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2.1 – Motor Speed Control (part 1 – general information & motor connections)

Electronic speed control is highly desirable for washing machine, and other series wound, commutator motors.

One of the down sides to modern series wound washing machine motors is that they are designed to go very fast, and when run on unfettered mains current will screech along at about 8,000 rpm with a modest load (as I already said don’t run them unloaded). This uncontrolled speed is bad because:

a) it is going to make a lot of noise, annoying you and your neighbours;
b) to turn wood on a lathe we will need a much lower rpm – from about 400 – 2000. To achieve this without some sort of electronic control would mean an elaborate (read even more noisy) and large pulley system, and,
c) if connected straight to the mains the speed regulation will be poor when load is applied to the motor. In other words when you start turning your wood and you push the cutting tool into it, the speed will fluctuate wildly. Ideally you would like the rpm to remain predictably constant, rather than speeding up and slowing down as you make your master-works on the lathe.

There are various ways to build a controller – probably the minimum and most basic is to vary the voltage with a variable transformer. Although very simple, this is not a good solution because: when you set it to run at low voltage/low speed, it will also have very low torque – and it will still suffer greatly from point (c) above. This is a disaster because you often want the most torque and predictable performance at low speed.

A much better method is to make a very simple triac controller circuit, also referred to as a phase control circuit.

This stage required me to do fair bit of learning, but in the end I was able to build a really nice little controller that adjusts the speed quite accurately with a turn knob (an elaborately decorative new version of which I shall make once I have finished my new lathe). All that is needed are some quite cheap components (available from most electronics mail order places) and a soldering iron.

After reading lots of books and websites I found a fantastic chapter about speed control. Here is a link to the book – pages 59 to 73 are well worth reading.

The good thing about a triac system is that you don’t need to faff about with changing pulleys or drive belts. Also you get to learn some cool electronics jazz!

Working out which connections do what on the motor, is one of the first steps (see the photo below in which one of the brushes has been unbolted for clarity). Also check out the diagram that shows what bits of the motor are what.

There will likely be 3 stator coil connections,
2 brush contacts,
2 tachometer contacts,
and an earth connection on the motor frame.

If you feel like checking out the inside of the motor – I know I did – then unbolt the three long bolts and carefully pull the two outer casing halves apart.

Here you can get a good look at the field coils.  You should also note the three field coil connections. Two one side (the right of the photo) – one does clockwise rotation the other anticlockwise – never connect them both at the same time! The white device on the right hand side is a built in heat sensitive safety switch, which cuts out if the motor is overheating – useful.

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Responses to 2 – Motor speed control

  1. craig k.

    Hello, great job on the lathe! I live in the U.S. It is hard to find 220v washing machine motor. Using 110v motor, what change is needed for the speed control?

  2. Bongo

    Hi Craig, I think you could still use a very similar circuit – just a few modifications. You may want to change the values of C1, the main timing capacitor in this circuit. At the beginning of each half cycle of mains alternating current it is charged before the current goes on to reach Q1 and Q2 (which in turn send power to the motor for the rest of that half cycle). You may have to experiment a bit here, but you might start by lowering the value of C1 by about 50%. This will mean current would reach the motor sooner (for the same setting on VR1 – the speed control knob).

    That may be all you need to do, but if you are keen, you may also find it useful to change the value of Q1 – this diac’s breakdown voltage is 32V. So it is acts like an open switch until the voltage reaches 32V in either direction – when that happens it suddenly ‘breaks down’ and passes current, which is sent through into the gate of the triac (Q2), turning it on. When the speed is set appropriately (VR1’s resistance is set low) this happens every half cycle. When VR1 is at highest resistance the diac may not ever reach ‘break down’ voltage. As we slowly turn VR1’s resistance down (turning the speed control up) breakdown voltage is achieved towards the end of the half cycle, and for the remainder of the cycle current is sent to the motor.
    So anyways, this is a long winded way of saying try a lower break down value for Q1 – say 15 to 20V or something.

    Other than that I think it should be all good. Let me know how it goes.

  3. Selim

    I wonder if I could find a speed reducer from an old washing machine ? or similar appliances ?

  4. Bongo

    You can, but almost all of them use ICs (integrated circuits) to control speed these days. Figuring out the wiring and controls for these is quite a task – you might start by looking up the datasheets of the chips on the main control board. Doing this will take ages, and is defo not worth it unless you are sure the washing machine functioned properly (at least electronically) in its former incarnation.

    If you do get it working you would have excelent speed regulation – as the ICs can maintain the RPM of the motor under varying loads by continuously monitoring it with a little tachometer on the end of the motor. Unless you salvaged the main board and motor together and know how it was connected, personally I would make one of the little boards above – they don’t give as good speed regulation, but they sure are simple.

    Good, luck, let us know how you get on with this.

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