230 volt convertor

[mcclary's electrical]

Who is he?
And why would anyone do that?

Post #6 said 4kw would run the motor. I think he's wrong and the only explanation I can think of is he figured 1hp=746 . X that by 5 for 5hp you get 3730 watts. But I think he's wrong in figuring this way. It should be 6440 watts for a 5hp at 230 volt.

 

[Besoeker]

It should be 6440 watts for a 5hp at 230 volt.

5hp is not 6440W

 

[mivey]

5hp is not 6440W

So he says.:grin:

 

[mivey]

Sure they do, in theory,. They work great on big compressors.but not the cheap ones, by the time most compressors make two turns, it's backed up again. Big commercial compressors won't load for 4-5 seconds of starting, but these smaller ones aren't that sophisticated.

So look at LRA:
LRA = 5 HP x 7.09 kVA_LR/HP (code H) x 1000 / 230 = 154 amps

Assume the generator can deliver 300% current for 10 seconds to start the motor means we need: 154 / 3 = 51.4 amps. At 230 volts this is 11.8 kVA so we need an 11.8 kVA unit or a 9.5 kW unit rated at 80% p.f.

Sound reasonable?

 

[Jraef]

Post #6 said 4kw would run the motor. I think he's wrong and the only explanation I can think of is he figured 1hp=746 . X that by 5 for 5hp you get 3730 watts. But I think he's wrong in figuring this way. It should be 6440 watts for a 5hp at 230 volt.

The watts-to-HP conversion has nothing to do with voltage.

4kW = 5.36HP

That's not to say you could START a 5HP motor with a 4kW converter, that is an issue having to do with the overload capacity of the converter.

Back to the OP;
You can buy or build a rotary phase converter for a lot less than a VFD capable of converting for a 5HP 230V motor, because to do it with a VFD, you would actually need to buy a 10HP drive*.

You can see plans to make your own, including formulas for sizing components, on this forum for machinists. But I agree with the comments that it would likely be less expensive and more reliable to just swap out the motor with a single phase one, especially when you factor in the value of a 5HP 230V 3 phase motor on eBay.

* There is possibly one manufacturer who makes a 5HP VFD capable of 1 phase input without de-rating.

 

[mivey]

The watts-to-HP conversion has nothing to do with voltage.

4kW = 5.36HP

That's not to say you could START a 5HP motor with a 4kW converter, that is an issue having to do with the overload capacity of the converter.

Back to the OP;
You can buy or build a rotary phase converter for a lot less than a VFD capable of converting for a 5HP 230V motor, because to do it with a VFD, you would actually need to buy a 10HP drive*.

You can see plans to make your own, including formulas for sizing components, on this forum for machinists. But I agree with the comments that it would likely be less expensive and more reliable to just swap out the motor with a single phase one, especially when you factor in the value of a 5HP 230V 3 phase motor on eBay.

* There is possibly one manufacturer who makes a 5HP VFD capable of 1 phase input without de-rating.

Seems like the idea from #7 (installing a 1-ph 5HP motor) would be easier.

 

[mcclary's electrical]

The watts-to-HP conversion has nothing to do with voltage.
4kW = 5.36HP

That's not to say you could START a 5HP motor with a 4kW converter, that is an issue having to do with the overload capacity of the converter.

Back to the OP;
You can buy or build a rotary phase converter for a lot less than a VFD capable of converting for a 5HP 230V motor, because to do it with a VFD, you would actually need to buy a 10HP drive*.

You can see plans to make your own, including formulas for sizing components, on this forum for machinists. But I agree with the comments that it would likely be less expensive and more reliable to just swap out the motor with a single phase one, especially when you factor in the value of a 5HP 230V 3 phase motor on eBay.

* There is possibly one manufacturer who makes a 5HP VFD capable of 1 phase input without de-rating.

I realize that, 1 hp=746 watts, no matter what the voltage. But my point was a 230 volt, 5 hp motor, will not run properly off a 4kw inverter. That's my point

 

[Besoeker]

I realize that, 1 hp=746 watts, no matter what the voltage. But my point was a 230 volt, 5 hp motor, will not run properly off a 4kw inverter. That's my point

They have and they do.

 

[mivey]

They have and they do.

Will the ones you are thinking of start an unloaded compressor?

 

[Besoeker]

Will the ones you are thinking of start an unloaded compressor?

All the ones we have made so far worked just fine.

 

[Besoeker]

There is possibly one manufacturer who makes a 5HP VFD capable of 1 phase input without de-rating.

I don't know who you have in mind, but Siemens and Vacon have drives in that range that don't need to be derated for single-phase input.
There may be others.

