dc motor brake problem

[manic mike]

Hi Guys
I have a problem I'm trying to understand. I haven’t had a great of experience regarding dc motors and control so I'm hoping someone can help. I have been trying to understand an old amusement ride. It has cars powered by a 110V dc motor. Its a fairly simple pick up power from a rail and it go's straight to the motor. It is very old and I believe the motor is a neco. The motor has 3 wires black, red and grey. The black and red are connected together and wired to the ground and the grey is wired to the +110V. Now the motor works ok and drives around the track, but part of the track go's up a slope. If there is a power loss, the cars can roll back at speed. The cars have a diode connected between the +110v and ground with the anode to ground so it’s reversed bias. As I understand it, this blocks the +110V shorting to ground during normal operation. When the power is lost and the car go's backwards and the motor acts as a generator and reverses the polarity. The diode then is forward biased and completes the circuit. This causes the motor to act as a brake and slow the car down. Am I correct with my theory or is the diode just to prevent spikes? I have a couple of cars that dont slow down if rolling backwards! I have checked the diodes and connecting wire which are good, so I believe the problem is in the motors. Any help greatly appreciated.

 

[OperaHouse]

Are you sure the motors are wired that way? I've just never seen a DC motor use that color code before, not that I have seen a great many. It would be interesting to know the resistance from the black or red to the grey wire. That may indicate which is the field.I would imagine one is the field and the other the armature. You are correct that the motor would slow down in the reverse direction due to conduction of the diode. That effect would be much greater in a permanent magnet motor than one with a field winding. Don't know why there would be a difference in motors unless they possibly had an internal brake.

 

[manic mike]

Hi OperaHouse
Thanks for the reply. the resistance
Grey to red is 1.4 ohms
Grey to black is 189 ohms
Red to Black is 190 ohms
So I guess the Grey to red is the armature and Grey to black is the feild? These measurements are from a good motor that does slow down. The only slight differance on a bad motor that dose not slow was the Grey to red was 4.7 ohms. Thats what made me suspect the motor. I checked the brushes which seem ok.

 

[OperaHouse]

You have identified the armature and field. Armature resistance is hard to measure. It can vary a bit due to oxidation. I usually rotate it to different positions and get many readings. So, twice the resistance may not be all that telling. If you have seen the brushes, hoe many are there? If it is a four brush motor and one had a bad connection, that would explain the double resistance on the bad one. If you get consistently higher readings, it could be a short brush, old grease sticking or a weak spring.

Just how vintage is this? Is the diode a modern type or one of the flat plate selenium types?

 

[Compositepro]

Could the problem be with the residual magnetism in the motor? If it is not a permanent magnet motor then there has to be some residual magnetism of the correct polarity in the motor for it to start generating and energize the field windings.

 

[OperaHouse]

That was also a thought that crossed my mind. I've had to zap generators and alternators before. Never a motor because I wasn't concerned about regeneration. Some motor guy might know if that normal 110V would have been enough to re magnetize it or do you need like a 250V zap from a stiff capacitor bank to get it up to full potential.

 

[waross]

You may try lifting the brushes and measuring the resistance directly on the commutator bars. Measuring resistance through brushes is not a good idea.

Bill
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"Why not the best?"
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