Run away shunt wound motor

[S'mise]

I wired up a 6 lead 1hp DC Baldor motor to a KBIC240 speed controller the other day, and when I powered it up it had run away RPMs like it had an open field.
I quickly shut it off, and when I power it back up... Nothing. I presume because the drive had fried.

I wired field terminals to F1 and F4 with F2 and F3 together (for high voltage) and then Armature terminals to A1 and A2 of motor.

What did I do wrong? I didn't pay attention to polarity with the assumption that it would only impact rotation.

Now I'm Scratching my head.

 

[GoldDigger]

One hypothesis is that the speed controller was defective and applied too low a field voltage.
A second is that it is a compound motor with a small series winding that is supposed to supplement the shunt field. In that case reverse polarity on the shunt field winding would cause the two field components to oppose rather than add.

Sent from my Pixel 4a using Tapatalk

 

[S'mise]

Thanks GD.
Another thing I didn't check was the value of the plug in resistor which is sized by horsepower. Oops.

Well, ....is it a compound motor?
The title on motor says shunt wound, so I thought it should be wired like the top diagram with separate field connections.

I also see in the bottom diagram, it shows field and Armature combined, wiring it as a 2 wire PM motor.

The Armature is 180v and the field windings can be parallel for 100v or series 200v.
I don't see that the KBIC240 can be used with a 2 wire connection. I assume if it did, it would specify and wither to wire it high or low voltage?

Confusing to this AC motor guy.

 

[paulengr]

Four F leads can be tricky. Sometimes they are internally meant to be parallel so since you wired in series you had half the voltage. Finding documentation is often challenging. I always call the motor shop on these (spread the blame). I have a way around this.

Also Baldor is notorious because unlike everyone else they use a different connection on the fields internally. I don’t remember the details but it had something to do with TIG vs crimped or soldered. Baldor factory motors are notorious for failed connections right at the field connections. We have had brand new ones fail. They don’t test in the factory. If this isn’t a hint that Baldor is one of the more cheaply made motors despite the gold paint jobs I don’t know what is. In our shop we routinely change them over to the same connections everyone else uses. Sorry I can’t explain any better but I don’t wind motors.

I’ve seen one over speed from a defective drive. It sounded normal at first but in something like 2 seconds before I could stop it, it got so hot it slung all the solder out. It was a Baldor motor. Since then if I have a suspect drive I learned to leave A1 disconnected.

Ohm the field and check. The name plate will usually gives Volts and amps and Ohms law won’t match up. Sometimes it gives ohms. So trust the amps and your ohms reading to check if it’s close. Counter EMF will give you a bigger VD ((loser amps) when you put power on it. It’s just checking for shorts/opens and things like making sure F1-F4 are wired correctly. Test the individual coils. This will detect a mislabeled S1/S2 or an individual short/open. Use your head. If something doesn’t seem right, it isn’t.

Power up strong field only. Test F1 and F2 for DC and true RMS AC while in strong field first. That way the motor can’t over speed. Don’t wait very long if you see AC which is a bad rectifier...it will fry the motor fairly quickly. A few Volts is fine. More than that and replace the diode/rectifier/SCR/drive.

As to “strong” and “weak” field those little KB analogs are strong field only. In strong field you use name plate field voltage like on a fixed output drive. You will get up to name plate torque and RPMs but that’s it. Now if you reduce the field voltage max RPMs will increase significantly but torque is also reduced because we are field weakening. But if field voltage gets too low, much less than 30-50% of name plate, the motor over speeds...the commutation can’t keep up and it starts arcing because there are limits on how fast the magnetic field can reverse on the rotor. Too fast and it arcs because the residual field is no longer passing through zero. Everything gets very hot very quickly to say nothing of what the load is doing. Or the bearings fail.

 

[S'mise]

Thanks Paul.
I notice larger motors require less resistance (resistor on DC drive) than smaller HP. I believe this resistor is in series with the field so that means more field current and less VD so a stronger field.
If resistor on driver was too big (more ohms) I think less of a field was there to hold back RPMs of a larger HP.

