Phase Converter Question

[sethas]

I am bidding on installation of a phase converter to operate some metal working equipment.

Vertical Milling Machine
230v-3Ph-2Hp

Metal Lathe
230v-3Ph-4Hp

The manufacturer recommends a 15Hp Phase Converter

How do I size the circuit for the motor correctly for the phase converter?

 

[sethas]

Okay, so check my math, and see if I am on the right path.....or if I am even interpreting the code correctly.

So if it is a 15hp 240v motor then...

15hp *746w = 46.625a

47a * 125% = 58.75

58.75 * 250% = 146.875

So I would need #6 conductors and a 150a circuit breaker?

Or am I way off?

 

[gar]

130303-2331 EST

At most you probably have a 6 HP load plus inefficiency and some loss in the phase converter.

However, in normal operation it is extremely unlikely that the average load would be close to that. But, you might be concerned with peak inrush current.

Power factor won't be very good so current will be above that predicted by the actual load power under light load conditions. These machines under typical usage run at relatively light load.

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[sethas]

So how do I calculate the load, and what references in the NEC would I use for sizing everything?

I am thinking a VFD would be a better option?

I still want to know how to properly calculate it though!

 

[ActionDave]

So how do I calculate the load, and what references in the NEC would I use for sizing everything?

I am thinking a VFD would be a better option?

I might agree. Depends on the motor. Many can handle being hooked up to a drive; some cannot. My first choice would be to get a single phase motor.

I still want to know how to properly calculate it though!

I have always used the FLA on the nameplate of the phase converter. Never multiplied anything times 746W. 125% is the limit for overcurrent protection.

 

[sethas]

I don't have the FLA at this point. This is two fold.

First, to see if he can even install this equipment.

Second, to provide an estimate.

If my calculations are correct on the phase converter, he can't go that direction, as his service will not allow it. So......now we are in the VFD scenario.

How do I properly calculate the VFD to the equipment? How do I properly size the circuit for the VFD?

I'll be calling some of the VFD companies today, but I really want to know and understand the calculations behind it, not just be told this is what you need.

Thanks!

 

[gar]

130304-0910 EST

The phase converter is being oversized for inrush current reasons. This occurs on motor startup.

If you have ever run a lathe or milling machine you will realize that these machines are mostly run under light load conditions. If you have a heavy hogging operation that runs for a continuous long time, then you might fully load the machine's motor. That is why I suggested using the two motor ratings plus inefficiency and losses in the phase converter.

You could put a 100 HP phase converter in the circuit and not change the required input power other than by the increased losses in the phase converter consisting of core losses and windage.

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[sethas]

Gar,

Would I then calculate the load as if it was 6hp 3ph motor? Sizing the single phase circuit based up one that?

I have only hooked up a phase converter once, and I wasn't the one who calculated anything for it.

 

[gar]

130304-1215 EST

If it were my own installation, then I would do this:

Size the wiring and breaker on the 3 phase side based on the NEC requirements for the 6 HP worth of load. This implies I would use a breaker on the output of the phase converter.

On the input side of the phase converter I would assume a power load of 8 kW and a PF = 0.8 . Thus 10 kVA. Size the 240 supply at 10,000/240 = 42 A. The only question will be if a 50 A breaker will tolerate the phase converter inrush, and will voltage drop be so great from the inrush as to cause startup problems with the phase converter.

The phase converter is simply a mechanically unloaded three phase motor excited as a two phase motor with one coil directly connected to the single phase source, and a capacitor to the other motor terminal to supply a phase shifted current for starting.

The phase converter needs to be started with no electrical load.

Note: above I assumed quite a bit of excess power over the 6 HP (4500 W) of maximum load power. Also have assumed a rotary converter. If you can find a used converter there should be a big cost saving, even if the bearings need replacement. Your description might imply a home shop. Also I doubt that a 15 HP converter is needed.

Anyone else have other suggestions?

If there is really a power source problem, then get a converter with an external shaft, then use a rope pull method to start, or a small motor to bring the converter up to speed, and then apply input power.

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[gar]

130304-1330 EST

Since the application is a home shop the 15 HP converter is overkill. But if a 5 HP air compressor was added, then the 15 HP converter might have been a good selection.

For the home application and a single user I would probably used a 5 HP converter. I don't remember the size converter that I had in my garage, but it was probably a 5 HP unit and I powered one manual mill, one CNC mill, and an air compressor from it. That CNC mill was essentially equivalent to a second manual mill.

Even if both machines are run at the same time, but are not started at the same time then the machines themselves are acting as phase converters.

One purpose of a three phase breaker at the output of the phase converter is to provide a convenient switch to unload the converter before starting the phase converter. The converter should have no electrical load when it is being started. The output breaker also allows a smaller breaker that can be sized closer to the needs of the actual load. With an output breaker an overload on the converter output should not shutdown the converter.

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[gar]

130304-2350 EST

sethas:

Since your potential customer already has the phase converter you can run an experiment on whether or not a 50 A breaker will easily trip on the inverter startup. You could also try smaller breakers and see where tripping occurs. Probably you would want to try 10 or more starts per breaker. If any trips occur, then the breaker is too small.

I would add some resistance, possibly a #12 extension cord of 50 ft, to lengthen the converter startup time. Just a guess on an experiment. I would also guess an initial inrush current of possibly 300 A for the inverter. With said extension cord the inverter input voltage may drop to about 210 V during startup.

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[ActionDave]

Here is my rule of thumb. Double the size of your three phase load, round up to a standard horse power. This equals the size roto-phase you need.

Multiply the FLA on the roto-phase (use the tables in 430 if you don't have one picked out) times 125%. That gives you the conductor size and breaker size. The actual load will never be that much. As long as what feeds the roto-phase is not less there won't be a problem.