Aligarh Muslim University
From the SelectedWorks of Mukhtar Ahmad
December, 2006
A Doubly Fed Induction Motor as Very Low
Speed Drive
Mukhtar Ahmad, Aligarh Muslim University
Available at: http://works.bepress.com/mukhtar_ahmad/9/
A
Doubly Fed
Induction Motor as High Torque
Low
Speed Drive
Mukhtar Ahmad, Senior Member, IEEE, M.Rizwan Khan, and Atif Iqbal
Abstract--For high torque low speed applications in pulp and
paper and cement industries a dc motor or a cage type motor
with reducer has been used. In this paper use of a doubly fed
induction motor as a high torque very low speed drive is
presented. It is shown that such a motor works as constant speed
motor without any problem of stability.
Index Terms-- AC motors, low speed drives, sensorless
control.
I. INTRODUCTION
BEFORE 1980, DC motor with reduction gear was the
main machine used in high torque low speed drives.
Later, with the availability of power semiconductor devices
for variable frequency applications cage type induction motor
was also used. These motors were connected to the drive
equipment through a gear box. The higher speed motor and
gear box provided a cost effective solution for low speed
drives. However, the maintenance and reduced efficiency of
such drives makes them unsuitable for high torque drives. In
this paper a doubly fed induction motor is proposed for high
torque and very low speed operation.
The recent interest in the study of doubly fed induction
machines is mainly as variable speed wind generator or for
driving fan and pump loads [1]-[2]. However, if in the doubly
fed induction machine the stator is supplied from the 50 Hz
Grid supply and the rotor is connected to a power converter
with 49.5 Hz frequency., the motor will run at very low speed.
For a 4 pole motor the speed is 15 rev. per minute.
Presently, active research is also going on to eliminate
position / speed sensors from most of high performance drives
[3]-[5]. In doubly fed machine, the rotor field is explicitly
defined
by the external source which requires
synchronization with respect to stator field. The vector control
therefore, without a position sensor is more complicated in a
doubly fed machine compared to a single fed machine.
In this paper a scheme for low speed drive of a doubly fed
induction motor is presented. It is shown that the machine
runs as synchronous motor at very low speeds and the vector
control for the speed is possible with feed back from rotor
M.Ahmad (e-mail:mukhtar aieee.org) M.R. Khan (km_rizwan a
rediffmail.com) and A. Iqbal (atif iqbal orefiffmail.com) are with the
Department of Electrical Engineering, Aligarh Muslim University, Aligarh
India.
0-7803-9772-X/06/$20.00 (©2006 IEEE
Fig. 1. Doubly fed machine configuration.
voltages and currents only.
II. MACHINE FOR LOW SPEED OPERATION
A doubly fed induction machine is shown in Figurel. As
shown, the stator of the machine is connected to 50 Hz ac
source directly. The rotor winding is connected to an ac/dc/ac
converter with bi-directional power flow. Considering the
power balance equation of the system, the electrical power
from the stator and rotor must be equal to the mechanical
power output plus losses. Since the motor is working at very
low speed, the slip is high and the voltage at the rotor circuit
is very nearly equal to the standstill voltage. The steady state
behavior of the machine can be easily understood from the
equivalent circuit as shown in Fig. 2. Unlike doubly fed
induction motors working with low value of slip, the rating of
the converter here will be comparable to the motor rating.
When an induction motor is supplied from both sides, there
will be fluxes produced by both stator and rotor winding. In
case of motor running at near synchronous speed the rotor
voltage is very small and the torque developed by the motor is
due to induction only. However, if the rotor supply frequency
is very nearly equal to the supply frequency of the stator, the
rotor voltage is also comparable to stator supply voltage. In
such cases the induction torque is very small as s is nearly
equal to 1.
torque = Ir2rr (I1- s)lse)
co,
As the motor now behaves as a synchronous machine, the
torque is produced by virtue of the tendency of stator and
rotor air gap fluxes to align their axes. Now if the motor is
running on no load with rotor connected to 49.5 Hz supply,
the speed of the motor is 15 rpm and slip s =0.99, the power
required to run the motor is low. In this condition the power
is supplied from rotor and stator sides almost equally. The
Aqdm
= imd, and Aqm = 0
The torque equation therefore reduces to
(9)
Te
(10)
Iqdr
Y4idm
Also the relationship between the rotor speed, rotor current
frequency, and stator current frequency is given by
cos
Fig. 2. Steady state equivalent circuit.
motor runs exactly like asynchronous motor without
problem of stability.
any
DYNAMIC PERFORMANCE OF DOUBLY FED INDUCTION
MACHINE
III.
The dynamic performance of the doubly fed motor can be
conveniently understood from the well-known d-q-0
transformation. The voltage equations in terms of flux
linkages for the stator and rotor can be written in matrix form
as:
V
abcs
vabcr
=rsiabcs
+
dk abcs
dqs
Vdqr
=
rr idqr +
dt + Ce x kdqs
dt
+
(6)
(C)e -(!)r ) kdqr
X
(7)
where Woe is the synchronous speed and or is the speed of
rotor. Normally the term (Coe -)r )
case
it will have
a
small, but in this
high value. The electromagnetic torque
is
very
developed by the motor is-
Te =
Aqdm iqdr sinfl
For vector control
must be maintained at 90 degrees or
the current vector is to be maintained orthogonal to the air gap
flux.
