Motor Starters
Small motors can be started by connecting them to a power source using a switch or circuit breaker. Larger motors require a special switching unit called a motor starter or motor contactor. When starting, a direct-on-line (DOL) starter connects the motor directly to the power source. Reduced-voltage starters, Y-Δ switching starters, or soft starters connect the power source to the motor via a voltage-reducing circuit, after which the voltage gradually or in stages increases. Smaller motor starters are manually operated switches, while larger motors or applications requiring remote or automatic control typically use magnetic contactors. Medium-voltage power supplies (approximately several kilovolts) use circuit breakers as the switching element.
Reduced-Voltage Starters
Two or more contactors can be used to provide a lower voltage during motor startup. Using an autotransformer or series inductor, a lower voltage can be input to the terminals during motor startup, reducing starting torque and inrush current. Once the motor speed reaches a certain percentage of its rated speed, the starter automatically switches the motor terminals to the normal voltage input. Because autotransformers and series inductors only experience a heavy starting current during startup, which may only last a few seconds, their rated voltage is lower than their rated voltage for continuous use at the same current. The switching from lower voltage to normal voltage can be timed or achieved using a current sensor, switching occurs when the current begins to decrease. The Korndörfer autotransformer starter was patented in 1908.
Adjustable-speed drive (ASD)
A variable-speed drive (VSD) is a set of devices that can adjust the operating speed of a mechanical load. An ASD includes a motor, a speed controller or power converter, plus auxiliary equipment or instruments. However, generally, the term "drive" is used to refer only to the controller. Many newer ASDs can also perform soft starts on motors.
Intelligent Controller
An intelligent controller uses a microprocessor to control the power components used in motor control. It can also monitor the motor load and match the motor torque with the load. This is done by reducing the AC voltage supplied to the motor, while simultaneously reducing current and ineffective power. Therefore, if the motor operates under light load conditions for a long time, the above method can measure the degree of improvement in energy efficiency. At this time, the generated heat, noise and vibration are also reduced.
