Energy and Power En gi neering, 2011, 3, 107-112
doi:10.4236/epe.2011.32014 Published Online May 2011 (http://www.SciRP.org/journal/epe)
Energy Efficient Control of Three-Phase Induction
Motor Drive
Hussein Sarhan
Faculty of Engineering Technology, Al-Balqa’ Applied University, Salt, Jordan
Received March 7, 2011; revised March 25, 2011; accepted March 29, 2011
Abstract
Induction motors are extensively used in industrial and household appliances and consume more than 50% of
the total generated electrical energy. The need for energy conservation is increasing the requirements for
saving the electrical energy. It is therefore important to optimize the efficiency of electrical drive systems
under certain operating conditions. This paper proposes a new control scheme based on search method taking
advantage of the fact, that at a certain torque and speed (operating point) there is only one value of stator
voltage that operates the motor at optimum efficiency. Simulation performed and results are presented.
Keywords: Induction Motor Drive, Efficiency Optimization, Slip Compensation
1. Introduction
Induction motors are the most used in industry since they
are rugged, inexpensive, and are maintenance free. It is
estimated that more than 50% of the world electric en-
ergy generated is consumed by electric machines. Im-
proving efficiency in electric drives is important, mainly
for economic saving and reduction of environmental
pollution [1,2]. Induction motors have a high efficiency
at rated speed and torque. However, at light loads, motor
efficiency decreases dramatically due to an imbalance
between the copper and the core losses. Hence, energy
saving can be achieved by proper selection of the flux
level in the motor [3,4 ]. The main inductio n motor losses
are usually split into: stator copper losses, rotor copper
losses, core (iron) losses, mechanical and stray losses. To
improve the motor efficiency, the flux must be reduced,
obtaining a balance between copper and core losses.
Many minimum-loss control schemes based on scalar
control or vector control of induction motor drives have
been reported in literature [4-8]. Induction motor drive
can be controlled according to a number of performance
functions, such as input power, speed, torque, airgap flux,
power factor, stator current, stator voltage, and overall
efficiency [9]. Basically, there are three strategies, which
are used in efficiency optimization of induction motor
drive: Simple state control, model based control, and
search control. Search strategy methods have an impor-
tant advantage compared to other strategies. It is com-
pletely insensitive to parameters changes, while effects
of the parameters variations caused by temperature and
saturation are very expressed in two other strategies
[10-12]. In this paper, an efficiency optimization con-
troller of induction motor drive system, based on search-
ing the value of stator voltage that maximizes the effi-
ciency, is developed. Then, the validity of the proposed
controller and the performance of the drive system were
analyzed by simulation results. To reduce the slip at light
loads and low frequencies, a slip compensator has been
introduced.
2. Modeling and Simulation of Drive System
The original drive system studied in this paper consists
of IGBT-inverter-based AC to AC converter, three-phase
squirrel cage induction motor and Vf
controller. In
order to analyze the system performance, all of these
components should be modeled (mathematically de-
scribed). The inverter-based AC-to-AC converter is con-
sidered to be an ideal system, where the DC voltage at
the input of the inverter has no AC component, and the
output voltage of the filter at the output of the inverter
has no harmonics. For sinusoidal pulse width modula-
tion SPWM, the ratio of the amplitude of the sinusoidal
waveform to the amplitude of the triangular waveform
is called the modulation index , which can be in the
range of 0 to 1 [5]. The stator voltage
m
s
V can be de-
fined as:

108 H. SARHAN
s
n
VmV (1)
where = nominal value of stator voltage.
n
The frequency of the stator voltage f equals the fre-
quency of the sinusoidal input waveform
V
in
f
.
in
f
f (2)
Varying the modulation index and the sinusoidal
waveform frequency will change the RMS value of the
stator voltage and frequency, respectively. Equations (1)
and (2) constitute the steady-state model of inv erter.
The controller with constantVf

must apply the
following function:
,0
1,
n
n
K
ff
mf
f
f



 (3)
where 1n
K
f, and n
f
= nominal frequency.
Based on the flux linkages and voltages equations, the
electrical and mechanical models of the squirrel cage
three-phase induction motor with respect to a synchro-
nously rotating coordinates were adopted for
simulation [5,13]. The parameters of modeled and simu-
lated induction motor are given in Table 1.
dq
3. Design of Efficiency Optimization
Controller
The efficiency of the drive system at any steady state
operating point can be calculated as:
m
s
P
P
  (4)
Table 1. Induction motor data.
Parameter Value
Stator resistance 1
R
1.230

Stator Inductance 1
L
0.006 H
Mutual Inductance m
L
1.172 H
Rotor resistance referred to the stator 2
R
 1.395

Rotor Inductance referr ed to the stator 2
L
 0.006 H
Nominal voltage
n
V230/400 V
Nominal output power
n
P4 kW
Nominal power factor cos n

