Single Phase Power Supply of 3 Phase VFDs

[R_HATT10]

I was looking for some more information on applying a single phase input (power supply) for 3 phase VFD. I have been beating my head on this for weeks now, I have done some basic math (on derating the drive for single phase power input), and I am now at the point where I want to tell the customer to go with a single phase 240 V motor.

The basic synopsis of the customer's needs are as follows:

- The customer only has 240 V Single Phase, 200 A Service.

- The customer needs 3 Phase 240 V or 480 V to drive a 240 V/460 V, 76/37 FLA, 60 Hz, 1755 RPM, 0.875 P.F. Motor (to drive a pump for a hydraulic compressor).

- I have supplied the customer with a 60 HP VT Variable Frequency Drive, which is as follows:

77 A Input Current, 87 A Input Current
380-500 V Input Voltage
0-500 V Output Voltage

The Problem:

I obviously do not have enough input voltage available from the Customer's Service Panel.

I - think I - need at least 480 V input voltage and at least the available input current of the drive (77 A) to run the motor somewhat efficiently.

I do realize that I will not be getting near the DC Bus Voltage (with single phase power input) since only half the inverter section with be getting AC input. So, there will need to be a substantial amount of input power available to compensate.


My question is: Can I use a single phase 240 V to 480 V Transformer (or even a single phase 480 V to 240 V transformer, by back-feeding 240 V through the secondary to get 480 V out of the primary) with at least 77 A available from the transformer?

Or in other words, would I need a transformer KVA Rating of: (77 A x 480 V)/1000 = 36.96 or 37 kVA?

I also realize that this seems quite complicated and silly to be doing but, I purchased the drive for a good price and would like to get it to work.

Any help at all would be greatly appreciated.

 

[wireguru]

nevermind........

 

[quogueelectric]

Callthe drive mfg I am sure they will tell you.

 

[Jraef]

Drives can't create voltage, so yes, if you want to run that motor at 460V, you will need to transform up to 480V ahead of the drive. No problem doing it, but your sizing is a little sparse. You may need a little extra starting power, especially with a hydraulic pump. So I would go with a 50kVA transformer.

You don't need to worry about the 1 phase with regards to the bus voltage, as long as the input voltage is high enough. But what you do have to do is double the size of the VFD compared to the motor. Yopu never say, but it appears you have a 30HP motor, so a 60HP VFD is the right choice. Should work fine.

The only possible glitch is that SOME 480V drives are not capable of accepting a single phase input. The manufacturers didn't think of that application and in order to protect the diaodes on the front end from overloading if a phase is lost, they put in Phase Loss Detection. Some who do that make it so that you can defeat that feature, but some, such as Allen Bradley, do not. So in many 480V A-B drives, you cannot use a 1 phase supply to feed it even if you double the size of the drive. The only way to check is to ask the manufacturer.

 

[Lcdrwalker]

There are several Mfgs that supply VFDs with 240 single phase input and 240three phase output.

 

[Besoeker]

I was looking for some more information on applying a single phase input (power supply) for 3 phase VFD. I have been beating my head on this for weeks now, I have done some basic math (on derating the drive for single phase power input), and I am now at the point where I want to tell the customer to go with a single phase 240 V motor.

The basic synopsis of the customer's needs are as follows:

- The customer only has 240 V Single Phase, 200 A Service.

- The customer needs 3 Phase 240 V or 480 V to drive a 240 V/460 V, 76/37 FLA, 60 Hz, 1755 RPM, 0.875 P.F. Motor (to drive a pump for a hydraulic compressor).

- I have supplied the customer with a 60 HP VT Variable Frequency Drive, which is as follows:

77 A Input Current, 87 A Input Current
380-500 V Input Voltage
0-500 V Output Voltage

The Problem:

I obviously do not have enough input voltage available from the Customer's Service Panel.

I - think I - need at least 480 V input voltage and at least the available input current of the drive (77 A) to run the motor somewhat efficiently.

I do realize that I will not be getting near the DC Bus Voltage (with single phase power input) since only half the inverter section with be getting AC input. So, there will need to be a substantial amount of input power available to compensate.


My question is: Can I use a single phase 240 V to 480 V Transformer (or even a single phase 480 V to 240 V transformer, by back-feeding 240 V through the secondary to get 480 V out of the primary) with at least 77 A available from the transformer?

Or in other words, would I need a transformer KVA Rating of: (77 A x 480 V)/1000 = 36.96 or 37 kVA?

I also realize that this seems quite complicated and silly to be doing but, I purchased the drive for a good price and would like to get it to work.

Any help at all would be greatly appreciated.

At 37 kVA, the transformer would be big enough. From the figures you have given, the motor input power would be about 26 kW and the inverter input maybe 3% more.

However, I see a couple of potential problems.
On the input rectifier stage of the inverter, only two out of the three legs will conduct. To operate at the same mean on-state current in the remaining legs, you would have to reduce the rectifier output current to two thirds of its rating. Combined with the reduced voltage obtainable from the rectified 480V single-phase, you would be some way short of the 26 kW motor input power.

