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From: Bored With the Bitch
Subject: Re: DC Injection Brake for Induction Motors???
Date: Tue, 01 Oct 2002 03:51:16 +0100
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On Mon, 30 Sep 2002 06:37:04 GMT, "DeepDiver" wrote:
>I want to modify a fractional HP induction motor to add an electric brake. I
>understand that the best way to do this is to inject DC current into the
>stator windings (after de-energizing the AC). I've seen this discussed, but
>the details have always been either sketchy or speculative. Some of the
>methods discussed (and my questions):
>1. Draw 120VAC off the line and use a single in-line diode to rectify the
>current to half-wave pulsed 120VDC. My questions: Is 120VDC too much? Should
>one use a current limiting resistor and, if so, what resistance?
>2. Use a transformer to step down the AC line voltage to 12.6VAC and then
>rectify (either with a single diode or a bridge) to DC. My questions: Should
>I use a current limiting resistor and, if so, what resistance? If I used a
>transformer with a rated secondary current of 1.2A, will that automatically
>limit the current to the stator.
>3. Use a capacitor to supply the DC current. Charge up the cap during the
>motor operation (by tapping off the current going to the motor) using an
>appropriate AC-to-DC circuit. Then, when switching off the motor, dump the
>cap into the stator windings. This seems to be the most complicated design,
>but it has the elegance of being automatic: no need for a momentary switch
>to apply the DC injection current. If using this method, what voltage should
>I use and what "size" cap? My guess is that I'd need a current limiting
>resistor (and possibly a diode to protect the cap), but I don't know what
>resistance to use.
>Also, do I have to worry about back EMF or currents generated in the stator
>by the braking effect? If so, what rating should I use for the diode?
>These are the specs of the motor I intend to use this on:
>Max developed power: 1 HP
>Current: 7.7 Amps
>If I were to use method #1 or #2, then I'd need a momentary switch to apply
>the DC injection current. The best way would be to have a DPDT, 3-position
>toggle switch, with one throw "On" (for running the motor) and the other
>throw momentary (for braking); the center position would be "Off". Does such
>a switch exist? If so, can anyone recommend a source?
>Note 1: Please try to limit the discussion to DC Injection Braking. All too
>often, I've seen these discussions wander off on threads about resistive
>braking and other related--but irrelevant--topics.
>Note 2: I am aware of the potential safety issues regarding braking an
>electric motor and stuff flying off the arbor; no need to rehash here.
>Note 3: This is a single phase motor so a typical variable frequency drive
>(VFD) will not work.
>Thanks in advance!
Here is a method that works for me. First, measure the DC resistance of your
motor's running winding. Multiply that by the amperage the motor consumes
while it is running. (The idea here is to have the DC injection current no
more than the motor running current.) This will give you the amount of DC
voltage to inject into the motor in order to stop it. Build (or buy) a DC
power supply that can handle the load. It doesn't have to be anything more
than a transformer and fullwave rectifier for this application; it doesn't
even require a filter capacitor. Use a momentary DPDT switch or a relay with
a timer to apply the braking voltage to the motor. Test the circuit on the
unloaded motor first.
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