12-04-2012, 04:45 PM
SPEED CONTROL OF INDUCTION MOTOR
[attachment=20021]
The speed of standard induction motors can be controlled by variation of the frequency of the voltage applied to the motor. Due to flux saturation problems with induction motors, the voltage applied to the motor must alter with the frequency. The running speed is set by the frequency applied to it and is independent of load torque provided the motor is not over loaded.
In the ideal motor, the magnetic circuit would be purely inductive and keeping a constant V/Hz (voltage and frequency )ratio would maintain a constant flux in the iron. The real motor has resistance in series with the magnetizing inductance. This has no bearing on the operation at line frequency, however as the frequency is reduced, the resistance begins to become significant relative to the inductive reactance.
Design
Our design comprise an AC rectifier to convert the incoming power from AC to DC. This is followed by a power DC Filter which comprises a number of high voltage high current DC capacitors commonly in a series parallel arrangement. The DC filter will commonly include one or two DC chokes in series with the rectified DC.
AC to DC Converter
The AC to DC converter is a full wave bridge rectifier, single phase or three phase depending on the input requirements. The rectifier can be controlled using a combination of SCRs and Rectifiers, or more commonly uncontrolled using rectifiers only. Because the output of the rectifier is connected to a large capacitive filter, there must be a means of providing the initial charge to the capacitors without damaging the rectifier. - The initial charging current for discharged capacitors connected to the full rectified voltage is very high and would cause rectifier failure.
DC Filter
The DC filter provides smoothing of the DC bus applied to the output DC to AC inverter. There must be sufficient capacitance to provide the smoothing required for the output current required. The capacitors must have sufficient ripple current rating to avoid excess heating and life shortening and voltage rating to withstand the maximum expected input voltages. There are two types of DC filter used, a capacitive input filter and an inductive input filter. The capacitive input filter comprises a capacitor bank and an inductive input filter has an inductor in series with at least one of the DC inputs to the capacitive filter.
DC to AC Output Inverter
The AC output inverter for a three phase output stage comprises six solid state switches. In small low voltage and low current application, the output stages will typically be MOS FETs and in larger voltage and current, they are typically IGBTs.
Requirements
SINGLE PHASE IM is used for most of the heating, ventilation, air-conditioning (HVAC) applications. Normally, it has two windings: main and auxiliary while auxiliary winding has more turns than main winding has. To do the adjustable speed control, two voltages must be supplied to the main winding and auxiliary winding with variable frequencies and variable amplitudes and they are subjected to the following constraints:
• Voltage ratio Vaux/Vmain is approximately equal to the effective turns ratio a, Naux/Nmain (»1.37 for the motor used for this project).
• Current ratio is Iaux/Imain=1/a,Vaux leads Vmain by 90º at rated frequency.
• Constant V/Hz (voltage frequency ratio) for adjustable speed control.
[attachment=20021]
The speed of standard induction motors can be controlled by variation of the frequency of the voltage applied to the motor. Due to flux saturation problems with induction motors, the voltage applied to the motor must alter with the frequency. The running speed is set by the frequency applied to it and is independent of load torque provided the motor is not over loaded.
In the ideal motor, the magnetic circuit would be purely inductive and keeping a constant V/Hz (voltage and frequency )ratio would maintain a constant flux in the iron. The real motor has resistance in series with the magnetizing inductance. This has no bearing on the operation at line frequency, however as the frequency is reduced, the resistance begins to become significant relative to the inductive reactance.
Design
Our design comprise an AC rectifier to convert the incoming power from AC to DC. This is followed by a power DC Filter which comprises a number of high voltage high current DC capacitors commonly in a series parallel arrangement. The DC filter will commonly include one or two DC chokes in series with the rectified DC.
AC to DC Converter
The AC to DC converter is a full wave bridge rectifier, single phase or three phase depending on the input requirements. The rectifier can be controlled using a combination of SCRs and Rectifiers, or more commonly uncontrolled using rectifiers only. Because the output of the rectifier is connected to a large capacitive filter, there must be a means of providing the initial charge to the capacitors without damaging the rectifier. - The initial charging current for discharged capacitors connected to the full rectified voltage is very high and would cause rectifier failure.
DC Filter
The DC filter provides smoothing of the DC bus applied to the output DC to AC inverter. There must be sufficient capacitance to provide the smoothing required for the output current required. The capacitors must have sufficient ripple current rating to avoid excess heating and life shortening and voltage rating to withstand the maximum expected input voltages. There are two types of DC filter used, a capacitive input filter and an inductive input filter. The capacitive input filter comprises a capacitor bank and an inductive input filter has an inductor in series with at least one of the DC inputs to the capacitive filter.
DC to AC Output Inverter
The AC output inverter for a three phase output stage comprises six solid state switches. In small low voltage and low current application, the output stages will typically be MOS FETs and in larger voltage and current, they are typically IGBTs.
Requirements
SINGLE PHASE IM is used for most of the heating, ventilation, air-conditioning (HVAC) applications. Normally, it has two windings: main and auxiliary while auxiliary winding has more turns than main winding has. To do the adjustable speed control, two voltages must be supplied to the main winding and auxiliary winding with variable frequencies and variable amplitudes and they are subjected to the following constraints:
• Voltage ratio Vaux/Vmain is approximately equal to the effective turns ratio a, Naux/Nmain (»1.37 for the motor used for this project).
• Current ratio is Iaux/Imain=1/a,Vaux leads Vmain by 90º at rated frequency.
• Constant V/Hz (voltage frequency ratio) for adjustable speed control.