Motor speed controller using LM324

Posted on Mar 12, 2013

Most battery motor speed controllers are under manual control and the operator automatically adjusts speed to match demand. Under these conditions closed loop motor speed control is an unnecessary expense. However, when it is required, a tachogenerator can easily be added. True tachogenerators are usually expensive - which puts off most potential users. The expense is because they are very accurately manufactured and calibrated. They are also low volume items. However for most uses a small permanent magnet motor is perfectly adequate. It won't come with an accurate 'volts per rpm' calibration and it may (just possibly) drift over several years. For most purposes this simply does not matter!

Motor speed controller using LM324
Click here to download the full size of the above Circuit.

With any tacho feedback system, the loop gain (controller, motor, tacho, tacho amplifier) must not be more than unity or instability results. To put that more simply: the feedback system must alter the output voltage to the motor so that the motor voltage is raised to exactly match the drop in the motor armature caused by the current flowing, so the mechanical speed remains constant. If the loop over compensates, then the motor will go too fast if loaded and will 'hunt' - the speed will be unstable and will bounce about. If the compensation is too little - perfornance will be less than optimum. However - there are a lot of variables, including battery voltage and motor temperature. An ideal system would measure everything and compensate for it (and be very expensive!). In practise, adjustment is quite easy: you will simply include a pot in series with the motor used as the tacho generator and will adjust this until the motor starts to hunt (show speed variation), then back it of a bit until it is slightly below optimum. The board is built in two versions (Pro-120 and VTX or NCC) but the only difference is the connector - 9 way for Pro (black numbers) and 6 way for VTX. The circuit uses a quad op-amp type LM324. The first two stages are are straight forward difference amplifiers that compare the two wires from the tacho generator. One works for positive voltages and the other for negative. Now the circuit is...

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