Price comparison of rotary, VFD, and Phase Perfect converters

The increasing use of increasingly low cost VFDs by many members, plus the new-on-the-scene arrival of Phase Perfect solid state converters, prompted me to do a price comparison of these with a rotary which could be built by someone with electrical savvy.

This comparison is not valid for someone wanting to run only one or two 1 – 3 hp three phase machines as cheaply as possible from residential single phase. It’s meant for several members who have recently posted, who plan to have a moderately big shop with mills 10 hp or larger.

The assumptions used in the comparison (so don’t compare apples to oranges) are:
A) Want to run 10 hp motor (nameplate hp) off 240 volt single phase.
B) Want power quality good enough to run CNC (voltage imbalance < 4%,
or voltage range among all three legs is between 230 and 250 volts at
all motor loads including no load).
C) All components are brand new and never used.

Here’s the comparison:

Type-------------Manufacturer----------Model---------- Vendor-----------Price

VFD-----------------TECO------------FM100-220-------Dealers Elec------$1,207
Solid State -------Phase Perfect---------DPC-10--------CNCPros.net-------2,250
Rotary Converter------You (see chart below for gory details)----------------1,547

VFD ADVANTAGES: great regulation, best regenerative braking, variable speed
control, smoothest start/stop forward/reverse motor control
VFD DISADVANTAGES: basically a one motor device (cannot run whole shop off
single unit)

Phase Perfect ADVANTAGES: best regulation, excellent regenerative braking, output
in phase with single phase input, can run entire shop off one unit
Phase Perfect DISADVANTAGES: no variable speed control, BUT you can run in
conjunction with VFD WITHOUT derating VFD

Rotary Converter ADVANTAGES: still cost competitive (don’t know how long though),
can run entire shop off one unit, can add additional idler motors easily to increase capacity, very easy to repair
Rotary Converter DISADVANTAGES: poorest regulation by far, OK regenerative
braking if sized big enough, no variable speed control, big units may need starting compensator


