Transcript powerquality

Papers on the
Induction Motor Transients
Common Types of Transient Phenomena in Electric Power Systems

Lighting strokes on or near transmission lines
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Energization of transmission lines (closing and reclosing operations)
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Capacitor switching.
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Interruption of small inductive currents (switching off reactors and
unloaded transformers)
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Series capacitor switching and subsynchronous resonance
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Load rejection
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Transient recovery voltage across circuit breakers
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etc.
Analysis of induction motor starting transients
Torque and Rotor Speed
4
Electromagnetic torque [pu]
3.1
2.2
1.3
0.4
Time [sec.]
-0.5 0
0.3
0.6
0.9
1.2
1.5
1
0.8
0.6
Rotor speed [pu]
0.4
0.2
0
0
0.3
0.6
0.9
Time [sec.]
1.2
1.5
Per Unit a, b, and c Phase Currents
6
6
Phase "a" current
4
4
2
2
0
Phase "b" current
0
0
0.3
0.6
0.9
1.2
-2
1.5
Time [sec.]
-2
-4
-4
-6
-6
0
6
0.3
0.6
Phase "c" current
4
2
0
-2
-4
-6
0
0.3
0.6
0.9
1.2
1.5
Time [sec.]
0.9
1.2
1.5
Time [sec]
d- and q-Axis Components of Stator Current
6
d-axis stator current [pu]
5
4
3
2
1
Time [sec.]
0
0
0.3
0.6
0.9
1.2
1.5
0
0.3
0.6
0.9
1.2
1.5
Time [sec.]
0
-1
-2
-3
-4
-5
-6
q-axis stator current [pu]
d- and q-Axis Components of Rotor Current
0
0
0.3
0.6
0.9
1.2
-1
1.5
Time [sec.]
-2
-3
-4
-5
d-axis rotor current [pu]
-6
4.5
q-axis rotor current [pu]
3.6
2.7
1.8
0.9
Time [sec.]
0
0
0.3
0.6
0.9
1.2
1.5
d- and q-Axis Components of Stator Flux
0.6
d-axis stator flux [pu]
0.5
0.4
0.3
0.2
0.1
Time [sec.]
0
-0.1 0
0.3
0.6
0.9
1.2
0.3
0.6
0.9
1.2
1.5
-0.2
-0.3
0
0
-0.2
1.5
Time [sec.]
-0.4
-0.6
-0.8
-1
q-axis stator flux
d- and q-Axis Components of Rotor Flux
0.2
0.1
Time [sec.]
0
-0.1
0
0.3
0.6
0.9
1.2
1.5
-0.2
-0.3
-0.4
-0.5
d-axis rotor flux [pu]
-0.6
0.2
0.1
Time [sec.]
0
-0.1 0
0.3
0.6
0.9
1.2
1.5
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
q-axis rotor flux [pu]
Conclusions

Line start produces higher currents, flux linkages, and dc
offsets in them.

The torque pulsations are very severe, and repeated line
starting could endanger the mechanical integrity of the
motor.

Higher stator and rotor currents also produce resistive
losses that are multiple times the design limit.
A Fast Recursive Solution for Induction Motor
Transients
IEEE Trans. on Ind. Applications, 1988
S. Ertem and Y. Baghzouz
Detail Induction Machine Model
Detail Induction Machine Model
Simulation Algorithm and Test Results
Synchronous Machine Models for Simulation of
Induction Motor Transients
IEEE Trans. on Power Systems, 1996
R. Hung, Student Member IEEE
H.W. Dommel, Fellow IEEE
The University of British Columbia
Department of Electrical Engineering
Vancouver, B.C., V6T 1Z4, Canada.
Similarity between synchronous machine and induction
motor models
Comparison…..

Since there is practically no saliency in an induction motor, the
equivalent circuit shown in Fig. 2 is valid for both d- and q- axes.

Comparing Figs. 1 and 2, it can be seen that the synchronous
machine model is almost identical with that of the induction motor
model. In fact, if the field winding of the synchronous machine
model is short-circuited, the synchronous machine model will
become the induction motor model.

