Simplest DC Motor Speed Controller Circuit Diagram

The effect can be eliminated by means of Ri compensation, which essentially entails measuring the motor’s current consumption, relating this to the motor’s instantaneous drop i across Ri, and increasing the supply voltage accordingly.

The load current is measured as the drop across sensing resistor R3. For optimum results, this impedance must be kept about equal to that of the motor. Figure 2 shows the practical circuit of the motor driver based on a power operational amplifier.
Simple DC operated motors with a permanent magnetic stator behave as an independently energized motor. The speed of an ideal motor with an, infinitely low internal resistance is in direct proportion to the voltage applied, irrespective of the torque.
The motor thus runs at a speed at which its reverse electromotive force (e.m.f.) equals the supply voltage.
The reverse e.m.f. is directly proportional to the force of the (constant) magnetic field, and the motor speed.
In theory, therefore, the motor speed can be held constant with a constant supply voltage. The speed reduction observed in practice arises from the voltage drop across the internal resistance, Ri, of the armature winding. Thus, when the motor is loaded, its current consumption, and hence Vai, increases, reducing the effective supply voltage.
The Type Ll65 £rom SGS can supply up to 3 A at a maximum supply voltage of 36 and is therefore eminently suitable for the present applications. cation. Capacitors C1 and Cz suppress noise on the reverse e.m.f. from the motor.
The basic set-up of the supply required here is shown in Fig. l.

In· fact, this calls for a voltage source with a negative output impedance, since it caters for a higher output voltage when the . load is increased.

Due care should be taken, however, in so l extending the circuit, because this readily leads to instability. The motor itself already forms a fairly complex load, since the revolving rotor winding is mainly inductive, and the rotor itself represents a fairly large capacitance.

Simplest DC Motor Speed Controller Circuit Diagram

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PWM Based Speed Control for DC Motors

There are several methods for controlling the speed of DC motors. One simple method is to add series resistance using a rheostat. As considerable power is consumed in the rheostat, this method is not economical. Another method is to use a series switch that can be closed/opened rapidly. This type of control is termed as chopper control. We’ve described here a PWM-based chopper schema that smoothly controls the speed of general-purpose DC motors.

Fig.1: Block diagram of PWM-based speed controller

Fig. 1 shows the block diagram of a basic PWM-based chopper. The schema shown in Fig. 2 is designed as per this diagram. A dual timer IC (NE556) is used to configure both the astable as well as the monostable multivibrator. Timing components for the astable are chosen to provide a frequency of 546 Hz, while the monostable components are selected to obtain a maximum pulsewidth of 2.42 ms. Diode D1 improves duty factor of the astable oscillator output, whereas D2 acts as a free-wheeling diode. Transistor SL100 drives the motor, while the 22-ohm, 2W resistor (R4) serves as a current limiter, avoiding overheating of the transistor. The DPDT switch enables direction reversal of the motor, as desired.

Fig. 2: The schema of PWM-based speed controller

The speed can be varied by adjusting VR1, which changes the threshold value to which capacitor C1 in the monostable schema is charged. This, in turn, determines its output pulsewidth and hence the average voltage applied to the motor. Waveforms shown in Fig. 3 depict the average voltage for controlling various speeds.

For effective speed control, ‘on’ period (TON) of the astable should be equal to the maximum pulsewidth (TON) of the monostable.

Fig. 3: Waveforms at different conditions of VR1

For higher voltage and power requirements, SL100 can be replaced by an appropriate MOSFET or IGBT with relevant changes in the drive schemary.

The schema costs around Rs 75.

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Two Basic Motor Speed Controllers

Here are two simple 12V DC motor speed controllers that can be built for just a few dollars. They exploit the fact that the rotational speed of a DC motor is directly proportional to the mean value of its supply voltage. The first schema shows how variable voltage speed control can be obtained via a potentiometer (VR1) and compound emitter follower (Q1 & Q2). With this arrangement, the motor’s DC voltage can be varied from 0V to about 12V. This type of schema gives good speed control and self-regulation at medium to high speeds but very poor low-speed control and slow starts. The second schema uses a switchmode technique to vary motor speed.

Circuit diagram:

Fig.1: a very simple motor speed controller based on a compound emitter follower (Q1 & Q2).

Here a quad NOR gate (IC1) acts as a 50Hz astable multivibrator that generates a rectangular output. The mark-space ratio of the rectangular waveform is fully variable from 20:1 to 1:20 via potentiometer VR1. The output from the multivibrator drives the base of Q1, which in turn drives Q2 and the motor. The motor’s mean supply voltage (integrated over a 50Hz period) is thus fully variable with VR1 but is applied in the form of high-energy "pulses" with peak values of about 12V.

Circuit diagram:

Fig.2: this slightly more complicated schema gives better low speed control and higher torque.

This type of schema gives excellent full-range speed control and gives high motor torque, even at very low speeds. Its degree of speed self-regulation is proportional to the mean value of the applied voltage. Note that for most applications, the power transistor (Q2) in both diagram will need to be mounted on an appropriate heatsink.

Source by : Streampowers

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