Two single-phase transformers as a filter for 208 VAC, three-phase, 10 HP motor

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moonshineJ

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USA
Recently I saw two smaller size (about 2-5 kVA) single-phase 208/36 VAC transformers connected to three-phase 208 VAC line, which supplies 10 HP motor controller. Would it be some sort of a filter for the cancellation of triplen harmonics? What puzzles me, why only two transformers used, not three. How they would be connected? I assume the 36 VAC secondaries are not in use.
 
Larry, while this makes perfect sense, I don't see what would be the reason to implement buck-boost in this case. All three phases at a knife switch were/are 208 VAC, and the motor is also wired for its "low" 208 as per data plate. Maybe I will take another look in a week or so.
I popped up a cover on one of transformers just to be sure that wiring is not just transiting thru from a knife switch to the motor (I've seen it once before), but didn't have time to investigate how connections were made.
 
moonshineJ said:
Would it be some sort of a filter for the cancellation of triplen harmonics?
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Triplen harmonics problems only exist when the loads are connected L-N. They were really only a potential problem back in the days of electronic fluorescent ballasts, CRTs and tower style computers without switched mode power supplies.
 
My quick thought was that it's some inductive filter. But almost 30 A FLA for a 10 HP one don't match with the small size of transformers. There is no way they could handle that much current. So I laughed on this idea of myself.
 

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Still sounds like it is boosting 208 to 240 even though the motor apparently is marked suitable for use on 208 and likely doesn't really need the boost transformers. Maybe original motor has been replaced at some time? Not really hurting anything yet doesn't seem it is needed either. Current beyond the boost transformer should be a little less than it is on supply side is about the only real gain here, maybe about 10% less if motor is operating at full load rating.
 
moonshineJ said:
My quick thought was that it's some inductive filter. But almost 30 A FLA for a 10 HP one don't match with the small size of transformers. There is no way they could handle that much current. So I laughed on this idea of myself.
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No need for any "inductive filter". What you might run into IF you were using a VFD on this motor is line reactor on input, output of the drive or even both in some cases. Input is usually to lessen fault current if there is a short in the drive. Output is usually to protect the motor from high voltage spikes produced by the drive, usually only needed for motors not designed as "inverter duty" or for long runs of conductor from drive output - usually over ~ 100 feet.
 
kwired said:
Still sounds like it is boosting 208 to 240 even though the motor apparently is marked suitable for use on 208 and likely doesn't really need the boost transformers. Maybe original motor has been replaced at some time? Not really hurting anything yet doesn't seem it is needed either. Current beyond the boost transformer should be a little less than it is on supply side is about the only real gain here, maybe about 10% less if motor is operating at full load rating.
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Well, this sounds like a good possibility. The motor in my opinion indeed looks newer than the skid itself. If it was equipped with a 240, a buck-boost setup would have a perfect sense, since there was no 240 VAC in the building. Many newer motors don't differentiate between 208 and 240, they can run on either one using the same set of lids.
Thank you kwired!
 
moonshineJ said:
Well, this sounds like a good possibility. The motor in my opinion indeed looks newer than the skid itself. If it was equipped with a 240, a buck-boost setup would have a perfect sense, since there was no 240 VAC in the building. Many newer motors don't differentiate between 208 and 240, they can run on either one using the same set of lids.
Thank you kwired!
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My guess is that whoever replaced the motor saw that the incoming was 208 and either didn’t realized the transformers were there, or didn’t understand how that were being used. Probably said “240V motor on 208…no wonder it failed. We’ll fix that!”
 
moonshineJ said:
Well, this sounds like a good possibility. The motor in my opinion indeed looks newer than the skid itself. If it was equipped with a 240, a buck-boost setup would have a perfect sense, since there was no 240 VAC in the building. Many newer motors don't differentiate between 208 and 240, they can run on either one using the same set of lids.
Thank you kwired!
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Many 230 volts will run just fine on 208 volt systems. If they are marginally sized and always operate at full output rating it may shorten their life if they are used in fairly continuous duty situations.
 
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