 

[mivey]

So look at LRA:
LRA = 5 HP x 7.09 kVA_LR/HP (code H) x 1000 / 230 = 154 amps

Assume the generator can deliver 300% current for 10 seconds to start the motor means we need: 154 / 3 = 51.4 amps. At 230 volts this is 11.8 kVA so we need an 11.8 kVA unit or a 9.5 kW unit rated at 80% p.f.

Sound reasonable?

That was for single-phase but the 3-phase yields the same result:
LRA = 5 HP x 7.09 kVA_LR/HP (code H) x 1000 / 230 / sqrt(3) = 89 amps

Assume the generator can deliver 300% current for 10 seconds to start the motor means we need: 89 / 3 = 29.7 amps. At 230 volts this is 11.8 kVA so we need an 11.8 kVA unit or a 9.5 kW unit rated at 80% p.f.

 

[mivey]

All the ones we have made so far worked just fine.

Is it the way the models you reference have the ratings defined or are you saying that, in general, a 5 kW generator will have no problem with a 5 HP compressor, loaded or unloaded?

Why do you think these generator sites show a bigger unit is needed?

http://www.gillettegenerators.com/sizing/sizing03.html
yields a 28.4 kW unit

and is similar to the result from:
http://www.cumminspower.com/en/technical/application/t030/chapter7.jsp

Here is a calculator for 50 Hz:
http://www.macgen.com/calcul.html#genkva
and it shows 6 kVA to start (4.8 kW?)
and recommends a 7 kVA (5.6 kW?)

http://www.jobsite-generators.com/generator_sizing.html#motor_starting
indicates 35 kVA for 5 HP

 

[Besoeker]

Is it the way the models you reference have the ratings defined or are you saying that, in general, a 5 kW generator will have no problem with a 5 HP compressor, loaded or unloaded?

I'm saying we have not had problems. On any size from 3kW to 6,600kW.
We've been doing it for a while......:grin:

Why do you think these generator sites show a bigger unit is needed?

Because they refer to full frequency and either DOL or Star-Delta starting methods.
Thus, somewhat irrelevant in the context of variable frequency drives.

 

[mivey]

I'm saying we have not had problems. On any size from 3kW to 6,600kW.
We've been doing it for a while......:grin:


Because they refer to full frequency and either DOL or Star-Delta starting methods.
Thus, somewhat irrelevant in the context of variable frequency drives.

Somehow I wandered off on a generator tangent. I see Mcclary's post #2 was a rotary phase converter. I guess these are labeled by the amount of load they handle. So a 4 kW unit would handle 5 HP, depending on the model purchased.

For the link, a standard A-5 would start a 5 HP compressor with no head pressure. You would need a heavy-duty HD-5 to start a 5 HP with head pressure.

Not sure how a VFD would be relevant for the compressor load with a rotary converter.

 

[Besoeker]

Not sure how a VFD would be relevant for the compressor load with a rotary converter.

It wouldn't be at all.
They are alternative solutions.

 

[winnie]

Does anyone market a small VFD specifically targeted for compressor applications. By this I mean a VFD with the necessary integrated controls to take a pressure input and adjust motor speed in order to maintain a desired tank pressure?

-Jon

 

[mivey]

Does anyone market a small VFD specifically targeted for compressor applications. By this I mean a VFD with the necessary integrated controls to take a pressure input and adjust motor speed in order to maintain a desired tank pressure?

-Jon

I know Sullair makes them down to 15 HP.


add:
http://www.sullair.com/corp/details/0,,CLI1_DIV61_ETI8139_SIT64,00.html

 

[Jraef]

Does anyone market a small VFD specifically targeted for compressor applications. By this I mean a VFD with the necessary integrated controls to take a pressure input and adjust motor speed in order to maintain a desired tank pressure?

-Jon

I don't think anyone makes a VFD with a direct pressure input. But you can just get a cheap little pressure transducer that gives a 4-20ma signal proportional to pressure and use almost any VFD that has a PID function built-in. Most mid to high end drives do that now.

Just be forewarned, messing with compressor speed is not for the faint hearted. Some compressors have mechanical lube systems and lowering the speed can make them stop functioning or reduce the lube enough to cause rapid damage to the compressor. Know what you are dealing with before trying it.

 

[winnie]

Just be forewarned, messing with compressor speed is not for the faint hearted. Some compressors have mechanical lube systems and lowering the speed can make them stop functioning or reduce the lube enough to cause rapid damage to the compressor. Know what you are dealing with before trying it.

So for a small compressor, even if you use a VFD, would you recommend using 'bang-bang' control.

With the lubrication issues that you describe, it seems to me that a VFD would still have substantial value, but it might be limited to 'full torque soft starting'. Though I doubt that this would be worth the cost of the VFD unless you had severe voltage drop issues (eg. large compressor in a home shop )

-Jon