Aidan.

You are correct with your understanding of a static converter, they are a fudge.

 

The rotary once running can be left running and you start and stop the mill motor on it’s normal controls. the rotary converter doesn’t replace the machine controls more live it supplies your 3 phase electrical supply.

Can be noisy but they are needed for things like 2 speed motors which require a true 3 phase supply.

 

Digital Phase converter or Inverter or VFD depending on who’s terminology you use take in single phase and spit out 3 phase but at 240 volts and for this you need a motor that can be rewired from star to delta. Basically the terminal box on the side of the motor needs 6 terminals.

 

3Hp or 2.2 Kw is about the maximum you can get on single phase and a quick look in a manual rates one of these at  11 amps input current.

 

One advantage of these inverters is that it’s possible to get variable speed and so save changing belts as much.

 

They also have the added advantage of soft start and breaking and are a lot better for the motor, they also monitor the motor and provide overload features far better than the old thermal trips as fitted to the push button startes of old.

 

John S.

…. oh, and here’s a sketch of the control wiring required. Use at your own discretion and risk.

 

 

You may find that there is a transformer in the machine which supplies the 110v from a 415v input, Sometimes these transformers have other voltage taps, it might be worth a look

HI Aidan, I also have an AJt4, I fitted it with 2 inverters from Direct drives, one to run the spindle and the other to run the feed motor.

I utilised the existing control ppod and aded some relays to do the switching. If anyone is interested I will produce a wiring diagram of how to achieve this.

The suds pump is currently unused as I use an air powered fog free coolant spray system of my own design however, as the suds pump motor is only 1/8 hp, it can easily be run on a single phase supply with a phase capacitor supplying the ‘dummy’ 3rd phase, again, if anyone needs detailse let me know and I will provide the necessary info

 

regards

 

coalburner

 

 

 

 

 

A really useful thread btw folks as I plan to change my lathe and mill to 3 phase for the variable speed at some time in the future so this is great for my research

 

One thing that hasn’t been touched on yet is replacing 1 phase motors to be able to take advantage of this. I know old motors (1940’s on my lathe) have a footprint – something along the lines of B56 for mine I seem to recall – and then there is the spindle (5/8″ on my lathe) whereas a lot of the 3 phase motors seem to have bigger spindles

 

But if I can summarise what has been said I will need an inverter with a speed controller, a switch or isolator to switch between lathe and mill, a new motor for each machine. I like the idea of re-wiring the controls on the mill though. Mine is a Chester Champion with start/stop/reverse/isolate whereas the lathe just has a Dewhurst forward/stop/reverse lever switch

 

Keep on posting and I’ll keep reading

 

John

They are an “own fix” and as such because many of the components were never intended for 440 v operation they can never be CE approved.

 

This is not my take on it but from a drives technician who has works for many  of the large manufacturers.

When you get one of these guys on-site to sort a problem out and they can clearly explain what EVERY one of the 400 odd parameters does and how to set up for the optimum result they are worth every penny. 

 

John S.

Well, you could be right that there is a problem with them, although I think they would be pretty brave going to market with a product if there is any doubt. I have been told that such an inverter is available locally in NZ, so I will follow that up and see what I can find out. Perhaps somone at the UK end of things would like to check up on what sort of guarantee is offered with the particular devices. I can tell you a few things:

 

1/ In principle it is possible to do this safely. It will be more complex than a standard inverter. it will either need a voltage doubling rectifier on the mains, which calls for some expensive capacitors, or else a switch mode power supply to change the rail voltage internally from about 325V to about 650V. Rated of course for the full power of the drive. So it would be more expensive to do.

 

2/ It may occur to someone to use a transformer to step up to 230 Volts to 415 V and apply this to a three phase to three phase inverter. THIS WOULD NOT BE A GOOD IDEA! Apart from the size and cost of the necessary transformer, the rectifier in the motor control is not designed to be used in that way. A three phase rectifier fed with three phase power has a ripple frequency three times that of the equivalent rectifier on single phase, so there is much less ripple. Also the peak currents through the diodes are less. So using a device meant for three phase input on a single phase would not be wise. (If the  device is well designed, it might detect the absence of the extra phases and shut down.) Converting the rectifier would call for bigger filter capacitors and bigger diodes, since the peak current through the diodes will be about three times as high

 

3/ I would not expect an inverter for this purpose to be very  commonly made, since the main market would be people like ourselves trying to upgrade older machines, or machines where the motor is not easily changed, and where three phase power is not readily available. In industry, three phase power is more commonly available than not. So it is not too surprising if there are not too many sources for them.

 

 

John Coates….Your inverter needs to have a capacity large enough for the horsepower of the motor you use with it. It does not need to be oversized. Since a three phase motor has better starting torque than a single phase motor, you don’t need to put on anything more powerful than you already have. If you don’t know what it is, someone with experience around motors will be able to tell you from the frame size.  The 1425 RPM speed means that it is a two pole motor. If you put on a two pole three phase motor, you will get essentially the same range of speeds with a controller, although the controller can run the motor faster if you wish. (They often come set as a default to allow you to run the motor up to 60Hz.) If you decide to set the controller up to allow running the lathe faster than before, bear in mind that the spindle bearing may not be meant for this, so don’t overdo things. The three phase motor of the same power will be smaller and lighter and will vibrate less, so if anything will remove problems from your lathe.

