Wiring Dual Voltage Motors - Electrical - DIY Chatroom Home Improvement Forum

 DIY Chatroom Home Improvement Forum wiring Dual voltage motors
 Register Blogs Articles Rewards Search Today's Posts Mark Forums Read

07-08-2009, 07:34 AM   #1
Newbie

Join Date: Jul 2009
Posts: 5
Rewards Points: 10

## wiring Dual voltage motors

Hi all,

Can anyone help?

I have a 20hp 230/460 3ph volt motor 50/25 amp, upon receiving the equipment not having a wiring diagram, we hooked up 208 volt 3 phase to it, took an amp reading, it was around 7 amps/leg. Contacted the motor mfg (Baldor) for a wiring diagram which they provided, switched wiring to low (230 volt) motor runs but amperage way up above 60 amps.

Quiery 1: when a motor is wired for high voltage and you hook it up to low voltage should the amperage drop or increase.

Quiery 2: Given the motor is wire for low voltage and hooked up to low voltage what would cause the high amperage (even when not under load)

Tiq

07-08-2009, 08:20 AM   #2
Member

Join Date: Feb 2008
Location: Atlanta, Ga/Hamilton, Al
Posts: 2,487
Rewards Points: 2,350

Quote:
 Originally Posted by tiquilagold Hi all, Can anyone help? I have a 20hp 230/460 3ph volt motor 50/25 amp, upon receiving the equipment not having a wiring diagram, we hooked up 208 volt 3 phase to it, took an amp reading, it was around 7 amps/leg. Contacted the motor mfg (Baldor) for a wiring diagram which they provided, switched wiring to low (230 volt) motor runs but amperage way up above 60 amps. Quiery 1: when a motor is wired for high voltage and you hook it up to low voltage should the amperage drop or increase. Quiery 2: Given the motor is wire for low voltage and hooked up to low voltage what would cause the high amperage (even when not under load) Tiq
Amperage will go up as the voltage is lowered, like the name plate data you posted said: 230 V, 50 A.... 460 V, 25 A.

At 208 V, which is even lower than the rated 230 V, it will draw more than 50 A. Does the name plate say "208-230/460" or just "230/460"? I ask because the motor may not actually be rated to run on 208. It usually will with no problems, but if there is ever a problem, Baldor will always fall back on the voltage as the issue.

P.S.: It shouldn't be a very large amount over 50 A. It will be higher, but I don't think it should be alot more. Micromind may chime in here with some more knowledge on this. But if the draw is way above 60 A, it may be a problem. Is it coming up to full speed? If it isn't getting up to rated RPM, then the current draw will be high and stay high. Start by checking your connections, and making sure they are correct. Then check them again.

Is the wiring sized right for the number of amps, and how long is the run? For a motor rated to draw 50 A, the circuit conductors should be at least #6. You may end up having to get a buck/boost transformer to get the voltage up to 230 V after all.

Last edited by InPhase277; 07-08-2009 at 08:28 AM.

 07-08-2009, 08:36 AM #3 Newbie   Join Date: Jul 2009 Posts: 5 Rewards Points: 10 Thank you Inphase277, Your thoughs are like mine, i will agree that the lead line is too light & too long for the amps drawn, I was puzzled as to why it was only drawing 6-7 amps when wired for 460 @ 208 volt feed. I thought it was too low. But in any event, I will take your advice & check wiring & check again. Tiq

 07-08-2009, 09:02 AM #4 Member   Join Date: Feb 2008 Location: Atlanta, Ga/Hamilton, Al Posts: 2,487 Rewards Points: 2,350 Wired at 460, the coils for each phase are in series, a higher overall resistance. Because the motor could not even start, the impedance was quite high as well, so the current draw was way down. I believe there is a certain amount of reaction between the rotor and stator that's needed before the impedance begins to fall and current begins to rise. Wired at 460 and connected to 208, I guess the sweet spot hadn't been hit, so current remained really low. I'm just guessing.
 07-08-2009, 11:05 AM #5 Newbie   Join Date: Jul 2009 Posts: 5 Rewards Points: 10 Maybe, but it certainly seemed that it was running @ full rpm, but then again seemed is the operative word here, maybe just my perception. Thanks for all your help. Tiq
07-08-2009, 07:11 PM   #6
Member

Join Date: Nov 2008
Posts: 89
Rewards Points: 75

Quote:
 Originally Posted by tiquilagold Maybe, but it certainly seemed that it was running @ full rpm, but then again seemed is the operative word here, maybe just my perception. Thanks for all your help. Tiq
....per the low voltage applied to the high voltage hookup question......

A motor generates a counter emf (electromotive force) which is proportional to it's rpms. Another way of saying this is it's impedance (mostly reactive) increases with it's speed (I'm talking unloaded here...not referring to additional burden) so the current should typically taper off as the unit comes up to speed.

My first thought would be just like InPhase277's...the motor shouldn't have started at essentially half it's nameplate voltage.

