Phase Converter Which Type?

Edited By joegib on 01/09/2010 12:09:41

I will agree with you on the size of the market. However plans are cheap and easy to sell. Part kits (plans and boards) are not so easy, full kits are a bit more difficult and ready to run even more complex in terms work contents etc. There is a very nice place to trade it is a niche market one which can provide a small company with a nice living.

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

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

Which I've just completed and variable speed is fantastic