View Full Version : Rotary Phase Converters

08-12-2004, 12:13 PM
When using a rotary phase converter to run your machines motor does this increase the amps used? Two motors vs one motor.

J Tiers
08-12-2004, 12:29 PM
Yes, more power and hence more amperage is drawn two ways.

One because the "extra" motor has losses and draws some "no-load" current.

The second because the power sent to the third phase by the converter has to come from the two input wires, so the current in them increases.

Of course it's hard to compare anything, because you are using 3phase motors with single-phase input. So it is higher compared to what?
I'd figure that yes the total power is indeed larger than if you used the machine directly on three-phase. And the amperage may be higher than expected because of the extra motor.

Mike Burdick
08-12-2004, 06:55 PM

Just to add a little…

Since the rotary phase converter is basically “idling” (no load), the power consumed by it is only what’s needed to maintain the inertia of the rotor, internal resistance of the wiring and friction. This value will be very small. It is not like you are running two motors at the same time at nameplate amperages. Cost of running a rotary converter will not add significantly to your electric bill.

Mark Jones
08-13-2004, 01:16 PM
mmm well I did some measurements one day on how much power I used over a 4 hour period of non stop milling.
I have a 240 volts single phase domestic supply.
The rotary converter was a 5hp design
and I was using a bridgeport with a 1hp head motor and a 1/4 hp feed motor.
I noted the units (READING) on my electricity meter before and after the session.
it showed that I was using 2.5 units an hour..10 units consumed total..so that equals 2.5 kw
now then........I know that a 1hp single phase motor is about 750 watts........but I dont know what the 1hp three phase brodgeport motor is.
all I can say is that 2.5kw is twice what I expected.
all the best....mark

J Tiers
08-13-2004, 05:09 PM
There is also the fact that motors draw at a lousy power factor. They draw more "KVA" than they do "watts".

They may draw 1/3 to 1/2 their full-load current at idle. This represents relatively little actual power in watts, because it is drawn "lagging". It probably won't register much on your meter. It still gets the motor almost as hot as under load, though, because of the resistive losses.

But if your meter is sensitive to reactive loads, it may register higher. The power company hates lagging reactive loads because they tend to reduce the amount of delivered power per watt lost in transmission. So they are looking at charging extra for them.

An old-fashioned house meter generally has no added sensitivity to reactive power. In fact, the old ones had an adjustment to allow them to read correct "real" power with a reactive load. That "real" power is far less than the current would indicate.

A motor might draw 6A full load at relatively high power factor. So it may be drawing 6 x 120 x 0.85 = 612 watts of real power, although the KVA is .72 (720 VA)

At idle, it may draw 2.5A, at a low power factor. 2.5A x 120 looks like 300W, but is in fact 300 VA (0.3 KVA). The VA (volt-amperes)or KVA (kilo-volt-amperes) is more than the real watts.

The "real" power drawn may be more like 2.5 x 120 x .2 = 60W.

Mind, now, the power company has to size the wires for the KVA, not the real power. So they naturally want to use their wires efficiently, and want to reduce the KVA to equal the "real" "watts resistive".

Mark Jones
08-13-2004, 06:45 PM
The above is.. totally mind bogling to me .
all im interested in is the amount of power consumed......and how much I have to pay for it.
in a nutshell the units im charged for.
are you saying i should call the power company in and ask them to install a new meter that calculates the actual power IM using ......will this result in a lower bill.
If this is so .......I will get on to them as soon as possible.
all the best....mark

08-13-2004, 08:03 PM
Mark,was the mill the only thing running?If you have lights,frig and or waterheater,etc going the readings will be bogus.

Like said above the loads found in electric motors makes little difference on the bill.I have one friend with a 20hp converter running all day and he only noticed a $10.00 increase after a typical 40 hour week.

The converters will draw more on start up since both motors are under load for a breif period,basically the rotor of the RPC loses a bit of speed when the machine motor is started.I just today finished a small 3hp converter for a friend.Auto start,no run caps and a heavy flywheel on the shaft.The flywheel virtualy eliminates the starting lag by adding enertia to the rotor,the result,nice smooth crisp motor starts,perfect.

Mark Jones
08-13-2004, 08:31 PM
I dont get any noticable strting lag with my converter....
the only thing i have is sort of lag on the lathe.. were the lathe has a motor wound for different speeds.......going from one speed to the next I notice in-between the changeover the converter sounds like its starting one phase at a time .
but the lathe has a sort of rotary drum switch for changing speeds ....me thinks that some of the contacts are not always in alinement properly when rotating the switch.
...mmmm forgot about my immersion (WATER) heater.......that comes on for half an hour during the period I was opperating the milling machine......
next time I will do the test more scientifically me thinks,with all lights and household appliences turned off......think I may have been jumping the gun a bit.
Another thing I want to add is that my rotary converter has the capicitors leaked to earth thru resistors deliberatly to comply with electrical guidelines.......would this cause anymore power consumption......the company that sold it to me stated that it was designed without this feature but they had to add it later to comply.
something to do with if the owner were to open it up ...say to change a fuse ...he would not get blown out of the workshop door if he was idiotic enough to touch the capacitor circuit without discharging it.
thanks wierdscience......would be able to say thanks "yourname" if you had signed your reply...feels a bit odd saying thanks wierd...lol
anyhows .will do a better test in the future .watch this space.

my convertor showing inside



[This message has been edited by Mark Jones (edited 08-13-2004).]

[This message has been edited by Mark Jones (edited 08-13-2004).]