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#174511 - 02/06/08 01:01 PM 208V feeding a 240V motor?  
mr_electrician  Offline
Joined: Jun 2007
Posts: 106
London, Ontario
Hello, I am looking for an experienced opinion which I am sure I will find in here. The boss bought a 240V 3 phase belt sander. Problem is that we have a 208V service. Should I buy a step up auto transformer or try the motor on 208V. I am thinking that the motor will run hotter and may have a problem with nucence tripping of the breaker if I apply 208 to it.

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#174528 - 02/06/08 10:17 PM Re: 208V feeding a 240V motor? [Re: mr_electrician]  
Sandro  Offline
Joined: Dec 2001
Posts: 444
Stoney Creek, ON, Canada
From my experience, it would be better off running proper voltage to the sander. We've had way too many issues in the past running 240V @ 208. Some of the equipment wouldn't even start at 208V, never mind run hotter.

#174568 - 02/08/08 11:59 AM Re: 208V feeding a 240V motor? [Re: Sandro]  
jay8  Offline
Joined: Jul 2005
Posts: 183
Vancouver, BC
Unless the equipment nameplate tells you it is suitable for 208 volt, then I would also suggest you stick to the 240 volt supply. If you assume at least 5% voltage drop to the piece of equipment at 208 volt then you are getting 198 volts to the equipment designed for 240 volt.

#174571 - 02/08/08 12:33 PM Re: 208V feeding a 240V motor? [Re: jay8]  
KJay  Offline
Joined: Nov 2007
Posts: 763
You will probably be replacing that motor often if you run it at low voltage.
A few years ago, I had a situation where I was called for problem at a school that was maybe 1-year old, which had 3-phase 208V service, and a large well water pump wired by the original EC that was designed for 240V.
The well pump was shot and the well company told the school they wouldn’t replace the pump under warranty because it was constantly being run at low voltage. The original EC never checked the nameplate voltage before connecting the pump.
The well company installed a new pump and at the same time I installed a boost transformer to bring the voltage up to spec and all has been fine for several years now.

#174595 - 02/08/08 06:04 PM Re: 208V feeding a 240V motor? [Re: KJay]  
Rewired  Offline
Joined: Jan 2006
Posts: 558
Hamilton, Ontario, Canada
If the motor is rated 208-230V then its fine but if not, I wouldnt try it, especially if the line voltage is known to run on the low side of 208.. My former highschool had this problem with a few window shaker A/C units, they wound up installing "buck / boost" transformers for them


#174609 - 02/09/08 01:22 AM Re: 208V feeding a 240V motor? [Re: mr_electrician]  
u2slow  Offline
Joined: Sep 2003
Posts: 200
Salt Spring, BC, Canada
I had a similar thing with a 7.5HP air compressor. I called the manufacturer. I didn't want to risk anything. Last thing I need is to be on the hook for a burnt-up compressor.

They said NO - and suggested the customer take it back their dealer and exchange it for the 208V version.

#174617 - 02/09/08 08:22 PM Re: 208V feeding a 240V motor? [Re: u2slow]  
twh  Offline
Joined: Mar 2004
Posts: 915
Regina, Sask.
The main concern with a motor is heat. If it fails to start quickly, runs too slow for the fan to keep it cool, or is overloaded for a period of time, it will overheat. The overload protection may not act quickly enough to protect the motor, but, I've seen many 240 volt motors running on 208 volts.

If your boss insists that the motor be connected, he could very well be telling you "I told you so", in a couple years.

#174664 - 02/10/08 10:44 PM Re: 208V feeding a 240V motor? [Re: twh]  
frank  Offline
Joined: May 2001
Posts: 376
windsor ontario canada
As long as you stay within 10% an EE with tell you it's ok.I have seen them trip and weaken the winding thermals and crack klixon o/l when ran at rated load.Ive also seen them have a long happy life.Check current draw for a few cycles and you should have your answer.

Last edited by frank; 02/10/08 10:49 PM.

#174686 - 02/11/08 07:59 PM Re: 208V feeding a 240V motor? [Re: frank]  
mikesh  Offline
Joined: Jun 2006
Posts: 613
Victoria, BC, Canada
A 10% drop in voltage is still 226 volts which is still a higher voltage than 208 would deliver. A motor run undervoltage will in fact run at the rated speed as motor speed is related to the number of poles and frequency of the supply. Universal motors are voltage sensitive for speed but not a squirrel cage.
Your problem is going to be reduced torque and horse power. If the motor was oversized and the torque requirements over sized then maybe the motor will hum along just fine. How much the motor needs to be oversized is unknown to me.
Another issues is going to be efficiency. So if your belt sander runs 1000 grit on balsa you might be ok but if there is any 120 grit or courser and anyone pushes hard on the belt the motor will run hotter because it will run higher than rated current and will not meet your production or performance requirements. The cost of this motor might be very much higher than the cost of getting the right motor installled. Add it up. reduced efficiency, lower Hp = slower sanding. More electrons to pay for too. Cost of replacement before amortizing the purchase cost. Down time when the motor smokes at a high demand moment in production, etc.
Get a new motor. A boost transformer would work but you are again adding to the complexity and cost of what is probably not that expensive a motor to begin with.

#178753 - 06/10/08 08:47 PM Re: 208V feeding a 240V motor? [Re: mikesh]  
homerjones  Offline
Joined: Dec 2007
Posts: 35
seems a little strange for it to be 240v three phase that would be delta which is very rare to findive only seen 208/347 hydro can give you 240 three phase but virtually unheard of

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