Does that make sense?

Motor is A=180 and F=100/200.
So the KBIC puts out 200vdc field. Wouldn't higher voltage mean wire windings in series?

I'll check windings and compare ohms tomorrow.

As for voltage testing, it had 40v at Armature on DC scale but my fluke meter was indicating AC signal.
Perhaps my fluke was reading switching DC voltage as AC. Motor wasn't moving with 40v so something is fried.

How can motor even run with A1 disconnected?

By Strong field.. I think you mean high voltage (200v)?

I suspect I'll be ordering a new drive.

 

[paulengr]

You leave A1 disconnected while you straighten out any field problems. Remember a DC motor is variable speed. Zero is fine. Unlike AC even though it’s not much current it is critical to the motor to have a rock steady field. If it’s too low or not steady (AC), you should not energize the armature.

In strong field (name plate current) armature voltage minister counter EMF (less than 10%) as a percent of name plate voltage equals RPM. Armature current is torque. Unlike AC where flux and torque are combined DC is far easier to troubleshoot.

 

[S'mise]

OK, Armature is 180v 5a nameplate and field is .3A @200v.
That means i should wire motor as high voltage with the field windings in series, correct?

I imagine it would read close to the 200v field with speed pot turned all the way up but how can you accurately measure current without the motor running (A1 disconnected)? Surely the Armature spinning has an effect on it.

 

[gar]

202025-1415 EDT

You need to understand the basics of a DC motor.

A basic DC motor consists of a magnetic field that is fixed in space, of constant magnitude, and does not rotate in space. This produces a constant magnetic field fixed in position in space. In a wound field vs a permanent magnet motor the current to the field can can be changed so the field magnetic intensity can be changed. This means I am talking about an original type DC motor, not a DC brushless which is really a synchronous AC motor. A basic DC motor will have an armature with a number of coils wound on an armature that is mounted on bearings to allow rotation, and the armature coils are connected to a commutator that provides a synchronous switch via fixed brushes. The fixed magnetic field interacts with the armature field to produce a force that produces a torque to rotate the motor.

Because of armature rotation there is also a voltage induced in the armature coil. This induced voltage is in opposition to the voltage applied to the armature, and is called counter EMF. This counter EMF is directly proportional to armature speed, RPM. In an ideal motor, no losses of any kind, this counter EMF would exactly equal the applied armature voltage.

Thus, for a fixed field intensity, constant current to the field, the motor RPM is directly proportional to the applied armature voltage.

Now, if you reduce the fixed field intensity to 1/2 its previous value (1/2 the current), then the armature has to rotate 2 times as fast to produce the same counter EMF. Any good shunt wound DC motor control circuit must have a loss of field excitation circuit to cut armature power on loss of field current. In our DC machinery labs one person in the group was assigned the task of being that field monitor. Typically we worked with 5 HP motors, and if one ran away it could fly apart and kill someone.

In a test machine that was used to test Dana transfer cases for the Ford Aerostar I supplied the testing gear, and I had to interact with 3 50 HP shunt wound motors. These were GE motors with blowers so full torque could be supplied at zero RPM. The motor controls were equipped with loss of field detection. With full field excitation maximum RPM was about 2000. By reducing the field during part of the machine cycle we could increase RPM to 3000, but at reduced torque.

This transfer case had three shafts, a differential, a locking clutch, and means to detect differential speed between front and rear drive shafts. If angular difference between front and rear was excessive, then the clutch was engaged making it a solid transfer case. A very effective automatic transfer case. In normal operation torque to the rear was about 65%, and front 35%, but the front and rear wheels did not have rotate at precisely the same speed. When locked up the torque balance was road determined. On a wet pavement with plastic marking strips I could accelerate across these with no apparent slip. That was the speed of the automatic lock up. An ordinary solid transfer case forces both front and rear to be exactly the same speed. That results in tire slip on dry pavement, tire wear.

