# VFD motor leads connected to a transformer



## MotoGP1199 (Aug 11, 2014)

I have never seen this, and I am curious to see what others think. Just off hand I think it would be harder on the transformer as it is getting higher voltage as the output of the drive will be 1.41 times voltage in DC. Drives put out pulsed DC voltage that simulates an AC sine wave. I would assume the drive output for 600AC would be around 846 volts DC and much more likely to cause Corona discharge on the windings insulation and burn through it. Like I said, I am merely speculating as I have not seen or done t his before. 

I would assume the drive would also only work in V/hZ mode and not any type of sensorless vector mode. I would also think you might need other overloads for the motor as the drive would be reading current for the transformer and not the motor. This makes me really curious to see others input.

I am sure someone much smarter than my self will chime in.

Edit: I personally would put the transformer before the drive. Not only would this be less stress on the transformer, it would also help with harmonics to protect the drive and reduce harmonics going back into the system .


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## MoscaFibra (Apr 15, 2021)

MotoGP1199 said:


> I have never seen this, and I am curious to see what others think. Just off hand I think it would be harder on the transformer as it is getting higher voltage as the output of the drive will be 1.41 times voltage in DC. Drives put out pulsed DC voltage that simulates an AC sine wave. I would assume the drive output for 600AC would be around 846 volts DC and much more likely to cause Corona discharge on the windings insulation and burn through it. Like I said, I am merely speculating as I have not seen or done t his before.
> 
> I would assume the drive would also only work in V/hZ mode and not any type of sensorless vector mode. I would also think you might need other overloads for the motor as the drive would be reading current for the transformer and not the motor. This makes me really curious to see others input.
> 
> ...


I too would have personally put the transformer before the drive, and ordered the the proper voltage drive for the task at hand. 

The only reason I am even looking into this is I was asked to make it safe after they found 120V arcing off the frame from an abandoned piece of field equipment that no one actually disconnected from power.....


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## just the cowboy (Sep 4, 2013)

Never seen it in step down, and yes I would think the insulation on the transformer would not like the peaks same as a non VFD rated motor unless it was rated for it. On the other hand we run 480 outputs from VFD to sine wave filters then step-up transformer to run 2,300 volt motors all the time but the primary is rated for VFD.


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## MoscaFibra (Apr 15, 2021)

just the cowboy said:


> Never seen it in step down, and yes I would think the insulation on the transformer would not like the peaks same as a non VFD rated motor unless it was rated for it. On the other hand we run 480 outputs from VFD to sine wave filters then step-up transformer to run 2,300 volt motors all the time but the primary is rated for VFD.


This is pretty much how my stance is going with management when they are asking me why.

There is 0 documentation about the transformer. Even the manufacture website show's nothing so more than likely it was custom made for a different process somewhere else (the rest of the machine is hobbled together)

I cannot guarantee any safety or likelihood of anything working with this setup. The transformer is not meant to be used as a varying load with in rush. I hadn't even thought of the fact that it couldn't truly monitor the amperage on the motor! 

This place might just be the end of me with finding these gems everywhere. How many corners can you cut ffs


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## just the cowboy (Sep 4, 2013)

I would just tell them for $158 US to install a 120V input to 240 Volt 3 phase drive and do away with current drive. If no 120 on machine use existing transformer for feed 240 volt drive, problem fixed.


https://www.automationdirect.com/adc/shopping/catalog/drives_-a-_soft_starters/ac_variable_frequency_drives_(vfd)/micro/cfw300a01p6s1nb20


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## gpop (May 14, 2018)

MotoGP1199 said:


> I have never seen this, and I am curious to see what others think. Just off hand I think it would be harder on the transformer as it is getting higher voltage as the output of the drive will be 1.41 times voltage in DC. Drives put out pulsed DC voltage that simulates an AC sine wave. I would assume the drive output for 600AC would be around 846 volts DC and much more likely to cause Corona discharge on the windings insulation and burn through it. Like I said, I am merely speculating as I have not seen or done t his before.



Vfd's do not make voltage they simply convert it. Rms voltage may read 600v but peek to peek ac voltage would be at 846.


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## MotoGP1199 (Aug 11, 2014)

gpop said:


> Vfd's do not make voltage they simply convert it. Rms voltage may read 600v but peek to peek ac voltage would be at 846.



