Working principle and circuit of permanent magnet commutator motor

Permanent magnet commutator DC motor is a widely used one. Just put the appropriate voltage on it. The motor turns. Figure 9 is the symbol and simplified equivalent circuit of this motor. Working Principle: 　　This kind of motor consists of a stator, a rotor, a commutator (also called a commutator), brushes, etc. The stator is used to generate a magnetic field. Under the action of the stator's magnetic field, the rotor obtains torque and rotates. The commutator changes the direction of the current in time so that the rotor can continue to rotate. That is to say, DC voltage is applied to the brushes and applied to the rotor coil through the commutator. Current flows through and generates a magnetic field. This magnetic field interacts with the fixed magnetic field of the stator, and the rotor is forced to rotate. When it rotates, due to the interaction of the magnetic field, a back electromotive force will also be generated. Its magnitude is proportional to the speed of the rotor, and its direction is opposite to the applied DC voltage. Figure 9(b) shows the equivalent circuit. Rw represents the total resistance of the rotor winding, and E represents the speed-related back electromotive force. Characteristics of permanent magnet converter motor: When the motor load is fixed, the motor speed is proportional to the applied power supply voltage. ·When the DC power supply of the motor is fixed, the operating current of the motor is proportional to the size of the load transferred. ·The effective voltage applied to the motor is equal to the applied DC voltage minus the back electromotive force. Therefore, when driving a motor with a fixed voltage, the speed of the motor tends to be self-stable. Because when the load increases, the rotor tends to slow down, so the back electromotive force decreases and the effective voltage increases, which in turn makes the rotor tend to speed up, so the overall effect is to stabilize the speed. ·When the rotor is stationary, the back electromotive force is zero and the motor current is maximum. Its maximum value is equal to V/Rw (where V is the power supply voltage). The maximum current occurs at just starting conditions. ·The direction of rotor rotation can be controlled by the polarity of the voltage applied to the motor. ·Small size and light weight. The starting torque is large. Due to the above characteristics, it has been widely used in various aspects such as medical equipment, small machine tools, electronic instruments, computers, meteorological sondes, mining logging, power tools, household appliances and electronic toys. The control of this permanent magnet motor mainly includes motor start and stop control, direction control, variable speed control and speed stability control. 1. Motor start/stop control. The simplest and most primitive method for motor start/stop control is to add a mechanical switch between the motor and the power supply. Or use relay contact control. Everyone is familiar with it, so I won’t give an example. The more popular method now is to use switching transistors instead of mechanical switches, which have no contacts, no spark interference, and are fast. The circuit is shown in Figure 10(a). When the input terminal is low level, the switching transistor Q1 is turned off, and the motor has no current and is in a stopped state. If the input terminal is high level, Q1 is saturated and turned on, and there is current in the motor, so the motor starts and runs. Diodes D1 and D2 in the figure are protective diodes to prevent back electromotive force from damaging the transistor. Capacitor C1 is added to eliminate radio frequency interference. R1 base current limiting resistor limits the base current of Q1. At 6V power supply, the base current does not exceed 52mA. In this case, the maximum current supplied by Q1 to the motor is around 1A. 　　The circuit in Figure 10(a) requires an external drive circuit due to the base current. If another level of buffer amplification is added, such as the circuit in Figure 10(b), the driving current is reduced to 2mA. R3 limits Q1's base current to a safe value. The functions of other components are the same as in (a). 2. The direction of the motor controls the rotation direction of the hydromagnetic inverter motor. You can reverse the motor by changing the polarity of the power supply. If a positive and negative bipolar power supply is used, a single pole can be used for conversion, as shown in Figure 11(a). Because the current of the motor passes directly through the switch, it is easy to burn out the switch contacts. Therefore, power switching transistors can be used instead of mechanical switches to overcome the above shortcomings. The circuit is shown in Figure 11(b). 　　Circuit working principle: When switch SW1 is placed in the "forward" position, bias current is added to the bases of Q1 and Q3; the bias circuits of Q2 and Q4 are disconnected. So Q1 and Q3 are on, Q2 and Q4 are off. The current forms a loop from V+→Q3 emitter→Q3 collector→motor positive terminal→motor negative terminal→ground, and the motor rotates forward at this time. In the same way, if SW1 is placed in the "reverse" position, Q2 and Q4 receive bias current and are turned on; 01 and Q3 are cut off. The current flows from the ground terminal of the power supply → the negative terminal of the motor → the positive terminal of the motor → Q4 collector → Q4 emitter → The negative terminal of the power supply forms a loop, so the motor power supply is opposite to the above situation, so when SW1 is turned off, the motor stops rotating. In the circuit of Figure 11(b), SW1 is connected to the positive and negative power supplies. In application, it is difficult to replace SW1 with an electronic switch. In order to overcome this shortcoming, the circuit in Figure 11(c) can be improved. SW1 in Figure 11(c) can be easily replaced with an electronic switch in this circuit. , When SW1 is placed in the "forward" position, Q1 and Q3 are turned on, and Q2 and Q4 are turned off. When SW1 is placed in the "reverse" position, Q2 and Q4 are turned on, and Q1 and Q3 are turned off. 3. Unipolar power supply. Direction control If the power supply is unipolar, the switch to control the direction must be double-pole and three-throw, as shown in Figure 12(a). However, the most basic and common form of the circuit is to use a transistor connection. As shown in Figure 12(b). 　　It can be seen from the circuit that when SW1 is placed in the "forward" position, Q1 and Q4 are turned on, and Q2 and Q3 are turned off when SW1 is placed in the "reverse" position. Q3 is turned on, and Q1 and Q4 are turned off. Diodes D1-D4 are protection circuits to prevent the back electromotive force of the motor from damaging the transistors.