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Real Time Rotor Bar Current Measurements Using a Rogowski
Coil Transmitted Using Wireless Technology
Ehsan Abdi Jalebi, Paul Roberts and Richard McMahon
ea257@cam.ac.uk, pcr20@cam.ac.uk, ram1@cam.ac.uk
Engineering Department, University of Cambridge, Cambridge, CB2 1PZ, UK.
KEY WORDS: Rotor bar current measurement, Rogowski coil, Bluetooth wireless technology
ABSTRACT
Rotor bar current measurement is a
valuable step in verifying the theory of
electrical machines design and control.
However, because the rotor is moving, the
approach is difficult to implement. This
paper presents the design and evaluation
of an approach to real-time rotor bar
current measurement using the Rogowski
coil as a current transducer. Rogowski coils
have become an increasingly popular
method of measuring current within power
electronics equipment due to their
advantages of low insertion loss and
reduced size compared to an equivalent
current transformer. The design of the
associated integrating amplifier is
discussed in detail, including the
advantage, tradeoffs and limitation of the
approach. Bluetooth wireless technology is
employed to transmit data between the
moving rotor and the computer which logs
the data. Experimental results from a
system for measuring the rotor bar current
in a Brushless Doubly Fed Machine
(BDFM) are presented.
1. INTRODUCTION
In electrical machines it is very often
desirable to measure rotor bar currents in
real time for purposes of machine design
and analysis. The study of the Brushless
Doubly Fed Machine (BDFM) is a particular
example. The BDFM shows promise as a
replacement for induction generators with
wound rotors in wind turbines, but there is
a need to measure rotor bar currents to
verify models for the machine.
However, measuring rotor bar currents is
difficult because the rotor is moving. Since
the measurement apparatus must be
installed on the rotor, the dimensions must
be minimized. Moreover, it must be
possible to read the transducer data from
the machine when the machine is moving.
There is also likely to be a high level of
electromagnetic (EM) interference,
especially if the machine is inverter-fed.
Furthermore, mechanical considerations
must be taken account in the installation of
the apparatus since, even at modest rotor
speeds (1000 rpm), the centripetal
acceleration is of the order of 1000 m/s2 for
a frame size D180 machine.
This paper presents the design and
evaluation of a rotor bar current
measurement setup which employs a
Rogowski coil for bar current sensing. A
block diagram of the measurement
apparatus is shown in Figure-1.
While the Rogowski coil sensing technique
has not been previously employed for such
an application, it is advantageous in this
application due to its high accuracy and
bandwidth, low weight and cost, and low
sensitivity to parameter variations [1,2].
The integrator is necessary since the coil
provides a voltage proportional to the rate
of change of measured current. The
integrator output can be connected to an
analog to digital converter (ADC).
Recently introduced Bluetooth wireless
technology (BT) is used to send the ADC
output data to a PC. BT is the radio
technology that allows devices to
communicate with one another within a
range of ten meters. BT therefore enables
the data to be transmitted from the moving
rotor without the need for slip-rings or other
mechanical connections. The link speed,
communication range, and transmit power
level for BT were chosen to support low-
cost, power-efficient, single-chip
implementation of the current technology
[3].
The setup is intended for measuring rotor
bar currents in a Brushless Doubly Fed
Machine (BDFM). The measurement of
rotor current is an important step in
verifying the theory of this machine, which
has been proposed for use as an
asynchronous generator for wind turbines
[4]. The measurement setup is designed to
measure the rotor bar current with the
range of 10A to 3000A peak-to-peak and
over a frequency range of 1Hz to 100Hz.
2. ROGOWSKI COIL DESIGN
A Rogowski coil is a low-noise air-cored
current transducer. Rogowski coils have
become an increasingly popular method of
measuring current within power electronics
equipment due to their advantages of low
insertion loss and reduced size as
compared to an equivalent current
transformer [5].
Most commonly, Rogowski coil designs are
one of two typical kinds: those which are
wound on a rigid toroidal core former, and
those which are wound on a flexible belt-
like, or sometimes a worm-like core former.
Both kinds may be made to be openable
which makes placing them on a current-
carrying conductor convenient.
Figure-1: Block diagram of the measurement apparatus
In the application of rotor bar current
measurement, the coil is wound on a
flexible polyethylene former, such as is
used for a coaxial cable, of sufficiently
small cross sectional area to enable it to be
threaded around a current carrying
conductor and to have its ends clipped
together. The end winding needs to be
returned to its start along the central axis of
the coil (i.e. in the middle of the former) to
avoid sensitivity to magnetic fields parallel
to the conductor which could be caused by
other currents.
