Single phase power to 3 phase motor

[Besoeker]

If I may, I think what ActionDave meant in asking what type of compressor was the mechanical design, as in reciprocal, screw, centrifugal etc. If it is a 7-1/2HP basic shop compressor, I'd guess there is a 90+% chance it is recip. You can reduce the speed of a recip, but you can quickly run into trouble if it uses a mechanical oil pump or splash lubrication, because with reduced speed you get reduced oil flow and can damage the machine. So you must thoroughly investigate your compressor before reducing the speed and if it does have an electric oil pump, make sure it can be powered separately from the compressor motor as well.

For 7-1/2HP you must double the size of the VFD for the reasons Besoeker mentioned, so you need to buy a 15HP 230V drive to do phase conversion. Be careful in that there are a few brands of VFD that do not allow for single phase input because they have phase loss protection that cannot be disabled. If your vendor doesn't know, you can look in the operating manual to see if there is a fault code for phase loss and if there is, then is there a way to disable it. If you can't determine it, don't use that brand.

Also keep in mind that most compressors start unloaded via an unload valve but again if that valve is powered from the same line power as the compressor, ramping the compressor voltage may cause that valve to fail to open and your compressor will not start.

So bottom line it can be done, but you must do it carefully with full knowledge of what you have and all of the ramifications.

Good points.
I don't routinely deal with the low power end and had not thought about shaft driven lubrication.

 

[Rick Christopherson]

To add to what Jraef said, the default oversize (derating) of the VFD is by √3 (pretty close to double). However, you can tweak this value down a little by taking a closer look at the specifications. Amperage through the input rectifier is the limiting factor, not the rated horsepower. For example, most 5hp motors can be operated from a 7.5 hp VFD (single phase) instead of requiring the 10 hp default size.

Check the motor nameplate and get the FLA amperage (which is given as "per-phase"). Multiply this by √3 to get your total DC bus amperage load. Then choose the VFD specific to this amperage for a single phase input. The VFD input amperage rating will also be "per-phase", so you don't need to convert this number because you are feeding it with single-phase power. In most cases, you will find that you can use a smaller VFD than the default 2X.

I would be leery about putting too much of a start-up ramp speed on the motor unless the compressor has head-unloading. That's because without the unloader, the motor will have a full compressive load in just a few revolutions. You need it to get up to speed quickly to maximize its torque output at a reasonable amperage.

What would be "ideal" is if the compressor had an electrically controlled unloader valve. Then you could use a VFD output to trigger the head unloader to disengage when the VFD/motor reached full speed regardless of ramp time.

Stepping back for a minute to the RPC option. The RPC will work nice for the motor, but the problem is that because the motor starts automatically, you either need to leave the RPC running all the time, or add an on-delay relay to start the compressor motor after the RPC has been started by the compressor's on-board control signal. In other words, the compressor's pressure switch would trigger the RPC to start, and the On-Delay relay would trigger the compressor motor a couple seconds later.

Alternatively, the RPC will have a voltage-sensing relay to disengage the start capacitors when the idler motor is up to speed. If you can tap into this signal, you could have the compressor motor fire up when the RPC's start circuit drops out.

 

[Besoeker]

To add to what Jraef said, the default oversize (derating) of the VFD is by √3 (pretty close to double). However, you can tweak this value down a little by taking a closer look at the specifications. Amperage through the input rectifier is the limiting factor, not the rated horsepower.

I'm sure such a general statement like this has a not a lot of validity. Rectifiers, usually modules on all but quite large ratings, come in discrete sizes. It would be something of a coincidence if the input rectifier was rated to exactly match the required DC current for a three phase input. There are other components to consider like the input wiring rating and the ripple current rating of the DC link capacitors. On single phase these will be presented with a much lumpier ripple at twice the supply frequency instead of six times supply frequency that they would get with a three phase supply and a much smoother voltage. Both work against the capacitor rating as a rule and in combination.....

 

[Rick Christopherson]

I'm sure such a general statement like this has a not a lot of validity. Rectifiers, usually modules on all but quite large ratings, come in discrete sizes. It would be something of a coincidence if the input rectifier was rated to exactly match the required DC current for a three phase input. There are other components to consider like the input wiring rating and the ripple current rating of the DC link capacitors. On single phase these will be presented with a much lumpier ripple at twice the supply frequency instead of six times supply frequency that they would get with a three phase supply and a much smoother voltage. Both work against the capacitor rating as a rule and in combination.....

I think you misread what I said. It's simple conservation of energy from 1-p input to 3-p output. The math works.

 

[Cow]

I would think a starter and a single phase motor is cheaper and more cost effective in the long run. You won't have a drive using electricity sitting idle all the time just waiting for a lightning strike to take it out in the middle of the night either.

