208V Motor on 240V Delta. VFD?

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TallTimber

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Electrical Engineer
Hello,

I am currently trying to get a 208V motor to function with an existing 240/120V Delta. My first thought is to use a VFD as I can lower the output voltage to match that of the motor. However I have no need to control the speed of the motor. From my understanding a RVSS is essentially a very limited VFD. Does anyone know if an RVSS can be used to lower the line voltage all the time? Or does it switch over to full line voltage following its soft start? Thanks!
 
If by RVSS you mean reduced voltage soft starter, I don't know of any that you can do this with. Usually by the end of the starting sequence there is a bypass contact or that comes on to provide full voltage to the motor.
 
petersonra said:
If by RVSS you mean reduced voltage soft starter, I don't know of any that you can do this with. Usually by the end of the starting sequence there is a bypass contact or that comes on to provide full voltage to the motor.
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Thank you, I figured that is how the soft start functioned. Have never directly dealt with the operation of one though. That makes sense.
 
TallTimber said:
I had considered that. My only concern was the size needed. We are hoping to contain this within an existing enclosure. However. A buck-boost may provide a cheaper solution even if an additional enclosure is needed.
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Buck boost will be in it's own enclosure. If needed bolt it to the the other control enclosure so it essentially is all one piece?
 
TallTimber said:
I had considered that. My only concern was the size needed. We are hoping to contain this within an existing enclosure. However. A buck-boost may provide a cheaper solution even if an additional enclosure is needed.
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You only need two small units mounted in an open delta. What size is your motor?
 
TallTimber said:
Thank you, I figured that is how the soft start functioned. Have never directly dealt with the operation of one though. That makes sense.
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They aren't designed to dissipate heat from continuous use. Also reason they are rated for a number of starts per hour for a given starting characteristics.
 
jim dungar said:
You only need two small units mounted in an open delta. What size is your motor?
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It is a 15hp Motor, 46A FLA. Jumping on the first site I saw, Federal Pacific, And using 200% of FLA I would need a pair of K1XGF16-3 transformers. to handle this. I typically use 200% for sizing standard transformers so I figured that was an appropriate number here to avoid saturation during start up.
 
TallTimber said:
It is a 15hp Motor, 46A FLA. Jumping on the first site I saw, Federal Pacific, And using 200% of FLA I would need a pair of K1XGF16-3 transformers. to handle this. I typically use 200% for sizing standard transformers so I figured that was an appropriate number here to avoid saturation during start up.
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for something supplying a single load I see no reason to oversize by that much unless maybe it is a high inertia load with a long acceleration time, but for that situation you possibly need to derate the motor as well.
 
kwired said:
for something supplying a single load I see no reason to oversize by that much unless maybe it is a high inertia load with a long acceleration time, but for that situation you possibly need to derate the motor as well.
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Should have elaborated. This is a 208V 3 phase motor that would be bucked down from a 240V Delta supply.
 
jim dungar said:
Why not use a buck-boost transformer?
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Just wondering about buck boost transformer when used in three phase loads. I was told that it was better to always use three single phase buck boost transformers rather then two slightly larger single phase transformers. Believe we were told that you get better voltage regulation with three transformers verses two transformers. It's been years since I did a price comparison so wonder what most electricians use nowadays. Did a lot of work for a machine shop that had a 800 amp 120/208 volt service that had a few a used CNC machines that required 240 volts and always went with three buck boost transformers.
 
garbo said:
Just wondering about buck boost transformer when used in three phase loads. I was told that it was better to always use three single phase buck boost transformers rather then two slightly larger single phase transformers. Believe we were told that you get better voltage regulation with three transformers verses two transformers. It's been years since I did a price comparison so wonder what most electricians use nowadays. Did a lot of work for a machine shop that had a 800 amp 120/208 volt service that had a few a used CNC machines that required 240 volts and always went with three buck boost transformers.
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Two single phase are great for motor loads. Voltage regulation problems are not typical with standard induction motor loads.
 
You CAN absolutely get Soft Starters that will operate continuously and not require a bypass.

You CAN absolutely get some that have a “voltage limiter” function which can be programmed to put out only 208V from a 240V input. This feature has been around since the beginning of soft starters.

What it will COST you is ventilation, lots of ventilation. SCRs reject about 1-1/2W of heat per running load amp per phase, so with a 46A motor running full load the soft starter would reject 207W of heat. If you seal up an enclosure and don’t ventilate it, the soft start will roast itself in short order.

If you ventilate it and put in fans to move air, you introduce dust, oil, moisture and other contaminants. If you filter it, someone has to change filters on a regular basis. MOST preppie don’t want to add that hassle to their work life, so they use a bypass contactor. But I’m just pointing out that the bypass is not mandatory.

Personally, I would connect the motor and check the running load amps. If, under the load YOU HAVE, it’s below the nameplate FLA, do you care?
 
LarryFine said:
You're suggesting running the motor directly on the 240v?
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Depending on circumstances I would.

Motor that doesn't run long periods of time, is running near full load, or maybe is short cycle but very frequent starting or even reversing- may get more consideration to apply a bucking transformer. Of course all those conditions also have good reason to possibly de-rate the motor in the first place.
 
LarryFine said:
You're suggesting running the motor directly on the 240v?
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Yes. Look, voltage alone is not the enemy of a motor, the EFFECTS of out-of-tolerance voltage generally translate to changes in current and torque, which THEN causes heating effects in the motor. If the load is lighter than the motor is capable of, these effects might be irrelevant. I have come across 480V motors that were connected to 240V and ran for years, because the load on them was so light that the loss of torque and increased slip still had the load current way below the nameplate FLA. Conversely I have seen a 200V rated motor run on 240V for years, where the motor would have been running in near saturation, but again, the motor was so oversized that the amps were still only 60% of the FLA, so it didn’t really care. That’s my point. He already HAS this motor and we don’t know what the LOAD is, so why not hook it up and OBSERVE? Costs nothing to check it out at this point.

Now if, after connecting it, the motor is at 90% plus of the Service Factor Amps, that’s cause for concern and expensive corrective actions. But if the running load amps are at 75% of FLA, why be concerned?

zbang said:
No one seems to have asked if this is a dual-rated 230/208v motor and we're assuming it is not. Is it?
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If it was, it would say it on the nameplate. I suppose we are assuming that the OP would have told us that, but that’s a fairly safe assumption. Still… OP, is it by chance a dual rated motor? Can you attach a pic of the nameplate?
 
Thank you all for the discussion. This has been very insightful
Jraef said:
If it was, it would say it on the nameplate. I suppose we are assuming that the OP would have told us that, but that’s a fairly safe assumption. Still… OP, is it by chance a dual rated motor? Can you attach a pic of the nameplate?
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I do not have a name plate handy but I confirmed with the nameplate and manufacturer that this is only a 208V motor. Due to concerns of warranty it has been decided that a buck transformer is going to be the best course of action. I am going to remember this conversation for the future. Good to hear about the large ranges a low loaded motor can operate in. For future reference, I will plan to get the motor running and see the draw. Thanks!!
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