1 DC motor starting. The DC motor starts to rotate from the power supply until it reaches a certain fixed number of revolutions. This process is called the starting process of the motor. DC motors have three methods: full-voltage start-up, varistor start-up and step-down start-up.
a. Full pressure start. Full-voltage start-up is to start the motor directly to the rated voltage power supply. Since the motor is supplied with a rated power supply, and the motor starts to be powered on, the armature does not move, and the armature counteracts E. It is zero, so the current is very large at startup. The maximum current of the motor at start-up is positive because the starting current of the motor is large, so the starting torque is large, the motor starts quickly, and the starting time is short.
However, once the motor starts to operate, the armature winding has induced electromotive force, and the higher the number of revolutions, the larger the armature counter electromotive force. As the number of motor revolutions rises, the current drops rapidly and the electromagnetic torque decreases. When the electromagnetic torque of the motor is balanced with the load resistance torque, the starting process of the motor ends and enters a stable operating state.
The advantage of full-voltage startup is that it does not require other equipment and is easy to operate; the disadvantage is that the starting current is large. It is only suitable for small motors, such as DC motors in household appliances.
b. The varistor starts. The varistor start is to start a group of starting resistors RP in series with the armature circuit to limit the starting current, and when the number of revolutions rises to the rated number of revolutions, the starting varistor is removed from the armature circuit.
The advantage of the varistor starting is that the starting current is small; the disadvantage is that the varistor is cumbersome and consumes a lot of energy during the starting process.
. . Buck start. Buck start is to limit the starting current by temporarily reducing the motor supply voltage at startup. Of course, the step-down start requires a variable voltage DC power supply. This method is only suitable for high-power motors.
2 DC motor speed regulation. Changing the number of revolutions of the motor under the condition that the mechanical load of the motor is constant is called speed regulation. The method of speed regulation can be seen from the mechanical characteristic equation of the motor, that is,
L"LMp, where, when the torque M is constant (that is, the load is constant), there are the power supply voltage U, the armature resistance that affect the number of revolutions of the motor: and the main magnetic flux, as long as one of them is changed, the motor The number of revolutions can be adjusted.
a. Change the speed regulation method of the series resistance of the armature circuit. After the motor is made, its armature resistance is two. It is certain, but a variable resistor RP can be connected in series in the armature circuit to achieve speed regulation, as shown in the figure. This method increases the loss in series resistance, which reduces the efficiency of the motor and softens the characteristics. If the load is slightly changed, the number of revolutions of the motor will vary greatly, which is disadvantageous for loads requiring constant speed.
b. Change the speed regulation method of the excitation loop resistance. In order to change the flux, a speed-regulating resistor RP is connected in series in the excitation circuit. As shown in Figure 4-98, changing the resistance of the speed-regulating resistor RP can change the excitation current, which in turn changes the main flux. , to achieve speed control. This kind of speed regulation method can only reduce the magnetic flux to increase the number of revolutions, and the characteristic is that the mechanical characteristics after the speed regulation become hard.
c. Change the terminal voltage speed regulation method. Changing the terminal voltage U of the armature can also increase or decrease the number of revolutions of the DC motor accordingly. Since the voltage of the motor must not exceed the rated voltage, this method of speed regulation can only reduce the number of revolutions and cannot increase the speed.
3 DC motor reversal. The motors in the recorder and recorder must be both forward and reverse. The positive and negative rotation of the DC motor is very easy. As can be seen from the foregoing, changing the direction of the armature winding current or changing the direction of the stator magnetic field can change the steering of the motor. However, for permanent magnet DC motors, the purpose of changing the direction of the motor can only be achieved by changing the direction of the current.
The figure is the principle circuit of DC motor forward and reverse control. In the figure, RP1 and RP2 are adjustable resistors. Changing the resistance of RPl can change the current of the field winding to adjust the strength and weakness of the magnetic field. Changing the resistance of RP2 can change the speed of the motor. The double-pole double-throw switch S in the figure is a control switch for changing the direction of rotation of the motor. When the switch S is turned to "1", the current flows from the a brush and flows out from the b brush; when the switch S is turned to the +2, the current flows from the b brush and flows out from the a brush. It can be seen that changing the state of the switch S can change the current direction of the armature winding, thereby achieving the purpose of changing the steering of the motor.
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