I am currently studying for a electrical engineering related exam and came across this problem.

What exactly is the difference between the nominal and idle RPM of a DC generator, and which one is higher?

To my understanding the nominal speed of the generator denotes the speed needed to match the power supply frequency (e.g. 50 Hz in Austria), if the generator is supposed to feed back into it.

When talking about a DC motor with shunt, I think understand the principle: Idle speed is reached when the rotor induces the same voltage in the stator as is applied to the rotor itself. Nominal speed is the RPM at which the motor delivers the nominal torque. Here, idle speed is higher than nominal speed.

Are those assumptions correct?


Idle speed usually means the speed at which the generator should be driven by a prime mover when there is no electrical load. A generator with no electrical load is easier to turn mechanically, so unlike your car, its idle speed is faster than its nominal speed.

An unladen generator is faster, but not as useful because it does nothing. You will want to run it at nominal (laden) speed and torque (regardless of whether it is a European or African generator), which is the speed-torque combination that provides peak efficiency for a given electrical load. The speed of the prime mover and the impedance of the electrical load are the variables you can adjust to hit maximum efficiency. Often, one cannot control the impedance, so the speed of the prime mover (perhaps a petrol engine) may be the only adjustable variable.

The nominal speed of a DC generator does not necessary match the national power supply frequency, because a DC generator outputs DC. If you use a DC generator, it's to create a DC supply for some reason, such as generating a high voltage supply to create an ex-parrot (the technical term being "voom").

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  • \$\begingroup\$ As you made me realize, I confused the part about the frequency with a synchronous generator. But other than that I think I understand it now. Applying excitation in addition to the prime mover "starts" my generator and puts it at some lower RPM than idle RPM. Would that be correctly formulated? \$\endgroup\$ – pat3d3r Sep 16 '15 at 6:52
  • \$\begingroup\$ Basically but not quite. The generator will spin at whatever speed the prime mover drives it – although the generator will fight the rotation via torque, ultimately it can't stop a prime mover that's determined to run it at a given speed. Idle and nominal speeds answer the question: what is the recommended speed to run this particular motor at? \$\endgroup\$ – jbarlow Sep 16 '15 at 8:04
  • \$\begingroup\$ Nominal speed is given at 5000 RPM. I have a voltage induced due to the prime mover of 494 V. Nominal voltage of the generator is 460 V. According to my textbook, I have to calculate idle speed out of those three values. I would do that by comparison of ratios 494/5000 equals 460/idle RPM. Would that be correct? \$\endgroup\$ – pat3d3r Sep 16 '15 at 8:20
  • \$\begingroup\$ For a separately excited DC generator, yes, the ratios are "good enough", neglecting magnetization effects. In a real motor, the armature reaction/demagnetization effect is nonlinear and ratio comparison will be wrong. In a shunt excited DC generator the generator modulates its own field, also throwing off ratio comparison even if magnetization effects are ignored. \$\endgroup\$ – jbarlow Sep 16 '15 at 8:42
  • \$\begingroup\$ You are right, it actually stated to neglect the armature reaction and demagnetization effects among other losses. Thank you for the hint about the real motor/generator though, your help is much appreciated. \$\endgroup\$ – pat3d3r Sep 16 '15 at 19:29

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