a modified torque control approach for load sharing application...
TRANSCRIPT
A Modified Torque Control Approach for Load Sharing Application
Using V/F Induction Motor Drives
Abstract: The volt-per-Hertz (V/F) drives of induction motors (IMs) are widely used in industrial applications because these drives are simple and cost effective. V/F drives operation is based on speed command and unable to toque control. In industrial applications the IMs are coupled to feed heavy loads. Coupling the IMs in addition to economic advantage leads to reduction of energy consumption. When the load becomes heavier the number of IMs that feed the load will be increased and vice versa. However with coupled IMs, the load is not shared properly and some of the IMs may be overloaded. For a proportional load sharing using of the torque control drives seems to be necessary, but these drives are complicated and expensive. This paper describes a torque control approach for coupled IMs V/F drives which is accurate and inexpensive and also uses minimum number of motor parameters.
Keywords: induction motors, V/F control, load sharing,
variable frequency drives
1. Introduction
Mechanically coupled induction motors (IMs) are
widely used in industrial applications such as conveyor
belts for transportation of raw material, mill motors used
in iron and pulp and paper industries, mining drills, etc.
“Load sharing” is a term used by many to describe a
system where multiple drive and motor sets are coupled
and used to run one mechanical load. In the strictest
sense, load-sharing means that the amount of torque
applied to the load from each motor is prescribed and
carried out by each drive and motor set. [1], [3]. Each
drive and motor set must provide its proportional share of
power to the driven load. In load sharing, for torque
control possibility of individual motors, each motor must
have an individual drive set. The drive sets must be
interconnected, interconnecting the drive sets make it
possible to have a comparison between the drives and
generate an error signal, which is used to compensate
unbalanced load sharing of the drive sets.
The drive sets of motors range from the more
advanced and expensive vector-controlled schemes to the
conventional Volts-per-Hertz (V/F) control. The vector-
controller is capable to speed and torque control and can
implement load sharing schemes such as torque-follower
or trim control; hence this controller excels to the others
[1]. Therefore, the V/f controller is widely used in many
industrial applications mainly due to its simplicity and
low cost. Since a volts/hertz drive does not have the
ability to run in “torque mode”, a more loose
interpretation of the term “load sharing” is sometimes
used. Load sharing on a volts/hertz drive is much less
controllable and to a large extent dependent on the motor
and type of load coupling.
The properties of a load sharing system also depend on
the type of coupling used between the motors [2].The
focus in this paper is on the cases that the load sharing is
carried out merely through rigid couplings, although the
proposed concepts may be extended to other cases.
Fig. 1: block diagram of a conventional V/F speed control scheme
General diagram of the conventional V/F drive is
shown in Fig. 1. Reference and actual speeds are applied
to speed control block, then resultant speed reference is
converted to the voltage reference according to the
following equation [4]:
bs ref
b
VV
(1)
Where, bV and b are the base voltage and base angular
frequency of the machine respectively.
The torque-speed characteristic of the IM depends on
the applied voltage, frequency and the rotor resistance. In
practice, the rotor resistance changes with load variations,
temperature and frequency of rotor current [5], [6].
Furthermore the equivalent rotor resistances may be
different even in two identical motors. Consequently the
torque-speed characteristics of two identical machines
may be non-identical. Different torque-speed
characteristics cause improper load sharing of the IMs. In
Mohammad Amiri*, Mohammad Reza Feyzi **, and Hossein Saberi*** *University of Tabriz, [email protected]
** University of Tabriz, [email protected]
*** University of Tabriz, [email protected]
a load sharing system composed of several machines,
such deviations in the rotor resistance value has undesired
effects in the load sharing.
A modified scheme for decreasing motor parameter
deviation effect is proposed in [8]. This scheme relies on
output torque equation in steady state and can decrease
deviation effect and consequently an approximated
balance load sharing is achieved. It should be noted that
this scheme is based on approximated equation of output
torque that can be used just in low slip values. This
equation is also dependent to motor parameters and so
accurate values of motor parameters are needed in this
scheme. When heavy loads are applied to the system, slip
increases and causes large error in the approximated
equation of torque output. This error results in improper
load sharing between the motors. The problem can be
more severe if a number of various machines with
different parameters are interconnected. This issue is
further discussed in the next section and a new and
modified V/F control scheme is proposed to ensure a
proportional load sharing of the motors with respect to
their rated power
2. Problem Definition
The V/F drives are generally investigated in steady
state, where the torque equation is given by [5] 2
2 2
/3
2 ( / ) ( )
th re
e th r th r
V r sPT
R r s X X
(2)
thV , thR and thX are the Thevenin equivalent circuit
parameters and is the electrical frequency of the source.
