Phase Converter Which Type?

[John OlsenParticipant @johnolsen79199]

Hi All,

 

I think the original poster was looking for a way to control the speed of  the existing single phase motors. While in principle some degree of control should be possible, I doubt if it is worth the effort. Once the motor is running it should, over at least some speed range, adjust its speed to follow the frequency of the supply from an inverter. However the torque of a single phase motor is not good, so it will tend to drop out at lower speeds. If we use another inverter generated phase on a starting winding we could improve that, but would have to set the second phase up to suit the actual impedence of that winding, which will depend on what type of motor it is.

 

I wouldn’t care to try and publish and sell parts for a design of inverter suitable for this sort of application. Power electronics circuits are once properly designed and debugged very reliable, but they are very unforgiving of mistakes. The one I built at university went through 8 output switching devices in the development process, they tended to go in pairs since if one fails to turn off when the other turns on, they form a dead short across the internal DC bus. For a development project that is OK, but you don’t want your customers going through devices like that when they try to get your kit going. Fuses are not fast enough to protect this sort of device.

 

I dunno about the UK, but here in New Zealand the main barrier to installing three phase is the sheer cost. It cost about NZ$6000 for a friend of mine when he did it to power a rather large lathe and mill in his home workshop. (probably around GBP2000 or so) That was needed for the size of the machines, there was no way that phase converters could have done the power. But as you will see, for machines up to the power of a single outlet, say 3 hp, an AC motor controller is an attractive proposition.

 

regards

John

 

 

[Anonymous]

My experience in the UK mirrors that mentioned by Alan and John above. I simply rang the electricity company, they sent a man out to do a survey, they quoted a price and you either agree or otherwise. However, the cost is highly dependent upon the location of the existing three phase supply. The total cost to me was £2500. The existing street supply is on the far side of the road to me, and I’m told by a friend in the house building trade that this probably accounted for £1000 of the bill. The reason being that if the electricity companies dig up the road, then they have to gaurantee the repairs for a year. Consequently they bump up the price, just in case their sub-sub-contractors make a mess of it.

 

Although expensive, I think that having the three phase supply installed was worthwhile in comparison to the time wasted in mucking about converting motors from star to delta, buying inverters (this was 10 years ago, so the electronics wasn’t as cheap then) and sorting out the contactors, wiring etc. The other factor was that the existing single phase supply was below par, and needed replacing according to the survey report done before I bought the property.

 

I can categorically state that no license is needed, and it is nothing to do with the council. I’ve never had a visit from revaluation people or aliens. I pay exactly the same per kWh as I would if it was a single phase domestic supply. The house runs on a single phase anyway!

 

Regards,

 

Andrew

[John RuddParticipant @johnrudd16576]

Aidan,

 

I like many others have bought from Drives Direct. I spoke to Dave on the phone, found him very helpful and informative. The motor/vfd package he supplied does the job for me..( Chester 9*20 vfd conversion) I’m really pleased I went that route.

[Richard ParsonsParticipant @richardparsons61721]

 

In 1975 I went to the local electricity board showroom. It was at lunch time I asked about 3 phase supply (which was hitched to my house from an underground cable which ran under my back garden without any way-leave}. The girl went to a cupboard looked in and came back. “Have you got a licence?” She asked “no” I said.  “You have to have a licence” she retorted. Before I could ask here where do I apply for a licence? She had vanished. I spent the next few years on and off trying to find out where I got a licence. There was no point in trying to go ‘higher up the tree’ as you would never get past the ‘flack stoppers’.

I would like to read MEWS 168 but it is the end of the holidays so I doubt if I will receive it. I may get 169 in October if ‘they’ have been issued with their winter loin cloths and cleft sticks.etc, but 168 I doubt it.

John Olsen many thanks, I understand your points. I may well look into an auxiliary countershaft, but I am writing a set of logical ideas for such an electronic device and the way it should be used. It would have a limited speed range. Yes It would have one relay yes but that is it. There is a company that provides speed controllers for large AC fans

[Richard ParsonsParticipant @richardparsons61721]

   

Objectives

The speed controller is designed to control the speed of a single phase motor by steps within a limited range by changing the frequency of the power supplied to the motor.