The flux and current relationships for both machine types are of the
same form. For the synchronous machine, the relationship is
Flux and current relationships
For the synchronous machine,
Simulation results

The minor modifications in the synchronous machine model
to make it behave as an induction motor model have been
implemented in the EMTP version MicroTran® of the
University of British Columbia.

To show its usefulness, the start-up of a large induction motor
is simulated.

The data required for this simulation are provided in the
Appendix; and the results of this simulation are given next.
Specifications of the Induction Motor
Simulation Results
Conclusions

The current envelope in Figure 5(a) shows the large inrush
currents which exist during a motor start-up. The amplitudes of
these currents remain practically unchanged until the motor has
reached its rated speed.

In addition, d.c. offset currents are present in the beginning of
the start-up process.

The torque curve, on the other hand, shows that there is a large
oscillatory torque immediately after the motor is energized.

Comparing Figures 5(a) and 5(b), it is seen that this oscillatory
torque decays with the d.c. offset currents.

Finally, the speed curve shows that the rotor speed climbs up
steadily to its rated value. In particular, a small overshoot is
observed before the rotor speed settles down to its rated value.
Starting High Inertia Loads
Robbie McElveen
Reliance Electric Rockwell Automation
101 Reliance Road Kings Mountain, NC 28086
Mike Toney
Amoco Corporation 3700 Bay Area Blvd.
Houston, TX 77058
Objectives
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Many methods are used to reduce the current draw during startup of
high inertia applications such as centrifuges, hammer mills, or large
fans.
Reduced current conditions are desired to decrease the strain on both
the motor and the connected mechanical system.
This reduction in starting current leads to a corresponding reduction
in the starting torque available from the motor.
This reduction in torque leads to longer acceleration times and the
potential for increased heating during startup.
The goal of this paper is to evaluate the "conventional" methods of
starting, and to compare each of the methods for temperature rise,
acceleration time, and economical considerations.
Methods of starting AC induction motors

Full Voltage (across-the-line) starting

Electro-mechanical reduced voltage starting
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

auto-transformer starting
wye-delta (star-delta) starting, and
resistor/reactor starting

Solid-state reduced voltage starting, and

Variable frequency drive starting
Full Voltage (across-the-line) starting

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Of the many methods, full voltage (or across-the-line) starting is
typically used
This method of starting results in a large initial current surge, known as
inrush, which is typically 600% to 700% RMS of the full load current
drawn by the motor.
Power companies may apply restrictions as to how much current draw is
allowed.
These restrictions are typically specified as the maximum allowable
voltage drop at the incoming power connection point or the maximum
allowable kVA which may be drawn by the plant.
By limiting the inrush, the corresponding voltage drop will be reduced.
Brownout or other associated problems may be experienced if the
voltage dips too much.
Furthermore, this inrush current induces large magnetic forces in the
stator windings which actually try to force the windings to move and
distort.
Full Voltage starting (cont’d)
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

Full voltage starting produces greatest amount of starting torque
High starting torque is desired when starting a high inertia load in
order to limit the acceleration time
Beyond the initial shock of inrush current and torque, this type of
starting result in a smooth acceleration characteristic with the
shortest acceleration time
Electro-Mechanical Reduced Voltage Starting



This is a popular method which is used to reduced inrush current
With this type of starting, the current drawn by the motor decreases
linearly with decreasing voltage
Torque is reduced by the square of the percent voltage ratio
V

I reduced   reduced 
 V rated 
1.1
V

Treduced   reduced 
 V rated 
I rated
2.2
V rated
Electro-Mechanical Reduced Voltage Starting can be
achieved in three ways:

Auto-transformer: as the motor gains speed, taps are changed to
increase the voltage at the motor terminals. Autotransformer
starting is a more costly method than either wye-delta or
resistor/reactor starting.