By my wording of the drive putting out voltage I didn't mean to infer that it creates voltage. However I do believe that the output is actually a series of DC pulses in a sieries of positive pulses followed by a sieres of negative pulses made possible by the rapid on/off switching of the IGBT's to form a pattern that looks like a sine wave. I don't believe it's actually true AC, but DC mimicking AC well enough to work on motors. And yes the 846 volts would be peak before being averaged out to RMS.

I am not an electrical engineer and my training has all been on the job with lots of reading. If I misunderstood how this works I sure would like a better explanation as this is how I learn.


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## SWDweller (Dec 9, 2020)

this sounds a lot like a dv/dt filter, 








MTE dV Sentry dv/dt filter | dv output filter | Motor Protection Filter


The dV Sentry's Patented design is the first proven filter that provides common mode reduction, peak voltage protection and rise time reduction – all in one unit.




www.mtecorp.com





I hope I helped and not confused the situation.


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## JRaef (Mar 23, 2009)

It's done all the time for MV motors, i.e. you only have 600V available but you need to run a 4160V motor, you put a transformer to step UP the votlage on the output of the VFD.

I agree, it would have been FAR cheaper to put the transformer on the INPUT side and buy a cheaper 240V rated drive; they paid extra to get a 600V drive. More likely, this was a fix for an error in that someone ASSumed the machine's motor would be 600V, but it came in with a 230V motor and they already had purchased and installed the VFD, so they bought a transformer.

_Technically _is was unnecessary, because they could have just *REPROGRAMED the VFD to put out 230V at 60Hz* for the motor. You can always do that with any VFD; program it to LIMIT the voltage to the motor, you just can't make the VFD create voltage that isn't already there. There is a downside to that though, based on what gpop said in Post #7. The DC bus voltage will be 141% of the RMS line voltage, so with a 600V input, the DC bus is at 846VDC, which means the PULSES sent to the motor are at 846VDC, even if the eventual RMS voltage ends up as 230V. That puts a lot of stress on the insulation of the motor. If the motor was made with newer "Inverter Spike Resistant" insulation, typically 2,000V, then it will be fine. But if not, the motor may fail in short order. In this case the same will happen with that transformer, however transformer mfrs NEVER use that kind of insulation, so it's likely that the transformer will fail prematurely.

Personally, I would have just reprogrammed the VFD for 230V output, then got whatever life I could get from the motor and when it fails, replace it with a 600V version. Most likely the new motor will be less expensive and certainly less wasteful than the transformer was.


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## just the cowboy (Sep 4, 2013)

JRaef said:


> _Technically _is was unnecessary, because they could have just *REPROGRAMED the VFD to put out 230V at 60Hz* for the motor.
> Personally, I would have just reprogrammed the VFD for 230V output, then got whatever life I could get from the motor and when it fails, replace it with a 600V version. Most likely the new motor will be less expensive and certainly less wasteful than the transformer was.


I would of suggested that but I just don't that soma would not change it someday. I have done it many times with a 480 drive when needed. Just set 480v 120 hz clamp at 240 and away it goes. BUT I still never felt right,


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## gpop (May 14, 2018)

MotoGP1199 said:


> By my wording of the drive putting out voltage I didn't mean to infer that it creates voltage. However I do believe that the output is actually a series of DC pulses in a sieries of positive pulses followed by a sieres of negative pulses made possible by the rapid on/off switching of the IGBT's to form a pattern that looks like a sine wave. I don't believe it's actually true AC, but DC mimicking AC well enough to work on motors. And yes the 846 volts would be peak before being averaged out to RMS.
> 
> I am not an electrical engineer and my training has all been on the job with lots of reading. If I misunderstood how this works I sure would like a better explanation as this is how I learn.



You have it correct.


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## micromind (Aug 11, 2007)

JRaef said:


> It's done all the time for MV motors, i.e. you only have 600V available but you need to run a 4160V motor, you put a transformer to step UP the votlage on the output of the VFD.


I've done this a couple of dozen times over the last 20 years. 480 VFD, transformer usually in the same room as the VFD then out to a 4160 motor. 

I thought it was dumb until I saw the cost of a 4160 VFD............


