AC motor stop

[jack mcleod]

A shot of DC into an AC motor will stop the motor. Plan is to open one of the 240 V AC lines and insert a NC, SW in parallel with a diode. Opening the SW will send the DC shot to the motor; the regular motor SW is shut off at the same time. What should the PIV and A ratings of the two diodes be?

1) max draw 8A, operating draw 3-4 A (table saw)
2) " " 5A " " 2-3 A (radial arm saw)

How long would it take (us - several s) for the varnish on the winding to melt?
or is that even likely to be a problem?

I'm aware of the inertial problem hazard of the blade coming off - turn the saw back on quickly.

advice & experience welcomed

 

[Diver300]

I would suggest that you try to limit the current to about the rated current of the motor. If you go higher, you will saturate the magnetic circuit which won't do you any good.

How about at big resistor in series with the diode? The current will still rise quite quickly. The motor resistance won't be significant, and with no inductance, I would go for somewhere around 30 hm: to limit the current to about 8A. Without a resistor, the maximum current could be very large and difficult to measure or estimate.

At 8 A the motor will last for half a minute or more. You will get about 2kW of heat generated in the resistor, but wirewound resistors are very robust and have huge short-term overload capability. Depending on how many stops per hour, I would guess that a 100W resistor would be fine.

I have used some 25W resistors where the power peaked at about 20kW for about 5ms, and they don't give any trouble.

The diode will be quite cheap, so it is a good idea to aim for a big overrate, and go for somewhere around 600V and 25A. Diodes are much less tolerant of overload than the resistors.

 

[Roff]

Are you trying to build one of these?

 

[jack mcleod]

Roff -NO- The object is to cut down on lost time waiting for the blade to stop.
Appreciate your sending the video along.
Diver 300 (sky? or scuba? I know forum not about fishing.) Liked your response
will be a part of my spring experimenting on expendable induction motors.

Still open to input. Thanks, Jack