Wye-Delta: this method is used for motor voltages of less than 1000
volts. a normally delta-connected stator is connected in wye during
the initial startup phase. It is most common for the motor to reach
full speed before the transition to the delta connection is made. This
essentially applies full voltage to the motor at this point. The
advantage of connecting the stator in wye is that only 0.57 times
rated voltage is applied to the phase windings. This results in only
0.57 of nominal current draw, but reduces the starting torque by
factor of three as well. A drawback of this method is that it requires
the neutrals of the motor.
Relative magnitudes of Torque and current for both wye and delta
configurations
Electro-Mechanical Reduced Voltage Starting can be
achieved in three ways:



Auto-transformer
Wye-Delta
Primary resistor/reactor: This kind of starting is achieved by
placing a resistance or inductance in series with the motor
leads in order to reduce the inrush current. When the motor is
nearly up to speed, the resistor or reactor may be switched
out of the circuit, causing transitory currents with their
corresponding torque pulsations. Energy is wasted as heat is
dissipated in the resistor during each startup cycle. Less
energy is wasted when using a reactor, but the magnitude can
still be significant.
Solid-state reduced voltage starting (Electronic Soft Starting)
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
Voltage ramp starting: Voltage ramp starting is the simplest form
of soft starting in which a microprocessor is used to control the
firing angle of pairs of SCRs, thus progressively increasing the
voltage supplied to the motor.
Current limiting starting: In this case the user can set a pre-defined
maximum current that will be supplied to the motor. The starter
control circuit will sense the load current or motor back EMF and
alter the firing angle of the SCRs in order to adjust the voltage at
every point to whatever value is necessary in order to maintain the
current at the desired level.
Data from the feedback circuit is used
to adjust the voltage in order to
maintain the current at a constant level
Below are graphs showing acceleration times and motor heating
results for various current limits. Figure 8 demonstrates how the
acceleration time increases with decreasing the current limit to the
motor. As less current is supplied to the motor, less starting torque
is produced, which results in increased acceleration time.
Figure 9 illustrates how both the rotor and stator temperature rises are affected
by decreasing the current limit. Because there is less available torque under
limited current conditions, the tendency is to believe that increased motor
heating may result. However, due to the fact that the current is reduced, both the
rotor and stator I2r losses are decreased. In addition to this decrease in losses, the
acceleration time is extended, allowing for more of the heat generated to be
dissipated to the frame and surrounding atmosphere. Thus, the reduced current
leads not to increased heating, but rather to a cooler acceleration for the motor.
Variable frequency drive starting

Starting a motor using a variable frequency drive provides maximum control
over the starting characteristic.

Because the frequency is varied, the motor operates only on the right side of
breakdown on the speed-torque curve.

Thus, any torque value from full load to breakdown can be achieved across the
entire speed range from zero speed to base speed assuming that the drive has the
necessary current capability. The load can be accelerated as slowly as desired,
thus virtually eliminating mechanical stress.

The VFD is considerably more expensive than the other methods discussed and
takes up more space than the other electronic starter option (soft start)
TIME DOMAIN ANALYSIS OF INDUCTION
MOTOR STARTING TRANSIENTS
R. Natarajan, V. K. Misra, and Mathew. Oommen
Mine Electrical Laboratory
Department of Mineral Engineering
The Pennsylvania State University
University Park, PA 16802
Objectives

In this paper, a simplified time domain analysis based on
equivalent circuit, differential equations, and Laplace
transform is presented for the calculation of the starting
current of an induction motor.

Experimental verification of the results were carried out on
a 2-hp, 220 V, 7 A, 3,600 r/min, Y-connected, three-phase
induction motor.
Introduction

Induction motors draw approximately 500% to 700% of rated
current during starting. This causes significant voltage drop at the
terminals, and affects the performance of other equipment connected
in parallel.

In industrial, and commercial distribution systems, the voltage dip
due to motor starting causes flicker of incandescent lamps. The
maximum voltage dip, and the frequency of dips, are dictated by
standards. For example, a 6% voltage dip is allowed for one
fluctuation per hour [1].

In order to reduce the intensity of the voltage drop in small and
medium size motors, wye-delta starters are employed. Still many
motors are started directly on-line.

