Rotary Phase Converter 380/400V

I am building a rotary phase converter to run my Deckel milling machine and other machine tools which require 400VAC 3 phase and because of multiple speed motors, they cannot be run with inverters.

My efforts have stalled because I am unable to source start and run capacitors which are rated appropriately. Start capacitors must be 440 VAC and run capacitors must be a minimum of 562 VAC(line voltage x 1.414). In South Africa I am unable to source such values. I am informed that this problem can be overcome by using multiple, lower value capacitors connected in series.

My question is, can anyone help with the method of calculating the appropriate values I would require? The idler motor is a 5.5kw, 400V, 3 phase motor and It will run from 400V single phase supply. Alternatively, is there a supplier out there who can provide the correct rated capacitors.?

Hi

Have had a go at building a rotary phase converter without much success a couple of years ago, and sourced capacitors from Power Capacitors Ltd. Very helpfull but you need a thorough understanding of motor theory, capacitance, reactance etc. In the end I got a static phase converter off e-bay which works my Colchester 2000, and Bridgeport miller.

Depending on the motor size you might get a way with a 240V to 3 phase inverter which are much more efficient and controlable. Mitsubishi, Siemens are the main players but there are plenty more available with a reletavly cheap second hand market.

Regards

John

Hi

Just another point with regard to running multiple motors, providing you start the main motor first the fractional horse motors will run fine.

John

Why cant multi speed motors be run off an inverter?

How would the machine be originally meant to run, 400V 3ph? Inverters do that got two.

Ian,

IIRC the Workshop Practice book "3 Phase Conversion" by Graham Asterbury contains formulae to calculate what you want.

I found the book essential reading you might also.

Regards

Paul

Multi-speed motors run fine from inverters, as long as you have the switching interlinked so that you can't do a switch speed change whilst the inverter is running. Failure to do this leaves your inverter liable to being blown to bits by the reverse emf from the 'other' set of windings at switching time. The reports I've read say that you can do the switch in one direction, but not the other...

And if you are using those 240 to 415v inverters (which you'd almost certainly have to do with two star windings), then they are rather expensive to blow up!

Thanks Steve but you are assuming wired direct to motor.

Run the inverter like intended, it outputs the same as if powered from a proper mains 3ph 400V supply but regulated.

There's no such thing as an inverter that's safe from reverse emf at the levels you can get from a switched-winding motor - you still need to take the precautions. In principle (if you are very rich!) you might get away with using two inverters, permanently connected to both sets of star windings, because the reverse emf protection diodes might well be able to cope, as the winding will then be loaded, rather than having a massive potential at the ends of the disconnected winding. All you'd have to do then is to contrive a way to control them instead - probably a worse problem. And there's another problem with dual-speed motors and inverters on switch-over - going from high to low means that the inverter will, at the switching instant, have no way to control the over-speed. There's a very interesting Mycncuk thread about rebuilding a Hardinge HLV-H where the issues of controlling a dual-speed motor are discussed (and I think resolved along the lines I'm suggesting) - you can read it here: http://www.mycncuk.com/forums/showthread.php/566-Renovating-a-Hardinge-HLV-H

Thanks to all! I appreciate the suggestions with regard to the use of inverters and i already have 5 machines running in this manner. However I need to include more than one machine which requires 400VAC 3 phase and I have set myself the goal of completing the Rotary Phase Converter project because I believe it to be the best solution to my needs. I would therefore ask anyone with knowledge in the field of capacitors and/or Rotary converters to help me overcome this last hurdle.

Thanks to all! I appreciate the suggestions with regard to the use of inverters and i already have 5 machines running in this manner. However I need to include more than one machine which requires 400VAC 3 phase and I have set myself the goal of completing the Rotary Phase Converter project because I believe it to be the best solution to my needs. I would therefore ask anyone with knowledge in the field of capacitors and/or Rotary converters to help me overcome this last hurdle.

Thanks Paul, I have ordered a copy of the book and look forward to reading it!