0.850
Nominal frequency n
f
50 Hz
Number of poles P 4
Nominal rotational speed
n
n1430 rpm
Moment of inertia J 0.013 2
kg m

where m PL


, mechanical output power and

3
2

s
sdsdsq sq
Pvivi, total input power supplied to the
stator.
To design the efficiency optimization controller based
on search method, a matrix, as shown in Table 2, con-
sisting of loads, frequencies and stator voltage values
(optimal voltage), which maximize the efficiency, was
constructed using MATLAB Simulink model of the
studied drive system .
The relationship between load torque, frequency and
optimal voltage is shown in Figure 1.
Based on the data in Table 2 and Equation (4), the re-
lationship between the maximum efficiency, load torque
and frequency is shown in Figure 2.
For the original system with constantVf con-
troller, the relationship between efficiency, load torque
and frequency is shown in Figure 3.
The matrix shown in Table 2 was used to form a 2D
lookup module (table) in MATLAB, which serves as the
main part of the proposed efficiency optimization con-
troller, shown in Figure 4. The output of the controller is
the modulation index m, which correspondences the op-
timal value of voltage maximizing the efficiency under
certain operation condition.
Simulation results showed that the main disadvantage
of the proposed efficiency optimization technique was
the increase in slip, especially at light loads and low fre-
quencies, as shown in Figure 5.
So, there is a need to compensate (reduce) the slip ac-
cording to some reference (acceptable) value ref
s
. This
can be achieved by increasing the electromagnetic torque
Table 2. Optimal stator voltage as a function of load torque
and frequency.
Load Torque, N·m
Voltage, V 5 10 15 20 25
5 32.8 46.4 57.6 65.6 73.6
10 58.4 86.4 100 116 130.4
15 83.2 116 143.2 176 190.4
20 109.6151.2 185.6 220 246.4
25 137.6196.8 236.8 267.2308.8
30 160.8222.4 272.8 324 349.6
35 189.6276.8 326.4 369.6400
40 220.8295.2 353.6 400 400
45 243.2340 391.2 400 400
Frequency, Hz
50 272.8372.8 400 400 400

H. SARHAN
109
Figure 1. Optimal voltage as a function of load torque and
frequency.
Figure 2. The relationship between maximum efficiency,
load torque and frequency .
Figure 3. The relationship between efficiency, load torque
and frequency for the original system.
Figure 4. Block diagram of efficiency optimization control-
ler.
Figure 5. The relationship between slip, torque and fre-
quency for the optimized system.
of the motor by increasing the stator voltage in the opti-
mized system. In this case, the error in slip will be gained
and integrated to get the value of compensation voltage
that should be added to the vo ltage in the optimized sys-
tem. The value of compensation voltage can be
calculated by the following equation: com
V

d
comi ref
VKss
t (5)
where i
K
= integrating constant.
Equation (5) can be realized by adding an integrator as
a feedback loop into the optimized drive system.
The MATLAB Simulink model of optimization con-
troller with slip compensation is shown in Figure 6.
4. Analysis of Optimized Drive System
For quantitative analysis, the efficiency of the optimized
system with slip compensation was compared with that
in the original system with V/f = constant controller un-
der the same operating conditions by using the simula-
tion results of the MATLAB Simulink model of the drive
system, shown in Figure 7.
Figures 8-10 are examples of the system behavior
under different operating conditions. Figure 8 shows tha t
the efficiency optimization controller increases the effi-
ciency of the drive system, and makes it approximately

H. SARHAN
110
Figure 6. The MATLAB Simulink model of optimization controller with compensation.
Figure 7. MATLAB Simulink model of optimized drive system with slip compensation.

H. SARHAN
111
Figure 8. The relationship between efficiency and torque at
frequency = 10 Hz.
Figure 9. The relationship between efficiency and frequency
at torque = 10 N·m.
Figure 10. The slip response at torque = 15 N·m and fre-
quency = 5 Hz.
constant for all the range of load. Slip compensation re-
duces the efficiency of about 4%.
Figure 9 shows the relationship between efficiency
and frequency at load torque = 10 N.m, form which, it is
clear that optimization has not significant effect on the
drive system performance.
Figure 10 shows the slip response at load torque = 15
N.m and frequency = 5 Hz, from which it is clear that
optimization and slip compensation have significant ef-
fect on slip.
5. Conclusions
In this paper, an efficiency optimization controller, based
on search method has been developed. The proposed
controller manipulates the value of stator voltage that
maximizes the efficiency at any given operating point.
To reduce the slip to a certain value, a slip compensator
has been inserted into the drive system. The suggested
technique can be used in variable frequency, variable
load electrical drive systems. Based on simulation analy-
sis, it was noticed that the compensated optimized sys-
tem gives significant results at low frequencies and light
loads.
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