Although some researchers have
investigated multi-layer coils to increase
the sensitivity, single layer coils are more
convenient from several viewpoints – ease
of winding, better flexibility and relatively
smaller inductance which gives a better
bandwidth [6]. Figure-2 shows the
Rogowski coil which is made for the rotor
bar current measurement. The Rogowski
coil has the design parameters and
specifications shown in Table-1.
The output voltage e(t) at the open
terminals of the winding wound around the
toroid is proportional to the time derivative
of the current i(t) flowing in a conductor
passing through the toroid and is given by
Equation-1.
()
dt
di
l
AN
te 0
µ
= (1)
which N is the number of turns, A is the
cross section area, and l is the length of the
toroid.
Table-1: Rogowski coil specifications
Coil parameter Specification
Outside diameter (mm) 33.9
Inside diameter (mm) 25.7
Turns per meter (turn/m) 4090
Self inductance (mH) 9.18
Resistance (Ω) 2.68
Number of turns (turns) 380
Figure-2: Rogowski coil is used for rotor bar current
measurement
3. INTEGRATOR DESIGN
The current can be determined by
integrating the voltage at the terminals of
the Rogowski coil. As any real op-amp
used in an integrating circuit has an offset
voltage, the output of the integrator will
consist of the desired integral plus the
integral of this offset voltage. The op-amp's
offset is a constant quantity, and thus the
integral of the offset voltage, which is
added to the desired output of the op-amp,
is a ramp that will always grow to be larger
than the desired signal and will eventually
saturate the op-amp.
In the application of rotor bar current
measurement, as previously mentioned
there is likely to be a high level of
electromagnetic (EM) interference.
Therefore noise rejection should be
considered in order to filter all the
interference produced by the machine and
the inverter.
Due to the small voltages produced by the
Rogowski coil (about 1
µ
V/Hz for 1 A
flowing in a conductor), the design of a
suitable integrating amplifier circuit is not a
trivial matter. Furthermore, it is necessary
that the power requirements of the circuit
be kept to a minimum, as the device must
be battery powered.
A simplified representation of the circuit
used is shown in Figure-3. The AD8552, a
dual operational amplifier, which has the
correct combination of precision, low noise
and low offset with a high gain bandwidth
product and high slew rate, is used.
Since the integrator gain increases as
frequency decreases, thereby amplifying
the low frequency random noise and zero
frequency offset drift, it is necessary to
reduce the integrator gain for frequencies
below which measurement accuracy is not
affected. A large resistor R2 is put across
C1 to provide dc feedback for stable
biasing. The effect is to roll off the
integrator action at very low frequencies,
f<1/R2C1 [7]. The transfer function for the
integrator shown in Figure-3 is given by
Equation-2.
()
1)( )(
121
2int +
=sCRR R
sE sV (2)
The Butterworth filter is used as an active
filter which produces the flattest passband
response, at the expense of steepness in
the transition region from passband to
stopband. It starts out nearly flat at zero
frequency and bends over near the cut-off
frequency fc [7].
Figure-4 shows the pole-zero location of
the integrating amplifier circuit. The
integrating pole is placed at 0.02 Hz and
the active filter poles are placed at 1 kHz.
Frequency analysis of the circuit is shown
in Figure-5.
A 50 Hz sinusoidal signal with two different
noise signals super imposed is applied to
the circuit and the output is shown in
Figure-6.
U2
2
3
48
1
-
+
V-V+
OUT
0
E
U1
2
3
4 8
1
-
+
V- V+
OUT
R2 C2
R1 C2
0
R5
R4
R6
R3
0
0
Vout
0
Vint
C3
Figure-3: Integrating amplifier circuit
Figure-4: Pole-zero location of the integrating amplifier
(a) (b)
Figure-5: Frequency response of the integrating amplifier circuit (a) amplitude (b) phase
Figure-6: Real time simulation results
4. DIGITAL CIRCUITRY
The output of the integrating amplifier
circuit is connected to a 10-bit analog to
digital converter (ADC) which is contained
with a microcontroller. The microcontroller
is used to send the digital data over
Bluetooth via a RS232 serial connection.
