Troubleshooting a DC Compound Motor: Negative Effects and Solutions

[kenneth edmiston]

TL;DR Summary

DC Stab Shunt Motor. HELP please!!

What are the negative effects (if any) of running a compound DC motor with the series field open? I’m troubleshooting a 100hp recoiler motor on a metal slitter machine that constantly blows line and load fuses. It is on a DC drive. The drive has been replaced and the motor has been rewound.

The shunt field is connected to separate output from the drive (like normal), but the series field is open. Not connected. The motor blows the fuses seemingly at low speed. I’m curious if connecting the series field will help produce more torque and lower the armature current. I am uncertain if the drive contains field loss safeties so I’m worried if it looses shunt field the motor will take off down the highway.

I am just curious if it is possible that by not connecting the series field in a compound motor (using it as a shunt wound), the armature current could be effectively raised causing blown fuses.

Thank you in advance

 

[NascentOxygen]

Is the speed being controlled by varying the shunt field voltage and the armature voltage as one? Then at slow speed there'll be greatly reduced back emf, allowing high armature current.

 

[kenneth edmiston]

The DC drive has an output for the armature, and a separate output for the field. I’m not getting much information about the incidents, but I do know that at least 3 of the times it blew fuses at 300 feet per minute, 150 fpm, and 120fpm. All with “light load” as in, they were pulling thin gauge metal. Rpm of the motor is 650/1950 at 500v arm and 150v field.

I’ve disconnected the coupling and ran resistance with multimeter through the commutator, rotating the shaft. And did the same with a 1000v meg with respect to ground. I’ve megohm tested all the field windings also. As well as tested other components ie snubber circuit, reactor etc.

I know there is a culprit hiding somewhere, I just can’t put my finger on it yet.

This motor pulls sheets of metal through a splitter machine. Variable speed control is a must, but apparently starting torque is not since they left the series field open. I’ve verified another motor in a separate line is wired identical (open series field)

I was also told that the motor is dropping rpm uncommanded which led me to believe the drive is sending incorrect outputs, or the load was too high and torque too low. Or a short winding etc. ...the drive is 9 months old, the motor has been rewound in the last year, and I verified resistance.

Could connecting the (currently shunt wound) motor into compound potentially curb excess armature current while maintaining proper speed control?

 

[Asymptotic]

Rpm of the motor is 650/1950 at 500v arm and 150v field.

If this 650/1950 RPM is from the nameplate, 650 RPM shaft speed will be at full armature voltage at full shunt field current, and 1950 RPM at full armature voltage at minimum (fully field weakened) shunt field current.

650 RPM is "base speed" (full armature voltage at full shunt field voltage). Up to this point in the speed range a motor operates in a 'constant torque/variable power' regime. Multiply nameplate armature full load current by armature voltage from 0 to 500V, and you'll see available motor power follows armature voltage. To this point, shaft speed also follows armature voltage.

At base speed and above, armature voltage is limited to maximum nameplate value (500V). In order to run faster than base speed it is necessary to weaken the shunt field. This regime of operation above base speed is called 'constant power/variable torque'. If the shunt field controller is operating properly, shunt field voltage (hence, field current) will begin to fall off at base speed (650 RPM/500V armature), and continue to drop until the motor shaft is turning at the maximum speed (1950 RPM) specification.

Chicago Slitter has a brief overview of a typical recoiler application.
https://chicagoslitter.com/resources/tech-tips/recoiler-motor-drives/

Summary: DC Stab Shunt Motor. HELP please!
I am uncertain if the drive contains field loss safeties so I’m worried if it looses shunt field the motor will take off down the highway.

Double and triple check this. I'm not aware of any commercially available DC drive designed for field weakening control that doesn't have safety interlocks, and for the reason you've stated. If shunt field excitation drops too low at low shaft loading, the motor will overspeed, and quite possibly spin so fast that it rips itself apart.

What make and model drive is used? Can you post a legible photo of the motor nameplate?

I’ve verified another motor in a separate line is wired identical (open series field)

You've indicated this is a stab shunt motor, which means the series (stab; speed stabilizing) field was designed to provide just enough additional magnetic flux to overcome speed reduction caused by increased shaft loading.
https://chicagoslitter.com/resources/tech-tips/recoiler-motor-drives/
I've never worked on a recoiler, but if the stab shunt isn't used on either the properly functioning or problem-causing installation, it could be the speed stabilization it provides acts at cross-purposes in this application.