The output power reduction to possibly around 80% might be adequate for your application. But there is another problem.

Rectified single phase produces much greater ripple on the DC than rectified three-phase. The DC link capacitors in the inverter may not be rated to handle this.

 

[Lcdrwalker]

As I stated, I know that there are inverters for 240 single to three phase but I'm not sure about 480V because I've never used one. It seems to me though that the inverter would trip out on phase loss. As others have suggested, a transformer but with a phase generator feeding the VFD?

 

[ptonsparky]

Just went to a AB class last week where they were pushing VFDs for irrigation wells. Comment was all of their drives would accomodate single phase input but you needed to oversize by 50% and discuss the job with sales reps before ordering. I will assume they meant current products.

The instructor also noted that if anyone had need for a 5000 hp medium voltage drive, class was over and lunch for all was at the local steak house, instead of sandwiches.

 

[chaterpilar]

Schneider VSD's Altivars (ATV11 series) are for small motors and accept 240 vac (Single phase ) input and give 3 phase output, but i doubt they make the size you require.

Check their site for more details.

cheers.

 

[Jraef]

At 37 kVA, the transformer would be big enough. From the figures you have given, the motor input power would be about 26 kW and the inverter input maybe 3% more.

A 37.5kVA 1 phase transformer will put out 78A full load on the 480V side. His 30HP motor will will draw 37A 3 phase at 480V, x 1.732 = 64A full load 1 phase input. BUT, the VFD is going to need to be capable of 150% overload for 60 seconds just in case you need more starting current. That puts the transformer load at 96A and the 37.5kVA transformer will go into saturation. That's why I recommended a 50kVA transformer. The only way to get away with using a 37.5kVA would be to enable Current Limit on the VFD so that it never draws more than 78A input. If you do that, AND the hydraulic pump needs more than that to start, the application will fail.

However, I see a couple of potential problems.
On the input rectifier stage of the inverter, only two out of the three legs will conduct. To operate at the same mean on-state current in the remaining legs, you would have to reduce the rectifier output current to two thirds of its rating. Combined with the reduced voltage obtainable from the rectified 480V single-phase, you would be some way short of the 26 kW motor input power.

The output power reduction to possibly around 80% might be adequate for your application. But there is another problem.

Rectified single phase produces much greater ripple on the DC than rectified three-phase. The DC link capacitors in the inverter may not be rated to handle this.

All of this is taken care of by virtue of the fact that he doubled the VFD size compared to the motor size. That is why you do that. The rectifier is capable of the added input current AND the additional bus capacitance smooths out the extra ripple.

 

[Jraef]

There are several Mfgs that supply VFDs with 240 single phase input and 240three phase output.

This application is a 30HP motor, nobody makes a 30HP VFD that accepts 1 phase input without de-rating. So he would have to buy a 60HP (178A) 240V drive, which would be extremely expensive compared to a 60HP (94A) 480V drive.

 

[R_HATT10]

Thanks for all the responses, I appreciate it.

It is a Cutler-Hammer 60 HP Drive, the acceptable input voltage to this drive is 380-500 V AC, 77 A. So, as you can see, I am in a predicament as to supply voltage demand - of the drive - and available current.

Really my only choice is to step up from 240 V to 480 V to achieve acceptable input voltage while meeting the available current demand (at least 77 A).

Thank you to Jraef for recommending a higher rated 50 KVA transformer due to the demand the hydraulic pump may need.

The literature for the drive accepts single phase voltage input as long as certain criteria is met, such as:

Single Phase 480 V Input, 87 A required.

Thus,

X-FMR KVA = (480 x 87)/1000 = 41.6 kVA

So, compensating for additional start-up power & torque (required by pump), a 50 kVA XFMR seems to be the better choice.

Derating the output of the current (for the drive) would make it 2/3 of output current on the nameplate - since only single phase power supply is being used.

2/3 output current of drive nameplate = (2/3) x (87 A Output) = 56 A

Derating Factor:

VFD Input Current must be greater than or equal to Motor FLA x 1.73
(to achieve desirable output of the drive)

So,
77 A (at least) > 64.01

Supposedly, only 56 A (Max) will be available at the drive's output. So, I can see that I might be a little shy on Full-Load Operating current (since Motor FLA is 64.01 and available output current of drive is 56 A).

Does this make sense, correct me iif I am wrong. I would appreciate further inquires. Thanks again for the responses, very informative.

 

[Jraef]

... Derating the output of the current (for the drive) would make it 2/3 of output current on the nameplate - since only single phase power supply is being used.

2/3 output current of drive nameplate = (2/3) x (87 A Output) = 56 A

Don't know where you got that 2/3 issue from.

Don't over think it.
Your motor will require 37A at 460V according to the nameplate.
The VFD will draw 37 x 1.732 = 64A from a 1 phase source.
Your VFD is rated for 77A.
No problem.

