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Rotary phase converter VS VFD

jappietoutou

Aluminum
Joined
Dec 2, 2015
Location
Montreal
hello

So I've posted a video of my RPC ( not here ), and I get some comments like : your system is way too complex, you should have used a Drive and a transfo instead , at half the cost......

I have a very basic, regular, unusual RPC, the same as most of you guys build here ! I took the plans here !! except for the voltage, from 240v to 575v , i'm in Canada and most industrial machine are on 575v/600c. but that is not the point. It works really well, I have a pony motor for starting. 7.5 hp Idler, take approx 18 amp on the 240v side.

So my question, what can I answer back to people questioning the RPC way ? I'm kind of frustrated.....

Jean-Pierre
 
Well, truth be told RPCs are old tech and are burdened with certain innate issues and limitations. Whereas a drive is a nearly universal plug-and-play solution. Incandescent VS LED. Both have their advantages and shortcomings.

Part of it depends on your usage scenario. If you've got half a dozen machines to run then maybe an RPC will be more cost-effective than six drives. But if you need to start a 20 horse air compressor under load then a VFD will produce much greater starting torque at the motor shaft. Likewise if you've got loads that are sensitive to voltage fluctuations or imbalance then RPCs are not the answer. For people with only one or two three phase machines, drives are generally cheaper, easier and a lot less fussy.

But like I said, in the end it comes down to your unique situation. If you can't stand the noise of a 4-16kHz carrier wave or don't want to rely on semiconductor electronics then an RPC is a valid option.
 
Are transformers like that OK with being fed a VFD waveform?

Personally I would not feed a 240v:575v transformer a vfd waveform. the reason why is because various kinds of standing waves/resonances, in the bulk of the winding or located inside the winding could easily push the spikes to well over 1000 volts.

I am hesitant to recommend people place capacitors directly across the output of a transformer fed from a vfd.. because the core losses of the transformer may be be high enough to overheat the transformer.

However that is what many UPS manufactures do. They drive the input of a transformer via pwm directly from the batteries on the order of 10-40khz and they do not add an inductor.. they just put a 10-40uf cap across the output. (depending on kva rating). i have measured 20 watts power reduction in a 750watt rated ups simply by adding a good quality low loss inductor in series with the transformer.

If you do want to drive a 575 volt motor from a 240v vfd and transformer.. buy a line load reactor put it on the output of the vfd and then install delta or Y capacitors across the output of the 575 v transformer. preferably Y connected and ground the star point of the caps. this won't fully ground the secondary in the case of a delta transformer, but its good enough in my opinion.
 
Tell them it was cheaper than a drive and outputs a cleaner sine wave for the electronic controls of the loads?

VFDs are great but they are made to drive motors directly. They do not do well with motor loads being switched onto their output suddenly without their gentle ramp controlling the inrush.
 
Both ways work, under 5 hp vfd is cheaper over rpc is cheaper, vfd has better 3 phase control and veri speed and soft start, only can be used on one motor, rpc will run more than one motor, but needs to be dialed in, and sometimes needs a idler motor and no soft start...Phil
 
Modern VFDs are quite happy driving multiple motors and direct online starting them. Things have moved on a bit in the last 40 years.

Phase perfect do quite well in that respect, even if their drives only supply one extra phase.

Yes, I do use a single drive to run all the machiines in my workshop. I have used an old Danfoss one, I'm now using a more sensibly sized Teco unit.
 
Modern VFDs are quite happy driving multiple motors and direct online starting them. Things have moved on a bit in the last 40 years.

Phase perfect do quite well in that respect, even if their drives only supply one extra phase.

Yes, I do use a single drive to run all the machiines in my workshop. I have used an old Danfoss one, I'm now using a more sensibly sized Teco unit.


NO drive, modern or old, will like across-the-line starting of motors unless it is capable of supplying substantially more current than the motor full load current. The PP is not a "drive", it is a converter, and is far overbuilt for the nominal power it provides, in order to supply start current.