.

 

[S'mise]

I get the basics, I was just looking for some clarification on wither I wired it correctly to match specific driver KBIC240. YES, NO?

I mentioned that a incorrectly sized plug in resistor on the KBIC240 may have caused it.
YES, NO?

Looking again at motor wiring illustration again, it has all wiring scenarios tying A and F together (2 wire feed), instead of separately fed Armature and Field (4 wire) like the KBIC240 diagram shows. This leads me to believe the driver has to be hooked up as 2 wire.

 

[paulengr]

OK, Armature is 180v 5a nameplate and field is .3A @200v.
That means i should wire motor as high voltage with the field windings in series, correct?

I imagine it would read close to the 200v field with speed pot turned all the way up but how can you accurately measure current without the motor running (A1 disconnected)? Surely the Armature spinning has an effect on it.

By name plate yes “high voltage”. But obviously something is wrong so first we test the motor offline as much as possible then test the drive outputs before jumping into a running system. So take it one step at a time.

Offline we expect roughly 200 V / 0.3 A = 667 ohms. If as you described we would expect around 600-700 ohms total and about 300-350 ohms across each of the field coils (F1-F2, F3-F4). Start there. Check the open circuit output of the drive. Then hook up the fields only and test again DC and AC ripple. If it is close to 200 VDC with only a few Volts of AC at most we move on to full motor testing. Check armature ohms too although you can only really do this with a milliohm meter. Don’t forget to take the covers off the Comm, clean out any carbon build up. Do NOT take off the film on the commutator. But check brush wear, springs, and if it has been running, film pattern on the comm. Leave the doors off for now.

Repeating...the field and armature interact magnetically but electrically they really aren’t connected much at all. So unlike AC we can work on the field without powering up the armature but not the other way around. The speed pot should have NO effect on the field with a fixed field drive. On a more fancy drive at high speed the field voltage will decrease so you set it to zero if it does not use a fixed field.

It is also possible someone messed up the field labels but this is a little more rare. Use a cheap magnaprobe to measure this with the field powered up, the magnetic pattern should be even all the way around. Unlike an AC motor the frame of the DC motor is part of the magnetic circuit.

Once the field supply is verified then hook up the armature and have at it because there are no longer any field concerns. As you start up and run watch for arcing either from a rough spot or a brush issue if the neutral has somehow shifted which is more common in larger motors. Some minor arcing is normal but arcs extending out past the brushes isn’t.

There is a lot more with DC motor maintenance but this is basic troubleshooting and startup.

 

[S'mise]

Thanks for your help and explanations.

The field coils were indeed 350ohm, 700 together. With amature disconnected it only had 100v after replacing bad drive (with similar model) so I wired them parallel for low voltage in 2 wire configuration as motor states. Obviously it was a LV output drive.

It's running fine now.

I believe the on board resistor was the original cause of failure. It had a .025 ohm for 1/2hp when it should've been a .01ohm for a 1hp.

I think I'll stick with the motor diagram in the future.
Had I wired it 2 wire as opposed 4 wire (separate F and A) the resistor value wouldn't be an issue.

Paul, I'll definitely use your undo A1 wire trick to double check field voltage in the future.

 

[paulengr]

As I said DC is much easier to work on but techs that understand them are retiring out. I work for one of the largest motor shops in the country. I think five of us out of about 75 employees really know DC. Only 2 of us deal with it frequently.

 

[kwired]

Also Baldor is notorious because unlike everyone else they use a different connection on the fields internally. I don’t remember the details but it had something to do with TIG vs crimped or soldered. Baldor factory motors are notorious for failed connections right at the field connections. We have had brand new ones fail. They don’t test in the factory. If this isn’t a hint that Baldor is one of the more cheaply made motors despite the gold paint jobs I don’t know what is. In our shop we routinely change them over to the same connections everyone else uses. Sorry I can’t explain any better but I don’t wind motors.

