P5 Unit 5 Research Paper

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Task 3 (P5.3)
When speed control is used within induction motors then the overall efficiency of the motor increases and the power factor is also lower. The speed of an induction motor can be changed from the stator side or the rotor side of the motor.
Stator side speed changes include:
- Frequency control
- Changing amount of stator poles
- Controlling amount of supply voltage
- Adding rheostat in the stator circuit
Frequency control
This is where the frequency of the synchronous speed is changed. This will result decreases in frequency and increases in flux, which will also cause saturation of the rotor and stator cores. Due to this increases in no load current on the motor will occur. In this method both frequency and voltage have to be changed at same time in order to create no difference on the flux. This is due to them cancelling out the difference in flux.
Decrease frequency = increase flux
Decrease voltage = decrease flux
This shows how decreasing both will not alter the value of flux. Also in this method you must have a variable supply voltage and frequency therefore if need be a converter / inverter set can be used to allow change of voltage and frequency.
Controlling supply voltage
When controlling the supply voltage it is important to realise that decreasing the voltage will also result in a decrease on the torque. When the same load is …show more content…
The injected EMF can work by either helping aid the rotor induced EMF or by opposing it. If the EMF is injected to help aid the rotor it will decrease the resistance on the rotor therefore meaning speed is increase. The opposing EMF works by increasing the resistance therefore leading to a speed decrease in the rotor. This type of speed control is more useful than just adding an external resistance as this method allows for speed control in decreasing and increasing the

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