Replies

1. 
   Rich14 | Mar 30, 2008 03:31pm | #1

   Bill,

   In the way you are going to use the equipment, you should be ok.

   Every time I have heard of someone running a 230 V motor at 120 V, the report has been that it ran at full RPM under no load. But it always took longer to spin up and had less torque. Seems like every time that condition existed, it was a mistake, of course, and the operator slapped himself on the head when he realized it. That is, it was never desirable to do that, as full torque was always needed.

   On a lathe you might need more torque than you anticipate, though. Yeah, it should run REAL quiet. And very economically!

   Rich

2. 
   HowardAcheson | Mar 30, 2008 06:26pm | #2

   I think I would want to contact the motor's manufacturer and get them to answer your question. Better than damaging a motor.

   Howie.........
1. 
   JohnWW | Mar 30, 2008 06:38pm | #3

   Any manufacturer would tell him it is a bad idea.John White

1. 
   WillGeorge | Mar 31, 2008 03:50pm | #13

   I agree! I have worked with motors all my life. from low voltage DC (such as steppers) and up the huge Hp 440 Volt. Never worked on any with higher voltage. My first thought was low voltage = FIRE depending on the circuit.

   Just an old mans opinion.. Pull a 220 line and be HAPPY with your lathe! If you have a fire the insurance company my have a issue on paying for your new shop/house...

3. 
   BruceS | Mar 30, 2008 06:58pm | #4

   Lets put it this way,  what happens when you have a brown out or loose one leg of the secondary feed to your house power??  You burn up well pumps, refrigerators, and any other motor.  Volts X Amps =Watts.   One half the Volts is 2X the amps to get the Watts,  Amps cause heat and burn up motors.

   I don't put diesel in my gasoline truck either ;-)

   Work Safe,  Count to 10 when your done for the day !!

   Bruce S. 

    

1. 
   Bill1608 | Mar 30, 2008 08:14pm | #5

   The reason motors burn up when the voltage is low is because the load is still the same. If the speed is reduced too much, the motor actually draws more current and overheats. If a motor were like a resistor, which it isn't really, half the voltage would mean half the current, which equals 1/4th the power. I am only wanting to use 1/4th the power. I am reasonably sure this will work ok, I just want to make sure there isn't something else I am overlooking. Someday when I get more time, I want to experiment and record speed vs torque at 1/2 voltage to see what the actual numbers are.

1. 
   JerryPacMan | Mar 30, 2008 08:59pm | #6

   Check out table two for some plus and minus 10% numbers.

   http://www.eere.energy.gov/industry/bestpractices/energymatters/full_issue.cfm/volume=31 

   Life is what happens to you when you're making other plans .

1. 
   Rich14 | Mar 30, 2008 10:11pm | #7

   Bill,I think the cautions from the other posters should be heeded. And I think I need to modify my earlier encouragement.My only "experiences" with running a motor at "half voltage" are reports from others who have operated table saws that way. I've tried to remember if any other kind of equipment was operated that way, and have to say I can only remember stories about table saws. It is very possible that a table saw is a unique enough situation among power tools in that it runs under almost no load for a very large part of its duty cycle, experiencing load only when wood is pushed through the blade. And even then, may not be loaded very much.Your experience with supplying half voltage to a grinder may be a very similar situation. Under no-load situations, the under voltage may not result in excessive current flow, so no real damage to the motor. But damage may occur after long periods, or if any significant load is applied to the motor.I once bought a house which was 30 feet lower than the curb-level sewer system. It had a Little Giant 230 V grinder sewage ejection pump that had to be replaced. When first put into service, it had been mistakenly wired for 120. I don't know if it ran that way at all, but I do know that the wiring mistake had happened and was subsequently corrected. It ran for a few months after the voltage condition was corrected, but constantly burned out relays in the control box, requiring replacement about once a month at $100 a pop. It seemed to be working normally otherwise, but failed completely one day without warning. When it was pulled and replaced (a deLIGHTful job, being in a full-to-the-brim, raw sewage holding tank), the windings were toast.Rich

2. 
   Tinkerer3 | Mar 31, 2008 08:02am | #11

   I have on occasion run out of gasoline in the pickup and had to put in tractor fuel.  It runs okay if the motor is warm enough to start - maybe a little less power.  Diesel would be worse yet.  Of course the old tractor fuel tractors always started on gasoline then we would switch over when the motor warmed enough.  In fact some diesel tractors would do the same switch over from gasoline.  It is a lot better than walking several miles.

1. 
   BruceS | Mar 31, 2008 03:42pm | #12

   Used to do that too,  Think it was a Farmall M or H three wheeler.  Used it to gather maple sap. That old beast went through knee deep mud till we broke off the front trike wheels.   What a job gas welding it back on in the mud and cold.  Thanks for reminding me of that memory ;-)  Still prefer 3rd run dark amber over 1st run light.Work Safe,  Count to 10 when your done for the day !!

   Bruce S. 

    

1. 
   Tinkerer3 | Apr 01, 2008 05:39am | #14

   "3rd run dark amber"  Well I don't know anything about that.  I grew up and farmed a while in the eastern Wyoming - western Wyoming drylands.  Wheat, potatoes, livestock.  Didn't know what a maple tree looked like.  The only trees were planted.

4. 
   JohnWW | Mar 30, 2008 10:26pm | #8

   The fact that you did this once with a three phase motor does matter. A three phase motor has less trouble maintaining speed under load but more importantly a single phase motor has starter windings that kick in automatically if the motor speed drops. If the starter windings run for more than a few seconds they will fry.

   John White

1. 
   Bill1608 | Mar 31, 2008 01:39am | #9

   In my original post, I asked if there were any problems as long as I didn't go below the nameplate rpm. The starter winding would not kick back in at or above the nameplate rpm. I want to get up to 3/4 hp out of my 3 hp, but I don't know what I can actually get. The motor will run smooth as silk at 1/2 voltage, I just don't know how much hp it will put out at nameplate rpm.

1. 
   ndmatt | Mar 31, 2008 06:43am | #10

   Hi Bill,  Like John said, motors aren't designed to run on 1/2 voltage.  Most are only designed to run at nameplate voltage plus or minus 5 or 10 percent depending on how it's made.  So basically you were lucky to be able to run the 3 phase motor like you did.

   You asked:

   "In my original post, I asked if there were any problems as long as I didn't go below the nameplate rpm. The starter winding would not kick back in at or above the nameplate rpm. I want to get up to 3/4 hp out of my 3 hp, but I don't know what I can actually get. The motor will run smooth as silk at 1/2 voltage, I just don't know how much hp it will put out at nameplate rpm."

   If you halve the voltage you will definitely go below nameplate RPM WHEN UNDER LOAD.  Your torque will also drop (I wish I had the equations in front of me but I don't). 

   The reason the motor running at half voltage still runs at almost full speed under no load is that speed is largely dependent on frequency, NOT voltage.  However, the torque is dependent on voltage so that will drop.  This is why variable speed 3 phase motors only became feasible when power electronics became available.  In a nutshell, to reduce the motor's speed but keep the torque constant, you keep the ratio of voltage to frequency constant.

   Hope this helps.

5. 
   northhouguy | Apr 01, 2008 01:31pm | #15

   Where do you keep your fire extinguisher and insurance policy?