I'm sure a drive would probably lengthen motor life, but I wouldn't think the extra cost of replacing motors a little more frequently would make up for the extra drive costs. Keep it simple.

 

[kwired]

I would think a starter and a single phase motor is cheaper and more cost effective in the long run. You won't have a drive using electricity sitting idle all the time just waiting for a lightning strike to take it out in the middle of the night either.

I'm sure a drive would probably lengthen motor life, but I wouldn't think the extra cost of replacing motors a little more frequently would make up for the extra drive costs. Keep it simple.

I brought this idea up a few posts back, yet everyone seems to feel it is best to make it as complicated as possible, I hope I get to bid against them for this install:happyyes:

Seriously, How much does a 7.5 single phase motor cost? Probably 500-1000. How much does a phase converter, or 15 hp VFD cost? May be able to buy 2 or 3 single phase motors when all is done. Unless it is not a standard NEMA motor or if soft starting is a must I wouldn't even consider any other option.

 

[Cow]

I agree Kwired. I hope the service is fairly healthy because starting a single phase 7.5HP is probably going to make the lights blink!

 

[Sierrasparky]

I would go the route of replacing with a Single phase motor as it usually is the cheapest other than a phase converter. That is unless you want a tricked out compressor.

 

[kwired]

I agree Kwired. I hope the service is fairly healthy because starting a single phase 7.5HP is probably going to make the lights blink!

It will with the phase converter idea also. Vfd no.

I see more often the Vd problems are with excess length on service drop conductors (like before final span) or somewhat small transformers supplying several customers or both.

On farm places where there is no other customers served by same transformer it usually is not as bad unless it is just old distribution that is just undersized for what loads are on it today.

 

[Besoeker]

I think you misread what I said. It's simple conservation of energy from 1-p input to 3-p output. The math works.

I commented on this point:

Amperage through the input rectifier is the limiting factor,

What exactly has that got to do with conservation of energy?

 

[Rick Christopherson]

What exactly has that got to do with conservation of energy?

Because you can't get more out of the system than what you put into it. The conversion from single phase amperage at the input to 3 phase amperage at the output is √3. Since 2 of the 6 input diodes are not being used, the remaining 4 have to carry the full load. At single-phase, the input rectifier is the limiting component on how much power (or amperage) the VFD can handle. The input diodes are limited on how much current they can sustain without damage.

The reason why derating by rated amperage is any different than derating by horsepower, is because horsepower is too broad of definition. When a VFD is rated for a certain horsepower, its maximum amperage has to cover the full spectrum of FLA's that unknown motors could draw. Derating by amperage uses the motor nameplate amperage for the specific motor.

 

[Besoeker]

Because you can't get more out of the system than what you put into it.

I made no claim to the contrary.

The conversion from single phase amperage at the input to 3 phase amperage at the output is √3.

Not really. The input is at about 1.0 displacement power factor. Distortion power factor depends on a number of factors including supply impedance and the inclusion or otherwise of supply side and DC link reactors. The output power factor depends on motor characteristics and loading. There is no simple single number that relates input current to output current.

Since 2 of the 6 input diodes are not being used, the remaining 4 have to carry the full load. At single-phase, the input rectifier is the limiting component on how much power (or amperage) the VFD can handle.

I'm sure such a general statement like this has a not a lot of validity. Rectifiers, usually modules on all but quite large ratings, come in discrete sizes. It would be something of a coincidence if the input rectifier was rated to exactly match the required DC current for a three phase input. There are other components to consider like the input wiring rating and the ripple current rating of the DC link capacitors. On single phase these will be presented with a much lumpier ripple at twice the supply frequency instead of six times supply frequency that they would get with a three phase supply and a much smoother voltage. Both work against the capacitor rating as a rule and in combination.....

 

[texie]

Regarding my original post:

Clean sheet of paper-the compressor is not yet purchased, Cast in stone is 240 volt 1 phase power available. 7.5 or 10 HP TBD

Option 1:
Order it with a single phase motor and standard pressure switch w/mechanical unloader. In other words a common off the shelf unit.
Cons: single phase motor is more expensive than a 3 phase and not as reliable, high start current, need a starter.
Pro: simple to buy.

Option 2:
3 phase motor. Use VFD as convertor and as soft start. Use a pressure switch for on/off and a N.O. solenoid valve triggered by the VFD as an unloader.
Cons:??
Pro: save the cost of a seperate starter, less inrush current, motor cost less and is more reliable.

 

[Sierrasparky]

No. Because they have no bearing on the topic at-hand, hence the comment that you are arguing for the sake of arguing.

When supplying a 3-phase VFD with 1-phase power, is the input rectifier the limiting factor? Yes or no? If you say Yes, then the rest of your discussion is arguing for the sake of arguing. If you say No, then you are...well, you fill in the blank.