Two IMs and V/F drives are considered to provide the
mechanically coupled load as shown in Fig. 2. From (2)
the IM output toque relies on its parameters and rated
values. Impressing the same reference speed to V/F
drives will apply voltages with the same magnitude and
frequency to the motors. The IMs with different
parameters which have same speed reference, have
different torque speed characteristics. Coupling IMs with
different torque speed characteristics yields to different
torque values for the IMs. The torque value is determined
based on motor parameters therefore it is not proportional
to the rated power of the IMs. Consequently one or some
of the IMs may be overloaded.
Fig. 2: Load sharing between two traditional V/F controlled induction
motors
Fig. 3: Torque-speed characteristics of IM1 and IM2 using the
conventional V/F scheme
In this paper, two 5 and 10 horse power (hp) IMs are
considered. The parameters and rated values of each IM
are given in the appendix. A mechanical load of 60 N.m
is applied to the machines at the speed of 184.9 rad/sec.
The difference between motor outputs is shown in the
Fig. 3. The torque values of the IMs are given in Table II.
As shown in Table II, the 10 hp IM is overloaded by
5.05% but the 5 hp IM is operating 11.1 % below the
rated torque.
In load sharing applications to assimilate the IMs
parameters which lead to assimilation of torque speed
characteristics, the identical IMs have to be used.
When two low resistance motors with low slip in
operation region are coupled mechanically, small
differences in motor speeds result in large differences in
motor torques and consequently one of the motors will be
overloaded quickly. In other words, in high slip motors
the characteristic slope in operation region is lower than
high slip motors and changes in speed and torque are
small and load sharing can be implemented better. Hence
for load sharing applications, the high slip motors are
preferred [7]. On the other hand, high-slip motors have
higher copper loss and lower efficiency. Therefore, using
the traditional method for load sharing has the trade-off
between the proportional load sharing and high
efficiency.
Clearly load sharing has several advantages. Some of
the advantages include: limitless increasing of the power
and consequently increasing the effectiveness, optimum
energy consumption, and generally economic benefits.
However using identical, high slip and low efficiency
motors are the shortcomings of the traditional load
sharing methods. These shortcomings are originated from
this fact that a simple V/F drive is not able to torque
control. However drives with torque control ability are
complicated and expensive and sometimes the traditional
methods are preferred in trade off between traditional
methods and drives with torque control ability. This is a
reason for modifying the V/F drives in order to achieve
proportional load sharing among any number of different
motors with comparative price.
3. Proposed Scheme
n the proposed approach IMs torque control and load
sharing is performed by modifying the reference speed of
the IM drives. If different speed references are applied to
the IMs, different voltages with different frequencies will
be fed to the IMs and consequently from (2) the IM
torque values will be changed. So with increasing the
speed reference of the IM, which is operating below the
rated torque, contribution of the IM from load torque will
be increased and vice versa. In this method to determine
final speed reference of the IMs a speed correction block
is located next to the regulator block.
Speed correction block needs the speed reference
which is determined by speed regulator block and torque
values of IMs which are obtained from torque estimation
methods [9] to calculates the final speed reference for the
IMs. In this method just one speed regulator and speed
correction block is used to torque control of a number of
IMs and share the load proportional to their rated power.
It should be noted that the speed correction block just
needs the rated power of the IMs for load sharing and
does not rely on any motor parameter. However when
torque estimation methods are used the stator resistance
will be needed. Stator resistance value is almost a
constant value and simple to determine.
The new scheme is shown in Fig. 4. The speed
regulator block is used for adjusting the IM speed to the
speed command. In speed regulator block the speed
command is compared to IM speed feedback and an error
signal is generated. The PI controller converts the error
signal to a suitable reference speed. The obtained
reference speed is delivered to speed correction block. In
speed correction block, the total torque developed by the
IMs is calculated. Then the torque reference for each IM
proportional to its rated power is obtained from
aggregated torque. The torque reference of each IM is
compared to its developed torque and an error signal is
generated. A PI controller converts the error signal to
speed reference adjustment signal. The signal adjusts the
speed reference to suitable speed reference that shares the
load proportionally as shown in Fig. 5.
