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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
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:
Components in electric drives
• DC motors - permanent magnet – wound field
• AC motors – induction, synchronous , brushless DC
• DC – batteries, fuel cell, photovoltaic - unregulated
• AC – Single- three- phase utility, wind generator -
• 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
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
-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
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
– 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
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.
System efficiency can be increased from 15% to 27% by
introducing variable-speed drive operation in place of
For a large pump variable-speed drive, payback period
~ 3-5 years whereas operating life is ~ 20 years
DC DRIVES Vs AC DRIVES
Ac motors are less expensive as compared to dc
Ac motors require low maintenance as compared to dc
Power converters for ac drives are complex.
Power converters for ac drives generates harmonics in
the supply system & load circuit
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.