Series AC motor speed controller

Hi. I want to make a speed controller for my tool post grinder that has a series wound brushed AC motor. I have ordered from China a triac based dimmer switch as the basis. I have also ordered a three way (on/off/on) DPDT rocker switch that I want to use so that the motor is either off, on full speed or on reduced speed.

The circuit I propose to use means that the neutral will not be switched, so that with my rocker switch set to off, there will still be a continuous neutral path from motor to house wiring. Will this be a problem?

Thanks

Hmm... I've just been reading through what I've written and realised that most household appliances (or light bulbs, at least) only use a single pole switch on the live supply, leaving neutral continuously connected to the appliance. So my proposed arrangement will be just the same.

Could someone please tell me when DPST switches are used on appliances to switch both live and neutral?

Otley

DP switching on equipment totally disconnects live and neutral (considered live) conductors for greater safety. Many 13A sockets are available with DP switching for the same reason. Lessens the chance of neutral to earth faults on plugged in equipment tripping an RCCB.

Emgee

If you switch both the neutral switch could fail and leave the appliance live but switched off, a very risky situation.

Neil

+ 1 for what Neil said .

I would not bother with the switch as you can get full speed just by turning the dimmer on fully and only need an on/off switch on the active before the dimmer .

The dimmer is just a vriable chopper circuit and as you slow the motor down you will lose torque so you need to factor that in also .

Good question. When I were a lad, double pole switches were quite common, and sometimes equipment had fuses in both Line and Neutral. The latter arrangement is quite dangerous because  the neutral fuse blowing left the equipment live. Double-pole switches have the same problem, which is a fault can leave the equipment live even though it appears to be switched off and has stopped working. There isn't the same ambiguity in a single pole switch; if the equipment stops working, it's because live is disconnected. For that reason, single pole switching of domestic equipment was preferred (at least in the UK) rather than double pole switching.

There are exceptions, for example a double pole switch should be used if an isolation transformer is in circuit. That's because both sides of the transformer are hot - there's no longer a 'neutral'.

The question which type of switch is safest is controversial because neither single pole or double pole protect against all fault conditions. Each has advantages and disadvantages, and which is recommended varies depending where you are. For example, I believe that in the USA it is forbidden to ever switch neutral. In the UK, you can, but wasn't seen as 'best practice' in the past. Not sure about the latest thinking but both SP and DP sockets are available. (Switched sockets are themselves controversial: not all countries allow them.)

Whenever I work inside electrical apparatus I don't rely on the ON/OFF switch. I unplug it. I never work on permanently wired equipment, but if I really had to in a national emergency I would take elaborate precautions to make sure it was truly dead and that some other idiot couldn't switch it back on while I was inside!

Dave

Edited By SillyOldDuffer on 22/09/2017 21:56:53

If equipment fails for any reason the supply, both live and neutral lines should be isolated before any attempt to repair is carried out so how does a DP switch make it unsafe ?

Best look around your house to see how many DP switched fused spurs are fitted on fixed equipment.

Emgee

Thanks for the replies. To XD 351 - I had assumed that with the dimmer set to max, it would still not give the full 240v to the motor - hence my three way switch. I'll check the max dimmer output voltage and see it is 240; if so I'll ditch the rocker switch.

As long as you test it with a load ( a light globe will do ) as the voltage drop is affected by current flow .

Th voltage drop should be a few volts at most like most semiconductor junctions .

There are a lot  people on this forum that know much , much more than i about electrical / electronics than  so i hope they have some input here !

Wiring a dpdt  ( double pole double throw ) switch up to isolate the dimmer while the motor is powered from the mains is easy enough .

Question is why do you want to slow it down ? 

Ian.

Edited By XD 351 on 23/09/2017 10:04:55

A DP switch is two single pole switches operated in parallel by some form of linkage. The DP switch fails dangerously if the linkage breaks and only one pole disconnects. Operating the switch still turns the appliance on and off but it does so by disconnecting the neutral return. Although it appears to be 'safe' the appliance is still live and dangerous to work on.

On the other hand a single pole switch doesn't protect against Line/Neutral reversal faults. In that case a DP switch is safer. Otherwise I'm not completely convinced there's much advantage in disconnecting neutral if the equipment is still earthed. But I'm not an electrical engineer!

I don't know the original logic behind recommending SP rather than DP. Possibly because it's:

• Reasonable to expect that most of the time electricians won't reverse L and N on the supply side.
• Easy to test if L and N are reversed.
• Hard to test whether or not a Double Pole switch has failed, especially if the switch is fitted in an appliance. And there are millions of appliances!
• Easy to detect that a single pole switch has failed. It either works or it doesn't.
• Of itself an SP switch doesn't fail dangerously - a DP switch might.

Current thinking may have changed because of design improvements. I know some forum members like to think that anything made in the 'good old days' was high quality. It's not true; the mains switches used in old wireless and TV sets could be pretty nasty.

Dave

Edited By SillyOldDuffer on 23/09/2017 10:23:39

Ah but what happens if live and neutral are deliberately swopped? A SP switch then becomes unsafe. I wonder what they use in Germany, where the mains connector can be inserted in either direction, so you can't rely on live being live.

Andrew

Hi Otley, why not just use a standard plug on your grinder, then you can just unplug it when not in use and it will be totally disconnected from the house wiring. You then won't have such a worry about any switches failing unsafely and leaving it live when not in use.

There were various designs of speed controllers, back in 70's, in various electronic magazines and in a t least one of the Bernard Babani paperback books, which were aimed at DIY pistol drills. These were not much different than light dimmers, I built one or two and they worked at full speed without any problems when turned up to max.

Regards Nick.

Why would you switch active and neutral ? I can't see a practical reason why .

There are numeous designs for universal speed controllers on the web, plus one of our very own on here designed an add on for feedback from the motor comm....

Here's a design that works well as an example.....it suited my needs but food for thought.

https://easyeda.com/csz/Triac_based_lamp_dimmer-8nhzRL3lD

Edited By John Rudd on 23/09/2017 11:43:21

I think the Chinese dimmer I've ordered has a similar circuit to that - I can't see anything in the one in the link that has any motor speed feedback?

The tool post grinder motor has a no load speed of about 18,000 rpm and is fine for grinding chuck jaws (its original purpose) but too fast for any milling or drilling. I was hoping that a triac based controller would give a kind of PWM output that would maintain the motor rated torque down to low speed.

The dimmer and switch have cost £6.29 and I'll spend a bit more on box, flex and trailing plug and socket but the project will provide a little interest and education, so I'm happy.

If anybody believes they have isolated something by switching it off they have only done half the job. The bit that saves your life is testing to prove your isolation and ensuring that it cannot be switched back on.

Mike

According to the Paedia of Wick:

Schuko sockets are unpolarised, there is no way of differentiating between the two live contacts (line which is approximately 230 V to earth and neutral which is approximately 0 V to earth) unless the voltage to earth is measured prior to use.

The IEC 60906-1 standard was intended to address some of the issues regarding polarisation and replace Schuko, but the only country that adopted it is South Africa.

Neil

Thats because there isnt any feedback.....Like I posted earlier, a member posted up a sketch of an add on circuit to give better performance....have a search around....

OK, sorry - I misread your post.