Preparation Ass. 2
Reduced Voltage Starters • Autotransformer • Star – Delta • VFD • Solid State • Resistive Starting
Autotransformer Starter • Coil in each phase • Motor starts with an inductive load • As the motor gains speed the coils are removed from the circuit
Star Delta Starter • LC3-D Star/ Delta Motor Starters • Star Delta motor starter for squirrel cage motors • Fitted on open frame structure • Maximum frequency: 30 starts/ hour • Starting delay: 30 seconds • Mechanical interlock included • Auxiliary contact: 1N/C
Star Delta Starter • Star formation draws less initial current • Star will have less torque • Delta – more current and more torque • Motor starts in Star and then as near synchronous speed is achieved switches to Delta • Switch over can be done through a timer, rev counter or centrifugal switch.
Resistive Starting • Resistors are added to the windings • As motor starts and gains speed the resistances are removed
VFD – Variable Frequency Device
VFD • The frequency of the three phase current is varied • The starting frequency is low and gradually increased to the desired speed • Current into the motor is controlled by varying the frequency of the three phase current
Control Gear for three phase motors • Direct Online Starter • Emergency Stop • Fuses • Starters • Overload Protection • Single Phase Protection • Speed Changing Mechanisms (VFD, Gear Box) • Braking systems • Changing of direction of rotation http://www.ckit.co.za/secure/conveyor/troughed/electric_motors/alstom/alstom---motorcontrol.htm
Effect of Excitation • Excitation is varied and refers to the DC supply to the rotor • As the result the power to the armature can be leading, lagging or unity power factor • http://www.electrical4u.com/synchronous-motor-excitation/ • http://www.ece.umn.edu/users/riaz/animations/syncmotie.html • https://en.wikipedia.org/wiki/Synchronous_motor
3 Phase Motor Explanation • https://www.youtube.com/watch?v=Vk2jDXxZIhs
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