Transcript Document
Three-Phase AC machines Resource 4 Three-Phase Cage Rotor Induction Motor – Electronic Methods of Starting and Speed Control Three-Phase AC Machines Resource 4 Three-phase Cage Rotor Induction Motor – Electronic Methods of Starting and Speed Control Aim • To understand the operation of two electronic methods of starting and speed control of an induction namely the Soft-Start and the Inverter Three-Phase AC Machines Resource 4 Three-phase Cage Rotor Induction Motor – Electronic Methods of Starting and Speed Control 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 Voltage Ramping with a Soft-Start Voltage against time graph RMS voltage rises from V0 up to full voltage in time tS seconds Torque against speed graph Torque speed curve rises with voltage Not a good method of speed control Voltage Ramping with a Soft-Start Thyristors TH1 and TH2 connected in antiparallel 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 Voltage Ramping with a Soft-Start Normal uncontrolled AC wave has RMS value as shown (=70.7% of peak voltage) Voltage Ramping with a Soft-Start Voltage Ramping with a Soft-Start Voltage Ramping with a Soft-Start Voltage Ramping with a Soft-Start Full voltage supply to motor when there is no delay 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 Voltage Ramping with a Soft-Start Commercial Soft-Start Frequency Ramping with an Inverter Frequency against time graph Frequency rises from 0 up to 50Hz in time tS seconds Torque against speed graph Torque speed curve rises with frequency Frequency Ramping with an Inverter Frequency rises from 0 up to 50Hz 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 Frequency Ramping with an Inverter Inverter Output waveform Average voltage is near sinusoidal PWM wave PWM = Pulse Width Modulation Average voltage is sinusoidal Filter is needed to remove harmonics due to chopping 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