In applications like conveyor starting, solid-state starters are
employed to limit the magnitude of the starting current, while
developing adequate torque [2].
PROPOSED MATHEMATICAL MODEL
The sudden application of a sinusoidal forcing function to the
conventional per phase equivalent circuit is used in the
transient analysis. A closed form time domain solution is
derived for the switching transients using the following steps.

Differential equations from the equivalent circuit,
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Laplace transform of the above equations,

Solutions by partial fractions, and
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Inverse Laplace transform, and time domain solution.
Assumptions

The MMF waveform is assumed to be sinusoidal,
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Saturation is partly neglected,

The effect of eddy currents are neglected,
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The dynamics of the rotor is not accounted, and

The initial stored energy is assumed to be zero.
Taking Laplace of the voltage equations and solving for I1
Simulation: Determination of the inrush current
Current components during starting
Observation….
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

The magnitude of the natural response component is -35 A,
and this component decays rapidly with a very small time
constant.
The maximum value of the real current component is 30 A.
The reactive current component is appreciable during starting
(38 A), and can be supplied locally with the help of capacitors
Experimental Verification

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The starting of the motor was performed
by switching on a circuit breaker
To record the transient starting current, a
clamp-on current transducer was installed
on one phase.
A 10 ohm resistor was used as the
transducer load, and the transient output
was obtained in the form of voltage.
The transient signal was stored on a
Tektronix 5111A storage oscilloscope, and
was photographed using a polaroid
camera.
The transient stator current is 49 A under
noload condition (Fig.5) and the starting
time is 2.8 sec. With load, starting time is
3 sec.
Adjustable ac Capacitor for a
Single-Phase Induction Motor
Eduard Muljadi, Member, IEEE
Yifan Zhao, Tian-Hua Liu, Member, IEEE
Thomas A. Lipo, Fellow, IEEE
Abstract




The most common practice for starting a single-phase induction
machine (SPIM) is to install a starting capacitor in series with the
auxiliary winding.
In some applications, two capacitors are used. One is used during the
starting period to help create the starting torque. The other one is used
during the running condition to improve efficiency.
This paper discusses the possibility of using an electronic switch in
parallel with the running capacitor, thereby providing the equivalent
of a starting capacitor. The capacitor is shorted during each cycle to
vary the effective size of the ac capacitor.
By using this method, only one capacitor is used for both the starting
and running condition, and a similar starting performance can be
obtained when compared with the conventional method using two
capacitors.
Introduction






The capacitor size must be determined according to the terminal
impedance of the auxiliary winding.
Unfortunately, this impedance changes dramatically from the starting to
the running condition. Hence, it is not practical to use only one fixed
value capacitor for both starting and running.
If both the largest starting torque and best running conditions are
needed, at least two capacitors must be used with the auxiliary winding.
Motors with two capacitors are called capacitor-start, capacitor-run, or
two-value capacitor motors.
The larger capacitor is present in the circuit only during starting when it
ensures that the currents in the main and auxiliary windings are roughly
balanced, yielding a relatively high starting torque.
When the motor runs up to speed, the centrifugal switch opens, and the
permanent capacitor is left by itself in the auxiliary winding circuit. The
permanent capacitor is just large enough to balance the currents at
normal motor loads; therefore, the motor again operates efficiently with
good power factor.
Circuit layout with a switched capacitor
This system consists of three
major elements: the
induction machine, the
running capacitor, and an
inverse/parallel set of
bidirectional voltage
blocking switches (e.g.,
reverse blocking GTO's).
The main winding of the
SPIM is directly connected
to the supply mains.
The switched capacitor…

It is shown that the apparent capacitance of the running
capacitor can be made larger than its actual value if the
capacitor is shorted periodically.

Thus, by operating the switch on and off regularly during each
cycle and by changing the length of the shorting interval, the
effective capacitance of the capacitor can be enlarged and
adjusted to an optimal value to realize the maximum possible
acceleration torque for any rotor speed during run-up from
standstill.

By using this method, only one capacitor is used for both
starting and running conditions.