wiring Dual voltage motors

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

InPhase277

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.

tiquilagold

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

InPhase277

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.

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

WFO

....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.)

micromind

Assuming no load, 7 amps sounds reasonable for a 20 HP motor connected for 460 volts but with 208 applied. One problem here is that the 20 HP motor will be able to produce about 6 HP before it rapidly loses speed. Other than that, it'll run just fine!

If it's properly connected for 230 volts, and with 208 applied, the no-load current should be around 10 to 30. Since it draws more than rated current, I suspect that it's not properly connected.

Here's how to connect a dual-voltage single speed 3 phase motor of any size without a diagram.

If it has 12 leads, the low voltage connection is as follows; L1 goes to T1, T6, T7, and T12. L2 goes to T2, T4, T8, and T10. L3 goes to T3, T5, T9, and T11.

If it has 9 leads, you'll need to know if the stator is wound wye or delta. This is an internal connection, it has nothing whatsoever to do with the supply type. A delta motor will run just fine on a wye supply, and vice-versa. To determine the type of winding, you'll need an ohmmeter. Disconnect all 9 leads, and make sure they're all isolated from each other. If you have continuity between T1 and T4, but not T1 and T9; it's a wye. If there is continuity between T1, T4, and T9; it's a delta. Both wye and delta connect the same for high voltage, but different for low.

9 lead wye; L1 goes to T1 and T7. L2 goes to T2 and T8. L3 goes to T3 and T9. T4, T5, and T6 all tie together.

9 lead delta; L1 goes to T1, T6, and T7. L2 goes to T2, T4, and T8. L3 goes to T3, T5, and T9.

6 leads; rare in the USA, but common overseas. L1 goes to T1 and T6. L2 goes to T2 and T4. L3 goes to T3 and T5.

I realize you connected the motor per Baldors instructions, which were very likely accurate when it left the factory. It's possible that it was rewound to a different stator type.

I'd bet that this is a 9 lead motor, and it's connected for the wrong stator type. Easy to find, and even easier to fix.

Rob

tiquilagold

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

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

InPhase277

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?

Noel

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.

tiquilagold

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

micromind

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