Hi Ian,

I made a rotary phase converter 11 years ago before inverters were cheap and its been fine. The current running light is around 5a and I start the pilot (about 5hp) in delta (with contactors and interlock) and then switch to star using delay relays. The 400 supply is via an autotransformer. I used the normal motor run caps which are probably running near there top voltage but have survived up to now. It runs my DSG lathe with a 7.5hp motor but that tends to made things dip a bit on startup, in cold weather I run the mill (5hp) first to help.

I run various size 3 phase motors without a problem, think the smallest is a J&S tool & cutter grinder. I use a remote keyfob to switch on and off and I find that really handy so long as I dont lose it. Not sure what walues of cap i use as I found that it depends on the motor but I fond the best pilot motor was about 5.5hp and off a fridge unit. If you want I will try and find my notes.

Peter

Hi Peter,

Many thanks for the info, and I think I am getting closer to a solution! I have been told that I am worrying too much about the exactness of the capacitor ratings and values and your comments tend to reinforce this. I have also managed to source some capacitors which I believe will come closer to meeting my spec. I also think your Delta-Star configuration is correct and I intend to follow that route.

I will travel to Germany and the UK in June and hope to bring back the required capacitors so that I can begin the building and testing of my unit and if I win or lose I will certainly share my efforts on this forum.

Best regards to all, Ian.

I have made several converter using transformer primaries in parallel and secondaries in series to obtain the desired output voltage, forming an isolation transformer. Capacitor values are a bit, near enough, (approx) if the motor sounds right the value of capacitance is correct or so I have found.Don't forget caps are + - 10 or even 20% tolerance. Regarding working voltage, yes you can connect caps in series but put a resistor in parallel with each cap to share the voltage.. You will need two banks of caps, 1 a run group, 2 a very much larger group, initially in parallel with the first group until the motor is up to speed. Without going into theory, two caps in series don't necessarily share the voltage equally, for futher details have a look on Vintage Radio there are some brainy guys there on cap and transformers.For the rotary part I've used a motor with various contactors to give starting up procedure, never relied on timers, easier to hold the start push for 20 seconds or so. Ex power factor correction caps are very good if you can find them second hand.I have used an ex micro wave cooker with the internals remove for the cabinet. I made a sheet steel front in place of the door and same for chassis to mount things on,the chassis make life easier for the wiring and possible fault finding.Finally mount a 10k 1watt discharge resistor to each cap, you will be working with quality caps which can store a charge for a long time.Ted

> .Finally mount a 10k 1watt discharge resistor to each cap, you will be working with quality caps which can store a charge for a long time.

Don't spoil the surprise!

Neil

try this site, it tells you how to connect multiple capacitors together to achieve larger capacitance.

http://www.allaboutcircuits.com/vol_1/chpt_13/4.html

Hi Ian

Sorry to open up an old thread.

You seem to have a similar situation to mine. I own a Fritz Werner mill with a 2 speed 3 phase motor, but only have 240 VAC single phase supply. The machine has some 'interesting' control gear including a tachogenerator and a bank of resistors that look like electric fire elements, I have been led to believe that these are part of a soft start system.

My proposed solution is a 2.2 kw rotary converter which I plan to use to drive a Control Techniques Commander SE drive. This drive is rated at 4 kw. I have selected this drive because it offers the option of having two selectable sets of motor parameters. The manufacture's intention was that this would allow the drive to run two different motors (only one at a time). In their handbook CT shows the set up with a contactors located between the drive and the two motors. my reading of this is that it is not the presence of switchgear between the drive and the motor that destroys drives, rather it is the action of operating the switch whilst the drive is energised that is damaging to the drive.

So my plan is to use the existing rotary switch to handle the change over from 8 pole to 4 pole configuration. I will add a mechanical lock to prevent the switch being moved to either the off or the reversed position. I will then select either motor 1 or motor 2 on the drive and use the drive normally to control the motor (soft start, speed and direction)

I know this looks to be an expensive solution, but I can also use the rotary converter to power my Smart and Brown lathe that currently labours using a retrofitted 2 HP single phase motor. Luckily I still have the original three phase motor for the lathe.

Have you any updates on running your Deckel from a single phase 240 VAC supply?

Bob