1991 maxpak plus dc drive

[Abald88]

Hello all.

Im really hoping someone could help me shed some light on an issue im having with an older reliance DC drive. This drive runs a 50 HP DC motor for a main drive mtor for a machine in my facility. We started having issues with getting the machine to turn on. We would have to jog the machine then once it was moving we would hit the start button and it would run fine. Then it just stopped trying to turn on. We tested the motor leads and it tested like it was bad. We replaced the motor, same thing. So then we started assuming the main drive was bad. We had another sister plant with the same drive that sent it to us. Got it hooked up yesterday and same exact thing. This is a 500VDC armature drive with a field voltage of 300VDC. Were getting th 300VDC off the field. But were getting 24VDC leg to leg off the two wires from the drive. From ground to leg were getting 215VDC on one leg then 122VDC on th other leg. Im not sure you can really measure DC voltage like that though. Its 3 phase 480v into the drive. All of that checks out. No blown fuses anywhere. No fuses blew at all to begin with matter of fact. Ive had electricians in here and we cannot get voltage off this drive. Im at a loss guys. I really dont have a whole lot of expirence with DC stuff. Especially with it being this old and running through a drive. Any help would be greatly appreciated. Thanks in advance. I will try answering anything I can. In the picture named "Wiring diagrm" ignore the "40hp drive" Its actually a 50 hp drive and a 50 hp motor.

 

[blkfalc4]

Have you 'megged' the disconnected motor leads at the drive?

 

[GaryS]

From what you describe I would guess that you have a 3 SCR -3 Diode power section in the drive that limits the control. What you really need an scope to look at the output wave form. But my guess you don’t have access to one so we will have to try another way.
From your description I thing the motor s overloaded. When you run the drive from the start command the drive ramps up output voltage at the ramp rate when it output current reached the current limit point it stops ramping.
in your case 24VDC. But when you jog the drive the ramp is bypassed so the full jog speed / voltage is sent directly to the motor that current inrush would provide the necessary breakaway torque to start the motor moving then when you hit the start the motor is already moving so it runs normally. Your problem has gotten worse over time. Not the motor can’t provide enough break away torque to get thing moving.
To test this uncouple the motor from the load and see if you can run just the motor if it runs ok then you problem is mechanical , a bearing going out , a gearbox going bad.
Just a note before I would spend much to repair the drive or motor I would have replaced then with a new VFD and motor better performance lower Maintenace cost

 

[Abald88]

So we were able to get the motor running off the drive. I REALLY dont know if it was the combination of a bad motor or drive. But Friday night, we found the main contactor was faulty. The coil side was getting the voltage but wouldnt pull the contactor in. Which we didnt have any issue with that. Contactor up to that point was functioning correctly. But there was also a bad set of contacts on that contactor. But now our issue is the motor is turning on but is just running full speed. We have a potentiometer that controls the speed. Which is ran to the drive. But when checking the speed reference terminals, its working correctly. We have 0-10v while adjusting the knob. So thats not the issue. Has to be a jumper were missing in my opinion.

 

[Abald88]

I posted a quick reply. I meant to reply to you and the other guys comment.

 

[GaryS]

Check your field current you may have field voltage but if a wire is broken the motor will go into runaway the only thing that will limit the speed is the torque
I have seen motor with an open field have the RPMs ramp up to the point the motor explodes . it important when connection or checking a DC motor to check both the voltage and current on both the armature and the field.

 

[tmccormick]

A loss of speed feedback can also cause this to happen....

Could be an open wire, or even a jumper as you suspect...

1st question is does this DC drive utilize tachometer, encoder, or resolver feedback?

Also, you could actually have the feedback signal, but if the polarity or phasing is wrong the motor could still run away...

 

[GaryS]

I hate to point this out but if the motor had an encoder feed back and you lost that feedback the drive would output the full 500 volts to the motor but in this case the voltage in 24v while the motor it running at full speed it would lead me to be leave it an open field and the motor is in runaway speed limited by the load on the motor