In systems where there is back-emf induced to oppose the applied voltage, or to change it in cases of power converters, is there a direct way to cancel back-emf or harness it back?

The simplest way to cancel back-emf to me is to increase the supplied voltage from the supply, but is there another way?

Another complicated idea to me, might be an inductor, but what if the back-emf was quite high? Use multiple inductors?

Just wanted to know the possibility.


The back emf exists across the same terminals as the supply voltage, so it can't be separated out (I don't know why you might want to do that anyway, it isn't a free source of, otherwise wasted, energy). You can measure the back emf by driving the motor with PWM and measuring the terminal voltage during the 'off' periods - this voltage will be proportional to motor speed and can be used to produce a simple closed-loop velocity control system. Such a measurement would be via a high input impedance device (e.g. an analogue-to-digital converter) so that very little current is drawn during off-periods, otherwise the motor velocity would be compromised (any current would produce a viscous friction-type torque).

  • \$\begingroup\$ So there can't be a way to directly/indirectly separate back-emf from the same circuit? Just wondering. My calculations and studies showed there can't be a way to reduce back-emf without increasing the supply voltage, was wondering if there are other options. Or possibly harnessing it back is even possible. \$\endgroup\$ – AxtII Dec 2 '15 at 11:20
  • \$\begingroup\$ No. But back-emf isn't your enemy, it effectively reduces motor current to the level necessary to overcome friction torques. \$\endgroup\$ – Chu Dec 2 '15 at 11:29
  • \$\begingroup\$ I know it isn't it's just a curious question, I like to play around with these ideas. You say that canceling back-emf can't be done directly, well what if we used inductors? Lots of them in series. \$\endgroup\$ – AxtII Dec 2 '15 at 11:35
  • \$\begingroup\$ What is your goal? If you could somehow magically reduce a motor CEMF, then you would have to supply the locked rotor current to it all the time, reducing efficiency down about 0.0001%, and raising heating to destructive levels. \$\endgroup\$ – R Drast Dec 2 '15 at 12:01
  • \$\begingroup\$ It's neither possible nor desirable. The back emf is an inherent physical property. It ensures the current drawn is just enough to meet demands. \$\endgroup\$ – Chu Dec 2 '15 at 12:03

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