June 22, 2005
10 Motor Testing and CBM
Simple voltage readings can be used to perform some limited evaluation of the motor system. In particular, it can assist in the identification of power quality and motor mis-application. In this presentation, we will discuss how RMS voltage measurements can be used as part of your CBM program.
VP, Electrical Reliability Group
T-Solutions, Inc.
Voltage Measurements
Simple voltage readings can be used to perform some limited evaluation of the motor system. In particular, it can assist in the identification of power quality and motor mis-application. In this presentation, we will discuss how RMS voltage measurements can be used as part of your CBM program.
The primary application of voltage measurements for CBM are the identification of voltage mis-match (variation) and voltage unbalance. In order to properly identify both issues, the information must be properly collected. This involves taking phase to phase voltage measurements and NOT voltage to ground or neutral. That is used for a different kind of analysis that relates to current values and will be covered in the next part of this series.
Voltage information is taken from Phase A to Phase B, Phase A to Phase C, then Phase B to Phase C. The result will be three phase to phase voltage values that we will call A, B and C.
Voltage variation occurs based upon the maximum phase voltage difference from the nameplate voltage. In this case, let us assume that A is the greatest difference from the nameplate. Then (ABS(V-A)/V)x100 = %Variation, where V is the nameplate voltage. The maximum variation from the nameplate voltage is 10% with 5% recommended for greatest motor life. For instance, if a motor has a nameplate voltage of V=440Volts and the greatest difference is A = 490Volts, then (|440-490|/440)x100 = 11%, then the motor has been mis-applied. In this case, the motor will run very hot and less efficient with a reduced reliability. If the motor fails, due to this condition, the winding will appear as if it were overloaded. The correction opportunities are:
1) Install a buck-boost transformer for smaller applications. Less chance of a negative impact on other parts of the system.
2) Adjust transformer taps, if other parts of the system are affected.
3) Replace or rewind motor to the correct value.
Voltage unbalance occurs when the phase to phase voltages are not matched. Voltage unbalance will generate overheating in an electric motor due to unbalanced magnetic fields. The result is that, once unbalance exceeds 2%, the motor has to be derated up to 75% of nameplate rating (horsepower or kW) at 5% unbalance. Motor reliability is severely impacted beyond 5%, at which point a failure related to this issue will look like a single-phased motor.
Voltage unbalance is calculated as follows:
Ave = (A + B + C)/3
Then take the greatest difference between Ave and each phase. In this case, let us assume that it is B.
%Unbalance = (ABS(Ave – B)/Ave) x100
For example, if you have measurements of A = 460, B = 465 and C = 480 then:
Ave = (460 + 465 + 480)/3 = 468Volts
% Unbalance = (|468-480|/468) x 100 = 2.6%
Voltage unbalance can be caused by loose connections, power factor problems, unbalanced loads, and similar issues.
Caution on voltage test applications for troubleshooting, a great many conditions can cause failure in analysis. In particular, if testing from a control or motor control center, the distance from the test point, the size of the conductors, loose connections and other issues between the test point and motor can cause a great many faults to go undetected. One of the greatest potential conditions is voltage drop due to distance and wire size. In many cases, while working for a submersible pump manufacturer, we would see overloaded motors. The voltage at the control would appear to be correct, but the run might be about 300 feet with an incorrect wire size. When the wire size and distance was taken into account, it would be determined that there was a voltage drop of 100 Volts or more at the motor terminals.
In our next part of the series, we will discuss how to calculate voltage drop.
