# Motor Braking by Shorting Armature Windings

C

#### cyam

I have an AC motor with a Permanent Magnet rotor. I want the motor to come to a quick stop when power is removed. I have read the thread about injecting DC into the windings, but that would not work here because I will have no power whatsoever.

In my research I saw a few mentioning about wiring resistors across the armature windings (one resistor between each phase) would provide braking action. Since I have a Permanent Magnet, so I my magnetic field is obviously maintained, so this sounds like it might work. And I would accomplish this by using N.C. contacts on the same contactor that would cut power to the motor with its N.O. contacts.

I know the inertia of the motor+load assembly. I know the speed from which to stop from. I know
the time in which I want it to stop.

But, how do I size the resistors (resistance and power rating)?

Thank you very much for any information.

R

#### Roy

Cyam,
Shorting the windings with resistors should work well as your motor will try to act as an alternator. Start by assuming that your motor will generate about the same as line voltage and size the resistor for similar to line current. You can then go up or down from there.

You don't give a size so I assume it's quite small so perhaps off the shelf heater elements or lamps would make a suitable load. If you can use lamps it's easy to switch sizes until you get the right load then you substitute an equivalent fixed resistor.

Your power rating will be determined by the kinetic energy in the load you are trying to stop. Assuming it's only momentary it could be quite low

Roy

P

#### Phil Corso, PE

Responding to Cyam's 17-Nov (01:45)... I suggest the following procedure, but only if the manufacturer concurs that the rotor's magnetic performance will not be negatively impacted:

1) Develop a speed-vs-time curve beginning upon de-energization of the motor. The curve should be that of a decreasing exponential.

2) Develop a stator-volts vs time curve immediately upon de-energization of the motor. Depending on the magnetization properties, the curve could be a linear one or also that of a decreasing exponential.

3) Calculate the rate of deceleration from 1)!Because system inertia is, you can determine the kinetic energy in W-sec.

4) Using the data from 2) determine the average stator voltage, V, during deceleration!

5) Now, calculate a resistor value, whose dissipation, (V^2/R)xT, equals the kinetic energy determined in 3).

6) Now connect resistor ten times the one calculated in 5)! Then, decrease the value in 10% decremenrs until you reach the desired stopping time!

Cyam, if you want additional detail contact me, either on- or off-list!

Regards, Phil Corso ([email protected])

M

#### Michael Griffin

You will probably wish to keep the peak braking current within the normal motor operating parameters. Too high of a current can in some cases wreck the magnets (demagnetise them). If you wish to exceed the normal operating range of the motor, then talk to the motor manufacturer about this.