July 22nd, 2010 
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DOL reversing starter
Most motors are reversible or, in other words, they can be run clockwise and counter-clockwise. A reversing starter is an electrical or electronic circuit that reverses the direction of a motor automatically. Logically, the circuit is composed of two DOL circuits; one for clockwise operation and the other for anti-clockwise operation. The case of three phase motor inter changing of any two phases will do the same
Safety devices within the starting circuit
These devices are used to protect the motor as well as the user of the motor being operated:
1. Overload coil The overload coil, also known as a 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. The overload coil is a normally closed device 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. A bi-metallic strip located close to the 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 is basically a circuit breaker that will accommodate the brief high starting current of a motor whilst being able to accurately protect it from a running current overload. This is because the heater coil and the action of the bi-metallic strip introduces 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.
2. KM1 No-Volt coil The No-Volt coil serves the purpose of keeping the holding-in contactor closed, this is connected in parallel across the start switch (which is a normally open switch) in the control circuit. The no-volt coil is energized once the start switch is initiated and keeps current flowing through the control circuit. In the event of no power being supplied to the motor, the motor will stop. In order for the motor to be turned on again, the start switch must be initiated and therefore be the catalyst for closing the holding in contactor and hence energize the no-volt coil. Only once the no-volt coil is energized, will current remain flowing in the control circuit.
See also
Motor Soft Starter
Adjustable-speed drive
Variable-frequency drive
Thyristor drive
v d e
Electric motors
Broad Motor Categories
Synchronous motor AC motor DC motor
Conventional
Electric Motors
Induction Brushed DC Brushless DC Stepper Linear Unipolar Reluctance
Novel Electric Motors
Ball bearing Homopolar Piezoelectric Ultrasonic Electrostatic Switched Reluctance
Motor
Controllers
Adjustable-speed drive Amplidyne Direct torque control Direct on line starter Electronic speed control Metadyne Motor controller Variable-frequency drive Vector control Ward Leonard control Thyristor drive
See also
Barlow’s Wheel Nanomotor Traction motor Lynch motor Mendocino motor Repulsion motor Inchworm motor Booster (electric power) Brush (electric) Electrical generator Alternator
External links
EduMation contains a flash animation illustrating a DOL starter of a 3-phase, induction motor using a contactor.
Categories: Electric motors Electrical circuits Power electronics
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