Load distribution of DC motors.

[gearcutter]

This may be for another forum but it would be interesting to hear from someone that frequents the gear & pulley forum.
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 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

 

[eromlignod]

Are you sure they don't mean that each motor is connected in series with itself? Sometimes wound-rotor DC motors are wired with the field and armature windings connected in series, rather than parallel.

Don
Kansas City

 

[electricpete]

maybe they mean the motors have series-connected field vs shunt-conntected field?

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[Compositepro]

D.C. motors that have to share load are connected in series so that the same current will pass through each one. The supply voltage or rated motor voltage will be adjusted so that they work properly in series. Motors connected in parallel will turn at slightly differrent speeds and one motor will carry significantly more load than the other.
D.C. motors are great for variable speed drives because current increases rapidly when load increases and tries to slow the motor. Thus the motor speed stays fairly constant with constant supply voltage. But two motors are not identical enough to share load when in parallel electrically.

 

[sreid]

The most important thing that makes the motors share the load is that the drive usually controls the current to the motors.