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.

 

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

 

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])

 

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.

 

Hi Guys, Sorry about replying to such an old thread. But did anyone find a contactor to supply and short the motor?

 

Keiran1984
Your schematic represents a typical connection of power to the stator of a 3-phase Wye-connected AC motor. A permanent-magnetic-motor requires DC. However, it could be driven by an AC source if the AC is first converted to DC. Do you have other information, perhaps a photo of its nameplate?

Braking a Permanent-Magnet motor requires the introduction of eddy-currents to brake its rotor ! But size is an important issue. And, yes that an be done with simple, small relays as long as the conditions mentioned in the earlier reply are not ignored !

BTW. an aside, it is now possible to drive a permanent-magnet motor with VFD/VSD controllers! But not all manufactures are able to do it !

Regards, Phil Corso

 

Hi Phil
Sorry for my late reply. Have been away for work getting my hands dirty.
I was a bit sketchy with the details and I should have elaborated more.

Yes its a PSM motor that is controlled by a VVVF drive for a lift. In normal operation the drive does all the work for accel. and decel. and stopping. So for slowing to stop timing, the motor gets to zero speed by the drive, the motor is still excited by the drive through 5KM1 contactor, the brake drops onto the drum then the 5KM1 contactor de-energises. So in this case the bridging of the motor windings is trivial.
The purpose of the bridging circuit has 2 functions in this case:
1. When we want to move the motor by openning the brakes manually, motor unpowered, the weight of the lifts counterweight allows the motor to turn. With the motor windings bridged, the magnetic field is generated in the opposing direction of the motor which keeps its speed low and constant. Without the motor bridged, the motor runs away uncontrolled. So in this case only used from standstill.
2. When driving on maintenance function by push buttons on the cabin top. When stopping, the timing of the contactors, brake and drive, are simultaneous and the delay in actual brake drop to the drum lags. Without the motor being bridged the motor is pulled by the lifts counterweight being heavier and the lift can jump up 100-500mm depending on load difference. With the motor bridged its 0-50mm maximum.

The existing controller has a 5KM1 contactor which utilises the back of the main 3 phase contacts to bridge the motor. This way there is no real risk of having both input to the motor and short circuiting of the motor at the same time. This contactor is very difficult to get and haven't seen any others like it. I've used an auxilary block connected to a contactor but its possible to remove it, but I would rather it be mechanically coupled.
Just wandering if anyone had seen a contactor or device that would achieve this with mechanical coupling?

Cheers Keiran

 

Keiran...
Thank you for the additional information.
Frankly, I can't suggest a remedy for legal reasons. Have you contacted the Drive mfg?
Phil Corso