Hi Guys, I need some help with calculating size of phase converter, Line, load overcurrent. The Ice cream maker I am working this up for is 208/240v 30a I don't have access to the motor. It would require dismantling the unit. Any help would be apprciated.


Ob(Phil)

Phil,
Are you trying to get 3 phase,or are you trying to change Hz?

Trying to get 3 phase.

The easiest, best way is with a decent VFD .... it will convert single phase to true three phase.

The VFD will work fine on the motor, but the controls could be damaged.
If all you need the 3 phase for is the motor, give me the motor name plate information. Specifically HP, voltage and current. Or you can go to www.kbelectronics.com. Here look for the inexpensive chassis type control. The dimensions will be approximately 4"x4"x2" for controls under 5 HP.

Note: The output of a VFD is not a true sign wave. It is a simulated sign wave derived from the DC bus on board the drive. This output is not as clean as a true sign wave like you would see with a transformer output. That is why you must use caution using motors that are not designed to be used on VFD's. Motors that are VFD rated have magnet wire that is rated for over 1500 volts and in some cases special bearings. Do not let this over concern you. It is just a warning of possible motor failure.

Unfortunatly, I would have to dismantle the macine to get at the motor so a VFD would be out.

It sounds like you'll need a special motor-generator set; sometimes called a 'rotary phase converter'.
It's a motor turning a generator. In your case, it sounds like you would get one that has a single phase motor driving a three phase generator.

Phil"
Ronkelectrical.com is a mfg of rotary and static phase converters.

Situation may be (is) they require ALL info on your load, so you may have to take it apart. How can you hook it up if you don't have the nameplate info? Voltage, HP, etc.

Look for the THD rating; a solid state phase converter with an output filter will have low THD, and won't be an issue. They make them specifically for this purpose, it's no different than the inverter on a UPS.

A motor generator may be the cheaper solution, or may not be, solid-state is getting pretty cheap anymore.

Just an update,
I let this slide as there were at least 4 other contractors looking at it. Interesting to note I was the only one who questioned the service 100a 1Phase residential. along with this 30a unit was: centreal A/C, oven, full size copier, 1 deep freezer(she wanted to ad 7 more), and the 20 space panel was full. Someone has already and completed the job. She told me it onlt took him 1.5 hrs. She also told me she signed a contract to fill a large order based on this machine before it was even wired/installed.

This being said I would like to know more about VF drives being used as phase converter. Does it effect the control? What is a THD rating? Why would you want to ever use a rotarty phase convertor if you coul use a drive?

Ob

VF drives are inherently phase converters. They take 60Hz AC (or 50Hz) and convert into DC. They then turn around and convert that back to AC. Since the inverter is computer controlled and starting with DC, it doesn't care what was coming into the rectifier, and can precisely output whatever phases, frequency and voltage you want.

There is a certain economy of scale when it comes to this; VFDs are already mass produced, so it's often cheaper to adapt. Another common source are big UPS systems, which work the same way- only difference with a UPS is that there's a big battery bank attached to the DC bus. Which you'd just not install if you didn't need it.

The advantage here (using the UPS as an example) is that regardless of what's happening on the input- 45Hz 300V or 65Hz 600V (and I know one place where the line voltage DOES vary like this... grr...) that UPS will be pumping out pure, clean, 480V 60Hz.

I mention total harmonic distortion, because the rectifier is a switched power supply, and a big one, which is non-linear; the input power to the 3-phase rectifier looks more like a rectified 180Hz stepped wave than a 60Hz sine wave. As such, the current profile tends to be highly harmonic (EG, a lot of 60Hz, 300Hz, 420Hz, etc). The harmonics on a 1-phase input will have different harmonic frequencies, but the result is much the same. If the current draw is small, it matters little, but if that rectifier represents a large portion of the building load, voltage drop will cause the voltage to fluctuate the same way, and this can cause issues. It's even worse if you're coming off a generator, as the generator isn't as capable of coping with these frequencies as the utility is. So, if you have a model with a high THD (13-33%, sometimes higher), you can put on an input filter to reduce this to 5-7%, which is generally OK.

Also be aware that if you ever use this on a 3-phase wye circuit, you'll need a 200% neutral due to the triplen harmonics. Normally, the neutral just carries the imbalance, and the worst-case is a 100% imbalance, at 100% RMS load. For switched power supplies, though, it's not a sine wave, but a stepped wave- the current never drops to 0, it simply goes from high current one way, to high current the other, with harmonic current on top of that. IIRC, it works out to a theoretical max of 166% or so, but the cables don't deal with higher frequencies as well as with 60Hz (more skin effect), so a derating factor applies. 200% neutral is common.

Rotary phase converters are simple motor generators- quite literally an electric motor driving a 3-phase generator. There are pros and cons (moving equipment to break and maintain) but they're quite effective as well.