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## wiz1997 (Mar 30, 2021)

Are you sure it's a transformer and not a reactor?
A reactor looks like a transformer.
A quote from the internet.

A line reactor (also referred to as a "choke") is a variable frequency drive (VFD) accessory that consists of a coil of wire that forms a magnetic field as current flows through it. This magnetic field limits the rate of rise of the current, thus reducing harmonics and protecting the drive from power system surges and transients. There are two primary types of reactors used in variable frequency drives: AC & DC. When an AC reactor is placed between the power system and the drive, it is referred to as an AC line reactor. When a DC reactor is inserted into the DC link of a variable frequency drive, it is known as a DC link reactor.
Both AC and DC reactors limit harmonic currents. However, AC reactors have the significant advantage of protecting the entire VFD from power system surges and transients since they are placed between the power system and the incoming power terminals at the drive. Reactors can prevent overvoltage trips, increase the reliability and life span of the VFD, improve total power factor, and reduce nuisance tripping.


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## MotoGP1199 (Aug 11, 2014)

just the cowboy said:


> Never seen it in step down, and yes I would think the insulation on the transformer would not like the peaks same as a non VFD rated motor unless it was rated for it. On the other hand we run 480 outputs from VFD to sine wave filters then step-up transformer to run 2,300 volt motors all the time but the primary is rated for VFD.





micromind said:


> I've done this a couple of dozen times over the last 20 years. 480 VFD, transformer usually in the same room as the VFD then out to a 4160 motor.
> 
> I thought it was dumb until I saw the cost of a 4160 VFD............


When you guys do this do you add separate overloads after the transformer or do you calculate the amperage of the primary side of the transformer based off the secondary amperage?


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## MoscaFibra (Apr 15, 2021)

wiz1997 said:


> Are you sure it's a transformer and not a reactor?
> A reactor looks like a transformer.
> A quote from the internet.
> 
> ...


Oh it is a step down transformer, no doubt about that. I've built more than enough panels with reactors and filters to know their difference and help. 

The lack of current feedback on the motor, the spike on peak to peak is more than enough to scare management into ordering the proper parts. I have found a proper motor so that both motors are the same. Since there are two, one at 600V and one at 240V....why because its what they found on their shelf.


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## micromind (Aug 11, 2007)

MotoGP1199 said:


> When you guys do this do you add separate overloads after the transformer or do you calculate the amperage of the primary side of the transformer based off the secondary amperage?


I use the VFD for motor protection, it's turns ratio plus a bit for losses in the transformer. 

A lot of 4160 motors have stuff like thermistors embedded in the windings, they go to VFD inputs as well.


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## micromind (Aug 11, 2007)

wiz1997 said:


> Are you sure it's a transformer and not a reactor?
> A reactor looks like a transformer.
> A quote from the internet.
> 
> ...


A reactor will always have the same input and output voltage, if it changed voltage it'd be a transformer plus, a reactor is usually smaller than a transformer but they do indeed look the same.


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## just the cowboy (Sep 4, 2013)

MotoGP1199 said:


> When you guys do this do you add separate overloads after the transformer or do you calculate the amperage of the primary side of the transformer based off the secondary amperage?


All overload protection is done at the drive. It stays at 480 volt in the building or panel that house the drives, leaves building as 480 V to an outdoor rated transformer just outside the building then down hole to a pump motor 2000 Ft. away. This way for safety the electrician only deals with 480 Volts, the well company does the high voltage connections.
It is cool that the 400 hp motor is only about 12" around but 20 feet long. one day I will catch one out of a hole and post a picture


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## paulengr (Oct 8, 2017)

just the cowboy said:


> All overload protection is done at the drive. It stays at 480 volt in the building or panel that house the drives, leaves building as 480 V to an outdoor rated transformer just outside the building then down hole to a pump motor 2000 Ft. away. This way for safety the electrician only deals with 480 Volts, the well company does the high voltage connections.
> It is cool that the 400 hp motor is only about 12" around but 20 feet long. one day I will catch one out of a hole and post a picture


With well cable I’ve found the magic size is 2300 V. The pump itself doesn’t care…most are 4000/2300. But even if you buy unshielded 5 kV the unshielded 2.5 kV is smaller even considering larger copper size.


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