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 rotationhttp://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