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VFD and step up transformer?

Chris42

Aluminum
Joined
May 11, 2006
Location
York, PA
I have a TECO VFD for my Bridgeport 1 HP mill. Works great and I like it a lot. I am going from 220 single phase to 220 3 phase. I now have a Deckel tool grinder (beautiful machine) that is 440v, 3 ph, 1/2 hp.

Can I use a .5 kva step up transformer and step up the vfd output to run the Deckel?

Thanks, Chris
 
maybe a simpler way

alot of motors 3 phase are multi voltage so it might just be a case of altering the taps to the correct voltage
 
It's not a good idea to put a transformer on the output of a VFD. Transformers are designed to work on one frequency. When you change the frequency, you change the interaction of the steel laminations to the coil windings. The VFD output is also extremely rich in harmonics. The end result is usually a significant increase in heat in the transformer which causes rapid deterioration of the transformer winding insulation, followed by a short between windings and the probable destruction of the VFD output transistors. There are special custom made transformers designed for using below VFDs, but I doubt you can find one.

As mentioned above, check to see if your motor is dual voltage, which solves the problem anyway. If not, you could find a 480V VFD and put the transformer on the line side. A few 480V VFDs will not accept 1 phase input, but most will. Unfortunately, it will require that you double the size of the VFD, but then again, 1HP is usually the smallest 480V VFD you can buy anyway.
 
Sorry for not noting it above, but the motor is not able to be re-wired to 220v. That was the first thing I checked. It is a two speed, reversable motor of german manufacture. It is a nice setup and I really don't want to get into the machine and change anything. As for the VFD, the plan is to set it at 100% or 440V and not try and change motor speed. I would use the the VFD as a constant (non-variable) power source.

Possible?
 
It should be noted that the change of iron and copper loss characteristics with frequency and voltage of a transformer are identical to those of a motor.

The difference is that a motor has a fan that is less effective at low frequencies, whereas a transformer has convection cooling that doesn't know what frequency means.

In other words JFDI, Use the transformer. In large invertor plants, transformers are common, albeit designed for the duty. My 4 kVA doesn't even get noticably warm when running at rated voltage and frequency from the VFD and 2kVA average load for 6 hours at a time.
 
Mark - The transformer is pretty well matched to the German motor, larger capacity, but not by much. The motor is rated at .4kva and the transformer is .5kva

Will this work or should I find a transformer perhaps twice the capacity? I don't know if the transformer would allow a high current flow from the VFD, causing it to overheat, or if it will draw (and deliver) exactly what the motor needs?

Or, from a different angle, what would be the best way to go from 220 1ph to 480 3 ph?

I really want to set this up so there is no chance of damaging the motor. I think two speed, reversable motors (with metric shafts) are a little hard to come by and not cheap.
 
I would be all for trying the arrangement. That may be the only way to see how it will work. Probably won't have an issue but VFDs are known for sending frequency harmonics out on the line that could give a transformer fits. Maybe not.

What "I" would do is simply swap out the motor. All of a 30 dollar motor really. .5HP 3ph motors are dirt cheap used. The only thing you will have to contend with is the doubled amperage in the machine. Generally fusing will need upped and wiring needs checked to make sure capacity is acceptable. Nice thing here is no transformer to deal with.
 
You know, if this is a non-reversable, constant speed application, swapping to a single phase motor would sure make life easier!! I just do not see any reason to hassle with 3ph conversions for .5HP. Would sure save you some money and headaches. Your motor contactor is likely rated to handle a little more amperage. You would just dead head one lead of the three phases, and roll on. Would still have to adjust fusing but a much easier way to go here.


EDIT: kind of missed your reversing part. My bad. All 3PH motors are reversible though. Sounds like the transformer is the way you will go anyway. Good luck
 
Will this work or should I find a transformer perhaps twice the capacity? I don't know if the transformer would allow a high current flow from the VFD, causing it to overheat, or if it will draw (and deliver) exactly what the motor needs?


Wire it up. Run it for a quarter of an hour, under normal load if possible. Turn it off. Then feel the windings and the core. If you can keep your hand on it without getting burnt, you're probably ok. If it isn't ok, you'll smell the insulation getting hot before the magic smoke gets let out of the transformer.
 
Mark, I follow your logic and may well do it that way. If I understand it correctly, the odds are that I would cook the transformer before the motor? I can afford to lose a transformer.

Viper, this machine is built and wired around the base casting. Special switch for forward/reverse, another one for on, off, and speeds one and two. It all works as designed. I would rather not get in and rip it all out, although I could do that.
You mentioned swapping the motor. A quick search of an auction website showed you are completely correct about cheap 3 phase motors (thanks for the tip). An option along those lines would be to find a 220v 3phase motor and hook it to my VFD. Disconnect only the motor, swap it (leaving all other wiring untouched) and hook the 220v to the VFD. I'll keep my eyes open for a 3450 rpm, 220v 3ph motor. That might be the easy solution.

Another thought is that there is a Cabin Fever Show coming to York, PA on January 17 & 18. I can check on the 480v VFD with a supplier and see about their availability/practicality and cost. http://www.cabinfeverexpo.com/
 
I run 460 volt, 2 speed, motors with a VFD: Here is how I do it.

230 volt supply single phase to a single phase disconnect,
from disconnect to a single phase 460/230 volt transformer,
460 volt single phase input into a 460 volt, 3 phase VFD. The VFD must be rated for twice the motor amps (or Killowatts).
Connect directly form the VFD to the motor with no switches inbetween.

If the motor is a constant torque type (the low speed power rating is 1/2 the high speed power rating), you can connect to the high speed winding only. At 50% VFD speed, you will have the same speed and torque as with the motor connected for low speed. AT 100% VFD speed you will have the same performance as with the motor in high speed (as it actually is)

If the motor is constant horsepower type (low speed power is same as high speed power) then connect the motor for low speed operation then set the VFD maximum speed at 120 hz. This will give you the same performance as you would have had with the two speed motor.

Only trick here is to get a 3 phase VFD that will operate on single phase input.
I have an ABB model that will do this but can not remember the model number right now. I will need to look tonight.
 
Only trick here is to get a 3 phase VFD that will operate on single phase input.
As long as you derate the drive by 50% for single phase input, most drives will work perfectly. The exception are those that have phase loss detection, and even some of those can be reconfigured so the phase loss detection is not active. Check with the drive manufacturer to be sure.
 








 
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