I want to use universal motor as a DC generator. The motor specifications are

  • 180 Watt
  • 230 V
  • 10000 rpm

When I supply the motor with 3000rpm it gives 1.5V. I thought it was rpm problem but when I supply 10000rpm it only gives 4V. While exploring its theory I figured out that a universal motor is a DC series motor. As for DC series generator there must be load connected to its terminals in order for field circuit to be energized otherwise the circuit will be open (if I am not wrong). But when I connect load (incandescent bulb of 100Watt and 220V) it give me even less than 1V. I do not understand what is the problem. If someone has good undestanding of universal motors please provide guidance.


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    \$\begingroup\$ Post a link to the actual device please. \$\endgroup\$ – Andy aka Mar 16 '18 at 9:47

A series wound field generator needs not only a load to maintain the field current while it's running, but also enough residual magnetism in the iron to 'self-start'.

The amount of magnetism needed is dependent on the conductivity of the external circuit. Unfortunately the better the motor, the more likely the iron is 'good' for motors, which means very soft, with little residual magnetism.

One of your options is to whack the field windings with some DC to magnetise them as far as they can be. Then try starting. They may have stored enough, they may not.

Another option is to disconnect the windings, and separably excite the field with a power supply. Of course, once the generator is running, this can be powered from the output, so not much external power is needed for starting. This is not over-unity, as power is being supplied to turn the generator.

Another option is to connect a battery briefly to the field when starting, perhaps through a diode, to kick-start generation.

  • \$\begingroup\$ I think I put a high resistance at output so the current was not enough to excite the field. Should I check it with a very low resistance at output that develops enough current from voltage due to residual magnetism (4V in this case)? \$\endgroup\$ – Sohail Ahmed Mar 16 '18 at 12:51
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    \$\begingroup\$ check it into a short circuit, that's the optimal self starting case. Obvioiusly, limit the drive torque/speed under those conditions, perhaps with a clutch. Whether you can make an automatic self-starter that uses that principle is another matter. \$\endgroup\$ – Neil_UK Mar 16 '18 at 13:13

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