I started noticing failures on brand new Baldor motors the first time energizing them a few years ago. Kind of put two and two together and think it started happening about the time ABB acquired them. Seemed to be better products before then.

 

[paulengr]

I started noticing failures on brand new Baldor motors the first time energizing them a few years ago. Kind of put two and two together and think it started happening about the time ABB acquired them. Seemed to be better products before then.

Thats in general. The issues with Baldor started way before ABB. It really started before or during the Reliance acquisition. That’s when it got so bad for instance AB dropped them and went to Marathon for their private labeled motors.

The issue I’m referring to though is specific to their DC motors, not AC.

 

[kwired]

Thats in general. The issues with Baldor started way before ABB. It really started before or during the Reliance acquisition. That’s when it got so bad for instance AB dropped them and went to Marathon for their private labeled motors.

The issue I’m referring to though is specific to their DC motors, not AC.

Never used to have much trouble with Baldor AC motors until about 5-8 years ago, and yes possibly was about when Reliance acquisition occurred that I started to notice problems. I have only had one or two that were motors I happened to purchase and sell to a customer, but have had several others that were supplied by equipment sellers with the equipment that failed either immediately on first startup or within first few weeks as a result of poor construction/QC issues with some component. Those equipment guys situations are best for me - no extra cost or headaches for me since I didn't supply it.

 

[paulengr]

Never used to have much trouble with Baldor AC motors until about 5-8 years ago, and yes possibly was about when Reliance acquisition occurred that I started to notice problems. I have only had one or two that were motors I happened to purchase and sell to a customer, but have had several others that were supplied by equipment sellers with the equipment that failed either immediately on first startup or within first few weeks as a result of poor construction/QC issues with some component. Those equipment guys situations are best for me - no extra cost or headaches for me since I didn't supply it.

I work for a motor shop. The remnants of Reliance (IPI) are a thorn in our side but the Charlotte market is highly competitive even without them. We do business with Baldor. It used to be a lot more but when ABB acquired them they walked away from existing pricing structures that made them no longer competitive. We had to minimize our exposure and increased sales of others. We are still an authorized service shop. Within ABB the ACS and ACH drive lines were very popular but ABB got the JIT bug and now everything has ridiculous lead times so we had to rotate drive vendors too.

The other problem Baldor is having is that they’ve always been known for decent quality rolled steel motors. Not so much in the cast market where the two traditional big names (GE and Westinghouse) were highly competitive. Never mind US Motors (NIDEC), Marathon, Leeson, Oriental, and other lesser known names. Due to increased efficiency requirements most motors now require a cooling fin body instead of smooth which is far easier to manufacture in a cast frame. So the rolled steel frame is not as competitive as it once was.

 

[kwired]

Due to increased efficiency requirements most motors now require a cooling fin body instead of smooth which is far easier to manufacture in a cast frame. So the rolled steel frame is not as competitive as it once was.

I don't get this, if it needs to dissipate more heat how is that an efficiency improvement? I believe many higher efficiency motors have lower power factor than their predecessor, but do have less kW input rating. Heat lost is real power to the supply source.

 

[gadfly56]

I don't get this, if it needs to dissipate more heat how is that an efficiency improvement? I believe many higher efficiency motors have lower power factor than their predecessor, but do have less kW input rating. Heat lost is real power to the supply source.

Not a motor guy, but going back to the 80's, high efficiency motors became a big thing in the chemical industry. I think it was partly accomplished using higher density fields in a smaller form factor, leaving less metal to dissipate the heat. Hence the cooling fins.

 

[paulengr]

Not a motor guy, but going back to the 80's, high efficiency motors became a big thing in the chemical industry. I think it was partly accomplished using higher density fields in a smaller form factor, leaving less metal to dissipate the heat. Hence the cooling fins.

Yes. The goal is less of a core and packing more copper in to decrease copper losses (resistance) so poorer cooling thus increased cooling. Also the fan itself is now about 1/4 of the wattage it used to be as integral fans became a bigger loss than core/copper losses.