In the last Tip we discussed changing the test frequency to ensure the MCEmax® test equipment was optimized for testing DC or synchronous motor field poles. Continuing with the theme of change, this week we are going to focus on changing Field Pole Inductance. As mentioned in our previous tip, the classic approach to trending the health of DC or Synchronous motor fields is the intrusive AC Drop Test. This is a good test even today, but due to the heavy resource use of personnel and time required to perform this test we often reserve it for an end-of-life correlation to a far easier and less intrusive approach called MCE® Field Inductance trending.
First, to better understand the AC Drop Test, it is performed by applying an AC voltage across an isolated field. Then a volt-meter is applied across the input and output of each field pole to measure the individual voltage drop of each pole. An example is if 120 VAC is applied across a healthy 6-pole field you should see 20 VAC dropped across each field pole. A falling field pole will have <20 VAC dropped across it. Some of the difficulty is figuring out the best way to get through the insulation on the field poles to connect the volt-meter. Stabbing with a sharp probe or grinding through the field insulation are some options followed by re-insulating.
The MCE Field Inductance Trending is performed by connecting the MCE test leads to the already exposed field connections, either at the motor or the starter, and running a simple MCE Standard Test which automatically records resistance, inductance, and capacitance of the complete field circuit providing a perfect baseline trend point. A pre-defined setpoint for percent change from baseline in field inductance will alert you to a falling field pole.
To see a video showing the use of the MCE Field Inductance Trending to identify a falling field pole, visit the PdMA YouTube Channel at: https://www.youtube.com/watch?v=JrCQflS4cV4