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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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