IV. VECTOR DIAGRAM
The vector diagram of doubly fed induction machine is
shown in Fig 3. The rotor current can be changed by adjusting
the rotor voltage. Since the induction motor works at very low
speed the rotor voltage vector and current vector have
opposite direction. The power is therefore supplied to the
mains by the converter connected to the rotor.
(3)
dxdqs
dkdqr
(11)
is
dk abcr
=rriabcr +
dt
= rsidqs +
-)r
(2)
dt
(4)
i abcs f v abcs rs i abcs
i abcr f v abcr - rr i abcr
(5)
Since r, is very small and the applied voltage to the stator
is kept constant, the stator flux linkage remains constant.
When a doubly fed motor is running at near synchronous
speed, the rotor voltage and therefore the rotor flux linkage is
very small. However, if the motor runs at low speed the rotor
voltage is comparable to stator voltage and the flux in the
rotor circuit is also comparable to stator flux.
If d-q-0 transformation is used, the equations can be
written asV
CO e
a
Po
qdm
d
ir
Fig. 3. Vector Diagram.
V. CURRENT CONTROL METHOD
The inverter voltage vector supplying power to the rotor
has eight states; out of which only six are active the other two
are zero. The space distribution of the voltage vectors is
shown in Fig.4. The complete region of 360 degrees is divided
into six
regions 1,11,
III
.........
VI and each
region
lies between
the two adjacent voltage vectors. In order to change the
current 1r due to change in load, the phase angle and
magnitude of rotor current can be changed by controlling the
adjacent voltage vectors in the region where ir is placed. For
example if ir in region II, its phase angle and magnitude can
be controlled by V5 andv6 as is normally done with space
vector modulated PWM inverter These two voltages can be
applied one at a time for a fraction of the switching period
depending on the magnitude required. For remaining
switching period vO or v7 is applied.
In order to generate the required voltage vectors the
torque difference AT and the angle between rotor current
and flux vector is required. These quantities are calculated
.
(8)
where is the angle between air gap flux and rotor current
vectors. If the air gap flux is aligned with the d- axis of the
synchronously revolving reference vector
,
,
depending on the position of rotor current. The proposed
control system block diagram is shown in Fig.5. The block
diagram has two control loops. The outer loop is for the speed
and the inner loop is for torque control. The speed of the
motor is measured using a measuring system described in (6).
However, the scheme is being developed to have sensorless
system .The desired value of torque Te is determined based
on speed error. The machine torque is calculated using
equation (10). The angle a between rotor flux and rotor
current can be obtained as
a
=
aretan
+
Aqr
artani
(12)
'qr
The controller keeps the rotor flux linkage constant and the
angle a is forced to have 90 degrees value. Thus the torque
can be controlled linearly by controlling the rotor current.
Fig. 5. Block diagram of control circuit.
VII. REFERENCES
[1]
v3
[2]
ir
[3]
I
[4]
V6
VI
[5]
Fig. 4. Space distribution of voltage vector.
VI. CONCLUSION
A method has been proposed to obtain very low speed
drive using doubly fed induction motor. Since the problem of
estimating the value of flux accurately is very difficult in
single fed induction machines, this method will be very useful
for such drives. Now with the availability of fast switching
devices it is easy to control the rotor current quickly thereby
getting high performance torque control.
[6]
M. G. Jovanovic, R.E.Betz, and Yu Jian, "The use of doubly fed
reluctance machines for large pump and wind turbines" IEEE Trans.
Ind. Appl. Vol.38.no.6, pp. 1508-1516, Nov.-Dec.2002.
C. Abbey, and G. Joos, "Integration of energy storage with a doubly-fed
induction machine for wind power applications" in Proc. IEEE 35th
Power Electronics Specialist conference2004, vol. 3 pp 1964-1968.
Longya Xu, and Wei Cheng, "Torque and Reactive Power Control of a
doubly fed Induction Machine by Position sensorless Scheme" IEEE
trans. IndAppl. Vol.31 no. 3 pp636-642 May/June 1996.
O.A. Mohammed, Z.Liu, and S. Liu, "A novel sensorless control
strategy of doubly fed induction motor and its examination with the
physical modeling of machines" IEEE Trans. Magnetics vol.41 no.5
pp.1852-1855 May 2005.
Z.Wang, F. Wang, M. Zong, and F. zhang, "A new control strategy by
combining direct torque control with vector control for doubly fed
machine" in Proc. International Conference on Power System
Technology, 2004
Mukhtar Ahmad, "A digital tachometer for measurement of low speeds"
Proceedings IEEE, pp. 1096, 1984
VIII. BIOGRAPHIES
Mukhtar Ahmad received his B.Sc , M. Sc. And Ph. D from Aligarh
Muslim University Aligarh in 1969, 1972, and 199ldepartment of electrical
engineering A.M.U. Aligarh. He is currently working as professor in the
Department of electrical engineering AMU Aligarh. He has also worked in
Multimedia University Malaysia, and University Putra Mal;aysia as associate
professor He is a senior member of IEEE since 2002. His research interests
are in power electronics; electric drives and power system control.