Re: (ET) Deck Motor Alternate Use

["David Roden" <etpost drmm net>]

On 7 Oct 2013 at 23:30, Robert Troll wrote:

> I would like to power this one with house current.

Ah, then that's another story altogether.  Why didn't ye say so before, 
laddie? ;-)

You could rectify and filter the AC line, and use it as input to a DC 
motor 
controller which can handle ~120v input.  A pwm controller is in effect a 
DC 
transformer.

Then you need a way to limit the controller's output to 36-40 volts.  

If you were a EE you could probably come up with something that would 
dynamically monitor output voltage and adjust the control resistance 
appropriately.  Most (but not al) controllers use increasing resistance on 
the control potentiometer to mean "more output."  

But for a simple fixed limit, you could just use a lower value pot than 
the 
specified one.  A wild guess would be maybe ~1600 ohms (a typical 
controller 
pot is 5k).  You might also parallel a fixed resistor of maybe 2.7k with 
the 
5k pot.  In practice you'd probably have to determine the right value 
experimentally, using a light bulb in place of the motor, until you found 
the right value.

Now the rub is that while 36 and 48 volt controllers are almost commodity 
items because of their ubiquity in the golf car world, 120v controllers 
are 
not as common and thus cost more.  A Curtis PMC 1221C-7401 from 
evparts.com 
will run you almost $1400 plus shipping.  (By way of comparison, they 
offer 
an Alltrax 24-48v, 300a controller for $327.)

But I wonder if you really need anything that sophisticated.  

With a PM motor, RPM is proportional to voltage applied.  You have to 
limit 
voltage so you don't over-rev the motor, and limit current so you don't 
overheat it.

I'm not an engineer, but I wonder if you might try a simple series 
resistance between the motor and the rectified line.  You can calcluate 
the 
series resistance needed with Ohm's law, if you know the motor's running 
current.  You could also determine it experimentally, by starting with a 
very high resistance and gradually trying lower ones until the motor 
voltage 
was about right.

A light bulb makes a nice, cheap power resistor.  You could make a crude 
speed controller by using several bulbs in series, and having switches or 
relays that would bypass the lamps one by one to increase speed.

The downside of a series-resistance controller is that torque and speed 
regulation under load aren't good.  Under load, the motor wants to draw 
more 
current.  Its effective resistance goes down, so the voltage across it 
drops.

But then a lot of ETers are happy with their stock GE controllers - and 
that's how THEY work.  So ...


David Roden - Akron, Ohio, USA

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