This article posted with permission from EASA.
The majority of DC motors are now running from rectified power, rather than M-G (motor-generator) sets. That adds complexity to our job when trying to determine why fields overheat, or why the drive might shut down on field overcurrent or undercurrent.
Excess field motor temperature? Check these possible causes:
- Drive technician misunderstood the nameplate; make sure the field current setting is correct. Teach your customer about the effect of temperature on resistance, and Ohm’s Law. (Refer to July 2002 issue of Currents.)
- Blower operating in reverse; expect one-third to one-half the airflow. Visually verify rotation of blower.
- AC ripple from DC field card; measure both DC and AC current. Field heating results from the sum of both. Anything more than 10% AC content is too much. Not only do the fields overheat, the AC current could explain that “field overcurrent” fault code. (Refer to March 2016 issue of Currents.)
- Some manufacturers fields must be bonded to the pole iron using a thermally conductive potting compound, or they will overheat and may fail within weeks. This is essential for General Electric field coils, but it is also a simple way to reduce field temperature rise for most DC machines. (Refer to April 2008 issue of Currents.)
- For compound machines, use a megohmmeter, AC test light or 500v hipot to ensure there is no circuit between the series and shunt windings. Make sure the series are not “jumped out” – with AC ripple, that can cause overheating of the shunt fields. If the series are not being used, they should be open and insulated. Connecting the series leads together could trip the field circuit due to the inductive kick when fields are energized or de-energized. When AC ripple is present from the field supply, there is a continuous inductive kick / transformer effect which is exacerbated if the series fields of compound-wound machines are connected in anything other than a straight series connection. Bypass jumpers used to weaken the series, or any combination of parallel connections, will cause increased heating of the shunt fields.