 

[Besoeker]

A 37.5kVA 1 phase transformer will put out 78A full load on the 480V side. His 30HP motor will will draw 37A 3 phase at 480V, x 1.732 = 64A full load 1 phase input. BUT, the VFD is going to need to be capable of 150% overload for 60 seconds just in case you need more starting current.

Not sure that you have this right. You can't simply infer VFD input current from motor current.
From post #6, the motor input power would be about 26 kW.
Three-phase rectified 480V gives about 650Vdc so the 26kW would require around 40Adc in the DC link. Or about 33A line current into the rectifier.
Idc is about 1.225 Iac.
For a single-phase rectifier to produce 40Adc, the line current would be 44A.
Idc in this case is about 0.9 Iac.
44A and 480V gives about 21 kVA.

On starting current...
The application given (OP) is a pump although we don't know what kind of pump. If it is centrifugal, and many are, power is a cube law function of speed. The inverter input is mostly concerned with power. At low speed (starting) power is low also.

That puts the transformer load at 96A and the 37.5kVA transformer will go into saturation.

Saturation on a transformer is generally a matter of overvoltage, not overcurrent.

All of this is taken care of by virtue of the fact that he doubled the VFD size compared to the motor size. That is why you do that. The rectifier is capable of the added input current AND the additional bus capacitance smooths out the extra ripple.

The rectifier probably could. Peak currents would be significantly higher of course.

The capacitors........well maybe not.
DC ripple voltage for single-phase rectification is an order of magnitude greater than for three-phase rectification
Lop calcs might convince you that this not the best way to go.

 

[Besoeker]

Derating the output of the current (for the drive) would make it 2/3 of output current on the nameplate - since only single phase power supply is being used.

Ummm...
The problem is the input, not the output.

 

[R_HATT10]

Ummm... I realize the problem is input power, not output power.

Remember Kirchoff's Law.

 

[cschmid]

Drives can't create voltage, so yes, if you want to run that motor at 460V, you will need to transform up to 480V ahead of the drive. No problem doing it, but your sizing is a little sparse. You may need a little extra starting power, especially with a hydraulic pump. So I would go with a 50kVA transformer.

You don't need to worry about the 1 phase with regards to the bus voltage, as long as the input voltage is high enough. But what you do have to do is double the size of the VFD compared to the motor. Yopu never say, but it appears you have a 30HP motor, so a 60HP VFD is the right choice. Should work fine.

The only possible glitch is that SOME 480V drives are not capable of accepting a single phase input. The manufacturers didn't think of that application and in order to protect the diaodes on the front end from overloading if a phase is lost, they put in Phase Loss Detection. Some who do that make it so that you can defeat that feature, but some, such as Allen Bradley, do not. So in many 480V A-B drives, you cannot use a 1 phase supply to feed it even if you double the size of the drive. The only way to check is to ask the manufacturer.

A 37.5kVA 1 phase transformer will put out 78A full load on the 480V side. His 30HP motor will will draw 37A 3 phase at 480V, x 1.732 = 64A full load 1 phase input. BUT, the VFD is going to need to be capable of 150% overload for 60 seconds just in case you need more starting current. That puts the transformer load at 96A and the 37.5kVA transformer will go into saturation. That's why I recommended a 50kVA transformer. The only way to get away with using a 37.5kVA would be to enable Current Limit on the VFD so that it never draws more than 78A input. If you do that, AND the hydraulic pump needs more than that to start, the application will fail.


All of this is taken care of by virtue of the fact that he doubled the VFD size compared to the motor size. That is why you do that. The rectifier is capable of the added input current AND the additional bus capacitance smooths out the extra ripple.

this is good advice here. I do have a question though. if you installed a hydraulic load bypass solenoid that would not energize until the motor was up to speed would you still need to increase the transformer size.

 

[Jraef]

this is good advice here. I do have a question though. if you installed a hydraulic load bypass solenoid that would not energize until the motor was up to speed would you still need to increase the transformer size.

Maybe, maybe not. It depends on the type of hydraulic pump, but in general, all of them are impossible to start without bypass. Gear and lobe pumps can be somewhat more difficult to start than Recips., vane pumps are really easy but can't develop the same kinds of pressures etc. etc. There is no single answer.

 

[Besoeker]

Ummm... I realize the problem is input power, not output power.

Then why mention de-rating the output current of the drive (post #12)?
There are basically two stages in a VFD.
A converter that converts AC to DC and an inverter that converts DC to AC.
In your case, even with a single-phase input, the inverter stage is still three-phase.
Its declared output current rating is unaffected by going single phase on the input.

You should be careful about what you take away from this thread.
My calculations are shown in post #14. I have been in the field of variable speed drives and power electronics for all of my working life (40 years and counting :smile but that doesn't mean I can't get it wrong.

And Gustav Kirchhoff? Well, I first got acquainted with him around 1960...
:wink:

 

[R_HATT10]

You're an allstar in my book