By 'open', do you mean it isn't connected? If it were wired into the armature circuit but open, the motor wouldn't run at all. If disconnected, are the stab field leads taped off? If they can come into contact with one another (either directly, or through conduit box steel) I'm not certain what would happen, but doubt it would be good.

I was also told that the motor is dropping rpm uncommanded which led me to believe the drive is sending incorrect outputs, or the load was too high and torque too low.

Is speed control open loop (armature feedback) or closed loop (tach/encoder feedback)?
If open loop, shaft speed sags as mechanical load increases, which is why closed loop is often preferred.
If closed loop, tach feedback ought to maintain set point speed. If it doesn't, the question usually becomes "what other limits is this controller hitting up against?" Armature current limit is often implicated.

Another possibility is improper loop gain settings. A typical DC drive controller has two loops:
1. Speed loop (tach feedback compared against speed command)
Also known as "major loop", and "outer loop".

2. Current loop (current feedback compared against current command)
Also known as "minor loop", and "inner loop".

It depends on the manufacturer and when the drive was designed (digital drives usually have full PID (Proportional-Integrating-Derivative) options; older analog drives were often PI only) but if gain on either loop is set too high the controller will become unstable. Gross instability is rather easier to spot due to continually large shifts in output with little or no change in command. If gain is marginally too high (particularly current loop gain), the drive will run without excessively bad behavior when presented with low magnitude transient overloads, but commands massive current output on larger transient overloads.

... constantly blows line and load fuses. It is on a DC drive. The drive has been replaced and the motor has been rewound.

If it was blowing line fuses only it might suggest misfiring SCRs, or other power bridge-related issue. Load fuses clearing suggest high armature current.

Nailing this down is, at least in part, an exercise in minutiae.
First, get the drive and motor nameplate data, and find the drive and motor installation and operating manuals. It is difficult to troubleshoot a problem if you don't know what the manufacturer documented their equipment is capable of doing.

Second, measure, measure, measure!
This sounds like an intermittent problem, so if a suitable data recorder is handy (one capable of handling high voltage inputs; a 0-10 V recorder input connected to a 500V armature is a bad idea) it's a good time to set it up. If the drive features a communication link, monitor and log relevant performance data.

Suggested monitoring points
- Speed command
- Speed feedback (tach)
- Armature voltage
- Armature current
- Shunt field voltage
- Shunt field current

Let's say high armature current is involved. There are many ways high armature amps can occur. A few, off the top of my head:

- physical load with intermittent binding (a bearing just beginning to seize, etc.)
- uneven feed into recoiler (metal pulling taut upon occasion)
- tach to motor coupling slipping (causing motor to speed up, pull metal taut, and trigger armature overload).
- loose wire connection in shunt field (suggested if rapid and intermittent field voltage increases are observed in conjunction with matching reductions in field current).
- loop gain(s) set too "hot".

Another option, and a good one, is to call in a field tech from the recoiler manufacturer, and have him or her dig into the issue. Provided they have worked in the trenches for awhile they'll be experienced with both common and uncommon problems encountered in this application, and will make sense of it rather quickly. Have your best in-house tech unobtrusively bird-dog the factory service tech to follow what they are doing and pick up pointers (stress "unobtrusive" - nobody thinks straight if their elbow is continually joggled).

I'm assuming proper drive setup (particularly, loop gains) are critical in this application, and a good factory tech ought to rapidly ferret out any such issues.

 

[kenneth edmiston]

Tom, thank you very much for your reply. I’ve read about half so far, but I am currently working. Please allow me some time to review the rest of the information and I will get back promptly. Thank you for your speedy reply. I do have nameplate info and drive info. I will attach those in a bit

 

[Babadag]

Magnetic flux of the shunt field it is:

φsh=kφsh*V/Rsh

where kφsh it is a constant depending on shunt coil number of turns and its

magnetic circuit.

V=supply voltage

Rsh =shunt circuit resistance

Magnetic flux of the series field it is:

φse=kφse*Ia

where kφsh it is constant depending on series coil number of turns and the magnetic circuit

Ia=armature current

V=Emf+Ia*Ra

The counter electromotive force[Emf]:

Emf=ke*(φsh+φse)*rps

rps=rotations per second

If no φse exists then at low rps Emf is low.

Ia=(V-Emf)/Ra

If Emf is low then Ia is high.