Motor drive for big old shunt wound motor

[ArmchairEngineer]

Hello All,
I'm new to this forum so please excuse me if this an inappropriate place for this post

I've got the job of replacing the motor drive on an old piece of machinery.
The motor is a big old shunt wound thing, several hundred amps at about 140Vdc (from memory). It is constructed such that is not possible to insert a resistor in series with the field winding: the field and armature windings are hardwire in parallel.

Speed control was originally achieved by switching the dc bus voltage from low to high so it had two settings: slow and fast.

The load characteristic is such that the torque is proportional to the square of the speed, so at low rpm the torque load is very low.

I am hoping to find a motor controller that can run from the dc bus and allow linear control of the motor speed but I have had difficulty finding a suitable controller. There are several controllers available for series wound or seperately excited motors however the manufacturers are generally not interested in entering into dialogue on whether or not they will work with my shunt wound motor.

My question is this:
- Is there any reason I cannot use a controller designed for a series wound motor with this shunt wound motor? What makes a motor controller designed for a series motor unsuitable for a shunt wound motor? Do they not simply pwm the supply? Please excuse my ignorance on this subject.

Sadly, replacing the motor is not an option.

Thanks in advance for any help on this,

James.

 

[BradtheRad]

Since it has a field coil, then it would not be controlled the same way as a permanent magnet DC motor.

Speed control was originally achieved by switching the dc bus voltage from low to high so it had two settings: slow and fast.

This sounds as though you can alter motor speed (several hundred amps) by controlling a few amps (through the field coil).

Suppose you were to insert a heavy-duty rheostat where this hi-low switch is now?

Or a variable DC power supply? (I do not know how many amperes it would need to put out.)

 

[ArmchairEngineer]

Hi Brad,

Thanks for your suggestion.
Due to the way the motor is constructed it is not possible to disconnect the field coil from the armature - they are hard wired in parallel (the motor is arranged so that the field coils and armature rotate in opposite directions to provide concentric contra-rotating output shafts. Slip rings carry the DC to the moving field coil frame).

I guess my original question should have been phrased:
- How is an electronic controller for a series wound motor different from that for a shunt wound motor and can you use a series motor controller on a shunt motor?
- If not, why not?

Thanks again for any help

 

[BradtheRad]

Here are some articles I looked at:

https://www.brighthubengineering.co...s/123625-understanding-shunt-wound-dc-motors/



https://www.micromotcontrols.com/htmls/Motor characteristics.html

A shunt-wound motor is designed to maintain the same speed under all loads. One article suggests you can vary it 10-20% via the field coil. Another says there is risk of drawing overmuch current if you reduce voltage to the armature winding, with risk of overheating. I could have gotten it wrong.

Your motor is constructed to operate on enormous power, therefore its operation will be difficult to modify. Any components you install will need to be extremely robust.

What about the idea to PWM the supply? It's discussed frequently in threads here. However your controller would need to switch hundreds of amperes rapidly, and it will be exposed to severe inductive spikes.

Can you consult an expert about this motor. Conceivably he may know of a way to:
* add turns to the field winding
* add a winding at opposing polarity
* add a series winding (creating a compound motor)
* change the commutation somehow
* add high-current scr's which will allow you to PWM either field current or armature current.

These suggestions are off the top of my head. I do not know enough about motors to say what may or may not be feasible in your case.

Your boss has put you in a delicate position. Did you have a predecessor who was given the same task? Was he unsuccessful?

 

[mrinalmani]

Well the difference lies in the type of load the drive has to face in the two cases.
CASE 1 (Series motor drive)
The series motor drive has to supply current only to the armature winding. Although the armature winding is a coil, the current in this coil is not by the virtue of a voltage supply, but by the virtue of mechanically induced voltage. There is a big difference between these two types of currents. This current does not result in energy storage in the coil, in fact as soon as the load torque dies out, the current dies out. On the other hand if the current in the coil is by the virtue of an external voltage (not induced voltage due to changing magnetic field) then the coil will store energy, given by .5LI². In short, in the first case (induced voltage) the coil acts as a coupler for energy transfer but in the second case the coil acts as an inductor.
Coming back to series motor, the coil is a coupler (although a small leakage flux will result in leakage inductance). If we ignore the leakage inductance, then there is no problem of voltage spike arising from the armature winding. Because as I said earlier, the coil derives it current from a changing magnetic field and thus acts as a coupler, and NOT an inductor.
CASE 2 (Shunt motor drive)
The shunt coil clearly acts as an inductor and stores energy. Thus will result in voltage spikes. Appropriate snubbers must be used in this drive.

The above fact about the "types of current" can be better appreciated by the example below:
Consider a transformer having no leakage inductance and a magnetizing inductance L_m.
Let magnetizing current = 1A
Let load current = 20A
Now the energy stored in the magnetic system is .5L_m*1² and NOT .5L_m *20²

Conclusion:
Series motor drive sees a primarily resistive load.
Shunt motor drive has to face a large inductive load.

 

[ArmchairEngineer]

Dear Mrinalmani,

Thanks for a very thorough and informative response, and for filling a significant gap in my understanding.
So one good reason that a series motor controller may not be suitable for driving a shunt wound motor could be lack of suitable protection of the output bridge.

I think I need to do a bit more reading on this subject!

James.