Is it possible to allow a motor to turn freely while remaining connected to the circuit?

Question

When electric motors are disconnected from a circuit, they can spin relatively freely with little to no resistance, only the shaft friction causing resistance. Connect the motor to a circuit, and suddenly the motor prevents free spinning.

I understand the resistance is coming from the motor acting as a generator, creating a voltage and producing current.

My application ideas for this would be to allow for e-bikes to be used as a normal bike; a CNC table that you could manually move the arms when required; an electric skateboard that could be used as heavy normal skateboard.

What is required within a circuit design to allow the motor to remain "connected", however, be able to spin relatively freely? My thought process would be that a relay would be required to disconnect the circuit.

Are there any differences relating to this problem whether the motor is DC or AC?

Answer 1

For the motor to spin 'free' it needs to be generating zero torque. Taking or sending zero current to/from the controller is one way to get this.

You could achieve this with a series relay. Or you could do it by turning off all the power components in the drive / regenerate path so that the effect was the same. That's a sufficient condition.

The necessary condition is that it generates zero torque. You could manage that by alternately driving it and braking it so that the net torque was zero. That however would be a complicated and wasteful way to allow the motor to run 'free'.

It is a practical way to run the motor in some cars when charging. One of the components needed for the charger in cars is a large inductor. Various papers have been published on sequences of excitation for the motor so that it can be used as the 'large inductor' without creating any unwanted torque.