What are the methods for controlling the speed of a DC motor?

This article will primarily focus on DC motors, aiming to provide readers with an understanding of their characteristics and related information.

I. Three Methods for DC Motor Speed ​​Control

The three methods for controlling the speed of a DC motor are:

1. Electrode Switching Speed ​​Control Method: By switching the electrodes, the armature winding circuit is changed, altering the number of pole pairs of the motor, thereby changing the motor speed. The advantages of this method are its simple structure, high reliability, and low cost, but the speed control range is relatively small, generally only suitable for applications where high-precision speed control is not required.

2. Voltage Regulation Speed ​​Control Method: By changing the motor's supply voltage, the motor speed is adjusted. The advantages of this method are a wide speed control range and high adjustment accuracy, but it requires a dedicated voltage regulator, resulting in a relatively higher cost.

3. PWM Speed ​​Control Method: By changing the duty cycle of the motor, the motor speed is controlled. The input DC voltage is converted into a pulse signal, and the average voltage value of the motor is controlled by controlling the duty cycle of the pulse, thereby achieving motor speed regulation. This method offers a wide speed control range and high accuracy, but requires a dedicated PWM speed controller, resulting in a relatively higher cost. Simultaneously, PWM speed control generates high-frequency noise and electromagnetic interference, requiring appropriate measures to suppress them. The basic principle of Pulse Width Modulation (PWM): The control method involves controlling the on-off switching of the inverter circuit's switching devices to obtain a series of pulses with equal amplitude at the output. These pulses are used to replace the sine wave or the required waveform. That is, multiple pulses are generated in half a cycle of the output waveform, so that the equivalent voltage of each pulse is a sine wave, resulting in a smooth output with fewer low-order harmonics.

By modulating the width of each pulse according to certain rules, the magnitude of the inverter circuit's output voltage can be changed, and the output frequency can also be changed.

For example, if a sinusoidal half-wave waveform is divided into N equal parts, the sinusoidal half-wave can be considered as a waveform composed of N interconnected pulses. These pulses have equal width, all equal to π/n, but unequal amplitude, and the top of each pulse is not a horizontal straight line but a curve, with the amplitude of each pulse changing according to a sinusoidal law. If the above pulse sequence is replaced with an equal number of rectangular pulses of equal amplitude but unequal width, such that the midpoint of each rectangular pulse coincides with the midpoint of the corresponding sinusoidal segment, and the area (i.e., impulse) of each rectangular pulse is equal to that of the corresponding sinusoidal segment, a pulse sequence is obtained, which is the PWM waveform. It can be seen that the width of each pulse varies according to a sinusoidal pattern.

Based on the principle of equal impulse resulting in equal effect, the PWM waveform and the sinusoidal half-wave are equivalent. The PWM waveform for the negative half-cycle of the sine wave can be obtained using the same method. In the PWM waveform, the amplitude of each pulse is equal. To change the amplitude of the equivalent output sine wave, simply change the width of each pulse by the same scaling factor. Therefore, in AC-DC-AC converters, the amplitude of the pulse voltage output by the PWM inverter circuit is the amplitude of the DC side voltage.

II. Maintenance of DC Motor Commutators

(1) The commutator surface should be kept smooth and have a uniform, dark brown, glossy oxide film. If the commutator surface is contaminated with carbon powder or oil, it should be cleaned with a blower or wiped with a soft cloth dampened with alcohol to ensure cleanliness.

(2) If the commutator surface shows signs of deterioration, such as excessive sparking, roughness, unevenness, or burning, the motor should be stopped. The surface should be polished with "0" grade fine sandpaper to re-establish the oxide film. If the commutator surface is excessively rough, uneven, or has significant wear, the commutator should be re-machined. During machining, the armature winding ends and connecting tabs should be covered with paper to prevent metal shavings from entering. The cutting speed should be 2 meters per second, and the cutting depth and feed rate should not exceed 0.1 mm. After machining, the commutator segments should be chamfered, and if necessary, the mica between the segments should be undercut to prevent the mica from protruding above the commutator segments.

(3) Check that the mica grooves are clean, and the edges of the commutator segments should be smooth and free of burrs.

(4) While ensuring the quality of the commutator surface, it is also necessary to carefully observe and monitor commutation sparks during daily operation. Normally, pinpoint or granular sparks are sparsely and evenly distributed across most of the brushes, which is considered normal commutation sparking. However, crackling, fireballs, or splashing sparks are considered harmful. When ring-shaped sparks occur, the motor should not continue to operate.