bnelson

~~~~~
The stuff below is the gory details of how I would recommend building a rotary converter if asked by a machinist who had a shop with several machines.

This is NOT the cheapest way to build a rotary!!! Be aware all parts in the chart below are brand new, including the motors. The motors are the ones I use myself, made by Elektrim, built like a tank, with a service factor of 1.4 (so a 10 hp motor by Elektrim’s specs is actually a 14 hp motor, comparable to a 14 hp Leeson). Also, I’ve used TWO rotaries instead of one – definitely more expensive, but it gives the user the option of only running one rotary if that’s all that is needed, and gets the power company off your back for causing huge line sags in your neighborhood during starting of a big single rotary idler. You simply start the first 10hp, using a push button start cap circuit, and when that one is running, start the second 10hp across the line with no starting circuit.

To be brief, what I do is buy a Square D three phase panel, and use that panel for the main disconnect, the three phase bus for the entire shop, the run and start breakers for the rotary converters, and the main breaker for the three phase output for the whole shop. Spending about $200 up front, if you decide you want to go the rotary converter route, gives you, literally, infinite flexibility for future add-ons.

If anyone is interested in the wiring of this, please email me at: [email protected] and I will be happy to supply all details.

One final word: The price below INCLUDES the $90 breaker for controlling the three phase output of the entire converter bank to the shop. This breaker (or breakers) could be configured to protect the load motors.

If you know what you’re doing, you can get to <4% voltage imbalance with run caps alone. If that fails, it’s quite easy to put a buck/boost autotransformer (VERY cheap) on the offending leg to bring voltage into balance).

Item--------------------Manufacturer-------Vendor----Quantity----Total $

Idler motors, 10hp, SF1.4---Elektrim------Wholesale Tool---2----------$638
3 phase panel, 125a, 24sp—SquareD---------Grainger------1-----------189
Panel cover, surface mnt---SquareD---------Grainger------1------------17
Three pole breakers--------SquareD---------Grainger------3-----------270
Two pole breakers----------SquareD---------Grainger------2------------96
Start capacitor, 300mfd----Beyschlag--------Grainger------3------------38
Run capacitors, 50mfd,370vac-GE------------Grainger------8-----------165
Push button start switch—AllenBradley--Local Elec Supplier-1------------46
Box for start switch/caps---Carlon------Local Elec Supplier-1------------15
Box for run capacitors------Carlon------Local Elec Supplier-2------------38
Cable, lugs, connectors, ground bars----Local Elec Supplier -- -----------35
GRAND TOTAL ABOVE PARTS----------------------------------------$1,547

[This message has been edited by bnelson (edited 01-18-2003).]

Great Post bnelson.
It is useful to see all the poop in one place for all to see. I should be ordering my phase perfect unit next week. I will keep all informed.

Take Care

ARB

Doug,

Point well taken. Most rotary converters I've personally been involved with are in a person's garage or attached shop -- usually a one man operation, frequently just a hobby. These are OSHA exempt.

There are strict National Electric Code (NEC) rules for phase converters which I insist on observing when helping anyone build one. This would definitely help with insurance, even though a user-built rotary would obviously not have a UL rating, and legally does not need one.

While we're on your safety point, I'd like to mention two of the biggest problems I've observed with home-built rotaries which literally have life-and-death implications. One is failure of the builder to run an unspliced, unswitched, unfused ground as a home run back to the main feed panel ground bus and bond this ground to the frame of the idler motor(s) and the boxes of any motor controllers. The other is failure to use a three pole common trip breaker serving the motor loads on the output side of the converter. Many people do not realize that if you blow just one fuse on one of the non-generated legs of a rotary converter output, the load motor may still continue to run single phase, and there's 208 volts to ground from the generated leg. That can prove fatal if the reason for the fuse to blow in the first place was a short to ground of that non-generated leg!

Incidently, that also applies to the Phase Perfect output circuit which like a rotary, is in phase, and in continuity with, the power company supplied input.

Most of my referrals come from local electrical supply houses, where I get a contact because a customer cannot calculate the size of a start or run cap. At least that's not a life/death thing. But I see dangerous conditions which have nothing to do with the referral question.

Personally I believe you can build a rotary which is safe and will perform well if you understand three phase electricity. Your concern is very valid for some situations where the user had little knowledge of converter technology, and may have financial impact as well.

bnelson

Two of your assumptions are in conflict with the converter possiblities. You assume a 10 hp motor and then go on to assume power quality good enough for CNC. Well, the only reason someone would need power quality good enough for CNC would be if they wanted to use a converter ~for CNC~...i.e. not just the spindle motor, but the whole shebang, circuit boards, servo motors, the works. And in this case I can tell you from personal experience a VFD will ~not~ work. It will run the machine, but it will fry any RC Networks (resister/capacitor)~immediately~. The RC networks are not essential, as they are there to prevent sparking of relay contacts, so the machine can then be operated without them but a VFD will probably do other nasties to sensitive boards eventually. VFD's produce "dirty" power...only meant for 3 phase motors, nothing else.

I bought a new "cnc rated" 30hp converter with all new USA components except for the motor, right here in SC for about $1,400. The 30hp motor is made in China, but is compact in size, and runs smooth, cool and quiet so far. The package comes with pitiful instructions and the motor and control box are seperate, but works fine. Want the particulars ?

D. Thomas: you are right about my assumptions being in conflict. The voltage balance assumption would of course not apply to a VFD by its very nature. What I was getting at is power quality good enough to go into the front end of a CNC, but a VFD big enough to run a 10 hp motor off single phase. Running CNC machine up front on a VFD was not what I had in mind. It's akin to running a VFD on a VFD, which would confuse the ICs of both machines.

Please give us particulars on your 30 hp converter controller with the motor. In particular I'd like to know the ampere rating on the third leg. Have you ever measured the voltage imbalance?

Any members wanting additional details on the poor power quality of VFDs might want to read this excellent paper by Larry Meiners:

http://www.phaseperfect.com/Phase%20Whitepaper.PDF

bnelson

[This message has been edited by bnelson (edited 01-20-2003).]

bnelson - I'm starting to assemble a phase convertor with a 15 HP idler. Total cost is around $400 with mostly new components and includes contactor for the whole mess and adjustable overload relay for the idler motor.

You posted - <i>One is failure of the builder to run an unspliced, unswitched, unfused ground as a home run back to the main feed panel ground bus and bond this ground to the frame of the idler motor(s) and the boxes of any motor controllers.</i>

My RPC is being fed from a dedicated subpanel. Are you suggesting an unbroken ground conductor from that subpanel, or running from the ground/neutral bus back at the main service panel? Are the "motor controllers" you refer to the boxes for the RPC's idler motor controls or the motor control boxes of the 3-phase machinery?

