Dc motors types of compound

leitmotif

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In every class I have attended regarding DC motors the litany goes
series motors high torque poor speed control
Shunt motors lower torque but better speed control
Compound a compromise of preceding two. Then they say short shunt and long shunt.

I have never had the experience of working in a motor shop which as time goes on looks to be a mistake. I am sure I would have more knowledge if I had torn em down rewound and been able to test them.

Recently I have come across new terms stabilyzed shunt and compensated shunt.

I found a previous posting
http://www.plctalk.net/qanda/showthread.php?t=28274

Quoting from it (thank you Dick DV)
"stab shunt motor has less series field and the comp shunt has more"

To summarize and check my understanding:
There are basically two types of compound motors
stabilyzed and compensated shunt.
as Dick said
stabilyzed has smaller series field therefore probably better speed control and regulation (less speed droop more proper?)
Compensated has more series giving more torque and not as good speed control.

THEN both could be either long shunt or short shunt? OR shunt could be supplied by separate source ie sepex.

How come this is not stamped on the nameplate - I have never seen anything more than compound.

So here I am with an old DC compound motor how do I tell if it is stabalized or compensated? If I had the manual I would read it of course but,,, the motor is so old I can find no information on it. It is actually a military aircraft ground support generator made by Jack & Heinz who are no longer in business. All I have coming out of motor is two big terminals (armature) and two small ones for shunt field. It does have commutation windings and does run well. Rated for 30VDC and 400 amp.

I have the motor book written by Rosenberg - I like it a bunch great book for troubleshooting (told me how to find out shunt field correctly connected on compound motor) but maybe a little short on theory. Anybody able to recommend a good DC motor theory book that does not immediately dive into calculus?

Thanks for help

Dan Bentler
 
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Electric Machinery Fundamentals
Mc Graw Hill.

Not a bad theoretical book. Provided you're happy working with complex numbers.
 
leitmotif, at least in my world, the terms "short shunt" and "long shunt" are old and obselete terms dating to when the DC motor's armature and field circuit was powered by a single DC supply.

Being compound wound motors, they had a series and a shunt field. The series field was connected in series with the armature and therefore saw high current. A long shunt motor had the shunt field connected at the outside ends of the high current circuit so it saw the voltage that appeared across both the series field and the armature. A short shunt motor had the shunt field connected across the armature only so it saw only the armature voltage. Today, this is not done. The shunt field is a separate circuit powered by a separate power supply in the DC drive.

The different shunt, stab shunt, and comp shunt motors do have differing behaviors as to speed stability but not radically different. The really unstable rascal is the series motor. The main difference between the three kinds of shunt motors is the amount of additional torque developed in the overload range.

The straight shunt motor has a relatively small increase in torque as the motor is loaded down to stall. The Stab Shunt motor has somewhat more increase in torque as it digs into an overload. The Comp Shunt has a lot of additional overload torque really digging into an overload and pulling hard. The tradeoff for this higher overload torque is an increase in speed variation and instability. But, since most DC motors in industry have speed feedback (tach or encoder), the speed stability issue really fades into the background. It's mostly about overload torque production.

As I've mentioned on here before, the compound wound motors are intended for applications where there is no reversing, such as extruders. If you try to reverse a comp motor, the series field subtracts rather than adds to the shunt field and you end up with very little reverse torque and high armature amps.

Sometimes, a compound wound motor is used to replace a straight shunt motor. The thinking is that the series field is left unconnected. It is true that you end up with symmetrical torque that way but the field will never reach full strength since the motor was designed for both fields. The result is a lower torque motor with a higher speed range. This is more serious on a comp shunt motor than a stab shunt motor due to the larger series field component.

Hope this helps somewhat. I know many oldtimers prefer the DC machines but, for me, they've always been a high risk, caged tiger type of experience. Give me an AC motor that stays near synchronous to frequency any day!
 
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Thanks Dick.

Fully agree give me AC any day of the week. Got my experience on DC motors on Navy nuc submarine - also got my fill of brush changing, ground chasing, repairing fields and cleaning out carbon.

So it seems that the best way to figure out what kind of shunt motor (stabalize vs compensated) I may have is to get factory literature.
Difficult to do on bench with meter and especially so without a controllable load ie dyno.

I thought compound could be reversed BUT you had to reverse just the armature and commutating (interpoles) to do it. Reason I say this is on sub we had a 150 HP DC motor mounted inline on propeller shaft that was a compound (if recall correct) and it was reversible. The controller had more contacts than you could shake a stick at of course. That was Navy who did things just a bit different than industry of course.

Dan Bentler
 
Thanks Dick.

Fully agree give me AC any day of the week. Got my experience on DC motors on Navy nuc submarine - also got my fill of brush changing, ground chasing, repairing fields and cleaning out carbon.

So it seems that the best way to figure out what kind of shunt motor (stabalize vs compensated) I may have is to get factory literature.
Difficult to do on bench with meter and especially so without a controllable load ie dyno.

I thought compound could be reversed BUT you had to reverse just the armature and commutating (interpoles) to do it. Reason I say this is on sub we had a 150 HP DC motor mounted inline on propeller shaft that was a compound (if recall correct) and it was reversible. The controller had more contacts than you could shake a stick at of course. That was Navy who did things just a bit different than industry of course.

Dan Bentler

I have seen this done in industry but very rarely, as you mentioned you need a wack of contacts and extra leads out of the motor. I have also seen stab shunt motors reversed without reversing the series field (ie the fields bucked) in an application where the revers direction required much less torque than the forward direction, not something I would recommend but it worked in this particular application.

Thanks Dick for the excellent explanation.
 
If the internal A lead connection to the S lead is not made up inside the motor and instead brought out to some contactors, you can then reverse just the armature and the compounding will work both ways. But, as Allscott observes, its not done often and is a nasty mess when it is done.
 
To take that thought one step further, it is common on DC traction locomotives with dynamic braking to bring both of the a leads and both of the S leads out to contactors as well. These are series wound traction motors but, it seems, dynamic braking needs all four leads.

Even the locomotive folks are slowly migrating to AC traction. The only thing really holding them back is higher initial price for the locomotive. The railroads are notoriously stingy on paying for anything even when there are clear long term benefits.
 
To take that thought one step further, it is common on DC traction locomotives with dynamic braking to bring both of the a leads and both of the S leads out to contactors as well. These are series wound traction motors but, it seems, dynamic braking needs all four leads.

Even the locomotive folks are slowly migrating to AC traction. The only thing really holding them back is higher initial price for the locomotive. The railroads are notoriously stingy on paying for anything even when there are clear long term benefits.

As you may remember I intend to convert pickup to electric drive - will use AC though. Lot of EV guys claim impossible to do dynamic braking with series. I dont agree know it can be done but the main hurdle is regulation and control. Course there is a world of difference between a one ton car on even a 10% downgrade and who knows how many ton train on only a 3%. Series field would have to have separate control for effective dynamic braking - on train with qualified operator with lots of power its worth it - on car well probably not and difficult to automate to overcome operator lack of qualification.

I read recently where railroads are considering adding a battery bank to take advantage of saving energy from dynamic braking by recharging the battery and then use energy in battery to assist diesel on flat or next hill.

Thanks Dick. If I worked with this stuff everyday I would be more fluent and would not have to rely on memory banks.

Dan Bentler
 

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