Assuming a peak-to-peak rotor bar current
of 3000 A, an accuracy of 3 A/bit is
achieved. The baud rate of the data
transmission is set to 115200 Bits/s which
is limited by the COM port of the computer.
This baud rate allows the 144 samples per
cycle for a 50 Hz rotor bar current.
5. SERIAL CABLE REPLACEMENT
USING BLUTTOOTH TECHNOLOGY
Bluetooth (BT) technology is used as a
replacement for a serial cable for data
communication between the machine and a
computer. RS232 is chosen as a serial
standard for the asynchronous
communication.
RS232 is a widely used standard for
communications between industrial,
medical and scientific apparatus. Despite
growth in alternatives such as Ethernet and
USB, RS232 remains the only
communications standard which can offer
the widespread interoperability and cost
effectiveness required by manufacturers of
this equipment. However, the need for a
physical, wired connection between
devices can be inconvenient [8].
Bluetooth is a standard developed by a
group of electronics manufacturers that
allows any sort of electronic equipment to
make its own connections, without wires,
cables or any direct action from a user
[9,10].
The challenge in rotor bar current
measurement is to transmit the digitized
data from the moving rotor to the computer.
BT offers a solution to this problem. BT
was originally conceived as a replacement
for the IRDA protocol for communication
between laptops, PDAs, etc. However,
Cambridge Silicon Radio (CSR) have
produced software for their BlueCore
single chip Bluetooth and DSP which
implements an RS232 link over Bluetooth.
Compared to other wireless technologies,
BT is low cost and consumes very little
power.
Two CSR's BlueCoreTM RS232 cable
replacement modules are used for this
application. The schematic of the serial
communication profile is shown in Figure-7.
Figure-7: Serial communication profile
6. EXPERIMENTAL RESULTS
To establish the validity and degree of
accuracy of the proposed design, the
practical tests were conducted. Figure-8
shows the experimental rig which is used to
measure a current going through a multi-
turn wire to achieve a higher current level.
The actual current (CH2) and the output of
the integrating amplifier circuit (CH1) are
compared in Figure-9. Figure-10 shows the
current which is plotted with the data
received by the computer.
As can be seen from the figures, very
satisfactory performances have been
obtained. The phase shift of 7 degrees
produced by the integration amplifier circuit
can be seen more clearly in figure-11.
7. CONCLUSIONS
The paper presents the design and
evaluation of a real time rotor bar current
measurement technique which employs a
Rogowski coil current sensor. A coil and
integrating amplifier design suited to the
application is developed within the
constraints of high accuracy and good
noise rejection, low weight and cost and
low power consumption. Bluetooth
technology is used to transmit the data
from the rotor to a computer’s serial port.
The experimental results demonstrate the
feasibility of the approach. It is concluded
that the Rogowski coil with appropriate
integrator design using the Bluetooth
transmission medium offers a viable
method of measuring rotor bar currents in
moving machines. The approach can also
be used when electrical isolation is
required, as in high voltage machines.
Microcontroller
BlueCore RS232
Cable Replacement
Computer
B
luetooth
BlueCore RS232
Cable Replacement
R
S232
R
S232
Figure-8: Experimental apparatus
Figure-9: Actual and measured current comparison
0
100
200
300
400
500
600
700
800
900
1000
50 70 90 110 130 150 170 190 210 230 250
Time (3600 samples/sec)
Current (5 Bits/Amp)
Figure-10: Current plotted with the data received by the computer
Figure-11: Phase shift between the actual current and the measured current
ACKNOWLEDGMENT
The authors would like to acknowledge the
support of Cambridge Silicon Radio
(http://www.csr.com) for providing the
Bluetooth modules. We also gratefully
acknowledge helpful technical support of
Cambridge University Electronics
Development Group.
REFERENCES
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Rogowski Coil Current Transducers", Industry
Applications Conference, Conference Record of the
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[2] C.D.M. Oates, A.J. Bumett, C. James, "The
Design of High Performance Rogowski Coil",
International Conference on Machines and Drives,
IEEE2002, Conf. Publ. No. 487, pp. 568-573, 2002.
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[4] A. K. Wallace, S. Williamson, A. C. Ferreira,
"Generalized Theory of the Brushless Doubly-Fed
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1997.
[5] z. Mingjuan, D. J. Perreault, V. Caliskan,
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[6] J. D. Ramboz, "Machinable Rogowski Coil,
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[7] P. Horowitz, W. Hill, "The Art of Electronics",
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