DC Generators- Help!!

Thread Starter


Joined Dec 4, 2011
—A DC motor is rated at line conditions of 230 V and 27.5 A at normal full load. It turns at 1750 rpm under rated conditions. It has total armature circuit resistance of 0.803 ohm, and it is desired to hold its maximum starting current to 150 percent of normal full load current. Determine

The rotational speed that may be expected when the motor has accelerated sufficiently to reduce the line current to the normal value.

Thank you very much for the help folks.
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Joined Oct 26, 2011
This is a trick question--the answer is provided--before I realized it, I had gone through all the following stupid stuff:

Rated DC armature voltage is not provided, but can be assumed to be the standard 180VDC at base speed.

The DC armature current is not provided and cannot be realistically determined unless the AC line current is assumed to be average current (average line current is rather meaningless--analog meters measure average and scale it up to indicate RMS of a sine wave, which this is clearly not)

At base speed, it may be assumed that there is no free-wheeling diode current, or perhaps you have a full converter that has no free-wheeling diode--this avoids the variable of the armature inductance--at low speeds, no assumption may be made as there is significant free-wheeling diode current.

Using this convoluted logic, you may assume that the rated armature current = 27.5ADC = average AC line current.

Calculate the voltage drop across the armature resistance


Joined Mar 6, 2009
Perhaps the "line voltage" is simply a reference to the DC bus voltage - or 230V DC. There is no evidence the question is referring to AC conditions.

I think the question can probably be solved as a DC motor example.

However, it's quite unclear what the question is actually asking. Is the intention to find the speed at start-up, at which condition the motor starting current falls below 1.5x the rated current of 27.5A with a DC bus voltage of 230V. Is the bus voltage or current being controlled in some manner to limit the current? Is the motor being started with a mechanical load or not?

Perhaps the OP could come back with some clarification.
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Joined Feb 19, 2009
It's either a trick question, or the person who wrote it needs a beating on how to properly write trick questions.

A DC motor is rated.. It turns at 1750 rpm under rated conditions.

Determine The rotational speed that may be expected when the motor... reduce the line current to the normal value.


Joined Dec 26, 2010
Is the idea to find the starter resistance required to assure 150% current, at 230V input and nil back emf, from which to find the back emf required to drop to the rated current?

If the back-emf under normal conditions is calculated, would you be able to say that the rpm is pro-rata?


Joined Mar 6, 2009
If one ignores all the mechanical losses and assumes a constant armature voltage (emf) per rpm then presumably the motor would speed up until its armature voltage was 230V and there was zero power loss at equilibrium. All very unlikely.

I agree - the question wording is entirely ambiguous.
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