DC Motor Help

A member of my local metalworking club needs some help with a DC motor,
hoping one of you with the DC motor know how can offer up some advice.
Dear Marty
Thank you for the reply. The motor is a permanent magnet, 180 volt, 3 HP,
1750 RPM motor. It is very old and the name plate was missing when I found
it. The motor has been checked out by several people who do that sort of
thing, however I don't know if that really means anything.The controller is
a Baldor BC 160,( brand new). The problem- I can put on a pair of gloves,
turn up the RPM to full, grab the shaft and stop it! In other words I can
stop the wood lathe just by making a heavy cut. This slows down my
production. I have alternated the v-belt pully sizes on the motor and
I have also adgusted the approperiate trim pots in the controller to little
avail. I am afraid I need a new motor. Any thoughts?

Mike Blaze >
Reply to
Marty Escarcega
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How many leads does the motor have? I suppose the magnets could have been hammered some time in the past if everything else is correct. It should have a lot more power than what you have observed. If you short the motor leads, how hard is it to turn the shaft by hand? Respectfully, Ron Moore
Reply to
Ron Moore
Measure the current. If the motor has lost magnetism, I would expect the current to be very high, perhaps limited by the controler. It may be that the controler is defective, even though new.
Reply to
Hi Marty
Sibce nobody can really know what this motor actually is, without a nameplate, Mike's problem wont be easy to resolve *by mail*. If this motor is old and included in an operation where "production" is important, it might be worth considering an upgrade to anything thats considered "old". I have alot of doubt about the validity of the data in Mike's description of his motor. I would be impressed by the hand strength of a person who could stall a 3 HP motor spinning at 1750 RPM, by grasping its 1 inch diameter shaft with a gloved hand. Maybe Mike has a faulty 1/3 HP motor.
A 3 phase motor with a VFD would be my recommendation for this application.
Reply to
Jerry Martes
If your report is accurate, the only fault that could occur to cause this indication is faulty magnets. My gut feeling is that your indicator report isn't true. Steve
Reply to
Steve Lusardi
That's a good thought Jerry. It fits with an experience of mine. I had a good, almost new AC induction motor (now sold) which I used for a few science demonstrations at a local school. One of the things I did was to explain the calculation from the output power rating (1/3 hp), shaft speed (1425 rpm) and pulley diameter (about 2 inches I think) of an estimate of the minimum torque required to stall the motor. Most of the kids thought they could stall it with their bare hands when asked. I didn't let them try. If you stalled it gradually it was capable of smoking pieces of wood.
I seriously doubt Mike's data, unless the motor is connected in completely the wrong way. Mike might even have a faulty 1/8 hp motor. 1/8 could look like 3 in a faulty stamping.
Reply to
Christopher Tidy
If no-load speed is close to rated 1750 RPM at rated voltage (180VDC) then the field (wound or permanent magnet) must be OK. If the field were weak then the motor would overspeed. Yes, overspeed. With weak field it must go faster to produce near 180 volts of counter EMF.
Put a voltmeter on the motor leads, see what happens when the motor is loaded. If the voltage drops, the problem is in the controller. It could be just misadjusted, too small a controller for the motor, or it might be faulty.
Reply to
Don Foreman
This is one of those electrical things I'll never understand. I do know it to be true because my 10EE used field weakening to run the higher RPM's. Maybe that's why I tossed that terribly complex DC drive and put a VFD on. The OP should maybe look at the VFD route too.
Reply to
Karl Townsend
DC motors are easy, especially permanent-magnet ones.
The DC resistance of the armature winding is very low, such that if connected to power, at least ten times rated current will flow, and the rotor will soon burn.
That same armature, if in the motor and spun, will act as a generator. The faster it spins, the higher the generated voltage, in direct proportion.
So, if one hooks a PMDC motor to power, it will spin faster and faster until the generated voltage is almost equal to (slightly less than) the supply voltage. The difference between supplied and generated voltages will drive a current through the armature DC resistance sufficient to provide sufficient torque to maintain the rotation rate.
If one loads the motor down, the generated voltage will drop, the difference between supplied and generated voltage will rise, driving an increased current through the armature. This increased current will generate sufficient torque to keep the armature turning at a constant speed.
If one reduces the strength of the magnets generating the field in which the armature rotates, less voltage will be generated for a given speed of rotation, so the motor must spin faster to achieve the near-balance of supplied and generated voltages.
So, it's all in the simultaneous balance of current (proportional to torque) and voltage (proportional to speed).
Joe Gwinn
Reply to
Joseph Gwinn
specs for the baldor controller are here
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1. measure current at stall 2. measure armature and field voltage at stall 3. report results here.
my guess is that he's wired it as a series motor
Reply to
William Noble
There are very few motor faults that would make a permanent magnet motor behave in this way and most of them would result in smoke pouring from the windings.
The most likely cause is the correct voltage not reaching the motor - what happens when you hang a voltmeter across the brush connections?
If the voltage sags well below nominal it may be either a faulty controller or a mis set current limit.
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