Fig. 4: Block diagram of the proposed improved V/F scheme.
Fig. 5: Torque-speed characteristics of IM1 and IM2 using the proposed
improved V/F scheme.
The output torque values of the motors by using
proposed approach are given in Table III. Comparison of
Table II and III, shows capability of proposed scheme.
This method can be readily extended to several different
motor. As it is mentioned, this method does not need the
motor parameters and so it has impressive accuracy and
flexibility. The proposed method is very simple and easy
to implement.
4. Simulation Results
To provide a benchmark for evaluating the
performance of proposed scheme simulation results are
presented in two different parts. In the first section, two 5
and 10 hp IMs are considered. In the second section, the
simulation is done with three 5, 10 and 20 hp IMs.
The simulations are performed in MATLAB\Simulink
environment. The parameters and rated values of the IMs
are given in the appendix. In each section two types of
diagrams are presented as follow: (1) reference speed and
actual speed of the IMs, with and without the proposed
scheme (2) developed torque of each IM with and without
the proposed scheme.
4.1 Section 1
This section investigates the accuracy and performance
of the proposed approach and sharing the load between
different IMs. Two 5 and 10 hp IMs, IM1 and IM2
respectively, are considered. A mechanical load of 60
N.m has been applied to the machines in second one. The
simulation results are presented in Fig. 6-8.
0.5 1 1.51600
1650
1700
1750
1800
1850
1900
1950
2000
Time (sec)
Speed (
rpm
)
Actual Speed
Reference Speed
(a)
0.5 1 1.5
1600
1700
1800
1900
2000
Time (sec)
Speed (
rpm
)
Actual Speed
Reference Speed
(b)
0.5 1 1.51600
1700
1800
1900
2000
Time (sec)
Speed (
rpm
)
Actual Speed
Reference Speed
(c)
Fig. 6: Speed diagram of: (a) IM1and IM2 using the traditional V/F scheme; (b) IM1 using the proposed improved V/F scheme; and (c) IM2
using the proposed improved V/F scheme
0.5 1 1.5-10
0
10
20
30
40
Time (sec)
Torq
ue (
Nm
)
(a)
0.5 1 1.5-20
0
20
40
60
80
Time (sec)
Torq
ue (
Nm
)
(b)
Fig. 7: Torque diagram using the traditional V/F scheme for: (a) IM1;
and (b) IM2.
0.5 1 1.5-10
0
10
20
30
40
Time (sec)
Torq
ue (
Nm
)
(a)
0.5 1 1.5-20
0
20
40
60
80
Time (sec)
Torq
ue (
Nm
)
(b)
Fig. 8: Torque diagram using the proposed improved V/F scheme
for: (a) IM1; and (b) IM2.
Table I Reference speed of im1 and im2
IM No. Power (hp)
Speed reference using the traditional scheme (rpm))
Speed reference using the proposed scheme (rpm
1 5 1841 1845.5
2 10 1841 1839
Table II Electromagnetic torque developed by IM1 and IM2 using the
traditional V/F scheme.
IM No. Power (hp) Te (Nm) e
rated
T
T
1 5 17.95 89.75%
2 10 42.05 105.125
Table III Electromagnetic torque developed by IM1 and IM2 using the
proposed improved V/F scheme.
IM No. Power (hp) Te (Nm) e
rated
T
T
1 5 20 100%
2 10 40 100%
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4.2 Section 2
This section demonstrates ability of the proposed
approach for extending to any number of different IMs
with any arrangement. In this section, three IMs, 5, 10
and 20 hp are considered as shown in Fig. 9.
Fig. 9: Block diagram of the proposed improved V/F scheme.
A mechanical load of 140 N.m has been applied to the
machines in second one. The results are shown in Fig. 10-
12.
