EE462L, Fall 2011 Motor Drives and Other Applications

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24 Νοε 2013 (πριν από 3 χρόνια και 8 μήνες)

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1

EE462L, Fall 2011

Motor Drives and Other Applications

2

(Source: EPRI Adjustable Speed Drives Application Guide)

Three
-
Phase Induction Motors



Reliable



Rugged



Long lived



Low maintenance



Efficient

3

At no load, the motor spins at grid frequency, divided by the
number of pole pairs. Usually this is 3600 / 2 = 1800RPM

Slip frequency (about 5% of no load speed), so induction
motors are almost constant speed devices

4

High slip corresponds to
low efficiency

5

It’s much more efficient to reduce
operating speed by lowering the
frequency of the supply voltage.

But how?

6

Adjustable
-
Speed Motor Drives (ASDs)

(Source: EPRI Adjustable Speed Drives Application Guide)

7

Some Prices for Small 3
-
Phase, 460V Induction Motors
and ASDs

$50
-

$75 per kW

$150
-

$200 per kW

Power

Motor

ASD

10kW

$7
50

$2
,
000

100kW

$5
,
000

$15,000


For Comparison, Conventional Generation: $500
-

$1,000 per kW

Solar: $4,000
-

$6,000 per kW (but the fuel is free forever!)

14
-
8

Chapter 14 Induction

Motor Drives

Pump Application: Adjustable Flow rate



Fixed versus adjustable speed drive

Bad news


inefficient!

Equivalent to reducing the
output voltage of a DBR with a
series resistor

Payback in energy
savings is about 1 year

Source: Ned Mohan’s power
electronics book

14
-
9

Chapter 14 Induction

Motor Drives

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

Source: Ned Mohan’s power
electronics book

14
-
10

Chapter 14 Induction

Motor Drives

Torque
-
Speed Characteristics



The linear part of the characteristic is utilized in
adjustable speed drives

Source: Ned Mohan’s power
electronics book

14
-
11

Chapter 14 Induction

Motor Drives

Acceleration Torque at Startup



Intersection represents the equilibrium point

Source: Ned Mohan’s power
electronics book

14
-
12

Chapter 14 Induction

Motor Drives

Torque Speed Characteristics at various
Frequencies of Applied Voltage



The air gap flux is kept constant

For a constant
torque load

Source: Ned Mohan’s power
electronics book

14
-
13

Chapter 14 Induction

Motor Drives

Adjusting Speed of a Centrifugal Load



The load torque is proportional to speed squared

Source: Ned Mohan’s power
electronics book

14
-
14

Chapter 14 Induction

Motor Drives

Frequency at Startup



The torque is limited to limit current draw

Zero speed

An important property of ASDs is the
ability to “soft start” a motor by reducing
the applied frequency to a few Hz

Source: Ned Mohan’s power
electronics book

14
-
15

Chapter 14 Induction

Motor Drives

PWM
-
VSI System



Diode rectifier for unidirectional power flow

A three
-
phase DBR

A three
-
phase
inverter

Source: Ned Mohan’s power
electronics book

8
-
16

Chapter 8 Switch
-
Mode DC
-

Sinusoidal AC Inverters

Three
-
Phase Inverter



Three inverter legs; capacitor mid
-
point is fictitious

(called a six
-
pack)

Source: Ned Mohan’s power
electronics book

8
-
17

Chapter 8 Switch
-
Mode DC
-

Sinusoidal AC Inverters

Three
-
Phase
PWM
Waveforms

Source: Ned Mohan’s power
electronics book

8
-
18

Chapter 8 Switch
-
Mode DC
-

Sinusoidal AC Inverters

Three
-
Phase Inverter Harmonics

Source: Ned Mohan’s power
electronics book

8
-
19

Chapter 8 Switch
-
Mode DC
-

Sinusoidal AC Inverters

Three
-
Phase Inverter Output



Linear and over
-
modulation ranges

Source: Ned Mohan’s power
electronics book

16
-
20

Chapter 16 Residential and

Industrial Applications

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

Source: Ned Mohan’s power
electronics book

16
-
21

Chapter 16 Residential and

Industrial Applications

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

Source: Ned Mohan’s power
electronics book

16
-
22

Chapter 16 Residential and

Industrial Applications

Electronic Ballast for Fluorescent Lamps



Lamps operated at ~40 kHz save energy

Source: Ned Mohan’s power
electronics book

16
-
23

Chapter 16 Residential and

Industrial Applications

Induction Cooking



Pan is heated directly by circulating currents


increases efficiency

Source: Ned Mohan’s power
electronics book

16
-
24

Chapter 16 Residential and

Industrial Applications

Industrial Induction Heating

Source: Ned Mohan’s power
electronics book

17
-
25

Chapter 17 Electric

Utility Applications

HVDC Transmission



There are many such systems all over the world

Source: Ned Mohan’s power
electronics book

17
-
26

Chapter 17 Electric

Utility Applications

HVDC Poles



Each pole consists of 12
-
pulse converters

Source: Ned Mohan’s power
electronics book

17
-
27

Chapter 17 Electric

Utility Applications

HVDC Transmission: 12
-
Pulse Waveforms

Source: Ned Mohan’s power
electronics book

18
-
28

Chapter 18 Utility Interface

Reducing the Input Current Distortion

Like DBR current (high distortion)

Source: Ned Mohan’s power
electronics book

18
-
29

Chapter 18 Utility Interface

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

Source: Ned Mohan’s power
electronics book

18
-
30

Chapter 18 Utility Interface

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

Source: Ned Mohan’s power
electronics book

18
-
31

Chapter 18 Utility Interface

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.