Paired DC Motors 3

[gearcutter]

I'm in the middle of conducting a load analysis of a gear pair for a customer and am attempting to figure out the torque the gears are required to transmit.
Part of the information I've been given is that there are two motor & gear pair assemblies which both drive off a common rack. Both of the drives are attached to the same mechanism. I’ve been told that the total amount of power available is 70Kw and that both of the motors are 35Kw and connected in series and powered by the same drive.
I don’t know a great deal about DC motors or electrical circuits, but I’m having a problem understanding the information I’ve been given as I am under the assumption that two equal loads in series yields a total system power of half of one of the loads, or in this case 17.5Kw. For the system to be rated at 70Kw the two equal loads would have to be set up in a parallel configuration.
Would someone please explain the correct way of looking at this problem?
I have confronted the Electrical Engineer responsible for the system and he has assured me that what I’m saying is incorrect.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[waross]

If you have a 70 kW drive designed to power two 35 kW DC motors in series, the maximum voltage of the drive will be twice the rated voltage of the motors.


Bill
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"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

I usually hate to do mechanical comparisons. But, this time, it seems fit to do so.

Your DC motors can (in this case) be regarded as hydraulic positive displacement motors. Which means that their torque is proportional to pressure and speed is proportional to volume flow.

As you know, you can connect two such motors in parallel and have them rotate independently. Or you can connect their shafts via a gear arrangement and have them share the work. Doing so, you will notice that load sharing may be less than perfect.

Now connect these two motors in series and let them work on a shared load. What you get now is a system where you have the same flow in both motors and also the same speed. That means that each motor takes half the load. It is as simple as that. Your idea about halving the load and arriving at 17.5 kW is something you have to think about. It doesn't work that way.

Apply this to a couple of DC motors: Pressure is voltage (volts) and volume flow is current (amperes). Sharing same current (which they do when connected in series) means that they excert same torque. And their speeds are the same because of the mechanical coupling. So their loads are identical.(There is, for the purist, some concern about excitation being identical. We leave that for now.)

Of course, as Bill said, you need to add the two motor's voltage to make them run at full speed. But I think that that has already been taken care of.

And, yes. Your electrical engineer is correct; you are wrong in your assumption. I think that you perhaps are thinking about connecting resistors in series. Yes, then you will have half power in each resistor. But you must remember that motors are NOT resistors. Entirely different laws at work here. It is about torque constant, EMF constant and counter-EMF. Resistance has very little to do with this system.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[sreid]

Almost all drives for DC Motors control the current to the armature thus controlling the motor torque. If a drive is controlling two "Identical" motors in series, each will produce the same torque.

 

[roydm]

Gearcutter didn't say the motors were in series, perhaps in parallel.
I remember seeing a diagram showing the armature current of Motor A passing through the series field of Motor B to ballance the torque.
Roy

 

[davidbeach]

The OP clearly states that the motors are in series.

 

[gearcutter]

Thankyou all for your answers and stars for all those which I found most helpful.

Skogsgurra, thankyou for your excellent analogy using a hydraulic system; and yes I was thinking of the 2 resistors configuration.

I can see now why a series configuration for this application is so important if both motors share the same power source and are effectively mechanically linked.

It seems rather simple now, I feel stupid for having questioned the electrical engineer.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[Skogsgurra]

All is well that ends well..

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[roydm]

Davidbeach, you'r right, my new glasses are on order!