0.5 1 1.51600
1650
1700
1750
1800
1850
1900
1950
2000
Time (sec)
Speed (
rpm
)
(a)
0.5 1 1.51600
1650
1700
1750
1800
1850
1900
1950
2000
Time (sec)
Speed (
rpm
)
Actual Speed
Reference Speed
(b)
0.5 1 1.51600
1650
1700
1750
1800
1850
1900
1950
2000
Time (sec)
Speed (
rpm
)
Actual Speed
Reference Speed
(c)
0.5 1 1.51600
1650
1700
1750
1800
1850
1900
1950
2000
Time (sec)
Speed (
rpm
)
Actual Speed
Reference Speed
(d)
Fig. 10: Speed diagram of (a) IM, IM2, and IM3 using the traditional
V/F scheme; (b) IM1 using the proposed improved V/F scheme; (c) IM2
using the proposed improved V/F scheme; and (d) IM3 using the proposed improved V/F scheme.
0.5 1 1.5-20
-10
0
10
20
30
40
50
Time (sec)
Torq
ue (
Nm
)
(a)
0.5 1 1.5-20
0
20
40
60
80
Time (sec)
Torq
ue (
Nm
)
(b)
0.5 1 1.5-50
0
50
80
100
150
200
Time (sec)
Torq
ue (
Nm
)
(c)
Fig. 11: Torque diagram using the traditional V/F scheme for: (a) IM1;
(b) IM2; and (c) IM3
0.5 1 1.5-40
-20
0
20
40
60
Time (sec)
Torq
ue (
Nm
)
(a)
0.5 1 1.5-40
-20
0
20
40
60
80
Time (sec)
Torq
ue (
Nm
)
(b)
0.5 1 1.5-100
0
80100
200
Time (sec)
Torq
ue (
Nm
)
(c)
Fig. 12: Torque diagram using the proposed improved V/F scheme for:
(a) IM1; and (b) IM2.
Table IV Reference speed of IM1 and IM2
IM No. Power (hp)
Speed reference using the traditional scheme (rpm))
Speed reference using the proposed scheme (rpm
1 5 1832 1845.2
2 10 1832 1838
3 20 1832 1827.7
Table V Electromagnetic torque developed by IM1 and IM2 using the
traditional V/F scheme.
IM No. Power (hp) Te (Nm) e
rated
T
T
1 5 14.28 71.4%
2 10 33.85 84.625%
3 20 91.87 114.84%
Table VI Electromagnetic torque developed by IM1 and IM2 using the
proposed improved V/F scheme.
IM No. Power (hp) Te (Nm) e
rated
T
T
1 5 20 100%
2 10 40 100%
3 20 80 100%
The results and torque values given in Table IV-VI,
show the development ability of proposed approach. As
shown, by using the proposed scheme the load is shared
proportional to the IMs rated power with an impressive
accuracy. Also in this method gain values of the PI
controllers are constant and they should not be adjusted
with any change in arrangement and type of IMs. This is
another advantage of this approach
5. Conclusion
A new approach to torque control of induction motors
in load sharing application with V/F drives is proposed.
Performance and accuracy of proposed method for load
sharing between different IMs is investigated. Simulation
results verify effectiveness, accuracy and flexibility of
proposed method in different arrangement. This method
is more accurate and demonstrates better load sharing
than the previous methods.
The independency of the approach to motor
parameters is specified. Also it is mentioned that if the
torque estimation methods are used, the stator resistance
value will be needed. This parameter is almost a constant
value and simple to determine.
Feature works will devote to energy consumption
reduction and optimization. In feature schemes, the
quantity of load will determine the motor types which
provide the load. So with increasing the load the number
of IMs which provide load is increased and vice versa.
Appendix
Induction Machines Parameters:
A. IM1
5 hp, 460 V, 60 Hz, 1750 rpm, 4 Pole.
rs=1.115 Ω, rr=1.083 Ω, Xls=2.25 Ω, Xlr=2.25 Ω,
Xm=76.793 Ω, Treated=20 N.m, , J=0.02 N.m
B. IM2
10 hp, 460 V, 60 Hz, 1760 rpm, 4 Pole.
rs=0.6837 Ω, rr=0.451Ω, Xls=1.5653 Ω, Xlr=1.5653 Ω,
Xm=56.021 Ω, Treated=40 N.m, , J=0.05N.m
C. IM3
20 hp, 460 V, 60 Hz, 1760 rpm, 4 Pole.
rs=0.2761 Ω, rr=1645 Ω, Xls=0.826 Ω, Xlr=0.826 Ω,
Xm=28.7 Ω,Treated=80 N.m, , J=0.1N.m
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