Dimmer Switch

Hi

The speed of these motors is primarily determined by the applied frequency and the number of poles on the stator, this ignores the slip that is necessary to make them operate at their optimum/maximum torque.

Therefore speed control, without changing the percentage of slip of the rotor to the synchronous field, can be achieved in two ways. One is to change the frequency of the supply and the other is to change the number of poles being energised in the stator. It is quite easy to design an induction/synchronous motor that has a switch which changes the number of poles for different speeds. This gives a stepped change in speed, rather like changing gears, rather than the smooth change afforded by VFD supplies.

It is possible to vary the slip but the speed range is limited and it will affect the efficiency and torque of the motor and is probably not good practise.

Cheers

Martin

PS

Looks as if this posting has crossed previous entries on this thread.

Edited By Martin W on 11/11/2013 19:17:28

I think dimmers are designed for a resistive load, driving a motor from one may not be ideal although a poster says it has been done. The only motor that a dimmer may work on is a universal brushed type motor like a sewing machine or power drill. Not being familiar with the grundfos pumps I would guess the speed changing model may use pole changing to effect 3 speeds ie a 2,4or 6 pole selection it adds complication to the motor wind but the control is just a switch.

Mike

I see Andrew and Martin proposed the pole switching solution while I was typing.

Edited By Michael Poole on 11/11/2013 19:21:22

Edited By Michael Poole on 11/11/2013 19:21:49

Edited By Michael Poole on 11/11/2013 19:46:35

This is a useful and informative thread. Pity it will not come up in searches because the title does not really indicate the subject matter.

Please think of a descriptive and search friendly title when starting threads.

Induction motors can be speed controlled by series resistors - my warm air central heating fan is doing that right now. However the motor is a version specifically designed fo rthis mode. Not all would work well and could overheat or something else bad.
A centrifugal pump can be throttled without bad effect and uses less power when that is done or they are delivering to a high head which is the same.
Some of the newest CH pumps with variable speed use a built in VFD - saves power in theory but cost more and will only have a fraction of the life of a traditional one thus earning the manufacturer more money and not saving the planet at all.

CH pump bearings are lubricated by the fluid being pumped. They work fine with clean water but any contamination will soon reduce the speed to zero RPM.

Dave

Dave b

Have you seen the mess that some CH pumps have to circulate? Its more to do with lowered life of bearings before they actually stop.

I have a Grundfos CH pump on my Bridgeport clone although it's the 3 speed version. I run mine on the high setting to get it primed quicker. As noted, you need to mount the inlet below the fluid level. Being paranoid, I have a plastic box under it (a shopping basket) in case it leaks, which of course it now hasn't.

These pumps are fine with a static head ie no flow. The only downside would be that it (and the fluid) would get warm after extended periods but why would you run the pump with zero flow anyway? And it wouldn't get very hot even then.

I simply vary the flow with the nozzle. If you want to get really clever you can download the flow characteristics for these pumps which show that they can develop a decent head but nothing too silly. Trying to vary the speed would seem rather pointless given the ease with which you can achieve the same outcome(!) with a simple tap. I'd post some pictures of my setup but there is very little to see.....

Merry

You used to be able to get dimmers for fluorescent lighting(inductive load). I wonder if these could be matched to a motor? In my ten years as an electrician I think I only fitted two of these units for lighting control and they were very expensive.

Here's the flow characteristics for the Grundfos 15-50 pump in my system. BTW, it suggests you ensure at least 7.5% of the max flow as a minimum but as I said it's hardly critical. It can develop the best part of 5-6m head of water (suds) which was what I was checking when I found this.

When I worked for BR we fitted dimmers to florescent lights in signal boxes, bascically, they kept the tube end cap heaters on and lowered the voltage via a transformer.I'm sure there is now a non repairable digital gadget full of 16 leg chips which dims lights much better. Regarding pole changing motors it might be easy to design them, but try sorting out the switching arangement when some thing goes faulty. I've recently been sorting a two speed motor on a Smart & Brown lathe with several inter locks. Fortunately a reader on this ME site sent me a copy of the maual, but the circuitdiagram left a bit to be desired.Please to say all is now working.Thank you Richard. John

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I know it's off-topic, but Alan already has his answer; so what the heck.

