.

This explanation of the Test is a 3Phi Reliability perspective taken from many thousands of motor tests.

The standard test frequency of the I/F Current Frequency response is 200 hz, which stepped up to 400 hz in an alternating cycle.

This tone of 200 hz excites the motor winding between each Phase therefore, so one measurement per phase.

The protocol is to measure T1-T2, T1-T3, & T2-T3 in sequence.

The results are then compared and should be within specified limits. An imbalance above these limits indicates that a winding defect exists shown in a colour code within the MotorGenie App (free Download). Note: Red Alarm of the I/F (Current Frequency Response Imbalance)

Electrical theory states that Reactance of a coil is determined by XL = 2fL in Ohms

f frequency in Hz

L Inductance in Henry H

Reactance of a Capacitor is determined by XC = 1/2fC in Ohms

f frequency in Hz

C capacitance in F Farads

In a Motor Winding a minor amount of Reactance is generally Capacitance in the form of Insulation. XL tends to be the most dominant contributor to Impedance (Coil).

If you take XL = 2fL and you double the frequency, the relationship means the XL in Ohms doubles and therefore using V=I Z the current should halve if the voltage stays the same.

This is what the All TestPro instruments test for, the Voltage is known and the Frequency is doubled and the instrument measures the current in that change. Using current means testing can be conducted from the MCC (Unlimited Cable Length).

Using the XL= 2fL you would expect exactly 50% reduction in current if the frequency is doubled.

In practice this doesn’t happen as the Motor Winding has Capacitance which opposes XL therefore measurements of -15% to -50% are normal. Reading below -15% means Capacitance is high and test frequency needs to be adjusted upwards typically 400 hz to 800 hz.

The absolute reduction in current isn’t used but the imbalance between phases is very important.

If your readings are on the lower side -25% to -50% your winding is very inductive meaning any imbalance is likely to be Coil related as XL is dominant.

Under these measurements it is likely any imbalance is coming from a change in Inductance and hence a turn to turn short or Coil to coil short.

Higher I/F readings closer to -15% means Capacitance is relatively higher as the winding isn’t pure Inductance, any imbalance in these readings could be Insulation or Coil related. Therefore if the Phase Angle measurement are correlated an Insulation defect is developing.

If they are not correlated then the defect is likely to be Coil related and the motor decay rate is advanced. The explanation of Phase Angle is at https://www.3phi-reliability.com/blog/phase-angle-test-an-effective-means-of-determining-electric-motor-winding-health

Experience of 3Phi Reliability testing shows motors tend to develop a Phase Angle defect first as the insulation is stressed most likely from heat (Impedance Imbalance).

The I/F current frequency response then follows this initial Phase Angle defect with a coil defect to ultimate failure.

This can happen relatively quickly sometimes weeks.

If an I/F Current Frequency response defect is detected and the Phase Angle test is Green (Pass) it is likely to be manufacturing defect which is less likely if the winding has been VPI (Vacuum Pressure Impregnation) which eliminates much of the air bubbles between winding strands.

VPI is a process which for critical motors adds a higher level of reliability, but is more expensive hence normally conducted on larger motors.

This VPI process is recommended in 3Phi Reliability Purchase & Overhaul specification for critical motors or elevated temperatures.

This explanation may seem quite complex, but taking measurements and entering the data into the MotorGenie App (free download) is simple as the App calculates the imbalance and applies the correct limits giving you a Pass (Green), Warning (Yellow) or Alarm (Red) for the Motor Winding.

This method is quite rapid taking only a few minutes for a complete Motor Test, and because of the portability can be conducted in some challenging locations eg elevated motor bases, platforms, or pits.

The procedure is to start at the MCC cabinet and if a defect is detected then move to the motor, therefore not all motor terminal boxes need to be opened. Our experience suggests that it is wise to conduct testing at the motor for the first survey as many resistive defects are common across all phases, therefore imbalances are less dominant.

Note: Severe Resistance imbalances above 5% also result in Voltage drops to the motor greatly affecting Reliability & Energy Efficiency.

3Phi Reliability calculate the As Found versus As Left resistances and the I^2R loss or Power savings can be quantified. Termination guidelines are provided to all clients.

This Electrical Preventative Maintenance therefore “Gains Electric Motor Reliability & Returns Energy Savings”

www.3Phi-reliability.com