Control of Ac Drives

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.


Voltage control of permanent split capacitor 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 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.


Variable frequency converter block digram


Simplified single-phase cycloconverter


Single-phase cycloconverter waveforms


Solid-state speed control circuit.


Solid-state speed control voltage and current waveforms


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


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.


Simple single-phase inverter


MOSFET three-phase inverter for an induction motor


Simplified firing sequence of MOSFET three-phase inverter for an induction motor


Three-phase inverter output waveforms.


Three-phase six-step inverter circuit


Three-phase six-step inverterwaveforms


Line to neutral voltages for Y connected loads showing different combinations of winding connections achieved by the six-step inverter circuit


Three-stage ring counter


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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Control of Ac Drives