Here we go again.

 

[kwired]

Regarding my original post:

Clean sheet of paper-the compressor is not yet purchased, Cast in stone is 240 volt 1 phase power available. 7.5 or 10 HP TBD

Option 1:
Order it with a single phase motor and standard pressure switch w/mechanical unloader. In other words a common off the shelf unit.
Cons: single phase motor is more expensive than a 3 phase and not as reliable, high start current, need a starter.
Pro: simple to buy.

Option 2:
3 phase motor. Use VFD as convertor and as soft start. Use a pressure switch for on/off and a N.O. solenoid valve triggered by the VFD as an unloader.
Cons:??
Pro: save the cost of a seperate starter, less inrush current, motor cost less and is more reliable.

You really think single phase motor and starter will cost less than three phase motor and VFD?

What is so unreliable about single phase motors?

You can probably purchase 3 to 5 pressure switches with integral unload valve for cost of a solenoid type valve.

IMO unless soft start is a necessity it is not worth trying to use the three phase motor.

If you want to see severe voltage drop problems at motor start come to some of the grain storage bin sites around here and see how starting a 10 hp direct coupled fan with a fan impeller that is maybe 30 inches diameter and weighs maybe 150 -200 pounds drops the voltage as compared to the 7.5 compressor that is up to speed within a second or two. I'm talking single phase here, and it is a very common install around here. Worse yet we are starting to see 15 and 16 HP rated single phase motors on some of these fans. I replace start capacitors on these fans more often than any other motor, they take a lot to get started

 

[mivey]

This message has been deleted by iwire.

I like it. That's the way to cool a thread. :thumbsup:

No disrespect to any posters, BTW.

 

[Besoeker]

I like it. That's the way to cool a thread. :thumbsup:

No disrespect to any posters, BTW.

Well, I was cool with it anyway.
I just didn't agree that the input rectifier was THE limiting factor in all, or even most, cases.
As I noted, rectifier components come in discrete ratings. You wouldn't really pick devices that couldn't quite do the job. So you'd need to go up to the next available.
I checked with a couple of distributors and the cheapest 3-phase bridge was rated at 35A and priced at about five bucks in even modest quantities. And probably far less for the manufacturers who buy in bulk.

 

[mivey]

Well, I was cool with it anyway.

I know, but my comment was about the moderation in general, including the other "Reason: Keep the personal comments out of this" posts. I was just proud to see a mod deal with individual posts instead of killing a whole thread that is full of good info.

I just didn't agree that the input rectifier was THE limiting factor in all, or even most, cases.
As I noted, rectifier components come in discrete ratings. You wouldn't really pick devices that couldn't quite do the job. So you'd need to go up to the next available.

I checked with a couple of distributors and the cheapest 3-phase bridge was rated at 35A and priced at about five bucks in even modest quantities. And probably far less for the manufacturers who buy in bulk.

I understand what you are saying and think you make a valid point.

 

[Rick Christopherson]

Well, I was cool with it anyway.
I just didn't agree that the input rectifier was THE limiting factor in all, or even most, cases.
As I noted, rectifier components come in discrete ratings. You wouldn't really pick devices that couldn't quite do the job. So you'd need to go up to the next available.

Well that is why you go by the specific rating. However, I think you misunderstand how VFDs are manufactured....see next:

I checked with a couple of distributors and the cheapest 3-phase bridge was rated at 35A and priced at about five bucks in even modest quantities. And probably far less for the manufacturers who buy in bulk.

I get the impression that you are thinking these are discrete components. Possibly even thinking that the end-user needs to supply their own bridge rectification (but I may be misinterpreting what you said).

High power VFDs won't use composite (all-in-one) full wave bridge rectifiers. They will use discrete diodes that can individually be mounted to an effective heatsink. Manufacturers also don't shop at Radio Shack for their parts, so they are not limited to a few ranges of diode power ratings.

Nevertheless, what you failed to notice in my original posting was where I instructed the OP to size the VFD based on its published specifications for the rectifier amperage, whatever that rating may be. As a matter of fact, your notion that there are only a few discrete sizes of power diodes makes this all the more important. If, as you suggest, there are only a couple sizes, then it would be very likely that a 10hp and 15hp VFD both have the same size input rectifier.

Actually, I disagree with some of the moderating Iwire did with this thread, because I believe he removed a vary valuable question in the process because he didn't understand it. I asked you whether the input rectifier was the limiting factor for using a 3-phase VFD driven by 1-phase input. If you believe that is not what limits the available amperage for the VFD under these conditions, I would like to hear about it.

 

[mivey]

...However, I think you misunderstand how VFDs are manufactured...

That would be hard to believe since Besoeker both designs and manufactures VFDs.