# Thread: How to stop an electric motor

1. ## How to stop an electric motor

I have an application where I have to electrically/electronically stop/slow down a .75 Kw 3 ph electric motor as quickly as possible after dropping the contactor.
I know there are motors with magnetic brakes available but presently I do not have one.
I know it can be done as I have 2 EDM wire cutters with a similar function but I do not have a wiring diagram to trace this out.
Any ideas ?

2. Yes quite simple really. Apply a DC voltage to a winding for a short period once the main power is released.

If you just want to stop the motor and then leave it, have a bridge rectifier connected to a double timer connected to any winding of your motor. The first timer, is triggered when the contactor switches off, approximately 100 to 200milli seconds. You want this delay, as there is arcing across the contacts the moment the contactor releases the power to the motor.
The second timer is to allow the DC connected to the motor for a couple of seconds and then releases. The reason for this is that the windings will cook if left for a prolonged period. Use the same supply for the winding you choose to give DC as the supply to the rectifier. Calculate the rectifier rating based on your motor size.

If you wish to lock the motor rotor until the next cycle, then you require a more complex electronic circuit which then reduces the amount of DC to the windings, so that the windings do not over heat. I have made something like this for fan motors, to stop them spinning in the wind and reduce the starting torque when powered by the controls for operation.

3. ## Thanks given for this post:

Martinco (17-May-11)

Use the same supply for the winding you choose to give DC as the supply to the rectifier. Calculate the rectifier rating based on your motor size.
Thanks, just the above has me a little confused. Say I use a relay for the power switching to the motor, will the time that the relay restores to N/C be sufficient in time delay ? Also lets assume it is a 380 volt 3 ph 1,1 kw motor, can I then wire the DC through the N/C contacts and maybe reduce to 240 V using normal single phase ? The motor needs to be locked for about 3 seconds and can then be released via a timer. Will these standard square 25 amp bridge rectifiers do the job ?
With the above in mind, how long do you think it will take the motor to stop. The load is minimal. Also 4 pole motor .

I use the above on a cut-to-length line to measure as accurate as possible 30 meters of strap.

5. Hi Martin,

No you can not use the normally open contacts for this application. The back EMF of the motor lingers for around 100 to 200 mSecs, depending on whether the motor is fully loaded or not. You have to have this delay, or else there are huge surges which will damage the relay contacts and break the bridge rectifier. A bridge similar to this one will be adequate.

The motor depending on the inertia of the shaft, or if there is no fly wheel, should stop within a second once the DC is applied.

6. ## Thanks given for this post:

Martinco (19-May-11)

7. Ok, got it !

I shall report back as soon as I have done this.