Using 208VAC to power a 3watt 120VAC synchronous motor (add a power resistor ?).

Thread Starter

redwin88

Joined Dec 17, 2007
11
I’m in need of a cost effective way to reduce the 208 VAC (coming from 2 phases of a 3 phase line fused at 2 amps) for use with a small mechanical timer motor.

Can I just place a 4 ohm power resistor in series with the motor ?

It’s because we have 12 systems to run, that I’m looking for a cheaper way than buying 12 step-down transformers.
 

Thread Starter

redwin88

Joined Dec 17, 2007
11
Well yes I could do that, but these 3 phase circuits
don't have any commons (neutral wires)... so I would
have to send the 25mAmps down the ground wire.
Functional yes, but not safe if the ground wire ever becomes detached.

Sorry I hadn’t mentioned the lack of the "fourth" wire originally, and thank you for trying to save me some cash :)
 

richbrune

Joined Oct 28, 2005
106
You probably won't save much money trying a resistor, because it would need to be a 3000 watt resistor-- I paid $40 recently for a 200 watt, 1 ohm resistor. And I'm a little surprised that you arrived at a value of 4 ohms, that's 208v/8ohms, ----26 Amps (approximately) Usually when dealing with voltage above 50, you want to get a UL approved buck boost transformer. It seems you should be able to run several motors off one of them, if the job site wiring allows. If your'e a very CAUTIOUS and competent experiment oriented worker, you might figure out how to run two of the motors in series, but the phase shift caused by the inductors in series may cause problems if the motors are anything but very small, fractional horsepower motors--- Hope that helps.
 

Externet

Joined Nov 29, 2005
1,519
I would confirm first if one phase to ground (not neutral) shows or not ~120VAC.
What about connecting pairs of the small motors in series ? Six pairs total to 208V directly ?
Miguel
 

Thread Starter

redwin88

Joined Dec 17, 2007
11
Whoops...thank you richbrune, you're right about the 4 ohms.
It should have been 4,000 ohms (that way about half the power would be
dissipated at the resistor, and it being the "same" wattage as the motor).

Yes, the motor is a (sub)fractional type - it only drives a set of timing wheels
similar to a car's odometer.

The wiring for the 12 motors is all separate circuits (each using it's own
isolation transformer - in and out = 208VAC 3 phase).
 

Thread Starter

redwin88

Joined Dec 17, 2007
11
Thanks Externet for offering the series idea... I can't do that because each timer motor
has it's own 3 phase source (each floats with it's own isolation transformer).

Yes each leg does measure about 120VAC to ground (we don't have any neutral wires),
but (intentionally, 24hrs a day) sending current into a ground wire is not favored
because it can easily become a shock hazard.
 

Externet

Joined Nov 29, 2005
1,519
...And what is the problem to not use a 4.7K ohm / 3w resistor (or a plain 120V / 3W pilot lamp) in series with each motor into 2 - 208VAC phases ?
Miguel
 

gootee

Joined Apr 24, 2007
447
You probably won't save much money trying a resistor, because it would need to be a 3000 watt resistor-- I paid $40 recently for a 200 watt, 1 ohm resistor. And I'm a little surprised that you arrived at a value of 4 ohms, that's 208v/8ohms, ----26 Amps (approximately) Usually when dealing with voltage above 50, you want to get a UL approved buck boost transformer. It seems you should be able to run several motors off one of them, if the job site wiring allows. If your'e a very CAUTIOUS and competent experiment oriented worker, you might figure out how to run two of the motors in series, but the phase shift caused by the inductors in series may cause problems if the motors are anything but very small, fractional horsepower motors--- Hope that helps.
Hi richbrune,

$40 for 1 Ohm 200W? Was it an Ohmite WFH230 series, or maybe an Ohmite 210-series?

I'm not an expert with motors, and humbly seek enlightenment:

It seems like he's saying that, for voltage-divider purposes, his motor looks like a 4800 Ohm resistance, to 60Hz 120 VAC RMS.

If that is the case, then is there a compelling reason that he shouldn't just stick a 4160 Ohm (i.e. 4K) resistor in series with it, so that the motor and resistor act as a voltage divider for the 208 VAC, putting around 120 VAC across the motor and around 88 VAC across the 4K resistor, as long as the resistor is comfortable dissipating around 3.6 Watts?

If that would work, and whether or not it's the best solution, how important would the choice of resistor type be, and which type might be best? e.g. Could it be wirewound, or should it be non-inductive?

Thanks.

- Tom Gootee
 

Thread Starter

redwin88

Joined Dec 17, 2007
11
If I were to use a lamp in series with the timer motor, one day the
motor would stop running (burned open filament in the lamp) and a very large
amount of electricuty would be wasted by the timer NOT stopping that daily process, but I do thank you for that creative idea.

Tom was right on the money, when he asked explained (better than me) what it was I was asking.

Thanks for all that have contemplated my dilemma, and for offering ideas.

I plan to use a series circuit of 3 resistors with the small motor.
The only purpose of using three resistors is to spread the
heat load out a bit (I know it's only 3 watts, but it's easy
for me to do, the parts are available, and they will each remain
on the cool side.

The resistors are Ceramic Composition to avoid adding any inductance.
Ohmite P/N OY-122KE, 1.2Kohms, 2W max.

The 3.6Kohm resistance should drop the motor voltage to a reasonable
value (nearish to 120VAC), and only dissipate a watt each.

I hope to order the resistors this week, and some time in January
update this thread with a happy success story (or 5% chance maybe
a "Dhuoh!" - I didn't think of that issue thing to share).
 
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