1. DC Motor Starting: The process of a DC motor starting involves it rotating from the power supply until it reaches a stable speed. This is known as the motor's starting phase. There are three main methods for starting a DC motor: full-voltage start, varistor start, and step-down start.
a. Full-Voltage Start: In this method, the motor is directly connected to its rated voltage. At startup, since the armature is stationary, the back EMF is zero, causing a large initial current. This results in a high starting torque and a quick start. However, once the motor begins to rotate, the back EMF increases, reducing the current and torque. When the torque balances the load, the motor enters a stable state. This method is simple but causes high inrush current, making it suitable only for small motors like those in household appliances.
b. Varistor Start: A resistor is added in series with the armature circuit during start-up to limit the current. Once the motor reaches its rated speed, the resistor is removed. While this reduces the starting current, it also introduces energy losses and adds complexity to the system.
c. Step-Down Start: This method uses a variable voltage power supply to reduce the initial voltage, limiting the starting current. It’s typically used for high-power motors due to the need for specialized equipment.
2. DC Motor Speed Control: Adjusting the motor's speed while maintaining a constant load is called speed control. The speed depends on the voltage, armature resistance, and magnetic flux. Three common methods include:
a. Armature Resistance Control: Adding a variable resistor in series with the armature increases resistance, which lowers the speed. However, this method reduces efficiency and makes the motor less responsive to load changes.
b. Field Resistance Control: By adjusting the resistance in the field winding, the magnetic flux can be altered, allowing for speed control. This method is effective for increasing speed but cannot increase beyond the rated value.
c. Terminal Voltage Control: Changing the voltage supplied to the motor adjusts its speed. However, this method can only decrease speed and not increase it beyond the rated level.
3. DC Motor Reversal: To reverse the direction of a DC motor, the current in the armature or the magnetic field must be reversed. For permanent magnet motors, reversing the current is the only option. A double-pole switch can be used to change the direction of current flow through the armature, thereby changing the motor’s rotation direction.
The control circuit includes adjustable resistors for field strength and speed regulation, along with a switch that reverses the current direction, enabling bidirectional operation. This setup is commonly used in devices such as recorders and other applications requiring both forward and reverse motion.
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