control of ac drives
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Lecture 11
Control of ac motors
Brushless dc motors
Control of ac drivesBraking ac motorsSpeed control of ac motorsStatic frequency converters for ac motor control
Brushless MotorsHall effect sensors
Reference: Electrical machines and power systems: Nasar: McGraw-Hill
Control of ac drives
Braking ac motors
Speed control of ac motors
N = 120f/p
This formula shows that the supply frequency and the number of poles are the factors that determine the speed of the motor. Unlike in the dc motor, the speed of an ac motor is not changed by varying the applied voltage.
Reducing the applied voltage of a large motor in order to reduce its speed could damage the motor. This is due to the excess heat build-up inside the motor. Most ac motors are not designed to have their applied voltage vary more than 10 percent of the nameplate ratings.
Speed control of ac motors
Voltage control of permanent split capacitor motors
Speed control of ac motors
The polyphase (three-phase) motor utilises two separate and independent windings for each pole. With this arrangement, any desired combination of two, three, or four different speeds is possible
Speed control of ac motors
Speed control of ac squirrel cage induction motors can be accomplished if the frequency of the applied voltage to the stator is varied to change the synchronous speed. The change in the synchronous speed (stator rotating field frequency) results in a change in the motor speed. There are two basic methods used to vary the frequency of the applied voltage to the ac motor: one uses an inverter, the other a converter.
Speed control of ac motors
Variable frequency converter block digram
Speed control of ac motors
Simplified single-phase cycloconverter
Speed control of ac motors
Single-phase cycloconverter waveforms
Speed control of ac motors
Solid-state speed control circuit.
Speed control of ac motors
Solid-state speed control voltage and current waveforms
Speed control of ac motors
Motor speed control by thermistor (temperature)
Static frequency converters for ac motor control
There are several methods for controlling the speed of induction motors:
1. Variable-voltage constant-frequency or stator voltage control 2. Variable-voltage variable-frequency control 3. Variable-current variable-frequency control 4. Regulation of the amount of slip
Static frequency converters for ac motor control
The term inverter normally refers to equipment used for transforming direct to alternating current. A cycloconverter is used for transforming a higher-frequency alternating current to a lower-frequency without any intermediate dc link.
In most variable-frequency drives, a constant voltage per hertz is maintained up to the rated frequency of the motor, and then the stator voltage is maintained at its rated value as the frequency is increased. Failure to maintain a constant volts/hertz ratio affects the torque output and can cause an increase in stator current and may overheat the motor.
Static frequency converters for ac motor control
Simple single-phase inverter
Static frequency converters for ac motor control
MOSFET three-phase inverter for an induction motor
Static frequency converters for ac motor control
Simplified firing sequence of MOSFET three-phase inverter for an induction motor
Static frequency converters for ac motor control
Three-phase inverter output waveforms.
Static frequency converters for ac motor control
Three-phase six-step inverter circuit
Static frequency converters for ac motor control
Three-phase six-step inverterwaveforms
Static frequency converters for ac motor control
Line to neutral voltages for Y connected loads showing different combinations of winding connections achieved by the six-step inverter circuit
Static frequency converters for ac motor control
Three-stage ring counter
Static frequency converters for ac motor control
Waveforms from ring counter
Brushless dc motors
Schematic diagram of a dc motor
Brushless dc motors
Communtator wiring
Brushless dc motors
Brushless dc motors
a) A brushless dc motor. b) Static torque curve with no switching (one stator energised). c) Switching sequence for maximum average torque.
Hall effect sensors
VH = (R x I x B) / D
where VH = Hall voltage, V
B = Flux density, gauss (G) I = Current, A D = Element thickness, m R = Hall constant
Hall effect sensors
Simplified block diagram of brushless dc motor showing position of Hall effect sensors
Optical position sensors
Optical rotor position sensing
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