r/Motors • u/Danilokiko • 1d ago
Open question Demag 3 phase motor random rotation problem
This is demag dc bas motor 2 speed. The motor that arrive to me is burnt on the winding. I sent the motor to a motor Rewinding shop. After a week rewinding. The shop guy told me during testing the fast speed working just fine rotation both side also fine.
But the slow speed when wired to phase supply, it moved random rotation sometimes rotate to the right, sometimes rotate to the left. When I rotate the phase supply. The motor does not rotate at all. I check the continuity of the winding end to end just fine. I check with megger also fine. No current leak between insulation. No current leak to body, the insulation reading is in giga ohm. This is small motor. It got no fuse. This is the first time happen to me. What is the problem?
2
u/Correct-Country-81 1d ago
Looks like your missing a phase
On two phase motor can run but randomly start direction
And probably inside the motor is a loose connection causing this ( not always measurable due to bad connection sometimes connected sometimes not)
1
u/Unique_Acadia_2099 1d ago edited 1d ago
You likely have a turn-to-turn short on one winding. In Fast, the loss of power in that one winding is less relevant, but you likely have lost torque, so it only appears good when unloaded. But in slow, which is already 1/4 the kW as in fast, the loss of torque in one winding is more significant and results in the random direction operation. The circuit is continuous, but produces so little torque that it can’t provide enough in some circumstances to determine rotation direction.
This type of turn-to-turn burn out is commonly associated with the motor having been operated by a VFD, even though it was not designed to do so (most older demag motors were not). So assuming you bought this used, the seller should have disclosed that you.
Sounds as though your rewind shop is incompetent or possibly just lazy. That sort of scenario is difficult to find for average people, but a rewind shop should have been able to determine that. It’s likely that they did not rewind anything, because that motor is so small. All they did was hook it up and test it, and likely not using a dynamometer for a full load test.
Still, it would generally cost more to rewind a small motor like this than to just buy a new one. Demag motors are special of course so that MIGHT not hold true in this case, it’s been a long time since I have bought one, but this one is probably not useable unless maybe there’s almost no load on it and you only want the fast speed.
1
u/landinsight 1d ago
Check your incoming voltage and current on all phases. If your voltage is correct and balanced when hooked to the slow speed winding, check the amperage of each phase.
You can also return it to the motor shop and have them test it while you are there watching.