Efficiency of Asynchronous Generators
for Variable Speed Wind Turbines
Kali McLaughlin, Flowtrack Pty Ltd
P.O. Box 5 Nimbin 2480 N.S.W.
Greg Clitheroe, Biomass Energy Services & Technology
1 Davids Close, Somersby Industrial Estate, 2250, N.S.W.
Abstract
A modern high efficiency induction motor was tested as an induction generator over a broad range
of operating points far from the nameplate rating. Speed, voltage and power have been all varied
independently and the resulting data has been presented as a convenient design tool for optimally
matching a variable speed prime mover.
1
INTRODUCTION
In order to have fixed blade pitch in a wind turbine it is desirable to have variable speed operation of the generator so
as to maintain aerodynamic efficiency over a range of wind speeds.
Our wind turbine design uses a standard induction motor as a generator. It was necessary to determine the best
voltage at which to operate the machine as a function of varying speed and power. Previous design was based on
extrapolation from motor data and loss analysis (ref 1).
Recently the market for energy conservation has brought about a new product range of induction motors with
substantially higher efficiency. Induction motors do not have a reputation for high efficiency, but that is because they
have been optimised for price and operating parameters other than efficiency. Also the familiar single phase version
of the squirrel cage machine is a considerable design compromise.
A 4 kW motor was investigated which had specified efficiency above 90% for much of its load range and the question
was whether this would be maintained when the machine was generating, and whether it would be maintained at very
different conditions of speed and power.
We took a double approach to the problem. Firstly the machine was measured on a test bed in order to establish its
machine parameters in order to extrapolate from manufacturers data to predict generating performance. Then as a
model check we took the fundamentalist approach of running the machine on a dynamometer so as to measure the
input mechanical power and thus calculate the efficiency from the output power.
2
METHODOLOGY
The simple calculation of generator efficiency by dividing electrical power out by mechanical power is inherently
inaccurate because the energy lost in conversion is a small fraction of the total, and even small errors in the many
variables measured can lead to wide confidence intervals in the difference between power in and power out. We felt
however that an independent check of this sort was justified, and should give us more information about extreme
operating points.
The method of measuring power transferred to a load versus the method of analysing the losses in the generator was
thought to be simpler for comparing one generator to another, and safer from errors in modelling and extrapolation.
Also, our interest was not in finding the precise peak of efficiency. In a very variable application like wind power the
system would rarely operate at the optimal point. We were more interested in finding the areas where efficiency fell
sharply, and designing matching relations which avoided these.
Efficiency of Asynchromous Generators for
Variable Speed Wind Turbines
3
K. McLaughlin
EXPERIMENTAL SETUP
The mechanical drive was provided by a car with automatic transmission. The properties of this rig approximated a
turbine and was free from control loop problems, being very heavily damped. The throttle of the car was set by hand to
achieve various operating points.
The generator was belt driven in a cradle with torque electronically sensed. The balance and friction of the cradle were
both designed carefully to avoid offsets and nonlinearities over the wide operating range of torque.
Excitation of the generator was by capacitors. This meant there were no high frequency currents in the machine, and
the efficiency of a motor drive inverter did not have to be taken into account. Again we were making the test as
realistic as possible.
The load we used had to be active to stabilise some of the operating points, and injected some harmonic currents into
the machine, but this also was considered to be a recreation of the practical application. Our initial load was a pseudo
resistive load created by a 3 kW 20 kHz switchmode regulator. After destroying this several times we had to move to
one which clamped the three phase rectified voltage. The problem we were having was that the sudden voltage surge
at the moment of generator excitation was exceeding the 600 volt rating of the switchmode before its response time
acted to protect it.
Speed was measured by optical pickup on the generator shaft, electrical frequency voltage and current with good
multimeters. Power factor was calculated from the capacitors connected to the machine at that operating point, and
slip was calculated by recording a section of the output waveform which had synchronising pulses injected into it by
the optical shaft pickup. (fig #1). This proved necessary because considerable jitter of rotor phase was noticed,
probably originating from drive train irregularities.
