Please explain phase converters to outside-US

[usolutions]

I have a question. What is so special about phase converters? I get you get only 1-2 phase power to residential in US, needing to convert it to 3phase.
Question I have is why can not you just put a VFD instead of phase converters? It takes 220v single phase and outputs 3phase 220v. Disabling most of safeties it should be able to work as just a 3phase power source. They do run silent compared to rotaries and dont cost that much. EM enterference is only downside I see from where I am. If anyone can elaborate why not form that side of the pond I'd really appreciate.

 

[Trboatworks]

A person gets tired of buying and wiring in individual VFDs to each machine.
Life get must simpler when you have a PP with a circuit run around the shop.
Stand a machine up, plug it in and go to work.

 

[Gearclash]

Rotary phase convertors are so simple and inexpensive. I have 4 machines with 7 motors between them run by one $500 (used value) rotary convertor as I have no access to grid 3 phase.

 

[usolutions]

Why invidual vfds? You can get cheap but good enough 15kw unit for well under 1000 and use it to power all shop?

 

[Limy Sami]

Incoming spam !!!

 

[johansen]

Why invidual vfds? You can get cheap but good enough 15kw unit for well under 1000 and use it to power all shop?

Because you have to get lucky to find a 30hp converter for a grand, and your single vfd needs to be 4 to 6 times the "size" of the largest motor you need to direct online start.

Secondly, vfds produce a common mode voltage, at plus and minus 170 volts phase to "ground" at the pwm frequency for a 240vac system. This voltage is Not filtered out by the regular 3 phase line load reactors.

Individual motors dont care about that (other than it tends to destroy bearings slowly), but almost all smps power supplies and complex electronics do get affected, or destroyed.

 

[Bill D]

A vfd can also provide slow start and power braking. The slow start can allow big motors to start with limited power supply into a small shop. As well as 80-120% rpm with no special modifications. They also show rpm
Rotary converters use factory 3 phase wiring and switches. VFD must use separate switches to control them. They can control from the vfd box but it is nicer to add switches in a more convient location. VFD's are electronic so they need more protection from oil and coolant.
Some use a vfd even with full three phase from the power company.
Bill D

 

[scsmith42]

I have a question. What is so special about phase converters? I get you get only 1-2 phase power to residential in US, needing to convert it to 3phase.
Question I have is why can not you just put a VFD instead of phase converters? It takes 220v single phase and outputs 3phase 220v. Disabling most of safeties it should be able to work as just a 3phase power source. They do run silent compared to rotaries and dont cost that much. EM enterference is only downside I see from where I am. If anyone can elaborate why not form that side of the pond I'd really appreciate.

Great answers from others already in this thread.

To sum it up, for a single machine or two a VFD is a great option. However, if you have multiple pieces of 3 phase equipment that need powering, a RPC can be a less costly alternative.

 

[Mebfab]

Individual motors dont care about that (other than it tends to destroy bearings slowly), but almost all smps power supplies and complex electronics do get affected, or destroyed.

Whats with the bearing death?

 

[JST]

Whats with the bearing death?

Common mode signal (current) capacitively conducted into the rotor from the stator windings, going to ground via the bearings. Ball bearings get damaged by that.

The common mode signal is high frequency, so a significant current can be conducted even though the capacitance is often fairly small. Usually much worse with 480V motors and not too bad with 230V motors.

Ground contacts for the shaft, or insulated bearing holders are common countermeasures. Shielding has also been used, a former boss of mine had a patent on one form of such shielding.

SMPS "should" not be too sensitive to common mode voltage, but some can be. The important working parts of SMPS are all connected differentially (across the line) and should not be affected by the common mode voltage, but insulation to ground can be damaged by high common mode voltage and result in failure. That may be through the isolation transformer insulation (shorting to the secondary) , or direct from a component to ground.

 

[jim rozen]

Late to the party but if the question is "which is best?" Then the answer is easy.
Both.
My shop at home has a rotary converter that can run one of two lathes and a milling machine. The machines with VFDs are drill press, milling machine, two lathes.

Here at work we have utility 3 phase power, and the drill press still has a VFD.

So: both are best.

 

[Mark Rand]

Umm. Old argument. My shop has the 3 phase supplied by a single VFD working at a fixed (50Hz) frequency. It can DOL start any of my machines and run at least two, theoretically three of the three phase machines in the shop. It's got a delta/star transformer on its output, which allows me to have a grounded neutral. It's also got an LC sine wave filter which removes all of the irritating harmonics. Less noise, less cost and simpler installation than a rotary converter. You just need to find a VFD that is happy with downstream switching and transient loads, which most recent IGBT inverters will be.

