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Three-Phase AC machines

Three-Phase AC machines. Resource 4. Three-Phase Cage Rotor Induction Motor – Electronic Methods of Starting and Speed Control. Three-phase Cage Rotor Induction Motor – Electronic Methods of Starting and Speed Control. Three-Phase AC Machines Resource 4. Aim.

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Three-Phase AC machines

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  1. Three-Phase AC machines Resource 4 Three-Phase Cage Rotor Induction Motor – Electronic Methods of Starting and Speed Control

  2. Three-phase Cage Rotor Induction Motor – Electronic Methods of Starting and Speed Control Three-Phase AC Machines Resource 4 Aim • To understand the operation of two electronic methods of starting and speed control of an induction namely the Soft-Start and the Inverter

  3. Three-phase Cage Rotor Induction Motor – Electronic Methods of Starting and Speed Control Three-Phase AC Machines Resource 4 Objectives • To be able to describe the effects of voltage-ramping on the torque speed curve of an induction motor • To be able to describe how a soft-start ramps up the RMS stator supply voltage • To be able to describe effects of frequency-ramping on the torque speed curve of an induction motor • To be able to show how an inverter ramps up the frequency of the stator supply

  4. Voltage Ramping with a Soft-Start Voltage against time graph Torque against speed graph Torque speed curve rises with voltage RMS voltage rises from V0 up to full voltage in time tS seconds Not a good method of speed control

  5. Voltage Ramping with a Soft-Start Thyristors TH1 and TH2 connected in anti-parallel A = anode, K = cathode L1 – supply terminal T1 – motor terminal Gate terminal used to switch thyristor into conduction mode. Thyristor switches off when current reaches zero RMS voltage control by firing delay of gate pulse TH1 controls AC wave on positive half cycle, TH2 controls AC wave on negative half cycle

  6. Voltage Ramping with a Soft-Start Normal uncontrolled AC wave has RMS value as shown (=70.7% of peak voltage)

  7. Voltage Ramping with a Soft-Start

  8. Voltage Ramping with a Soft-Start

  9. Voltage Ramping with a Soft-Start

  10. Voltage Ramping with a Soft-Start Full voltage supply to motor when there is no delay

  11. Voltage Ramping with a Soft-Start Supply terminals L1, L2 and L3 connected via isolator, fuses, main contactor and overload unit Motor terminals T1, T2 and T3 connected to motor terminals U1, V1 and W1

  12. Voltage Ramping with a Soft-Start Commercial Soft-Start

  13. Frequency Ramping with an Inverter Frequency against time graph Torque against speed graph Frequency rises from 0 up to 50Hz in time tS seconds Torque speed curve rises with frequency

  14. Frequency Ramping with an Inverter Frequency rises from 0 up to 50Hz

  15. Frequency Ramping with an Inverter Inverter Block Diagram Diodes rectify AC into DC DC link feeds into inverter stage Inverter stage chops up DC into 3 phase, variable frequency output to motor

  16. Frequency Ramping with an Inverter Average voltage is near sinusoidal Inverter Output waveform PWM wave PWM = Pulse Width Modulation Average voltage is sinusoidal Filter is needed to remove harmonics due to chopping

  17. Frequency Ramping with an Inverter Commercial Inverter Adjustments made on a front panel menu include:- Maximum frequency Ramp up time Method of stopping Motor can be started and stopped locally from the front panel or remotely from external signals Speed can be adjusted using front panel or remotely from external signals

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