How different are dc motor designs from dc generator designs?

Question

Some dc motors can be used as generators as well by applying mechanical torque to the output shaft to induce a current. However, even if a dc motor can do this, I imagine they were not designed for this purpose and thus perform less efficiently when used as a generator rather than as a motor.

In my admittedly naive understanding, dc generators and dc motors are essentially the same machinery, but with inputs and outputs reversed. This leads me to believe that some other design considerations are used to make one direction more efficient than the other.

How differently are dc generators and dc motors designed to make one direction of input/output more efficient than the other? What can one do electrically or mechanically to improve the efficiency in either direction?

In particular, I'm interested in converting a dc motor into a generator and want to know how I can improve its efficiency in converting mechanical energy into electrical energy.

Answer 1

In Ye olden days DC generators were brushed commutated devices. They had a one or more stator windings and an armature winding. Field wound DC generators as well as motors were commonly connected in one of three methods: Series, Shunt and Compound. Without getting into details, each had its own set of strengths and weaknesses. But you only have to remember these two things: the voltage of a DC motor is dependent on its input shaft speed. Current is a function of torque. More voltage means more RPM's and more amps means more newton-meters (or foot-pounds).

So with all that, you need a constant speed source to get a constant voltage. And you need to ensure you have enough torque to satisfy the current demand of your load otherwise voltage drops off. Old automobiles had commutated generators. They couldn't regulate the voltage so they used a range of around 10-14 volts and used a relay that simply closed when the engines speed was within the voltage range. If the voltage went too low or too high, the relay opened. Primitive by today's standards. The Alternator in today's automobile uses a voltage regulation circuit that varies the armature current which changes the field strength based on the stators output voltage. Lower speed means more current to the armature and less current at higher speeds.

So how different were DC generators from motors? Not very different at all. If anything they mostly differed in mechanical design as they were to be coupled to a prime mover (steam, ICE, electric etc.). Though, in much larger dynamos they had adjustable commutator brushes to compensate for the shift in the commutation plane as a result of heavy load characteristics. A hand wheel would turn a worm gear which would advance or retard the commutation plane to bring the generator back into its normal operating parameters. You don't need to worry about this as I am sure you motor isn't megawatt capable.

I am guessing your motor is a permanent magnet type motor. Its nameplate RPM is what you need to spin the motor at to get the nameplate voltage. This means if you have a 12V motor that spins at 6000 RPM, you need 6000 RPM to get 12V. If you don't have a constant speed source you have no way to regulate the voltage. You would need a buck-boost switching regulator to get a constant voltage from your motor.

If you are using this for a renewable energy project like wind or hydro, a charge controller is usually designed for a wide input voltage swing via a buck/boost regulator. Solar panels are a close analogy to a permanent magnet DC generator, no internal voltage regulation and a varying amount if input energy. Sun might be shining bright one minute and a minute late, be blocked by a cloud. So the charge controller does its best to make a useful steady-voltage from its varying input. From there, use storage batteries to capture that power for later use and to act as a buffer for low input events.

And just for reference, an AC motor can also generate power if you spin it faster than its nameplate RPM, usually at synchronous speed. But again, no voltage regulation and a constant speed is needed. More trouble than its worth. Also of note: jet planes use a very elaborate mechanical speed regulator to produce constant shaft speeds which ensures a constant 60 or 400Hz AC frequency as the throttle is varied.

Answer 2

DC generators, or dynamos of any significant size are rather rare these days. It's much more common to use an AC generator (alternator) with an external rectifier.

I don't have an authoritative answer to your question, but I can't really see any reason that they'd be different in any significant way. All of the issues relating to mechanical strength, magnetic field design and the electrical characteristics are pretty much the same, regardless of which way the power is actually flowing.

Answer 3

Well, first, the dynamos do need either some kick-start power or permanent magnets, as they can't depend on electromagnets in the stator only; you don't generate any electricity by moving a wire without any current next to another wire without any current.

A permanent magnet DC motor will act as a dynamo if you provide rotation power to its axis, so there's not much difference here. The commutator may be aligned a little bit differently, ignoring the need to cut off early, when the magnetic field would brake the motor; there would be no concern about not creating a 'dead zone' where the motor wouldn't start, not pulled by a neighbor magnet. If anything, the device would be considerably simpler.

Now if you want to engage electromagnets for the stator, there will be some differences... usually they would be powered from an external source, which could be then charged from the dynamo.