The generator cradle was constructed of wood, and much of the instrumentation was left at floating voltage to
minimise interference and arguments about earth between the array of different test points. This proved a very bad
idea as the belt drive to the generator transferred charge to the extent of several thousand volts which arced across the
isolation of what used to be a very nice Tectronix oscilloscope.
4
RESULTS
4.1
Excitation
The initial question about this new high efficiency generator was its self-excitation properties. The answer was
interesting. Residual rotor flux was observed below 10 Gauss, but this did not prevent reliable self excitation. Notable
also was the very slow rate of excitation. The combination of the low residual field and slow build up was that the
machine could take ten seconds to come to life. This is a problem in self excited wind power systems as the aceleration
a turbine can be faster than this, causing runaway.
The low residual flux was expected because of the high quality of the lamination material, but the slow excitation rate
seems to be a result of the dual cage rotor construction that this type of motor employs to keep slip, and thus rotor
losses, low. A 50 kW machine of the same brand was found to have very similar properties, but was even slower in
its excitation response time. This can be calculated as a function of the electrical response time of the deep rotor cage.
4.2
Operating Points
The form of the data is that the variable of interest, generator efficiency, depends on three independent variables,
usually: voltage, current and frequency. Instead of using these variables we used the three variables: voltage,
mechanical power input and speed. We did this because we were interested in matching the generator to existing
turbine data, stated of course in speed and mechanical power. These three variables were enough to specify any
achievable operating point of the machine. Our data structure was able to yeild any convenient set of variables.
Presentation of the data was as slices of constant power but varying voltage and speed. This allowed optimisation of
the wind turbine at a particular windspeed. A series of slices at increasing power allowed an optimal turbine power
point track to be created.
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Proceedings for Solar ’97 – Australian and New Zealand Solar Energy Society
Paper151
Efficiency of Asynchromous Generators for
Variable Speed Wind Turbines
K. McLaughlin
We had intended to take data only on slices of constant power, thus reducing the three dimensional data to a series of
sets of two dimensional data. In practice this proved impossible because the continuously variable load was not
sufficient to traverse the operating space, and we found the need for continuously variable capacitors, which of course
are not possible in the sizes necessary. Furthermore the car throttle was difficult to set accurately enough to fine tune
the power level, and the automatic transmission had a tendency to kick down a gear at inconvenient times. The
designers of the gearbox had allowed for people to select a low ratio, but obviously had not considered that anyone
would want to hold the car in high range!
Thus the data came out as 150 points in a three dimensional space. We got the computer to first project the data
onto convenient slices of power, and then to draw contour lines of efficiency as presented . There is obviously a lot of
noise in the data, but the trend is clear, the areas to avoid are quite well delineated, and the areas of high efficiency
are gratifyingly broad.
4.3
Slip
Our plan to measure slip was to measure the difference between the optical shaft sensor and the electrical frequency
generated. This method worked for only part of the large range of slip we covered, from less than 1% up to 30% !
The slip data is not presented, as it is not germane to the matching problem, but it is worth noting that we had to take
recordings of the generators waveform with the shaft sensor output injected into it. Slip was calculated for every
operating point by counting the electrical cycles between phase concurrences. With enough data this was quite an
accurate, if slow, method. Typical recordings were as reproduced below. This buried cage motor showed very low slip
when running at good efficiency. This strong coupling to the stator field in the steady state was however at odds with
transient response. The rotor was very compliant to high frequency variation in rotor position, and this is possibly a
contributor to the phase jitter we encountered in the shaft position sensor, in spite of efforts we took with a hydraulic
drive and multiple large radius drive pulleys.
fig#1
Generator output
4.4
POWER FACTOR
As with efficiency and slip, power factor was calculated for each operating point, using the load current (resistive) and
the capacitor current (90 degree leading). The voluminous data is not presented here, but it indicated the expected
fall-off in the areas of poor efficiency. The only surprise was the poor power factor in areas of relatively high rotor
current, such as the full load operating point on the nameplate. This was consistent with the high rotor reactance
parameter as measured by Dr Peter Freere. The buried cage construction of the rotor could be expected to cause this,
but examination of the data for the full range of motors in the range indicates that this feature might be an artifact of
frame size quantisation. It is common practice to supply several different ratings of motor with the one frame size to
minimise the stock of frame sizes needed. This variation of machine design on the one frame will cause a change in
the proportions of the machine parameters from one motor rating to the next.
