DC Motor Speed Controller
A DC Motor Speed Controller is a device or group of devices that can coordinate in a predetermined manner the performance of an electric motor. A DC Motor Speed Controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and electrical faults.
The main objective of using a DC Motor Speed Controller instead of using a simple mechanical switch is to control the speed, start/ stop and rotation of the motor in a more accurate way. The limitation of a mechanical switch is the current limit. A big electric motor can draw up to 30Amp and above were most of the switches are unable to take it. Furthermore we are unable to control the motor speed using pulse-width modulation (PWM). The most common motor controllers in the market are using H-bridge circuit where we are able to control a large motor using a small signal.
A starter will contain protective devices for the DC Motor Speed Controller. At a minimum this would include a thermal overload relay. The thermal overload is designed to open the starting circuit and thus cut the power to the motor in the event of the motor drawing too much current from the supply for an extended time. The overload relay has a normally closed contact which opens due to heat generated by excessive current flowing through the circuit. Thermal overloads have a small heating device that increases in temperature as the motor running current increases.
There are two types of thermal overload relay. In one type, a bimetallic strip located close to a heater deflects as the heater temperature rises until it mechanically causes the device to trip and open the circuit, cutting power to the motor should it become overloaded.
A thermal overload will accommodate the brief high starting current of a motor while accurately protecting it from a running current overload. The heater coil and the action of the bi-metallic strip introduce a time delay that affords the motor time to start and settle into normal running current without the thermal overload tripping. Thermal overloads can be manually or automatically resettable depending on their application and have an adjuster that allows them to be accurately set to the motor run current.
A second type of thermal overload relay uses a eutectic alloy, like a solder, to retain a spring-loaded contact. When too much current passes through the heating element for too long a time, the alloy melts and the spring releases the contact, opening the control circuit and shutting down the motor. Since eutectic alloy elements are not adjustable, they are resistant to casual tampering but require changing the heater coil element to match the motor rated current.
Electronic digital overload relays containing a microprocessor may also be used, especially for high-value motors.DC Motor Speed Controllers model the heating of the motor windings by monitoring the motor current. They can also include metering and communication functions.
Starters using magnetic contactors usually derive the power supply for the contactor coil from the same source as the motor supply. An auxiliary contact from the contactor is used to maintain the contactor coil energized after the start command for the motor has been released. If a momentary loss of supply voltage occurs, the contactor will open and not close again until a new start command is given.
This prevents restarting of the motor after a power failure. This connection also provides a small degree of protection against low power supply voltage and loss of a phase. However, since contactor coils will hold the circuit closed with as little as 80% of normal voltage applied to the coil, this is not a primary means of protecting motors from low voltage operation.
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