Report by Stefan Spännare, January 2002
The author makes no warranties that this document is free of errors! Be very careful if connecting these circuits to larger lead accumulators! Very high short-circuit currents could be dangerous and destroy and burn your circuits! It is highly recommended to first test the circuits with a current-limited power supply and a small motor!
PWM (Pulse Width Modulation) is an efficient way to vary the speed and power of electric DC motors. Here two drivers are described for 24 V (15 V to 30 V) motors with a maximum current up to 80 A. These drivers can for example be used to vary the speed of small electric vehicles. The first driver use a power PROFET transistor switched at 500 Hz and the second use a power MOSFET transistor switched at 20 kHz. The second method is preferred due to its higher frequency.
The best switching conditions are when the switch frequency is much higher than the dynamics of the motor. The motor should “think” that it is powered from a true DC voltage. Due to different references the frequency must be at least five times higher than the rotation speed of the motor. For example, if the motor rotates at 6000 rpm (100 rps) the frequency must be higher than 5 * 100 Hz = 500 Hz. A theoretically better explanation is that the switching frequency must be much higher than 1/Ta where Ta = L/R is the electric time constant of the motor. Here L is the inductance and R the inner resistance of the motor. For a typical DC motor (200 W, 24 V) L = 1 mH and R = 0.5 Ω. This again means that the frequency must be much higher than 1/Ta = R/L = 0.5 / 0.001 Hz = 500 Hz. Usually a frequency around 20 kHz is used. This also avoids noise from the motor in the audio range.
The circuits below can quite easily be built on a standard prototyping card. It is a good idea to have the power component (i.e. PROFET or MOSFET transistor) on a separate card with the heatsink. See for example the photos below. The power component must be connected to the power supply or accumulator and the motor with quite thick wires and cables. The author used 1.5 mm (1.77 mm2) tin plated copper wires between the power components and the cable connectors. This is probably enough for motors that use a current of up to 20 A on average. The cables should also have an area of at least 2 mm2. For higher current motors the wires and cables must be thicker accordingly. As pointed out in the warning it is very important to not short-circuit, especially when connected to larger accumulators. Always test with a current limited power supply and a small motor first to see that everything works.
The original PWM circuit was found on the internet. Unfortunately the link seems to have disappeared when this was written. The PWM duty cycle can be varied from 0% to 100% with the 47 k potentiometer.
The following resistors and capacitors were used:
|f = 1 / (RC) (kHz)||R (kΩ)||C (nF)|
This power driver circuit is based on the PROFET transistor BTS 555 and is working at a switch frequency of 500 Hz. The reason for this low switch frequency is that the BTS 555 is a relatively slow component. This was the authors fist experimental PWM setup. However the power MOSFET version (below) is better. Note, the BTS 555 and the large diodes (DS75-08B and DSI75-08B) require relatively large heatsinks for high current motors.
This power MOSFET motor driver is better than the PROFET driver because it is working at a higher switch frequency of 20 kHz. This circuit also avoids the voltage drop and power loss over the power diodes present in the PROFET driver. The MOSFET transistor in this motor driver requires a special driver circuit between the PWM circuit and the MOSFET itself. The reason for this is that switching the gate voltage of the MOSFET transistor requires high transient current (2 A) due to relatively high capacitive load. The MOSFET transistor requires a relatively large heatsink for high current motors.
Presented here is some data (absolute maximum ratings) and approximate prices for the most critical and expensive components in the circuit layouts above. Imax and Pmax are given with appropriate heatsinks. Good heatsinks are quite expensive.
|Component||Umax (V)||Imax (A)||Pmax (W)||Ron (Ω)||Price ($)|
|EL7212CN||16.5||2 (< 1 µs)||1||-||5|
These images are taken with a Pico Technology ADC-212/3 oscilloscope for PC. The current was measured by measuring the voltage over a 0.1 Ω, 10 W thread winded resistor, which probably also has some inductance. The resistor was connected in series with the motor or lamp. The operating voltage was 18 V powered from a large power supply which can handle a maximum average current of 15 A. The small transient spikes that are seen at the positive flank of the voltage curves at 20 kHz probably occur because the power supply is not fast enough to keep the voltage stable for a short time. The DC motor (15 V, about 100 W) is running without load. The lamp is of normal car lamp type (12 V, 50 W).
Stefan Spännare (firstname.lastname@example.org)