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Old April 16th, 2018, 12:52 PM   #1
roxusa
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Dc drive

Having problems with a DC Drive Older BBC Veritron 10 hp that is running a 10 hp motor through a gearbox 5:1 ratio and drives a conveyor flat belt.
It seems the drive has no "balls" when the load increases on the belt and just stops. after a few minutes then it will trip the overload which I believe is on the armature leads (only one) I replaced the motor, checked the gearbox, and inspected the bearings on the belt it drives. I get no fault at drive or at least nothing I can see. Is it possible the drive is just failing (30 years) I am getting one in hopes of solving this but still am not convinced its the drive. Motor does not get hot, when gearbox is released from belt it will run with no load.
Any Ideas, Thanks
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Old April 16th, 2018, 07:40 PM   #2
jrwb4gbm
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If this motor has a separate field supply, verify that it is working properly and supplying the right voltage. I don't know if you used a new motor or a spare used one but if it was a used spare motor and uses a PM (Permanent Magnet) field, after 30 years the magnetism in the field could be weaker than needed.
A bearing on the conveyor may also be bad. It might be best to just replace them.

Last edited by jrwb4gbm; April 16th, 2018 at 07:46 PM.
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Old April 16th, 2018, 08:03 PM   #3
Gene Bond
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Without verifying the armature current with a DC ammeter, it's impossible to tell what's going on. The armature current is directly proportional to torque. The drive may be supplying full current.

Also measure the field current and voltage to determine if it is getting the proper excitation. The stronger the field (Watts / Ampere Turns), the higher the Kt (FtLb/Amp), Also, the lower the Ke (RPM/V). It's like a gearing change.

It could be that you have a partially shorted field, and a current regulated field supply. In which case, the voltage on the field will be low, but the current correct. If it's a voltage regulated or fixed (diode bridge) field supply, the voltage will be correct, but the current high.

You cant really troubleshoot a DC drive without a DC ammeter...
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Old April 16th, 2018, 08:24 PM   #4
GaryS
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Sounds to me like you lost the field power supply with out a filed the motor can't develop torque but it will overload the current in the armature and trip the overloads.
Verify the field supply if it's not there replace the hole thing with a good VFD and you don't have to be concerned about it. AC VFD is always better then a DC now
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Old April 16th, 2018, 10:41 PM   #5
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Quote:
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.... AC VFD is always better then a DC now
Oh oh ... you've opened the floodgates now...
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Old April 17th, 2018, 12:14 AM   #6
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Anything you can do with a DC drive system I can do with an AC drive system and a lot more.
I would welcome that discussion anytime There are many advantaged of an AC drive over DC.
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Old April 17th, 2018, 02:42 PM   #7
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I swapped the drive with an identical one from another line and had the same problem.
The next shift came in and the mechanic said even though this motor was sent out for repair its the same problem he had with it on the other line that is why he changed it. He put the original one back that was giving sporadic problems and it has ran 2 shifts without problems, I sent the motor back to motor shop (OEM Upgrade is to replace Motor/Gearbox and Drive to AC & VFD) I only need 1 more year on this and I have new machine coming. Hope they find something at the Motor Shop.
thanks
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Old April 17th, 2018, 02:53 PM   #8
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from what you say it looks like you have a wiring problem. A loose connection on the motor field wiring. Check the current in the field a Clamp On DC Meter will work Current in must = current out. it also must be with in the current range specified on the motor name plate
I the both the motor and drive have swapped out with known good then the problem must be in the wiring
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Old April 17th, 2018, 02:57 PM   #9
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Quote:
Originally Posted by jraef View Post
Oh oh ... you've opened the floodgates now...
Quote:
Originally Posted by GaryS View Post
Anything you can do with a DC drive system I can do with an AC drive system and a lot more.
I would welcome that discussion anytime There are many advantaged of an AC drive over DC.
I think that there are fewer arguments today than 10-15 years ago about the advantages of DC drives/motors.

I am just glad I no longer have to polish commutators and do PMs on brushes.
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Old April 17th, 2018, 03:27 PM   #10
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Let us know what they find. My money's on the field windings...
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Old April 17th, 2018, 03:30 PM   #11
Gene Bond
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Quote:
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Anything you can do with a DC drive system I can do with an AC drive system and a lot more.
I would welcome that discussion anytime There are many advantaged of an AC drive over DC.
360hz noise is hard to produce with an AC drive

For the most part, I agree. It's difficult sometimes to fit an AC motor in the limited space a DC motor used to fit, but usually it's not an issue.
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Old April 18th, 2018, 09:18 AM   #12
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My DC motor has 4 field wires marked f1-f2-f3-f4 and the wire diagram shows f2 &f3 wired together with my leads on f1 & f4. If they marked this incorrectly and I only had half of the field energized would I experience these symptoms. I didn't check the continuity before sending it back to motor shop. Just a thought
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Old April 18th, 2018, 09:31 AM   #13
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What dos the motor name plate show. i would bet it's a dual voltage field 250 -500 VDC
f1-f2 is one winding f3-f4 is the other. 500V wire the windings in series. From your description I would think its a 500V winding. the most important thing with the field is the current at rated field current the motor RPM max is base speed then you go into field weakening for extended speed ( speed above base speed) for field weakening you lower the field voltage this lowers the field current
Most DC motors show the field as voltage and min - max current
if you have no field current the motor will not develop torque
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Old April 18th, 2018, 09:35 AM   #14
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This is what I have, Thanks
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File Type: jpg IMG_6216.JPG (111.8 KB, 44 views)
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Old April 18th, 2018, 09:51 AM   #15
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There you have it 500V armature volts
Field 300/150 V
i can't quite read the field ohms
measure the ohms in the field from the drive side it should be close the plate value
don't forget series parallel resistance of the field winding in your case the windings are in series
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