There's a lot of, incorrect, 30-40 year old perceived wisdom about the vulnerabilities of VFDs based on the limitations of GTO thyristors.

 

[johansen]

It's got a delta/star transformer on its output, which allows me to have a grounded neutral. It's also got an LC sine wave filter which removes all of the irritating harmonics

This is the best way forwards for anyone who can get a 20 to 50hp 240v vfd and wants to run multiple machines.

With a little effort to measure the no load losses, the transformer can sometimes be used as the L of the LC filter. Just move the capacitors downstream of the transformer.

The vfd can also compensate for nonstandard transformers to a limited extent. A 380Y 220delta transformer can be paired with an oversized VFD programmed to deliver 140volts at 60Hz. You would get 240v out of the secondary.

 

[usolutions]

Umm. Old argument. My shop has the 3 phase supplied by a single VFD working at a fixed (50Hz) frequency. It can DOL start any of my machines and run at least two, theoretically three of the three phase machines in the shop. It's got a delta/star transformer on its output, which allows me to have a grounded neutral. It's also got an LC sine wave filter which removes all of the irritating harmonics. Less noise, less cost and simpler installation than a rotary converter. You just need to find a VFD that is happy with downstream switching and transient loads, which most recent IGBT inverters will be.

There's a lot of, incorrect, 30-40 year old perceived wisdom about the vulnerabilities of VFDs based on the limitations of GTO thyristors.

You precisely captured my point.

 

[friesen]

Biggest issue I see is that most vfds size the bridge rectifier to match incoming 3 phase. So to feed the converter with enough single phase power means that two of the six diodes are doing all the work then. Also, they are really designed with single loads in mind.

Rotary converters have their place, but the caps have to match the load, so rapidly changing loads can be problematic. The way I understand it’s because the output of the converter is 90 deg instead of 120, so you have to finish the shift with capacitors, depending on load. Motors are very forgiving, but some types of machinery aren’t so much. Think about an oven with rapidly changing pwm.

 

[JST]

....................

Rotary converters have their place, but the caps have to match the load, so rapidly changing loads can be problematic. The way I understand it’s because the output of the converter is 90 deg instead of 120, so you have to finish the shift with capacitors, depending on load. .....................

Not so.

The phase shift is "mechanical", due to the position of coils in the motor. Essentially, the generated output is the back EMF* of the motor, generated when the magnetic field in the idler motor comes around and induces voltage/current in the stator windings for that phase.

So correct phase is an inherent property of the motor.

The capacitors are used to counteract inductive reactance in the motor, and boost the output voltage. You can do the same thing with a transformer, or not do it at all, using no extra components, and still have correct phase.

* The "back EMF" is a voltage proportional to rpm, that opposes current flow into the motor. It is why the motor draws more current under load.... load slows the motor, reduces back EMF, and allows more current to flow.

 

[jim rozen]

Rotary converters that have a much larger idler than the load motor being run, don't need any extra components at all. As mentioned, the three phase from a rotary converter are exactly 120 degrees apart from the git-go.

 

[rons]

If you get a VFD then you have a quiet solution that is full of complicated circuits and software that will eventually kick your ass.

If you get a rotary convertor the constant idler motor whine will have you powering the thing off and then on all day.
This kind of bumps your ass across the floor to the on/off switch.

You need both solutions to save your ass.

 

[mksj]

If you get a VFD then you have a quiet solution that is full of complicated circuits and software that will eventually kick your ass.

If you get a rotary convertor the constant idler motor whine will have you powering the thing off and then on all day.
This kind of bumps your ass across the floor to the on/off switch.

You need both solutions to save your ass.

They will probably outlive us, MTBF of 20-25 years for a decent VFD and this is with constant use, the technology changes, and the replacement costs are low for most applications relative to the machine cost. That is the least of my worries.

 

[crickets]

Biggest issue I see is that most vfds size the bridge rectifier to match incoming 3 phase. So to feed the converter with enough single phase power means that two of the six diodes are doing all the work then. Also, they are really designed with single loads in mind.

Rotary converters have their place, but the caps have to match the load, so rapidly changing loads can be problematic. The way I understand it’s because the output of the converter is 90 deg instead of 120, so you have to finish the shift with capacitors, depending on load. Motors are very forgiving, but some types of machinery aren’t so much. Think about an oven with rapidly changing pwm.

I was also confused about how they do the conversion because of the language often used - "two legs are passed through, third leg is generated"... One then can [mistakenly] assume that split phase is used to simulate first two phases, and then the third phase is generated. In reality single phase is passed through, and two other phases are generated at the correct 120 offsets. So 90 degrees comes into the picture only if one is using the neutral wire to pull 120V as well, otherwise the output is proper 3 phase 240VAC with 120 offsets between the phases.