A DC starter is typically not used for speed control of a DC motor. DC starters are devices designed primarily for starting and stopping DC motors by controlling the voltage applied to them. They are not designed to provide precise speed control over a wide range of speeds, which is necessary for many applications requiring variable speed operation. For speed control of a DC motor, specialized control methods such as armature voltage control, field flux control, or pulse-width modulation (PWM) techniques are typically employed. These methods allow for precise adjustment of the motor speed by varying either the armature voltage or the field current, thereby controlling the torque-speed characteristics of the motor.
Speed control of a DC motor can be achieved using various methods depending on the motor type and application requirements. One common method is armature voltage control, where the voltage applied to the motor’s armature terminals is varied using a controlled semiconductor device such as a chopper or an adjustable DC power supply. By increasing or decreasing the armature voltage, the speed of the motor can be adjusted since speed is directly proportional to armature voltage (under constant torque conditions). Another method involves field flux control, where the magnetic field strength in the motor’s field winding is adjusted, thereby affecting the motor’s speed-torque characteristics. Modern methods like pulse-width modulation (PWM) provide efficient speed control by rapidly switching the supply voltage on and off with varying duty cycles, effectively controlling the average voltage applied to the motor.
Starters are typically not used for speed control of DC shunt motors because DC shunt motors inherently have relatively constant speed characteristics under varying loads due to their design. Shunt motors maintain a near-constant speed by adjusting their armature current with changes in load torque, thanks to the relatively weak coupling between the armature and field windings. Therefore, speed control of DC shunt motors is often achieved by adjusting the armature voltage using methods like armature voltage control or by field weakening techniques, rather than using starters which are primarily designed for on/off operation.
For controlling the speed of a DC motor on the field side (field control), a rheostatic starter is commonly used. This starter includes resistors that are connected in series with the field winding of the DC motor. By varying the resistance in the field circuit, the field current and hence the magnetic field strength can be adjusted, allowing for precise control of the motor’s speed. This method is effective for applications requiring smooth speed control and is commonly used in industrial settings where DC motors need to operate at different speeds depending on the application requirements.
The starter in a DC motor is typically used for starting and stopping the motor safely and reliably. It controls the voltage applied to the motor during starting to limit the inrush current and prevent damage to the motor windings and other components. Starters also provide protection against overcurrent conditions and can include features such as overload relays to disconnect the motor in case of excessive load or fault conditions. In essence, the starter ensures controlled operation of the DC motor during startup and shutdown phases, enhancing motor longevity and operational safety.