While winding resistance or Bobinage in french is a popular method of testing motors it is only one test in a suite of methods.

Circuit resistance if measured from the Motor Control Cabinet provides more rewards than probably any other Motor Test due to the fact that approximately 20% (1 in 5 ) motor circuits have defects.

The reason for Resistance Imbalance in a Motor Circuit

Those defects can be screw terminal looseness, Crimping Quality, Cable Joins, Terminal Arrangement, Isolator Contacts, and Motor Terminals.

A resistance test at the Motor usually returns resistive defects at the Links, with the arrangement of the motor lead, link, and supply having loose nuts, excessive washers and corrosion as the major causes.

The probability of a defect in the motor lead to the winding is lower, but this should be identified in an Acceptance Test.

The list of potential failure modes is quite high when compared to the Motor Winding resistance where most resistance defects originate in the Supply circuit to the motor.

The Winding Resistance Definition is the DC circuit resistance in Ohms between terminals on a motor when measured Phase to Phase.

This circuit can be either WYE or Delta connected meaning the connection arrangement of the Links. Links bridging three terminals across one side of the terminal block are WYE connected, while three individual links connecting across the terminal block are DELTA arrangement.

If an open winding circuit exists one of the measurements in WYE configuration will infinity as the Windings are in Series, and a Higher resistance if connected in DELTA due to winding configured in parallel.

On a larger motor connected in Delta and having one open winding may not be detected with a multi meter. The reason is each parallel circuit may return a reading below 1 Ohm which in most Multi Meters is the resolution.

If an Open winding exists it is recommended to remove the Terminal Links which isolates each individual winding. The DC resistance measurement is then taken on each Winding.

The problem is this takes TIME, and under the pressure of making a decision of replacing a motor in a breakdown situation this can be forgotten, resulting in the wrong decision.

This limitation of the Multi Meter is approximately 5.5 kW Motor Rating when connected in Delta. A WYE connected motor will measure a very high resistance on one winding.

The chart shows on average motor test a 5.5 kW Motor is just above 1 Ohm. The data is an aggregate collection and may vary based on Winding length or Frame length of the motor.

The resistance scale in the chart is a log scale meaning the Winding resistance of increasing Motor kW rating drops very quickly.

The instrument required to measure motors to approximately 90kW rating is an All TestPro with a resolution of 10 milli Ohms.

This measurement is conducted with a 4 wire DC Resistance method giving a much higher level of accuracy.

When a Motor circuit has a defect in Resistance this causes an imbalance in the Voltage, the losses in the Motor increase exponentially shortening the Motor Life and consuming more energy. Therefore Fixing resistance imbalances above 3% Warning limit "Gains Motor Reliability & Returns Energy Savings".

3Phi Reliability find approximately 20% of Motors on the first initial survey to have Resistance imbalances above this 3% limit. The opportunity to improve in nearly all facilities is easy to implement and immediate gains.

The question of what to do when testing Motors greater than 90kW rating as the Circuit resistance is most likely to be below 10 milli Ohms? Experience shows that a defect in  circuit resistance will result in that Winding being above 10 milli Ohms or 0.01 Ohms.

So the All TestPro instruments are quite suitable for all Motor sizes, but only show the imbalance. Note: Very high percentage imbalance occur at these levels as one small defect has a large affect on the result.

3Phi Reliability recommend that if an imbalance is shown on the All TestPro instrument for a Motor above 90 kW then re test with a Micro Ohm meter. These meters as the name states measure to 1 millionth of an Ohm.

Calculating the imbalance percentage with these readings give a far better result.

The chart above can be used to predict the Resistance results, if a motor has a balance resistance but all three Windings are above what is expected then it is quite possible that ALL TERMINATIONS are loose.

This type of defect is very common  especially when the Bottom nut between the Motor Lead and the Link is not torqued to specification. Approximately 94% of motor connections that are first surveyed have these type of problems. Some Sites this is 100%.

Note: Motor Manufacturers ship motors with the connections only finger tight, and often on installation are never checked. Implementing Motor Termination guidelines and torques is part of Best Practice Motor Management and these procedure provided to clients.

Conclusion: It is very important to know the limitations of the instrument you are using.

Measuring Circuit Resistances is the most rewarding activity you can implement in your Electrical Preventative Maintenance Program.

Both Gains in Motor Reliability and Energy Saving will be made immediately from the first Motor.

Motor Windings are Stressed when Motors are terminated poorly and in this example with debris in the terminal face resulted in the Motor lasting only months.