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View Full Version : Motor Wiring Question: How Does This Work?



john hobdeclipe
01-26-2007, 12:34 PM
Here is the wiring diagram from a "RobotCoupe" commercial grade food processor. This machine uses a standard nema frame, replaceable 1/2 hp capacitor start motor, and has a safety switch that won't let the motor run unless the bowl and cover are in place. And if you try to remove the cover when the motor is running, the safety circuit acts as a brake.

In this diagram, "gov" is actually the centrifugal switch inside the motor, and "phase" is the starting winding.

http://www.auldooly.com/imagehost/robotcoupe002.jpg

In normal operation, the safety switch connects #4 to #1. If the safety switch is activated, it breaks that connection and connects #2 to #1, bringing the capaciter back into the circuit. This acts as a brake, and stops the motor almost instantly.

My questions:

1. How does this work to stop the motor?

2. Could I use this same principle to set up a safety switch/brake circuit on other capacitor start motors, such as that on my bandsaw, or woodworking shaper?

Dawai
01-26-2007, 02:47 PM
Creates a dead generating loop? Is this a test??

Inverters work kinda like this, they shunt one pair of motor leads together, keep some power on the third.. called Dynamic braking.

If you do it a lot, you'd better have external resistors to dump to.. they heat up pretty bad.

J Tiers
01-26-2007, 05:42 PM
Induction generator action.....

Residual magnetism in the motor core starts teh generation by inducing a current in the rotor windings, which then sweeps a magnetic field through the stator windings, generating a voltage/current, and wasting enough power in the (shorted)stator resistance to stop the motor.

I don't think the braking current will heat the motor much.... it's already hot, and there is limited energy in it. Plus while stopped, its LOSING heat until started again. Starting will heat it more.

Dawai
01-26-2007, 06:20 PM
It also has the inertia load energy in it. A food chopper has what? blades? Little inertia

It'd be interesting to play with the idea, a brake thou is a easier proposition. A motorcycle caliper, some air and a solonoid to disable when on..

Trust me, I have smoked a few inverters over the 25 years of installation. I have seen the external resistors glowing red from a carpet roll up drive.

Something interesting? I built a 30 inverter drive cabinet. They had a common bus, all allen bradley inverters, when one was in DB others were pulling off the bus. The drive line paid back all installation and purchase costs over the dc drives in less than a year. NOW making it safe? I had to put removeable links for each drive to isolate the bus.
I got a picture somewhere of me in front of that huge for then Nematron monitor talking to the allen bradley Plc5's and inverters. Marilyn Monroe on on the monitor's screen.. whoo hoo..

The Allen bradley rep, brought me a couple cases of beer, but would not stay and drink them with me.

THE micro-motion rep, took his wife to the bahamas, Hawaii, and then somewhere in Mexico off the flow meters I installed.. he gave me a belt buckle.. I started looking at K-flow meters..

Mike W
01-26-2007, 07:50 PM
Years ago, Popular Science had a build it yourself circuit to inject a shot of DC into a motor to bring it to a stop quickly. This was a long time ago when they had useful information in the magazine.

Swarf&Sparks
01-26-2007, 09:56 PM
Yeah Mike. It's called plugging. Google "motor plugging". You should find a heap of info.