240 volt motor

robland

Is there a way to get a 1 phase 240 volt motor to run on 120 current with out using a step up transformer.

PaliBob

No,
Sorry

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Code05

Not that I know of . Why is 240 such a problem? What and where is the project?

Yoyizit

Run it off your 240v elec. dryer outlet?

Code05

What if it is 25 HP blower motor? Might not hold.
Just kidding.

Yoyizit

My lights just went out. He probably tried it.

Code05

LOL, I am still laughing.

Red Squirrel

It may run, but only at half speed, if less, and it may also be hard on the motor and draw more amps then it should.

I've run a desk fan on a dimmer before, works great. Not really suppose to do that though but I did not know any better as a kid and thought it was cool to be able to control the fan with a dimmer from my bed.

I would just run a 240v circuit though, any reason you don't want to?

nap

speed in an AC motor is not determined by voltage other than due to an extreme lack of power (horsepower) with the voltage being low, there will be an extreme amount of slippage and resulting from that, lots and lots of heat.

speed is determined by frequency.

LanterDan

In principle, you should be able to find a VFD (variable frequency drive) that can take a 120 V input and drive a 240 V motor. In practice, I suspect it will be more economical to replace the motor with one intended for a 120 V input.

In grad school I inherited an experiment set up by a previous student. I noticed that pump motor on one the chilled water lines was running rather hot. A commented on it to one of technicians a couple of times about it was told that many motors do run very hot to the touch. After about a year I some how discovered the motor was configured for 240V but connected to 120V (like many motors it could run off both depending on how you make the connection). Correcting this did make it run _much_ cooler. I never did notice any difference in performance though, although I wasn't measuring flow rates or anything.

J. V.

Speed is determined by the voltage and frequency applied proportionally. This is how a VFD operates.

Example: 1800 RPM motor @460 volts and 60 Hz.
Input 230 volts @ 30Hz = 900 RPM
Input 115 volts @ 15 Hz = 450 RPM




Yes, that would be possible for a short period of time, but not logical and would most likely burn the motor. When using a VFD you size it by input voltage and motor current. If you input 120 volts @ 60 Hz, to a 230 volt motor you are presenting the motor with a severe under voltage situation. Now, lower the Hz to 30 and your motor will run at 1/2 speed and run just fine. I have personally never seen a VFD that could take an input and double it. You can get more voltage out, than you put in, but it will be minimal. Not 230 from 120. But, you can over speed a motor driven with a VFD. You also cannot use a VFD to control speed on a single phase motor. One of the great things you can do is to use a single phase input and output three phase. This is a great function and we use it here all the time.

Most all single and three phase motors are designed to operate at different voltages via the external connections. There will be no operational differences when a motor is connected properly for the voltage available.

LanterDan

1) You'll note, I began my post with the words "in principle" a VFD could do what the OP wanted, but then suggested another course of action. But I do stand by statement this is entirely possible. Here (http://www.abb-vfd.com/files/manuals...l-Brochure.pdf) you'll note a drive which will happily take a 120V input and output up 240V 3ph. If you can produce 240V 3ph, then in principle you can make drive than produces 240 1 ph.

2) I can't think of any reason why can't have a single phase VFD. In fact I seem to see one here http://www.anaconsystems.com/data/eagledrive11.pdf.

3) If you were going to a VFD, it really would make sense to go with a three phase motor (and one invertor rated), which is probably why you don't see a lot of them designed a single phase one.

4) Its been about five years since I had anything to do with any VFD installations, but I do remember the ABB drives I used to use had a couple of different options in terms of scaling the output voltage with frequency, so its not always quite as simple of halving the frequency and halving the voltage. You are right that they are typically increased and decreased together. I'm sure there are those on this board that can shed much more light on the reasons for this than I care to get into at the moment.

nap

the frequency is what alters the speed with an AC motor unless you want to consider the slippage caused by a below rating voltage as speed control.

The formula to determine motor speed (without considering slip) is:


120X freq/ number of poles in the motor.

the freq is what determines the speed. The voltage is reduced as the freq is reduced to accommodate the reduced inductive reactance. If you did not reduce the voltage and still reduced the freq, you would smoke the motor due to high(er than designed) currents.

To further support my point, what do you think happens when you run over 60 Hz? You can't increase the voltage but you will still speed up the motor. As a result, you will get an increase in speed but a reduction in HP output so you will have a greater resulting slip so you cannot determine the exact speed simply by calculating the freq and # of poles

nap

really. I wonder what this is for then:

http://www.anaconsystems.com/text/eagle1.html

certain motor types do not do well on a VFD but there are most definately single phase output VFD's for single phase motors.

micromind

I've never seen a VFD designed for a single phase motor. I wonder just exactly how they deal with speed regulation when the start switch engages or disengages?

I didn't look at it any too closely, possibly it's designed only for motors without centrifugal start switches. Like PSC type, or A/C compressors, or submersible well pumps. Controlling the speed of these motors would be a cinch.

Interesting.

Rob