This is fascinating.

MichaelG.

Bazyle: I've racked my brain, but I still don't see how to speed control an induction motor with a series resistor? Could you explain in more detail please?

Regards,

Andrew

There is one way, it is sometimes used in fans to give two speeds, a choke (as used to be used for smoothing DC power supplies, looks like a transformer, but only one winding), with that in series the speed is dropped. Think (not sure) it's got something to do with altering phase angles or something like that. Ian S C

I think the answer lies in the fact that the torque required to drive a fan drops off rapidly with decreasing speed (something like a square law?) so the motor can be run with a large degree of slip without stalling. Trying this with most normal loads won't work and the motor will rapidly stall.

Russell.

Thanks chaps I have sorted out what I will do with it now, that's all I wanted to know in the first place you reply's have been gratefully received, well nearly all of them, KWIL you seem to challenge what I do for a living in that you imply I may not know what I'm doing, so for the record the 15-50 pump does have a 5 metre head as mentioned and the 15-60 has a 6 metre head, this is what I work to when I design a new heating system for any one of my customers, if my comment to you about answering the question I asked in the first place and not confusing the issue has upset you then I'm sorry, may be next time keep the toys in the pram.

Regards to all Alan.

Best debate I've ever read" Al""we will just put a valve on it.

rob

I did some experiments today. I used a 90W water pump from a washing machine, the only choke I have left was too big, and with it in series nothing happened, no rotation. Tried a transformer (230V : 36 V), tried the secondary first, and that worked OK, reduced speed by over 50%, it also reduces the torque by at least that (thumb and finger gripping the shaft), the system would do for a fan, but I doubt if it would be much use in a pump. I tried it with a 230V solenoid, and found similar results, but then I pulled the core out, and the speed changed over a range of 2 or 300rpm. I then put it all back in the junk box. Ian S C

I have been running my suds system based on an old CH pump for 6 years without trouble. Flow adjustment is a small tap in the line to give from a mere trickle to a full blown wash. I didn't seen the need for complicated electrical circuits. Keep it simple, just switch on and it goes. (having said that it is bound to break down tomorrow)

Ray

Hi Ray it seems the dimmer switch is dead in the water and your are right keep things simple and it should be OK, just a couple of things I would like to ask if that's OK the first thing is do you run the pump through a 3 amp or 5 amp fuse like you would on a normal heating system 3 amp is used on gas boilers and 5 amp on oil firing boilers, second thing the tap you have on your circuit is it done like a bypass straight back to the tank to alter the flow or have you got one pipe from the pump direct to machine and also do you run on speed 1-2-3, some one said earlier on this thread about the need to run clean water only through the pump I can safely say that is not the case as those of you that have looked in the pipe work from an old heating system will all ready know, don't get me wrong you wouldn't want to pump anything much thicker than the soluble oil though.

Al.

Alan, At the risk of restating the obvious, a central heating pump is a centrifugal pump. As has been stated, they can, indeed should, be started against a shut valve, and then the discharge valve can be opened. The degree of opening determines the quantity of flow. You do not need to by-pass any flow back to the tank IMO unless the flow is very very small and the pump is in danger of overheating, which is extremely rare, and in which case the pump is grossly oversized! Use either a needle valve to control or preferably a ball valve - the cheaper, easier option - to control what flow you need. Please do not use a gate valve, they are useless other than for scrap - at sea we used to call them dimmo valves because they were such a dim idea.

Basically, this is what KWIL originally said but in more detail - keep it simple, it's best each time!

Chris

Just as a matter of interest I forgot to switch off my Axminster coolant pump on my lathe and found it running after forty three hours. It seems to be none the worse so I have to say well done to Chinese manufacturing. I am just a satisfied customer, would not recommend using a circulation pump they can be touchy even on domestic heating and sieze if not in regular use.