 

As I understand it the traditional way to get 415V three phase from 240V single phase via an inverter is to put a three phase step-up transformer on the output of the inverter. Like some of the other respondents I did a quick internet search and didn’t find anybody else selling 240V single phase in, 415V three phase out inverters. Seems slightly odd, apart from the obvious point, already made, about a limited market.

 

One would assume that any half decent inverter should be using active PFC (power factor correction) in the initial AC to DC stage. Assuming that one wanted to build the said inverter I would have thought that the simplest way would be to adapt the active PFC to a incorporate a boost converter. This would be fairly efficient and since it should be switching in the 100kHz region and the inductors will be fairly small. After that just use the standard three phase half bridge, rated for the appropriate voltage.

 

Regards,

 

Andrew

 

 

 

 

 

 

Indeed an active PFC boost converter is a type of switch-mode power supply. The problem with a passive voltage doubler is that it doesn’t provide quite enough headroom on the DC bus to generate a three phase supply from a single phase supply, if you want to maintain the same phase to neutral voltages from input to output. Hence the need for an active boost converter.

 

I’m not sure where 100kHz gets fed to a three phase rectifier. The PFC converter runs at, say 100kHz, creating a DC bus which then feeds the three phase half bridges that generates the AC output.  The newer styles of PFC converter do away with the input rectifiers, thus saving the energy lost in the diode drops.

 

The informal group I work with designed what is essentially an AC to DC boost converter last year. It turned out to be quite a design challenge mechanically, as the unit is only 300x300x120mm and is rated at 30kW in an ambient temperature of 100°C. Liquid cooled of course. The unit takes the three phase output from a generator on a diesel engine and outputs 600VDC. Of course once the hardware is in place it is trivial (software changes) to run the converter in reverse and use the 600VDC to generate a three phase supply which uses the generator as a motor and starts the engine.

 

Regards,

 

Andrew

 

I wish someone would have a look at Single phase speed controllers for a ‘Squirrel Cage’ motors. Something with a limited frequency range say from 50Hz (CPS in old money) down to 20 Hz.. I do not want to run my machines at huge speeds nor do I want all of the bells, whistles, gongs and hooters nor can I afford the cost of replacing my perfectly good single phase motors with expensive 3 phase units.

My lathes have some blind spots in their speed ranges which are either very difficult to engage or make nasty grongeling noises.

Well, the speed control of single phase motors is something model engineers longed for for decades and indeed, there are now single-phase inverters available. But these command a premium price because the overall demand is relatively small. Predominantly, industry wants 3-phase inverters to drive/control 3-phase equipment. To a lesser extent inverters accepting 1-phase input and outputting a 3-phase supply are used in buildings only having a 1-phase supply. They’re used, for inststance, to enable sophisticated control of 3-phase fans and pumps (for environmental control) and other industrial processes. In short, the demand for 1-phase inverters is never likely to bring the price down to the level of 3-phase units.

 

On the ‘bells and whistles’ point, these represent the cheap elements in inverter design. They’re just a matter of relatively cheap signal level circuitry plus software (firmware). The expensive elements are the power components — power transistors, rectifiers, heat sinks etc. — capable of handling mains level voltages/currents. So, omitting bells and whistles isn’t going to materially affect the manufacturing cost and thus you’re unlikely ever to see cheap 1-phase inverters.

 

Nowadays, power provision and control seems to be going in a quite different direction for hobbyist machine tools. Pretty well all Chinese lathes below 10-inch swing now come with a relatively high-power DC motor under Pulse Width Modulation (PWM) control. The only exceptions I know of are older marques of the 9×20 and the Sieg C6. Even the German maker, Wabeco, now supplies DC motors.

 

You talk of ‘expensive 3 phase units’ but for a given power 3-phase motors are appreciably cheaper than 1-phase units. Indeed, depending on your equipment, you might be able to recover something from your old kit. I’ve seen the ‘must-have-Myford-gear’ crowd pay on E-Bay significantly more for a second hand 1-phase, resilient-mount, Crompton-Parkinson motor than I paid for a new 3-phase motor.

Joe

 

 

PS – Oops, I missed John Olsen’s post!

Richard

Read your letter “Speed Control” in MEW 168. Firstly you don’t need a licence to get 3 phase in the UK. Many homes already have a three phase head where there is a large heating demand e.g. electric underfloor heating or where the consumer has requested 3 phase.  Having  3 phase installed  can be expensive in rural locations but in urban settings may not be that expensive as the 3 phase supply is outside in the street. It is worth enquiring about after  looking under the stairs for 3 large sealed fuseholders above the feed.

 

There seems to be a lot of scare stories of invasive officials, revaluation and visitors from other planets… perhaps it sells more convertors.

 

Your request for a single phase convertor has already been well answered in MEW and in this forum. If you want to run variable speed then go to DC brushless for low speeds or 3 phase for higher torque. Both motors are better than single phase to drive machine tools.

 

Regards,

Alan.