From: John Larkin
Subject: Re: DC Injection Brake for Induction Motors???
Date: Mon, 30 Sep 2002 08:39:16 -0700
Organization: Posted via Supernews, http://www.supernews.com
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On Mon, 30 Sep 2002 06:37:04 GMT, "DeepDiver"
>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):
A lot depends on how severe you want the braking to be. Applying a
stiff 120 VDC would stop the motor dramatically, and toast the
windings if left for long. A few volts DC sould produce usable
>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?
This will work. The resistor depends on how fierce you want the
braking. Try it. 100 ohms, 50 watts might be a starting point.
Hmmm... how about a light bulb? Cheap, current limited, easy to tune.
>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.
If the transformer is rated at 1.2 amps, it will dump a lot more into
a low impedance load. But that's probably OK for a brief braking
>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.
Too complex, and would require a huge cap.
>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?
An AC motor will not make back EMF in this situation.
>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.
Really? I'm shocked, shocked to hear this. We will all try hard to
stay on topic in the future.
>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.
The best thing you can do is get in there and try things.
>Thanks in advance!