A/A Fuel GTX
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- Electrician
What are the consequences of applying 3 Phase 240V via a static phase converter to a 208V motor?
Motor Voltage Tolerance
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bphgravity
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From NEMA MG 1-2014:
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Under 10HP, usually none. If you want exactitude, you must ask the manufacturer of the motor (if they exist). But be forewarned, if you ask a salesman, and he gets a whiff of a possible sale, he might tell you that the motor is made for 230V and you have to buy a new motor. Here are the words of a chief design engineer at one of the largest industrial motor mfrs in North America said about it a few years ago, when asked how to deal with 208V systems and the problems with motor distributors having to stock duplicate inventories of 200V or 230V motor to try to cover it:
So if your motor IS in fact labeled as "208-230V", then it's what he described above and is fine. If the motor strictly says "208V", then it can handle +-10%, so down to 187V and up to 229V, which is not high enough to accept 240V without saturation, over heating and eventual winding damage and excess torque that may be hard on mechanical parts.
He then goes on to say this applies to motors under 10HP, by saying that 10HP and up you should use a 200V design.
So if your motor IS in fact labeled as "208-230V", then it's what he described above and is fine. If the motor strictly says "208V", then it can handle +-10%, so down to 187V and up to 229V, which is not high enough to accept 240V without saturation, over heating and eventual winding damage and excess torque that may be hard on mechanical parts.
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A/A Fuel GTX
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- WI & AZ
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- Electrician
Here is a photo of the nameplate. The motor is currently operating at 208V so it's been altered or a new motor was installed at one point. This is a Hobart mixer in a commercial food processing plant. When the mixing vat is loaded with whatever, I'm sure it's quite a load on the motor. The mixer is being moved to a location where single phase 240V is available. I bid the new install with a 5HP rotary phase converter and a 240/208 step down transformer. The owner tells me he got a bid from another contractor for about 1/5th of my bid but this guy is using a 3HP static phase converter without stepping the voltage down. My phase converter rep tells me that a static phase converter isn't true 3 phase power and the mixer motor under full load will probably not perform as needed. I don't want to jeopardize the operation of this business nor the function of a 10K dollar mixer to try to save a few bucks.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
1) Your phase converter guy is right, the motor would lose about 1/3 of it's capacity. A cheap Static Phase Converter (SPC) just makes it spin by faking it out, but the motor is really running single phased, with maybe a little less voltage imbalance because of the caps (depending on the design). Your cheap competitor apparently has no regard for the integrity of your customer's operation.
2) Why 5HP for a 1HP motor? Is there something else you have to run at 3 phase? If you go with an RPC, you don't need that much overkill.
3) For under $500 you can buy a 240V VFD that is capable of accepting single phase input, but putting out 230V 3 phase for the motor, and if the motor really is 208V, you can program it to put out only 208V 3 phase. It might be a less expensive way to get this done; no transformer, no RPC, and in fact this might actually be what the other guy was doing, but called it an SPC to keep it simple for the end user. But there is a problem with that.
4) THAT machine, depending on it's age, likely has a somewhat complex control system that operates a clutch/brake assembly for speed control, as well as possibly a timer. So sure, the motor runs at a constant speed and the clutches and planetary gears to the hard work, but that is NOT a good application for a VFD as a phase converter. Not only is engaging a clutch hard on the VFD, but the VFD output is not something that you can feed to control systems, timers, relays etc. it is ONLY good for motors. So to do that, someone would have to go into the controls and separate everything out to feed it with a SEPARATE source. Then there is the fact that there are safety interlocks inside that have to be changed to use with a VFD, because you cannot change speeds while moving, so it has a contactor that drops out if you move the handle. A VFD cannot have a motor starter down stream, IT has to do the starting and stopping itself. So someone would have to re-wire the entire system. Lastly, that motor has (should have) an electric brake on it for safety (so that it doesn't rip someone's arm off), and like the controls, the brake coil cannot be energized by the VFD, so it too would have to be separated out. And by the way, the SAME issues apply to an SPC as well. The 3rd phase is not real, it's just a trick to make the motor spin.
So is the cheap guy going to take on all the responsibility for ripping and replacing the controls to function with a Static Phase Converter or a VFD? If he screws up and some worker gets an arm torn off, is he going to answer the phone call from the lawyers?
Your RPC concept avoids all that, because it doesn't alter anything INSIDE of the machine, it just gets him 3 phase power from a single phase source. You will have to go through and make sure none of the controls are tapped off of the manufacture leg, but that's not hard and only involves moving a tap to one of the other two real phases. I think you have the right idea, just 5HP may be a little over kill if this is the only machine.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Oops, meant to say this about the voltage.
It's easy for a rewind shop to take a 440V wound motor and make it 220V, because most likely the internal windings are set up as pairs of coils for each phase, just without the connections between them brought out to the box. So the shop just has to find and cut those connections, then change them to be series vs parallel (or vice-verse depending on the configuration) to change the voltage by a factor of 2. But changing a motor to be 200V vs 440V would I think involve changing the actual turns ratio, and unless they had to rewind it for some other reason anyway, it would have been cheaper to just swap it out. So if it is actually a 220V motor now, it would have worked OK on 208V (220 - 10% = 198V), but 240V is fine for it now too (220 +10% = 242), so no need to changing anything. I would not buy the transformer yet. Hook it up to what you have, run it under load and watch the current. If it's still under the FLA rating, leave it alone. If not, then add the transformer.
It's easy for a rewind shop to take a 440V wound motor and make it 220V, because most likely the internal windings are set up as pairs of coils for each phase, just without the connections between them brought out to the box. So the shop just has to find and cut those connections, then change them to be series vs parallel (or vice-verse depending on the configuration) to change the voltage by a factor of 2. But changing a motor to be 200V vs 440V would I think involve changing the actual turns ratio, and unless they had to rewind it for some other reason anyway, it would have been cheaper to just swap it out. So if it is actually a 220V motor now, it would have worked OK on 208V (220 - 10% = 198V), but 240V is fine for it now too (220 +10% = 242), so no need to changing anything. I would not buy the transformer yet. Hook it up to what you have, run it under load and watch the current. If it's still under the FLA rating, leave it alone. If not, then add the transformer.
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Not necessarily. Lots of 240V motors will run with no trouble on 208V.
I agree with jref, skip the transformer unless the amps on the motor are out of line. I would check into swaping out the motor for a single phase. Hobart can be a pain though. At least around this area they are.
A/A Fuel GTX
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- WI & AZ
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- Electrician
Thank you for the detailed reply. It helps me a lot. I'm really tired of people always looking for the cheap way out but I won't compromise my reputation even if it means losing business, as this did. As far as the 5HP vs a smaller converter, I always figure on possible expansion or a larger machine one day. The difference in cost isn't that great.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Not only that, but that motor is wedged in there pretty tight and it's a custom design for that machine, so no way to replace it with something else other than their motor. I've only worked on one, I hope to never have to again. Check it out on this video at about 5:55 time mark. The one I worked on was NOWHERE this clean...
https://www.youtube.com/watch?v=YtQqn02eQlw
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- Licensed Electrician
Oopps, I meant dealing with Hobart sales and service reps can be a pain. Yes, working on a Hobart is a pain also, but not impossible, I have changed bearings on the motors a handful of times, and your right there is nary a one as clean as the one in the video outside of the showroom.
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