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 electrodes, the circuit of the armature winding is changed, thereby altering the number of pole pairs in the motor and thus changing the motor speed. This method has the advantages of simple structure, high reliability, and low cost, but its speed control range is relatively small, and it is generally only suitable for applications where speed regulation requirements are not high.
2. Voltage regulation speed control method: This method adjusts the motor speed by changing the power supply voltage. The advantages of this method are a wide speed range and high adjustment accuracy, but it requires a dedicated voltage regulator, making it relatively expensive.
3. PWM Speed Control Method: This method controls the motor speed by changing the motor's duty cycle. The input DC voltage is converted into pulse signals, and the average voltage value of the motor is controlled by adjusting the duty cycle of these pulses, thus regulating the motor speed. This method offers a wide speed range and high precision, but requires a dedicated PWM speed controller, resulting in relatively high costs. Furthermore, PWM speed control generates high-frequency noise and electromagnetic interference, requiring appropriate measures to suppress them. The basic principle of Pulse Width Modulation (PWM) is to control the on/off state of the inverter circuit's switching devices, resulting in a series of pulses of equal amplitude at the output. These pulses replace the sine wave or the desired waveform. In other words, multiple pulses are generated within half a cycle of the output waveform, ensuring 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 output voltage of the inverter circuit 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, it can be viewed as a waveform composed of N interconnected pulses. These pulses have equal widths, all equal to π/n, but different amplitudes, and the tops of the pulses are not horizontal straight lines but curves, with the amplitude of each pulse varying according to a sinusoidal law.
If we replace the above pulse sequence with the same number of rectangular pulse sequences of equal amplitude but unequal width, such that the midpoint of each rectangular pulse coincides with the midpoint of the corresponding equally divided sine wave, and the area (i.e., impulse) of the rectangular pulse and the corresponding sine wave are equal, we obtain a pulse sequence, 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 that equal impulse has the same effect, PWM waveforms and half-wave sinusoids are equivalent. The same method can be used to obtain a PWM waveform for the negative half-cycle of a sine wave. In a 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 proportional coefficient. Therefore, in an AC-DC-AC inverter, 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 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 hand blower or wiped with a soft cloth dampened with alcohol to ensure cleanliness;
(2) If the commutator surface condition is found to be deteriorated, with large sparks, roughness, non-roundness, burns, or other defects, the machine 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 excessive wear, the commutator should be re-machined. During machining, the armature winding ends and connector segments should be covered with paper to prevent metal shavings from splashing in. 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. If necessary, mica should be cut between the segments to prevent the mica from protruding above the commutator segments.
(3) Check that the mica channel is clean and that the commutator edges are smooth and free of burrs;
(4) In addition to ensuring the surface quality of the commutator, it is also necessary to carefully observe and monitor the commutation sparks during daily operation. Under normal circumstances, point-like or granular sparks (white or slightly blue and yellow) are sparsely and evenly distributed on most of the brushes and are considered normal commutation sparks. However, noisy, fireball-like, or splashing sparks (dark yellow, red, or green) are harmful sparks. When ring-shaped sparks occur, the motor should not continue to operate.
