Digital Phase converter…

[Robert Atkinson 2Participant @robertatkinson2]

Posted by Clive Foster on 13/12/2019 10:13:44:

Robert

SNIP

No way round the need for over-rating. Fundamental to the physics of motors. My 5 kw generator won't start a 3 hp compressor and lets you know about it if you try a naked motor!

SNIP

Hi Clive,

It's not really anything to do with motor physics, it's just marketing. They are selling on the continuous rating in hp but not providing enough overload capacitiy to start a motor of that size. It may sound a bit pedantic but if talking continuous ratings they should do it in kVA (apparent electrical power) not hp or kW (real power). If you buy a matched motor and drive from an OEM it does not say 4 times the motor hosepower of the drive.

Note that your generator is rated in kW but while 3hp is about 2.2kW  the motor does not have unity power factor. It's typicall 0.8 so even running that is nearer 3kVA and your generator may not be rated for a 0.8 power factor either.

Robert G8RPI.

Edited By Robert Atkinson 2 on 13/12/2019 21:32:19

[Matt HarringtonParticipant @mattharrington87221]

Clive, there's a new venture for you!!

Matt

[larry phelan 1Participant @larryphelan1]

I am surprised that the noise from the rotary converter is that bad. I have a 7.5 KVA unit and I don't find it too noisy, but then, I half deaf anyway, thanks to noisy conditions in a past life [Anyone else been there ? ]

Yes there is a some noise from them, but I built a cover around mine and can live with it.

Better than listening to S-W-M-B-O ,and can be switched off !!

[Bob WorsleyParticipant @bobworsley31976]

Interesting, not come across a digital phase converter before, but then I have three phase in the house.

As has been mentioned just seem to be a standard VFD with a voltage booster on the input. This is both a tribute to the amazing electrolytic capacitors now available, and a problem.

To step 220V up to 440V needs a capacitor, it is charged to peak volts on the negative half or the sine wave, and then lifted up to charge the output capacitor on the positive peak of the sine wave. This is identical to the dual voltage switch mode power supplies that have been around for years. The snag comes because the output capacitor is only charged once per cycle, so 50 times per second. And that charge is just a rapid dump of charge from the input capacitor to the output capacitor. If the VFD is running 5hp, then that is 3730W, divided by 0.9 pf divided by 0.9 efficiency giving 4600W. So this 4600W for one cycle needs to be dumped in the output cap in about 5ms to run the motor for the 20ms cycle. Simply put if the motor takes 10A then this current is 40A.

Certainly works the caps hard.

You can emulate this with a standard 400V 3ph VFD, the few I have seen all have terminals on the back to add in extra smoothing caps. Just make your own voltage doubler and attach the output cap to these terminals. It is seriously lethal, so perhaps not.

An induction motor will take an 8x overload to start. Don't forget that all that is limiting the current is the synchronous impedance of the motor. This is why a 5kW generator won't start a 3hp compressor. To some extent it is relying on the inertia of the rotor to keep turning to get the voltage regulator to crank up the excitation current.

If the rotary converter is noisy then switch the motor off as soon as the lathe is running, not then needed provided you check that there is enough running capacitance to keep the phases reasonably at 120 degrees.

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