Electric Motors and Power Drives Revision

What is an electric drive?

A system that uses an electric motor along with power electronic converters and control systems to control motion (speed, torque and direction) of a mechanical load

What is a DC motor?

A machine that converts DC electrical energy into mechanical energy, producing rotational motion

What is back EMF?

The voltage induced in a motor armature that opposes the applied voltage

What is a chopper?

A DC-DC converter that converts fixed DC into variable DC

What is a rectifier?

A device that converts AC into DC

What is an AC drive?

A system used to control the speed, torque and operation of an AC motor by varying the supply frequency and voltage

What is an induction motor?

An AC motor where current is induced in the rotor by the stator field

What is synchronous speed?

The speed of the rotating magnetic field in an AC machine

What is slip?

The difference between synchronous and rotor speed as a fraction of synchronous speed

What is open-loop control?

A system with no feedback from output to input

What is closed-loop control?

A system that uses feedback to correct errors

What is firing angle (α)?

The delay angle at which a thyristor is triggered in an AC cycle

What is duty cycle?

The ratio of ON time to total time in a chopper

What is V/f control?

Maintaining constant voltage-to-frequency ratio to keep flux constant

What is a step-down (buck) chopper?

A converter that gives output voltage less than input

What is a step-up (boost) chopper?

A converter that gives output voltage greater than input

What is a semi-converter?

A rectifier using both diodes and thyristors

What is a full converter?

A rectifier using only thyristors for full control

Why is V/f kept constant?

To keep flux constant and ensure stable motor operation

What happens if V/f increases?

Flux increases = magnetic core saturation and overheating

What happens if V/f decreases?

Flux decreases = reduced torque and motor becomes weak leading to poor performance

Why is slip necessary in an induction motor?

To create relative motion and produce torque

What is quadrant operation?

The ability of a chopper drive to operate in different combinations of voltage and current directions, enabling motoring and braking in forward and reverse directions

Two advantages of an AC drive?

Low maintenance (no brushes/commutator), more robust and reliable

Two disadvantages of an AC drive?

More complex control system, more expensive power electronic converters

What does a single diode in AC circuit indicate?

Half-wave rectifier = power halves

What controls voltage in a chopper?

Duty cycle (time ratio)

What controls voltage in a rectifier?

Firing angle

What controls speed in AC drives?

Frequency

What controls speed in DC motors?

Armature voltage

Difference: chopper vs rectifier

Chopper = DC-DC (duty cycle), Rectifier = AC-DC (firing angle)

Difference: squirrel cage vs slip ring

Cage = no slip rings, Slip ring = external resistance possible

Kramer vs Scherbius

Kramer = power used locally, Scherbius = power returned to supply

What must be constant in V/f control?

Flux

What does back EMF stand for?

Back electromotive force

What controls speed in a DC motor?

Applied voltage (via back EMF)

What controls torque in a DC motor?

Armature current

What happens to back EMF when speed increases?

Back EMF increases

Why does current decrease as speed increases in a DC motor?

Back EMF increases and opposes applied voltage

What controls speed in an induction motor?

Supply frequency

What controls torque in an induction motor?

Slip (and rotor current)

What happens if slip increases?

Torque increases

What happens if slip decreases?

Torque decreases

What happens when load increases in a DC motor?

Speed decreases slightly, current increases, torque increases

What happens when load increases in an induction motor?

Slip increases to produce more torque

What does a converter control in a DC drive?

Armature voltage

What does a converter control in an AC drive?

Frequency and voltage

What is a closed loop DC motor drive?

It controls the motor speed by comparing the actual speed with the reference speed and correcting the error using feedback

How does a closed loop DC motor drive work? (3 bullets)

The reference speed is compared with actual speed to produce an error signal. The controller processes this error and adjusts the converter output voltage. This continuously corrects the motor speed to match the desired value.

Two advantages of a DC drive?

Simple and precise speed control, higher starting torque

Two disadvantages of a DC drive?

High maintenance (brushes + commutator), less reliable (mechanical wear)

Explain why Volt per Hertz ratio is always maintained constant for a variable voltage variable frequency (VVVF) drive.

In a VVVF drive, both voltage and frequency are varied together to maintain a constant V/f ratio so that the magnetic flux in the motor remains constant. This ensures stable motor operation, constant torque capability and proper performance of the motor.

Name a constructional difference between a squirrel-cage and a slip-ring induction motor.

A squirrel-cage induction motor has rotor bars short-circuited by end rings and does not use slip rings or bushes, whereas a slip-ring induction motor has a wound rotor connected to external resistance through slip rings and brushes