Need help with stopping an electric motor?

Question

It is on a centrifuge. I need to set it up so that if the cover is open, the motor stops. I have a cutout switch that cuts the power to the motor, but in order to stop it, the centrifuge has to be put in reverse for approx. 1-2 seconds. Either that or find a mechanical brake of some sort.

Answer 1

The problem is the inertia of the rotor isn't it? You could try using regenerative braking. It is what they use to stop windscreen wiper motors quickly.

Wire a switch so that when the lid is opened, the motor input is shorted out (and isolated from the supply, obviously!). The energy of the rotor will be converted to heat in the motor coils.

Answer 2

The term regenerative braking is usually applied to systems in which the motor acts as a generator and returns the braking energy to the source. That type of braking requires a relatively complicated electronic control, but it is used for large centrifuges.

With dynamic braking, the motor acts as a generator, but the braking energy is transferred to a resistor and dissipated as heat. With a DC motor, it is relatively easy to disconnect the motor and connect a resistor across the terminals to provide dynamic braking. If the motor is short circuited after disconnecting the power, the braking energy is dissipated in the motor windings. That is OK for a small motor without a lot of inertia, but attempting to internally dissipate too much energy could damage the motor. However, if the motor is not damaged by reversing while running, it probably will not be damaged by shorting the terminals. Dynamic braking will probably work with a universal motor and may be possible with some types of AC motors.

Stopping the motor by putting it in reverse while running is called plug stopping. To do that automatically, a speed sensing switch is needed to disconnect the motor when it reaches a low speed so that it doesn't continue to run in reverse.

AC motors can be stopped by applying DC to the windings. That is called DC injection braking. With that method, the braking energy is dissipated in the windings, so it is not suitable high braking energy requirements such as with a high inertia load.

Edit 1

If your centrifuge can be plug stopped in 1 to 2 seconds, it must be fairly small. If it is an AC induction motor, not a universal motor (no brushes), DC injection braking might be a good alternative. Look at:

http://www.northstar-mn.com/ambitech1.ht...

If it is a universal motor (brushes) some kind of dynamic braking might work. Stopping the motor would require disconnecting the power, reversing the field connection and connecting a braking resistor

Answer 3

There are devices on the market for stopping band and circular saws that can be connected into the supply circuit and inject dc supply into the motor windings for a fraction of a second and bring the saw blade to rest within seconds.

Any reputable electrical supplier will have details.

Answer 4

Why not reverse this problem.... rather than stopping the motor if the cover is opened in operation, why not interlock the cover to prevent it from opening if the motor is spinning....

It would be safer to keep the cover locked until the motor shaft stops completely rather than open the cover and expect it to slow to a stop fast enough to prevent injury or damage to the centrifuge.  (what's its rpm 10000 to 20000 rpm? you expect to stop that in a second or two.)

Sensing shaft revolutions is fairly simple (hall effect sensor, quadrature encoder,rpm generator/tachometer)   a signal from a sensor would lock/unlock the cover to prevent opening until the shaft is stopped.  An alternative is to put the interlock on a timer that would keep the cover locked for a period of time after power to the motor is interrupted.  Allowing an adequate amount of time for the centrifuge to stop.