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Subject: Re: DC Injection Brake for Induction Motors???
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Mon, 30 Sep 2002 09:47:26 +0100
NNTP-Posting-Date: Mon, 30 Sep 2002 09:52:45 BST
Organization: (Posted via) VIA Net.Works UK Ltd.
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"DeepDiver" wrote in message
> I want to modify a fractional HP induction motor to add an electric brake.
> 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,
> 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?
> 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:
> 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
> 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
> 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
> 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
> 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
> Volts: 120VAC
> Hertz: 60
> Current: 7.7 Amps
> Single Phase
> RPM: 3450
> If I were to use method #1 or #2, then I'd need a momentary switch to
> 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
> a switch exist? If so, can anyone recommend a source?
> Note 1: Please try to limit the discussion to DC Injection Braking. All
> 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!
> - Michael
For greatest baking effect I would have +ve current in one phase and -ve in
another. Leave the other one shorted. Assuming 3 phase. I wouldn't pass
much more DC current that being passed at normal max load (rms current).
You will need a lower voltage since all you will have to do is overcome the
resistance which will be an order of magnitude or 2 less than the reactance.
I've never done it - good luck.
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