Using VFD in conjunction with rotay phase converter

[BobParticipant @bob17059]

Hi All

I am planning to run an old (1950s) Fritz Werner mill from a 240 V single phase supply. The motor is two speed 710/1430 rpm and rated at 1.2/1.6 kw 415 V 50 Hz.

I have a 2.2 kw rotary phase converter and a selection of 400 volt VFDs including a 4 kw drive.

I would like to take advantage of the speed control and soft start facility that the VFD offers. I plan to use the RPC to produce the necessary 3 phase 415 V to supply the VFD.

The reason for not wanting to connect the mill directly to the RPC is the original control circuits are not in good shape and there seems to be some damage to the original soft start system that appears to involve a switched bank of power resistors.

Has anybody any experience of this type of system or running dual speed motors from VFDs. It is interesting that the larger VFD I already own has a dual motor option that allows the drive to be connected to two different motors as long as only one motor is run at any time. This seems very similar to switching between a 4 pole and 8 pole configuration of a single dual speed motor.

Bob

[BobParticipant @bob17059]

VFD RPC single phase to three phase conversion

[Manny lambertParticipant @mannylambert86533]

Hi Bob, I am looking to run a smart and brown 1024 on a VDF, the motor is 400 volts/440 volts 2hp 3 speed. I was advised by drives direct that it would work just on the VDF. Direct drive assured me that the lathe circuitry for the 3 speed wouldnt change but I would need a new stop/start and forward/reverse switch and I could alter the speed with the VDF. I am new to this, I was advised not to use the rotary converter or normal phase converter. Any ideas? Would be great full for any help. I am disabled and needed to keep my mind occupied so decided to have a go at learning model engineering. Man

[John OlsenParticipant @johnolsen79199]

Hi Guys,

The VFD works by rectifying the incoming AC to DC. Now, if the VFD is made for three phase AC input, it will have a three phase bridge rectifier and it will be designed for all three phases to have the same Voltage. The potential problem that I see is that with a rotary phase convertor there is no guarantee that all the phases will be at the same Voltage. The phase that has been generated in the convertor may well be lower in Voltage than the other two. This would lead to that phase of the rectifier carrying less of the load and hence the other two carrying more. This might be within the ratings of the rectifier or it might not, and the only way to find out could cost a VFD.

The two speed motor should be OK, but I would not attempt to change speed while the system is under power. This is for the same reason that one does not usually put switches (or plugs and sockets) in between the VFD and the motor, eg breaking the circuit might let the magic smoke out of the inverter. Since the VFD would provide a wider speed range, it might be just as well to permanently wire the motor up for the fast speed setting and just use the VFD.

Usually the VFD has connections for the stop/start and forward reverse switches, as well as the speed control potentiometer. You can make a little box up yourself, or some VFD suppliers may sell a ready made box with these in.

John

[BobParticipant @bob17059]

Hi Manny/John

I understand the way VFDs convert the incoming supply to DC before synthesising the three phase output. I have found various information regarding running 400 V converters on supplies other than true three phase. Some authors suggest derating the VFD (by up to 50%) for rotary converter supply. Jim Cox in his book on Electric Motors (2nd ed. Workshop Practice Series) states that some VFD manufacturers accept that their product can be powered in unorthodox ways, for example using an autotransformer to step up a 240 V single phase supply to 480 V single phase, whilst others expressly forbid it.

Reading the manufacture's handbook (Control Techniques) I can find no mention of 400 V class drives being capable of running from single phase supply, but they do produce 200 V class drives that can be run from either single or three phase supplies.

The only reference I can find in the handbook is a statement that:

"(Input) phase imbalance not to exceed 2% negative phase sequence"

What sort of phase imbalance might I expect from a rotary inverter?

I have also noted that if an input phase is lost the drive will trip out and report the PH trip code. This suggests to me that the VFD should not destroy itself if supplied from the rotary converter, but it might trip out. The VFD is rated at 4 kW and since my motor is rated at 1.2/1.6 kW depending on its configuration I think the VFD is very conservatively rated.

Bob

[John OlsenParticipant @johnolsen79199]

I don't have any figures for rotary inverters but I think it could easily exceed 2%. However, it sounds like you would be safe enough to give it a try, since the vfd has protection for this situation. If you have a clip on ammeter you could check the current on each input phase to see if they are all contributing, with the motor on load.

I'd quite like to do the autotransformer idea myself since one of my shapers has a 460 Volt delta connected motor, so can't be restrapped to give a lower rating. I have run it in three different ways…First with just an autotransformer connected to two phases, which required kicking the pulley to start it, then with an autotransformer and an idler motor, which required manually starting the idler motor in the correct direction. Then lastly I have tried it with a single phase input VFD, giving 240 Volts on the output of course. Although this only gives a maximum power of 1 half of the rated power of the motor, it allows the machine to take remarkably heavy cuts, so for the moment that is all I do.

John

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