DIY Motor Speed Control

Over the years we’ve covered a few motor speed controls – but none have been as easy to use and effective as this one. But firstly, what is a motor speed control and how can it be useful in car modification?

As its name suggests, a motor speed control varies the speed of an electric motor. For example, you might be using an electric water pump (eg an ex-windscreen washer pump) to run an intercooler water spray. By using a motor speed control, you can vary the pump output.

Or perhaps you have a fan-forced intercooler – this works especially well for underbonnet intercoolers. The motor speed control can be used to vary the speed of the fan – for example, running the fan at a slow rate at low loads and then rising to full speed at high engine loads. Taking that approach reduces fan noise and gives a much better fan life.

Or maybe you’re building a car and want better control over the cabin ventilation fan – stepless control instead of the one, two, three knob position usually fitted. By using a motor speed control, you can have infinitely adjustable fan speed.

Other uses including dimming filament lamps and controlling fuel pump speed. In non-automotive applications, you can control the speed of electric toys and the like.

The Unit

The motor speed control covered in this story is available direct from China, either via eBay or through the Chinese retailer’s own website. It is rated for a maximum current of 15 amps, comes with a remote mount pot with a knob, and has very easy wiring connections. Best of all, it costs just AUD$26 plus $10 for postage.

We bought the motor speed control from VirtualVillage.com through their eBay shop. (Do a search under the ‘stores’ category of eBay.) The company calls the device “12V 15A DC Motor Speed Control PWM HHO RC Controller”.

The controller consists of a plastic box about 100 x 65 x 20mm. An aluminium plate is fixed to one side – this acts as a heatsink. The speed adjustment pot is connected to the box by about 70mm of wiring – these wires can be extended as required. In addition, there are power and ground feeds (red and green respectively) and motor connections (white and yellow). And that’s it!

The controller can be used in a few different ways – let’s take a look.

Manual Controller

When the controller is being used as a manual controller, motor speed is controlled directly by the user operating the knob. For example, in a water/air intercooler system, the controller can drive the water pump in the system. By mounting the knob on the dash, you can have the pump operating slowly (or even off) in cold weather, and then wind it up in hot weather or when track sprinting.

To install the controller, just connect the motor (pump, fan, etc) to the output of the controller (making sure that the pump or fan rotates the correct way – if this doesn’t occur, swap the connections to the motor), connect a fused power source to the controller and then mount the knob. As mentioned above, you can extend the wires going to the pot, but make sure that the different wires still go to the same pot terminals. (Click on any of the diagrams in this story to enlarge them.)

Note that as the controller outputs a voltage that varies from zero to a fraction under full battery voltage, it’s likely that the effective operating range of the knob will be from about half power upwards. That is, under half output, the fan or pump will not rotate. However, filament lights will glow right down to just above zero output – so the knob control range depends on what you are operating.

Automatic Controller

While in many situations having manual control is fine, in others it’s much better if the controller automatically varies its output. To achieve this, we can take advantage of how the control pot works.

If you connect a multimeter between ground and the centre terminal of the pot, you’ll find that as the pot knob is rotated, the voltage on that centre terminal varies from zero to 5.7 volts. That is, the motor speed controller’s output is set on the basis of the voltage fed in on the wire connecting to the centre terminal of the pot, with 0 volts giving zero output, and 5.7 volts giving full output.

This is really handy because if we cut the wire going to the centre terminal of the pot (green) and feed in a voltage on this wire, we can control the output of the motor speed control. For example, at 4.6V input, the motor speed control outputs about 98 per cent of full battery output. (The actual percentage will be influenced a little by the load.)

So why is this handy? Well, many vehicle engine management sensors work in the 0-5V range. For example, most airflow meters and throttle position sensors output a signal of between 0-5V. This means that to make the motor speed control work automatically with throttle position (TPS) or load, or that we need do is connect the wire to the signal output of the airflow meter or throttle position sensor. (Note that the wire to the centre terminal of the pot is cut – the centre pot terminal no longer connects to anything.)

Since almost no current flows in this wire, making this connection will not change the behaviour of the engine management system.

Before deciding whether or not to take this approach, you need to measure the output signal of whatever sensor you’re thinking of using. Use a multimeter to back-probe the working TPS or airflow meter – ground the multimeter’s black lead and probe with the red lead. Typically you’ll find wires with 0 volts (ground), 5.0 volts (regulated power supply) and a voltage that varies as you rev the engine or operate the throttle. There may also be other connections, so keep on probing until you find the right wire. Extend the probes and drive the car so that you can assess how much the signal varies.

If for example the signal never rises over 2.0 volts, the output of the motor speed controller will never exceed 25 per cent – no good! But if the output varies between say 1.2 and 4.5 volts, you can use it. (Note: voltage needs to rise with load, not fall with load.)

So what’s an example of this approach? Say you’re running an underbonnet intercooler with a smallish fan. The fan starts to rotate when fed 6V and then gets faster with increasing voltage. You’ve measured the output of the TPS and found that at idle it’s 1.4V and at full throttle, 4.6V. By connecting the motor speed controller’s pot input to the signal output of the TPS and running the fan from the controller, the intercooler fan speed will vary with throttle position. At low throttle positions insufficient power will get to the fan to turn it, but as throttle position increases, the fan will start up and then go faster and faster.

Automatic Controller with Delay

If you drive a car with a manual transmission, you might have read the above paragraph and thought: “Yep, but with the throttle lift of each gear-change, the fan will momentarily lose power!” And that’s true.

However, there’s a simple way of overcoming this problem, and the solution has other advantages as well.

You’ll need two additional electronic components – a 10 kilo-ohm resistor and a 1000 micro-farad 16V electrolytic capacitor. Both parts cost just cents from an electronics supply shop.

The resistor is inserted in the wire leading to the car sensor eg the TPS. The pot is wired from the module side of the resistor to ground, with the positive of the pot as indicated on the diagram.

So what do these additions do? In short, they slow the response time of the system. For example, with a quick burst of throttle, the motor speed controller will not react at all. Holding full throttle for 6 or 7 seconds will turn on the motor speed controller, and then, when the throttle is returned to zero, the output will stay on for about 5 seconds before dropping back to zero. Increasing the micro-farad value of the capacitor will further slow the response time, while decreasing its value will create a more rapid response.

If the module is running an intercooler fan or spray, for short bursts of throttle the fan will not run. That’s just what is wanted – the intercooler won’t have got hot. But if the throttle is nailed to the floor for (say) 10 seconds, the fan (spray) will come on after about 5 seconds and then, after the throttle is shut, will stay on for a further 4-5 seconds. That’s also what is wanted; the hot intercooler will have longer to be cooled.

The capacitor in the circuit above draws its current from the sensor that’s being monitored. In some cars, this tiny current draw might pull down the signal, changing car behaviour. If that’s the case with your car, stop using the circuit. In most cars, it won’t be an issue.

Conclusion

Lots of electronic projects end up being expensive and complex – but this isn’t one of them! Whether you use the motor speed control as a manual device or automate its operation, this controller can improve your modification outcome.