DC motor speed regulation principle and characteristics

DC motor speed regulation principle

DC motor speed regulation refers to artificially changing the motor speed according to needs under certain load conditions. DC motor speed regulation performance is good.

The so-called "speed regulation performance" refers to the ability to artificially change the motor's speed under certain load conditions according to needs. DC motors can achieve uniform, smooth, stepless speed regulation under heavy load conditions, and the speed regulation range is relatively wide.

The key to making the armature subject to an electromagnetic torque with a constant direction is: when the coil side is under magnetic poles of different polarities, how to change the direction of the current flowing through the coil in time, that is, to perform the so-called "commutation". For this purpose, a device called a commutator must be added. The commutator cooperates with the brush to ensure that the current in the coil side under each pole is always in the same direction, so that the motor can rotate continuously.

DC motor speed regulation characteristics

1. Good speed regulation performance. The so-called "speed regulation performance" refers to the ability to artificially change the speed of the motor according to needs under certain load conditions. DC motors can achieve uniform, smooth and stepless speed regulation under heavy load conditions, and the speed regulation range is relatively wide.

(ii) Large starting torque. It can achieve uniform and economical speed regulation. Therefore, all machines that start under heavy load or require uniform speed regulation, such as large reversible rolling mills, winches, electric locomotives, trams, etc., are driven by DC motors.

Small power DC motor speed control circuit

The low-power DC motor speed control circuit introduced in this article has the characteristics of novel circuit, convenient debugging, wide speed control range and low slip rate. It is suitable for various production machinery and equipment requiring high speed control accuracy. The circuit is shown in Figure 1.

Figure 1: A novel low-power DC motor speed control circuit

How the Circuit Works

The main circuit directly inputs 220V AC, which is supplied to the motor armature after thyristor half-controlled bridge rectification. The rectified voltage is relatively high, suitable for DC motors with rated voltage of 150 to 180V and power less than 2kw. Here, thyristor T1 and diode v3 as well as T2 and v4 use power semiconductor half-arm bridge modules.

The excitation winding L2 of the DC motor is powered by a rectifier bridge composed of diodes V1, V2 and V3, V4 of the armature circuit, saving two diodes. To ensure that the armature current is continuous and smooth when the thyristor is turned off, a freewheeling diode V5 and a smoothing reactor L1 are connected to the main circuit.

(1) Acquisition of synchronous voltage and pulse trigger circuit: Generally, single-phase half-controlled bridge rectifier circuits all use single-junction transistor triggering, and the synchronous signal uses the clipped voltage after voltage stabilization by the voltage regulator diode. The disadvantage is that the phase shift range of the trigger pulse is reduced. This circuit uses the zero-crossing moment of the power supply as the synchronous signal, and the synchronization range can reach more than 170°. The single-phase pulsating DC voltage output by the rectifier bridge of diodes V1, V2, V3, and V4 is divided by resistors R12 and R13, and ZD2 is used to obtain a 22V DC voltage as the working power supply for the trigger circuit, etc. The other path is divided by R21 and R22 and sent to the base of transistor BG6. Whenever the power supply passes zero, BG6 is cut off, while BG4 and BG5 are saturated and turned on. BG4 shorts capacitor C12 to ensure that charging starts from zero in any half cycle of the power supply. The low-level signal output by the collector of BG3 resets IC1, and its (3) pin outputs a low level to ensure that the trigger signal of the thyristor is based on the zero point of the power supply in any half cycle of the power supply. The collector current of transistor BG3 charges C12. When BG1 is turned on, C12 discharges to saturate and turn on transistor BG2. The low level output by the collector sets IC1, and its (3) pin outputs a high level. The trigger pulse is sent to the gate of the thyristor through the electric coupler. Here, the role of the time base circuit SE555 is to increase the pulse width, and the pulse width value is determined by the charging time constant of R6 and C7. The role of the photoelectric coupler is to increase the pulse amplitude to ensure that the thyristor is reliably triggered and turned on.

(2) Setting and feedback control: In order to improve the mechanical hardness of the motor and reduce the slip rate, the circuit adds voltage negative feedback and current positive feedback control. The setting potentiometer RP3 and the voltage sampling potentiometer RP1 and the current sampling potentiometer RP2 jointly control the conduction degree of the transistor BG3, thereby controlling the collector current of BG3. When the load changes, the phase of the trigger pulse changes accordingly, and the motor speed can be kept basically constant.