# Are DC induction motors possible?

Forgive me if I am asking something absurd, I really do not understand much about electric motors.

After reading the answers of Why does a Tesla car use an AC motor instead of a DC one? , I got curious:

Is it possible a DC induction motor? At least theoretically?

• Induction requires a change in magnetic field which in turn requires a change in current which in turn implies an alternating current. Feb 8, 2015 at 9:36
• Supplying DC input for induction causes the coil to burn out. This is because of the low/zero frequency of DC. This results in no induction of EMF to oppose the current flow and it increases to unimaginable levels and ends up burning out the coil. That is why commutators are used to reverse the direction of current every half cycle thus creating the back EMF.
– Sâu
Jun 13, 2018 at 10:41

## 4 Answers

No, a DC induction motor is not possible. Induction implies causing current thru a magnetic field. Only varying magnetic fields can do that. This is also the same reason that transformers don't pass DC. There is no on-going power associated with a fixed magnetic field, just some fixed energy to hold it there. If this weren't true, you could get ongoing power from a fixed permanent magnet.

However, true DC motors are possible. In fact, the first real continuously running electric motor was DC. You can also make a disk spin by pushing DC current thru the disk radially, with a fixed magnetic field perpendicular to the disk. The reason this is not usually done is because of the difficulty of electrically connecting to the outside edge of a spinning disk. However, it can be done and has been done.

The reverse also works. A spinning disk with a fixed magnetic field perpendicular to it will develop a radial potential (between axis and outer edge). There have been tachometers based on this principle.

• Thanks. Derived from this I now want to ask if it is possible a DC motor without magnets. Feb 8, 2015 at 1:29
• sure it is. series and shunt wound brushed DC motors
– user16222
Feb 8, 2015 at 1:42
• @JonRB technically, coils "are magnets" when energised. I think that sergiol wanted to know if DC motors could exist without magnetic field generation. But that's just me guessing. Sep 14, 2015 at 16:33

No (well ... sort of but it is a waste)

All electrical machines need an AC flux to couple the magnetic energy. DC brushed machines produce this via commutators. Basically these are also AC, just expose a DC accepting connection.

So why can't you have a "DC induction machine" Well the defining characteristics of an induction machine are: AC stator & AC rotor & torque generated at any other speed but synchronous.

If you were to feed the machine with DC you would need some form of commutator to take that DC and alternate it to produce the required AC. Could this AC be enough to excite the rotor with an AC field? ... sure something would be coupled across, but it would not be efficient as the airgap field of a DCmachine is more squarewave.

Imagine a mains transformer being excited with a squarewave, some energy does get transformed but not as much as if it was AC

Yes: but qualified. There is a class of induction motor called "Electrically Commuted Motor" or "ECM" for short. If you pound the string "ECM motor" into Google, you can learn how it's done and what the benefits are.

• Isn't that a DC brushless motor? Feb 8, 2015 at 1:45
• which is closer to a sync machine
– user16222
Feb 8, 2015 at 2:13
• I searched the Internet for 'sync machine' but the results where not pertinent to this discussion. What's that really? Feb 8, 2015 at 13:47

Yes,It's Possible,but not so efficient.You need two commutators to build such a motor.The first commutator receive current from source supply it to the rotating coil and second commutator.And the second commutator supply current to the stationary coil.The impression of this kind of motor is given in this image.