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High-Power Reversible DC Motor Speed Controller

Here's a 12-32V 30A speed controller that's easy to build. It's available in two versions ? reversible and non-reversible ? and features soft start, relay switching of motor direction and PWM speed control.

Words by Leo Simpson
Design by Branko Justic*
*Oatley Electronics

This reversible DC Motor Speed Controller uses a pair of high-power Mosfets connected in parallel to drive the motor and a unique relay changeover circuit to make it reversible. It can operate from 12-32V batteries at currents up to 30A. Logic control of the relay changeover circuit means that it can only change direction when the motor is stopped.

The unit comes in two kit versions. The first is the basic speed control with two paralleled Mosfets and a dual op amp to provide pulse width modulation (PWM). The second version adds the relay changeover circuit and its logic control. If you don’t need a reversing feature, you only need buy the basic kit.

Either way, the speed control can be via an onboard trimpot, via an external 5kΩ potentiometer, or via a motorcycle throttle based on a Hall Effect sensor. This could be ideal for a whe elchair controller or an electric bike.

Click for larger image
Fig.1: the scope grab illustrates the basic operation. The triangle wave from the oscillator is compared to a 3.5V reference (pink trace) and when it exceeds this reference, a corresponding motor drive pulse (blue trace) is produced.
Click for larger image
Fig.2: this scope screen grab shows the operation at higher throttle settings. The triangle waveform now exceeds the reference voltage for a greater proportion of the time and so the pulses fed to the motor are much wider.

Circuit description

Refer now to Fig.3 which shows both sections of the circuit. The lefthand side is the basic speed controller while on the righthand side are the relays and associated logic control.

First, let’s focus on IC1 (the LM358 dual op amp) and the 5kΩ potentiometer. Op amp IC1a and its associated components comprise a triangle wave oscillator. Its frequency is around 300Hz and its output amplitude is around 1V peak-to-peak. The mean DC level of this triangle waveform can be lifted up or down, dependent on the setting of the 5kΩ speed control potentiometer.

This output waveform is connected to the non-inverting input of IC1b, pin 5. IC1b is connected as a comparator and it compares the triangle waveform with the 3.5V fixed reference at its pin 6. When the speed control is advanced so that peaks of the triangular waveform at pin 5 exceed the 3.5V reference voltage at pin 6, the output at pin 7 goes high and this turns on two power Mosfets, Q6 & Q7.

This means that the Mosfets are pulsed on whenever the triangle waveform peaks go above 3.5V. Advancing the speed control increases the duty cycle of the pulses.

The circuit operation is demonstrated above in the two scope screen grabs of Fig.1 & Fig.2. In each case, the green trace shows the triangle waveform while the pink trace shows the 3.5V reference which is fixed. As you can see, each time a portion of the triangle waveform intersects the pink trace and is above it, there is a corresponding pulse to the Mosfet gates, as shown by the blue trace.

The voltage across the motor, between the positive supply line and the Mosfet drains, is shown in the yellow trace.

Fig.1 shows the operation at a very low throttle setting and so the pulses fed to the motor are very narrow and its speed will be low. By contrast, Fig.2 shows the operation at higher throttle settings. As can be seen, the corresponding pulses fed to the motor are much wider.

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