electrical ac & dc drives ppt

S.Sumalatha

G.Lakshmi prasanna

BY

Introduction: In some countries nearly 65% of the total electric

energy produced is consumed by electric motors.

DC motors play a significant role in modern industrial

drives.

DC motors can provide a high starting torque.

ELECTRIC DRIVES - A DEFINITION

About 50% of electrical energy produced is used in

electric drives today.Electric drives may run at

constant speed or at variable speed

Block diagram of Electrical drive:

• Small

• Efficient

• Flexible

Components in electric drives

Motors

• DC motors - permanent magnet – wound field

• AC motors – induction, synchronous , brushless DC

Power sources

• DC – batteries, fuel cell, photovoltaic - unregulated

• AC – Single- three- phase utility, wind generator -

unregulated

• DC drives can be classified, in general, into three types:

1. Single-phase drives

2. Three-phase drives

single-phase drives may be subdivided into:

Single-phase half-wave-converter drives.

Single-phase full-converter drives.

Single-phase dual-converter drives.

Single-Phase Half-Wave-Converter Drives:

The applications of this drive are limited to the 0.5 kW

power level.

Single-Phase Full-Wave-Converter Drives

It is a two-quadrant drive, and is limited to

applications up to 15 kW.

• Three-phase drives may also be subdivided

into:

-Three-phase half-wave-converter drives.

– Three-phase full-converter drives.

A three phase half wave converter drive operates in first quardant and is limited to applications upto 40kw.

A three-phase full-wave-converter drive is a two-

quadrant drive without any field reversal, and is

limited to applications up to 1500 kW.

Introduction: AC Motor Drives

• There are two types of ac drives:

– Induction motor drives

– Synchronous motor drives

• Ac drives are replacing dc drives and are used in

many industrial and domestic applications

• The speed and torque of induction motors can be

controlled by

– Stator voltage control

– Rotor voltage control

– Frequency control

Stator Voltage Control:

• The stator voltage can be varied by three-phase

– ac voltage controllers,

– pulse-width modulation (PWM) inverters

• They are used mainly in low-power applications, such as

fans, blowers, and centrifugal pumps, where the starting

torque is low.

Rotor Voltage Control:

• The typical torque-speed characteristics for variations

in rotor resistance

Frequency Control:

The torque and speed of induction motors can be

controlled by changing the supply frequency.

At low frequency, the reactances decrease and the

motor current may be too high. This type of frequency

control is not normally used.

Energy/Cost Savings System efficiency can be increased from 15% to 27% by

introducing variable-speed drive operation in place of constant-speed operation.

For a large pump variable-speed drive, payback period ~ 3-5 years whereas operating life is ~ 20 years

DC DRIVES Vs AC DRIVESAdvantages:

Ac motors are less expensive as compared to dc motors.

Ac motors require low maintenance as compared to dc motors.

Disadvantages:

Power converters for ac drives are complex.

Power converters for ac drives generates harmonics in the supply system & load circuit

Conclusion: In India, we are using three phase,50hz ac supply as an

input. There so many applications that require higher frequency and phases for better operations of industrial equipments. Even though we are with same frequency for particular applications keeping economy in point of view.