Control Bidireccional De Motor Dc 270

LANOITCERIDIBBIDIRECTIONAL

MOTOR SPEED CONTROLLER
This project allows you to control the speed of a DC motor in both forward and reverse direction, from fully off to fully on. It can be used for motors running at 12V or 24V and drawing up to about 5A. It runs in switchmode so it is quite efficient.

W

hile we have published a number of DC speed control circuits over the years, none hashad the ability to control speed over the full range in both forward and reverse (with the exception of the Bidirectional Train Speed Control in the April 1997 issue of SILICON CHIP). We have published a very popular 12V/24V 20A switchmode speed consiliconchip.com.au

trol (June 1997) but it works in only one direction. If you want it to change direction, you need a double-pole changeover switchor relay to change the polarity of the applied voltage and the motor spins the other way! However, this has the disadvantage that

Design by FRANK CRIVELLI

you then have two things to control the motor – a direction switch as well as the speed control. Also, it is not a good idea to suddenly reverse the voltage on a DC motor while it is spinning. It can cause a big current surge that couldburn out the speed controller, as well as causing big electrical and mechanical stresses on the motor itself. DECEMBER 2004 63

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Fig.1: four power Mosfets can drive the motor in either direction, the speed and direction set by potentiometer RV1.

This circuit overcomes both these problems. The direction and speed is controlled using a singlepotentiometer. Turning the pot in one direction causes the motor to start spinning. Turning the pot in the other direction causes the motor to spin in the opposite direction. The centre position of the pot is the “off” position, forcing the motor to slow and stop before changing direction.

Specifications
Voltage: both the control circuit and the motor use the same power supply. And while the maximumoperating voltage of the LM324 is 32V DC we would suggest the maximum operating voltage of the circuit is 24V DC, as supplied by a 24V battery. In practice, this means that the supply could be almost 29V. Any more than this means that there is very little safety margin (ie, below the maximum of 32V). Current: the IRFZ44 Mosfet can handle 49A and the IRF4905 can handle 74A. However, the coppertracks on the PC board that run from the Mosfet pins to the screw terminal block can only handle around 5A and the same goes for the terminal block itself. This could be increased by soldering wire links along the copper tracks and bypassing the terminal blocks with direct wire connections. If you do this, then the circuit can probably handle up to 10A or so. Check that the Mosfets don’t get too hot –if so, then bigger heatsinks will be required. In any case, the gate drive to the Mosfets does not ensure a fast enough switching speed to handle really high currents.

Speed control of DC motors
In essence, there are four ways to vary the speed of DC motors: 1. By using mechanical gears to achieve the desired speed. This method is generally beyond the capability of most home workshops. 2.Reducing the motor voltage with a series resistor. However this is inefficient (energy wasted in resistor) and reduces torque. The current drawn by the motor increases as the load on the motor increases. More current means a larger voltage drop across the series resistor and therefore less voltage to the mo64 SILICON CHIP siliconchip.com.au

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Parts list– K166
1 PC board, code K166, 93 x 42mm 1 IC socket, 14 pin (for IC1) 2 2-way screw terminal block (joined to make a 4-way block) 2 heatsinks for Mosfets 2 3 x 8mm screws and nuts Semiconductors 1 LM324 Quad op amp (IC1) 2 BC547 NPN transistors (Q1,Q2) 2 IRFZ44 N-channel power Mosfets (Q4,Q6) 2 IRF4905 P-channel power Mosfets (Q3,Q5) 2 1N4148 small signal diodes (D1,D2) 1 1N4004 power diode...