Generally, if the drive can supply 4x the motor current, AND it has a short term 200% overload, with a longer term 150% overload, it probably will start that motor. I'd go bigger.

Modern drives are "value engineered" to do exactly what they say, with no expensive extra capability. It's a tough market, and everyone seems to buy on cost alone.
 
As you well know, the PP has got pretty much the same output circuitry as any other inverter other than just having one third of it. Different control logic/circuit, but that isn't the bit that sees the load. So it's a drive. :). Calling one a converter leads to the thought that it is related to static or rotary converters instead of the inverter that it is.

As to drive sizing, that applies to all systems. My current drive is a 5kVA and its predecessor was a 23kVA. Largest single motors are 2hp and total possible load would be 5.5kVA if I could manage to run all three manual machines flat out plus the pedestal grinder at one time without crashing something. I'm happy to only run two machines at a time.

:cheers:
 
It is a converter simply because all it does is to provide the missing 3rd phase.

It does nothing with the incoming power at all, which is still connected to the motor etc. It is essentially an electronic version of an RPC, which nobody would claim is a "drive".

A "drive" separates the motor from incoming power entirely, reconstituting all 3 phases on its output (if it is a 3P drive).

Major difference.
 
VFDs are engineered with the intent of providing speed control and overload protection for one motor only. While it may be possible to connect multiple motors to one drive, that is not their intended usage.

A static phase converter is a different animal than a variable frequency drive. They both use rectifiers, DC busses and H-bridge inverter technology to emulate sine waves, but the design scope is vastly different. As JST mentioned, a static phase converter is designed to supply a manufactured phase to multiple loads through distribution equipment. I.E. it is intended to supply feeder conductors. The mention of connecting single phase, non-motor and motor loads protected by class 10 or faster heaters (i.e. separate motor controllers) in the PP manual corroborates this.


A VFD on the other hand is designed and intended to supply power, control, braking, overload & overtemperature protection and basic logic functions for ONE motor only. It is intended to be used *as* a motor controller, not to provide power to one.

The NEC also makes this distinction - for adjustable speed drives are covered under Article 430.120, whose rules are supplemental to the general rules preceding it in Article 430 (Motors, Motor Circuits and Controllers). Rotary and static phase converters on the other hand have their own separate article with separate rules - 455 (Phase Converters). Read through those articles and you'll quickly see that they're treated as completely different animals.

Also as JST mentioned, most drives have rather modest overload ratings. The ABB ACS150 for instance - one of their more capable value-added small drives - is only good for 150% for 60 seconds, or 180% for 2 seconds. A typical NEMA design B, C or D motor will pull up to 600-700% of it's nameplate FLA during startup. Most drives will shut down and throw a fault code instantly under that kind of overcurrent condition.
 
Most drives have a section in the manual regarding driving multiple motors. Generally though, they must start and stop simultaneously, and you add a bank of manual motor starters (MPCBs) with signal contacts to the drive, to protect each motor individually.

A common application is something like a cooling tower with multiple fans running at the same speed, varied by the VFD, and there is no need to start or stop motors individually (except when the overcurrent trip operates.
 
Modern VFDs are quite happy driving multiple motors and direct online starting them. Things have moved on a bit in the last 40 years.

Phase perfect do quite well in that respect, even if their drives only supply one extra phase.

Yes, I do use a single drive to run all the machiines in my workshop. I have used an old Danfoss one, I'm now using a more sensibly sized Teco unit.

I'm sure individual VFD manufacturers have sections in the manual regarding what is allowable in this regard, and as far as I know many of them are quite capable.

But no VFD cheaper than an RPC will like direct online starting at its output.
 
thank you guys ! I'm gonna do the Bubble head ( yes yes yes ) for now on when being questioned as to " Why did you do that ? I would have used a Drive instead " or " why don't you change the 600v motor to 240v instead ?" that is on facebook , where very few people know about RPC anyway.

Jean-Pierre
 








 
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