Constraints

The speed controller is subject to the following constraints

It will not be used to start the motor.

It will not output frequencies off more than 45Hz.

It will not output frequencies of less than 20 Hz.

The system will only be used in conjunction with the main ‘forward’ direction switch.

The system is designed to work with SPIMs (Single phase Induction motors). Where the motor has a centrifugal switch the speed controller will disconnect this whilst the motor is running under the control of the speed controller.

It will be designed to be operated in the way described below –see ‘method of use’.

It will be designed to ‘fail soft’.

Pre-requisites

The machine to which the device is connected will have one (or more) of the following. These are to allow the motor to start ‘light’.

A fast and loose pulley system.

A clutch.

A method of using the belt tensioner which allows the belts to slip.

The controls which operate these features will be fitted with a micro switch system which will be closed (passing current) if the motor is free from the machine it is driving (The clutch is open/the ‘fast and loose’ pulley is on loose/the belt tensioner is allowing the belt to slip). If the switch is open circuit (low) then the speed controller is inhibited.   The objective here is to allow the motor to be started using the normal 50 Hz supply.

The method of use

In use the motor would be started normally. If the motor is disconnected from the machine it is driving then the speed controller would become available. To use the speed controller the operator would select the speed required by turning a Yaxley/Rotoplex switch and press the start button. There would then be a small delay, which is designed to allow the motor’s starting surge to finish before speed control unit is called after starting the motor. The motor would then be disconnected from the main 50 Hz supply and a few milliseconds later switched over to the lower frequencies supplied by the speed controller. This procedure would be have to be followed each time the motor was stopped hence the need for the motor disconnecting mechanism.

How would it work?

Most of the answers I have read both here and in MEWS refer to digital solutions, but there are older (non digital ways) which do not need programming. They are ‘hard wired’ to do the job. When turning with a standard (belt driven or gear headed) lathe the turner selects the nearest appropriate speed and adjusts the feed accordingly.

The unit comprises 5 main assemblies which are : –

The Oscillator which out puts a pure sinusoid. The frequencies output are controlled by had wired components, which are selected by a rotary switch of the Yaxley type.

The power amplifiers which amplifies the input sinusoid from the oscillator and drives the motor.

The control relay/timer which controls the event sequence. I think such relays are still made as it is still necessary to start a 3 phase motor as ‘star’ and switch to ‘delta for running This may not be necessary if one uses a modified form of ‘drum starter’ which controls the change over from full mains power to power from the speed controller. The drum starter would be locked in the mains supply position if the clutch switch showed that the clutch was ‘engaged’.

For those motors with a centrifugal switches there would need to be a mechanism which would isolate the centrifugal switch when the speed controller was in use.

A no-volt release which will isolate the speed controller unit when the ‘forward direction run switch’ is turned off. 

With my limited knowledge of electronics, but long ago and far away I built a superhetrodyne radio with push button control for my granny. Oh yes it worked

Edited By Richard Parsons on 03/10/2010 17:10:58

[geoffParticipant @geoff]

this may or may not be obvious to all users of invertors but unless they are installed in the machines electrical enclosure they require an enclosure to house them to prevent contamination from swarf or metal filings or from direct contact with the supply or output voltages to the user which has been described in the iee regulations as having an ip protection value if you have a copy see setting up a home workshop autumn 2008 pages 46 47 & 48

[John CoatesParticipant @johncoates48577]

Posted by John Coates on 24/08/2010 12:24:17:

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

 

Which I’ve just completed and variable speed is fantastic 

Got a 1hp inverter from Drives Direct for £77 (ebay) then a 1hp motor  for £69 (ebay) and just finished wiring it all up today

 

And it’s so much quieter so will be able to be in the garage without annoying wifey or neighbours. No more whining speed wheels

 

Got a remote control pod to wire in but wanted to check it all worked properly 

[Author]

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