Ac drive

AC drive, importance of AC drive, different methods of control speed of induction motor and synchonous motor, variable frequency converter, speed control by varying frequency and voltage of stator.

1. AC DRIVE
SUBMITTED TO
Dr.Shimi S.L.
(Assistant Professor)
SUBMITTED BY
Sharvendra Omre
RollNo. 212504

2. Content
•Induction
•Induction Motor Drive and its Classification
•Pump Application with adjustable flow
rate.
•Per phase representation of IM
•Torque speed characteristic of IM
•PWM VSI
2

3. 3
AC Drive
An AC Drive is a device used to control the speed of an
electrical Motor in Order to:
•Enhance Process Control
•Reduce energy usage and generate energy efficiently.
•Decrease Mechanical Stress on Motor Control
Application.
•Optimize the operation of various application relaying
on electric motors.

4. 4
(Source: EPRI Adjustable Speed Drives Application Guide)
Three-Phase Induction Motors
•Reliable
•Rugged
•Long lived
•Low maintenance
•Efficient

5. 5
Induction Motor Drive Classification based on Application
•Adjustable –Speed Drive –One important
application of these devices is in process control
by controlling the speed of fans, compressors,
Pumps,blowersetc
•Servo Drives-By Means of sophisticated
control, induction motors can be used as servo
drives in Computer peripherals, machine tools,
and robotics.

6. 14-6
Pump Application: Adjustable Flow rate
•Fixed versus adjustable speed drive

7. 14-7
Per-Phase Representation
(assuming sinusoidal steady state)
Because of the shunt inductance term, we must reduce the
applied voltage magnitude in proportion to applied frequency
to avoid serious saturation of the iron near the air gap
This is what is called “Constant Volts per Hertz Operation,” which is
the standard operating mode for ASDs

8. 14-8
Torque-Speed Characteristics
•The linear part of the characteristic is utilized in
adjustable speed drives

9. 14-9
Acceleration Torque at Startup
•Intersection represents the equilibrium point

10. 14-10
Torque Speed Characteristics at various
Frequencies of Applied Voltage
•The air gap flux is kept constant
For a constant
torque load

11. 14-11
Adjusting Speed of a Centrifugal Load
•The load torque is proportional to speed squared

12. 14-12
Speed Control by Varying Stator
Frequency And Voltage

13. 14-13
Disadvantage of Stator Voltage
Control
•Disadvantage of this Method is that the smooth
speed control is not possible.
•This Method is more costly and less efficient
as two different stator.

14. 14-14
Variable-Frequency converter
/Drive
•Ability to adjust the Frequency according to the
desired output speed.
•Ability to adjust the output voltage so as to maintain
a constant air gap flux in the constant-torque
region.
•Ability to supply a rated current on a continuous
basis at any frequency.

15. 14-15
PWM-VSI System
•Diode rectifier for unidirectional power flow
A three-phase DBR
A three-phase
inverter

16. 8-16
Three-Phase Inverter
•Three inverter legs; capacitor mid-point is fictitious
(called a six-pack)

17. 8-17
Three-
Phase
PWM
Waveforms

18. 8-18
Three-Phase Inverter Output
•Linear and over-modulation ranges

19. 16-19
Improving Energy Efficiency of Heat Pumps
•Used in one out of three new homes in the U.S.
How does inserting an ASD save energy in single-phase applications?
Some losses
But a three-phase motor is 95%
efficient, compared to 80%
efficiency for a single-phase motor

20. 16-20
Loss Associated with ON/OFF Cycling
•The system efficiency is improved by ~30 percent
The big efficiency gain is here
•with conventional air conditioners, the first few minutes
after start-up are very inefficient as the mechanical
system reaches steady-state
•with ASDs, the air conditioner speed is lowered with
demand, so that there are fewer start-ups each day

21. 18-21
Reducing the Input Current Distortion
Like DBR current (high distortion)

22. 18-22
Power-Factor-Correction (PFC) Circuit
The boost converter is operated to
make the DBR current look
sinusoidal on the AC side
To be sold in Europe, this is a
necessary feature in high-current
single-phase power electronic loads
It also permits more power to be
drawn from conventional wall
outlets because the harmonic
currents are minimal

23. 18-23
Power-Factor-Correction (PFC) Circuit
•Operation during each half-cycle
The boost converter is instructed to
“close” when the current is below the
sinewave envelope, and
“open” with the current is above the
sinewave envelope
close
open

24. 18-24
Power Electronics Has Made Wind Farms
Possible
The choices used to be
•Use an efficient induction generator, which has very poor
power factor, or
•Use a synchronous generator, but constantly fight to
synchronize the turbine speed with the grid.
Now,
•Either use a DC bus and inverter to decouple the generator and
grid AC busses, or
•Use a doubly-fed induction motor, operate the wind turbine at
the max power speed, and use power electronics to “trick” the
wind generator into producing grid-frequency output. This is
what you see in West Texas.

25. 18-25
TYPE OF SYNCHRONOUS MOTOR
DRIVE
Based on Nature of voltage induced in the stator
classified as
Sinusoidallyexcited Synchronous Motor
•Stator has distributed winding.
•Stator induced voltage has sinusoidal waveform.
Trapezoidallyexcited Synchronous Motor
•Stator has Concentrated Winding.
•Stator induced voltage has trapezoidal waveform.

26. 18-26
Speed Control by Sinusoidal waveform

27. 18-27
Speed Control by Trapezoidal Waveform
•According to the torque equation torque is
proportional to Id.
•Regenerative braking mode operation is
obtained by reversing phase currents.
•This will also reverse the source current
Id.
•Now power flow from the Machine to
inverter and from inverter to dc source.

28. 18-28
References
•Power Electronics Converter,
Applications,and Design by Ned Mohan
•Electrical Acedimia
•Power Electronics Devices, Circuit, and
Industrial Applications. By V.R MOORTHI

29. 18-29
THANK
YOU