In this situation (i.e., stalled) the impedance to current flow is basically the DC resistance of the windings ( a mere fraction of it's running impedance) and I would have expected it to sit there growling like a big dog drawing roughly half it's rated locked rotor current until it tripped the breaker.

Since it did actually come up to speed, I'm going to assume that it's impedance by that time was at or near it's design rating. In that case, I would say ohms law took over and your were applying half volt across an impedance designed for twice that, so the current halved. (Yeah, I know.... 6 isn't exactly half of 25 amps, but I think those nameplate amps are assuming at rated load. Unloaded would be less anyway.)

Most motors are rated for plus or minus 10% voltage, so a motor rated at 240 volts would be marginally outside it's rating at 208 (unless it says on the nameplate otherwise.)

 07-13-2009, 03:04 PM #8 Newbie   Join Date: Jul 2009 Posts: 5 Rewards Points: 10 Thanks Micromind, The motor is a 9 lead wound motor, Upon doing some additional testing, when hooked up to 208, I found that the voltage was dropping to about 175 volt, that should explain the high amperage, this is a result that our test panel not having sufficient power, coupled with the #10 test cable, I would asume that this was the cause of the problem. Tiq
 07-13-2009, 08:28 PM #9 Idiot Emeritus   Join Date: Mar 2008 Location: Fernley, Nevada (near Reno) Posts: 1,849 Rewards Points: 1,492 If the voltage dropped to 175, and the motor was not turning any load (just the shaft spinning in free air), and it drew 60 amps, there's a problem. Most likely it's mis-connected. If the motor was loaded (like a fan or something), then 60 amps at 175 volts would be normal, if not a bit low. You're right, #10s are too small for this motor on 208. I'd use #4s. The codebook maximum breaker size is 150 amp. This is one of the few times it's legal to use a breaker that's larger than normal, but it's perfectly compliant to use #4s and a 150 amp breaker to power a 20 HP motor at 208 volts. The code doesn't list a minimum breaker size, but in my experience 80 amp would be the smallest, provided the motor comes up to speed in just a second or two. Longer than that, I'd use a 100 amp. Rob
07-13-2009, 08:52 PM   #10
Member

Join Date: Feb 2008
Location: Atlanta, Ga/Hamilton, Al
Posts: 2,487
Rewards Points: 2,350

Quote:
 Originally Posted by micromind If the voltage dropped to 175, and the motor was not turning any load (just the shaft spinning in free air), and it drew 60 amps, there's a problem. Most likely it's mis-connected. If the motor was loaded (like a fan or something), then 60 amps at 175 volts would be normal, if not a bit low. You're right, #10s are too small for this motor on 208. I'd use #4s. The codebook maximum breaker size is 150 amp. This is one of the few times it's legal to use a breaker that's larger than normal, but it's perfectly compliant to use #4s and a 150 amp breaker to power a 20 HP motor at 208 volts. The code doesn't list a minimum breaker size, but in my experience 80 amp would be the smallest, provided the motor comes up to speed in just a second or two. Longer than that, I'd use a 100 amp. Rob
My numbers don't work out to 175 V but that's probably because this is the real world. But varying the length of the circuit, I can get it well below 200 V. And it is hard to calculate because as the current rises the voltage drops, causing it to draw more current and drop the voltage even more. Depending on the length, this thing should be run with at least #6, and #4 would be better. And if the circuit is long, even bigger. I bet the drop in voltage has some factor to play in all this. Rob could it be that this is one of those rare times when a 230 V motor just doesn't play well with 208?

07-13-2009, 09:08 PM   #11
Newbie

Join Date: Jul 2009
Location: Kentucky
Posts: 14
Rewards Points: 10

Quote:
 Originally Posted by tiquilagold Maybe, but it certainly seemed that it was running @ full rpm, but then again seemed is the operative word here, maybe just my perception. Thanks for all your help. Tiq
Full nameplate rpm assumes that the motor is running with the proper load applied. I'd be really surprised if, even at 50 amps, the motor shaft even budged with load applied. The normal result would be burned motor windings.

 07-13-2009, 09:52 PM #12 Newbie   Join Date: Jul 2009 Posts: 5 Rewards Points: 10 Thanks for your replies, We were just testing this mixer, we really did not have a proper test set-up, just a small sub panel & about 30' #10 cable, it was not a fair test, unit was shipped, hopefully it will fair better when installed properly for use. Thanks, Tiq
07-13-2009, 11:24 PM   #13
Idiot Emeritus

Join Date: Mar 2008
Posts: 1,849
Rewards Points: 1,492

You might be right InPhase, I've seen a few times (as you have also) where 208 simply won't work. Rare, but it happens.

Like you stated a few posts ago, buck-boost transformers are the way to go here. Not all that expensive, and they work.

Rob

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are Off Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post mntnvew Electrical 15 06-08-2009 04:16 PM Malibujim Electrical 2 05-07-2009 08:52 PM schierle Electrical 2 03-29-2009 09:00 PM srwjr Electrical 14 01-24-2007 08:29 PM

Top of Page | View New Posts