<i>The other is failure to use a three pole common trip breaker serving the motor loads on the output side of the converter.</i>

Are you tlking about a breaker between the RPC and the distribution panel, or simply that a panel with 3-pole breakers should be used instead of hardwiring to a set of buss bars. FYI, my RPC feeds a SqD 3-phase breaker panel that's being used for power distribution to the machinery.

Thx - Rob

D, Thomas. I would be very interested in the pacticulars of your 30HP rotary phase coverter as that is what I am going to biuld. I will also be using this for CNC machinery.
Al

Rob,

It's fine to run an unspliced ground from the ground bus in the subpanel feeding the idler to the converter motor and its controller box. Best if you size the ground wire the same as the hot wires. ALL controller boxes must be grounded if metal, whether controlling the converter motors or the load motors.

I salute you for using a three phase panel to distribute the output of your converter. Wish more machinists did this. Doing so makes compliance with the NEC much easier as there's a REAL bus you can access easily (includes the ground bus) without using dozens of wire nuts. I like to use main lug panels instead of main breaker panels, as they're much cheaper. I wire the buses from left to right L1 - L2 - L3, where L3 is the generated leg. This is different from many commercial patterns, where the middle 'B' bar is often the high leg (wire colored red or orange). The output from your converter can be inputted to the panel by means of a three pole main breaker (in this case, current goes through breaker in opposite direction, from breaker lugs to the panel bus bars). You don't use the main lugs, but you must use three panel spaces.

The advantage of putting a three pole common trip breaker in the panel is that it not only protects the converter motor, but if there is an overload on only one or two legs of the three phase bus, the main breaker trips on all three hot legs so the entire panel is shut down and nothing single phases. Another real advantage is that you have an easy way to shut down three phase in your shop without having to shut down the converter.

If you have several rotary idlers you want to parallel, you can use a similar plan, except in this case the main breaker is a big two pole breaker attached to L1 and L2 buses, and the three pole breakers in such a 'converter panel' go to connecting one or more idler converter motors together. If you shut that two pole breaker off, EVERYTHING three phase dies! Often I'll use the bottom right three spaces in such a panel for an output breaker going to three phase machines, each with its separate overload relays. Or, you can wire output off the main lugs of the converter panel to a main breaker in a distribution panel.

Unfortunately all these three pole breakers are expensive (about $90 each); what you get is flexibility, controllability, and safety.

bnelson

Interesting - you're technique basically backfeeds the panel through a breaker. I see the benefit of it, though. I'll probably go snooping for a 45/50 amp breaker now ...

If you're patient, eBay is a great place to pick a lot of this stuff up. I paid about $50 net for my 18-slot 3-phase panel and that included (2) 30 and (1) 20 3-pole breakers. That seemed like a small price to pay to have the flexibility to properly breaker each machine.

bnelson, I have yet to measure amperages (lost the dang high amp meter). My single phase supply voltage is (an annoyingly high) 253 volts, the converter panel with generator motor running reads 251 volts between two single phase legs and 242 volts on generated third leg as measured between third leg and either of single phase legs.

The converter was furnished by Roto Master...here's their (lame) web site www.rotomaster.com. My unit looks like the one in lower right corner under "products" except the motor doesn't have fins. If you call you will get prices probably higher than I quoted, but they are subject to "talking down" when one's 'direct from factory purchase' aspects and general cheapskateosities are mentioned

Actually, I think this one on eBay http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3108291557&category=633 may be exactly the same as what I have. "American Rotary" is probably just a distributor for the Roto Master factory in SC. The "Auto Start" timer on mine wasn't adjusted quite right at first but a simple turn of the knob fixed that.

No safety switches, mag. starters or circuit breakers are included..at least not on mine anyway.



[This message has been edited by D. Thomas (edited 01-20-2003).]

D. Thomas: Don't need amps. Your voltages give imbalance of only 2.4% -- superb for a rotary.

If you are having no problems with your load motors, easy to see why. Every member of this forum should be so fortunate,

bnelson

Now if I could just talk the power company into lowering their dang voltage. It's not a problem for most 220 volt machines but is sometimes problematic for running 380 or 440 volt machines via step up transformer, where the "error" gets multiplied such that I can't get less than 520 volts when stepping up from my 250 volts via 220/440 transformer. Most transformers have taps for adjustments but often even the lowest taps give output still too high.

Unfortunately the 253 volts is within the specs the power company must provide.

You need a Square D Model 1S43F 1.0 kva buck/boost transformer, cost $201 at Grainger, which will subtract 12 volts from the 253 so that 241 volts single phase goes into your converter. Buck/boost is done on line going into converter. No adjustments needed at all in converter control panel. You get 240 volts three phase out of converter.

I'll email you the catalog page with picture.

bnelson

2.4% balance is great. Now for the question.
Is that at idleing, part load, or full load??? Some one must make a auto sensing transform to even the voltage out on all or one of the three legs as the load is applied??? Weather it would be cost affective or not I do not know.

[This message has been edited by Iceman1000 (edited 01-21-2003).]

[This message has been edited by Iceman1000 (edited 01-21-2003).]

There are devices you can purchase that go on the back (output) end of a rotary that improve line balance under varying loads. If anyone is interested I can provide an example or two.

A few words here about capacitor switching, which is a way to get better balance under different loads. You switch in or out some caps in your run cap bank using toggle switches mounted on the box cover for the caps.

This can really work, but DON'T DO IT with any load motors connected and running. It's like throwing gasoline on a fire to make it hotter -- works, but something's likely to get singed. Switching caps either in OR out causes huge voltage spikes.

If you do use manual capacitor switching on a home built rotary, disconnect the output from the rotary and then switch in or out with the idler still running. Reconnect the load and measure all three line voltages again. Repeat this process iteratively and you can get very good balance.

One reason I use the converter three phase panel mentioned above is that with one simple flick of a breaker you can disconnect the entire three phase bus in your shop from that rotary so you can make the run cap tweaking adjustment offline. This protects the rest of your shop from the gasoline dowsing they'd get if you switched caps in and out while online.

bnelson

UP DATE time,
The Phase perfect unit arrived today. It looks to be superbly made. Very high build quality. Should have it hooked up this weekend. We will see how the Haas likes it.

TAke Care

ARB

Arb, what capacity did you order and how much did that puppy cost ?

D. I sprung for the 20HP unit. It runs $3650 I think from cncpros.net
It looks sweet. Best part is the only moving parts are the cooling fans.

Take Care

ARB