This falling power factor at full load caused great problems in the use of this particular generator with our power point
tracking transformer as it forced a higher than desired rate of speed change with power. For this reason we ended up
using a different type of generator!
Proceedings for Solar ’97 – Australian and New Zealand Solar Energy Society
Paper151
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Efficiency of Asynchromous Generators for
Variable Speed Wind Turbines
4.5
K. McLaughlin
EFFICIENCY
The following graphs were computer generated from the 150 data points we recorded. In areas where scarcity of data
points make the interpolations meaningless the contours have been removed.
SPEED (Revolutions/sec)
0
50
100
rev/sec
150
200 Generator voltage, phase/phase
EFFICIENCY at 300 WATTS INPUT
Data Location
0
4
Data Location
100
200
300 volts ph/ph
EFFICIENCY at 600 WATTS INPUT
Proceedings for Solar ’97 – Australian and New Zealand Solar Energy Society
Paper151
Efficiency of Asynchromous Generators for
Variable Speed Wind Turbines
K. McLaughlin
rev/sec
0
data location
100
200
300
EFFICIENCY at 1 kW INPUT
Revolutions / second
100
150
Volts, phase/ phase
Data Location
200
250
300 volts (phase / phase)
EFFICIENCY at 1.5 kW INPUT
Proceedings for Solar ’97 – Australian and New Zealand Solar Energy Society
Paper151
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Efficiency of Asynchromous Generators for
Variable Speed Wind Turbines
K. McLaughlin
revolutions/sec
100
200
Data Location
300
EFFICIENCY at
revolutions / sec
100
5
400
500 volts, phase / phase
2 kW INPUT
Data Location
200
300
400
500 volts, phase / phase
EFFICIENCY at 3 kW INPUT
INTERPRETATION
At each power level there is a plateau of speed and voltage which yields maximum efficiency. The graphs indicate
that this is gratifyingly broad, making the job of power point tracking relatively easy. The trajectory of maximum
efficiency is seen to run from 75 volts and 12 rev/second in the 300 watt graph to 400 volts and 35 rev/sec in the
three kilowatt graph. The trajectory seems approximately linear, as suggested by our earlier theoretical analyses.
Below 300 watts the data became unreliable as relative errors rose, and the effects of friction dominated efficiency.
The analysis was not continued to higher speed and voltage because these areas were of little practical importance,
though it would have been interesting to see what happened!
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Proceedings for Solar ’97 – Australian and New Zealand Solar Energy Society
Paper151
Efficiency of Asynchromous Generators for
Variable Speed Wind Turbines
6
K. McLaughlin
REFERENCES
Kali McLaughlin 1992
A Wind Turbine with Power Point Tracking Induction Generator
ANZSES Proceedings, SOLAR 92
7
ACKNOWLEDGMENTS
This measurement of generator efficiency was done as part of a wind turbine commercialisation project conducted by
Biomass Energy Services and Technology (BEST) after an investment by Energy Research and Development
Corporation (ERDC).
Work was done by Monash university Dept of Electrical Engineering, both in measuring machine parameters at a
range of frequencies, and in validating our experimental apparatus. A report was produced showing the unusual
equivalent circuit of the high efficiency motor tested.
While not directly relevant to calculation of generator efficiency, data supplied by Newcastle University Engineering
Department on turbine characteristics gave us the curve which the generator had to optimally track..
8
CONCLUSION
It has been empirically verified that Induction motors work efficiently as generators over a considerable range of
voltage, speed and power. The trend of this three dimensional region has been presented in a series of graphs which
are useful as a design tool for power point tracking applications.
Proceedings for Solar ’97 – Australian and New Zealand Solar Energy Society
Paper151
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