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J. Electromagnetic Analysis & Applications, 2009, 2: 114-117
doi:10.4236/jemaa.2009.12018 Published Online June 2009 (www.SciRP.org/journal/jemaa)
Copyright © 2009 SciRes JEMAA
Air Compressor Control System for Energy Saving
in Locomotive Service Plant
Department of Control Science of Engineering, Huazhong University of Science and Technology, Wuhan, China.
Received January 2nd, 2009; revised March 12th, 2009; accepted March 20th, 2009.
The actuality and disadvantages of traditional high power asynchronism motor drive air compressor in locomotive ser-
vice plant are discussed. In order to reduce the energy consumption and obtain safe running, a variable frequency con-
trol method to the motor is sup plied. A PLC with touch screen is used for monitoring the status of th e compressor and
its control system. It also presents energy consumption analysis caused by the variable frequency control method in a
locomotive service plant.
Keywords: Variable Frequency, Air Compressor, Locomotive Service Plant
Air compressor is a key equipment to provide air power,
which is driven by asynchronism motor. Air compressor
is operated by adjusting rotation speed of the motor. In
general, the output air pressure in pip eline of the air tank
is acted as controlling object in the system.
The air pressure in the pipeline is controlled to be
fluctuate in a certain range. Its upper limit (0.7MPa) is
under the rating pressure of the pipeline, and its lower
limit (0.4MPa) is above the rating pressure of the equip-
ment using the compressive air. Usually, there are two
ways to control the air pressure in the pipeline to content
The first method is starting up and stopping the motor
continually for adjusting air pressure in pipeline. When
the air pressure is under the upper limit, the air compres-
sor operates until the air pressure goes up to the upper
limit, then the motor stops running. But the air pressure
will be lowered with air leak or air consuming equ ipment
operating. When the air pressure goes down to the lower
limit, the motor begins to work and air compressor oper-
ates again. The variable air pressure in the pipeline is
shown in Figure 1. This method is simple and low cost,
but it is suitable for small power motor because the motor
will be started up continually.
The second method is using pressure valve to limit air
pressure in th e pip elin e. When th e p ressure go es up to th e
upper limit pressure, the valve will close entrance of the
air compressor, then the compressor is in idle state. In
this case, the compressor is still driven in operation by
the motor, but it does not export compressive air, so the
air pressure will not go up further. When the pressure
goes down to the lower limit, the valve will be open
again, and then the compressor exports compressive air
and the air pressure will be up again. The variable air
pressure is same as that of the first method. In this case,
motor is running continuously, which can be used for
high powe r motor.
In locomotive serve plant, the second method is widely
used for control the air pressure from air compressor be-
cause of high power motor being used, which rating
power is about 100KW. Although the motor is in running
operation, its starting up should be controlled. Tradition-
ally, there are two ways to fulfill the starting up, which
are linking series resistor in the rotor loop and converting
Y-connection of the starter loop . However, there △
are still some disadvantages in these two ways as follows.
1) The air pressure in pipeline fluctuates greatly be-
tween the upper and lower limits.
2) The con tinual upload and d ownload of air comp res-
sor causes voltage fl uct uati o n in electrical power supply.
Figure 1. Variable air pressure in pipeline
Air Compressor Control System for Energy Saving in Locomotive Service Plant 115
Copyright © 2009 SciRes JEMAA
3) The air compressor is in full speed rotation all the
time, which may lead to mechanical failure and tempera-
4) The air compressor and the air pressure valve in
their operations cause a great noise pollution.
5) The driving motor of air compressor is inefficient
and energy consumptive and cause low power factor.
So, it is necessary to change the traditional control
method of the air compressor operation for energy saving,
less pollution and low failure possibility.
2. Variable Frequency Control of Air
2.1 System Principle
With the rapid development of power electronics tech-
nology, transducer is widely used in adjusting speed of
AC asynchrony moto r . So, a tran sducer is design ed as
an executor in the air compressor control system to adjust
output air pressure.
In order to control the air pressure in the pipeline, the
motor’s rotation speed should be in control. However, the
motor disables to change its ro tation speed itself, the only
method is to adjust its frequency to change the rotation
speed. So, a transducer used to control the motor’s rotat i o n
speed, then, the flux from air compressor can be adjusted.
The system schematic diagram is shown in Figure 2.
After comparing enactment pressure signal with feed-
back pressure signal, a pressure signal error is obtained,
which is used for calculation of PID arithmetic and then
converted the error signal to a control value to adju st fre-
quency of AC power supply. Then the asynchronies mo-
tor will drive air compressor to an appropriate rotation
speed with variable frequency power supply to eliminate
the pressure error and ensure a constant air pressure.
The air pressure adjusted by PID arithmetic in the
pipeline is shown in Figure 3. In general, a transducer has
an inner PID control unit.
Figure 2. System schematic diagram
(=Enactment pressure △– Feedback pressure)
Figure 3. Air Pressure adjusted by PID arithmetic
0: negative effects, when △ > 0, transducer output fre-
quency raises; △ < 0, frequency declines.
1: positive effects, when △ > 0, transducer output fre-
quency declines; , frequency raises.△
When the pressure detected by pressure sensor is
higher than the enactment pressure, PID regulator output
signal declines and the transducer output frequency falls
down, then the air compressor rotation speed reduces and
the output air pressure declines; when the detected pres-
sure is lower than the enactment pressure, PID regulator
output signal raises and the transducer output frequency
increases, then the air compressor rotation speed in-
creases and the output air pressure increases. The system
controls the air pressure automatically through the above
In Figure 4, a transducer of YASKAMA 616PC5
forms the air pressure feedback control system. On the
transducer, FS, FV, FI and FC are ports of pressure en-
actment and feedback input signal. The FS provides
power supply (+15V). An input voltage, which deter-
mines frequency o f AC power supply from the transducer,
is linked to the FV port from a resistor (4.7K). A feed-
back voltage of air pressure in the air tank detected by a
remote pressure gauge is linked to FI port. These two
signals are compared in the transducer and an error can
be calculated, by which PID arithmetic is used for calcu-
lating control variable. The transducer has self-educated
ability, i.e. PID parameters can be adjusted automatically
in terms of actual pressure change characteristic in the
S1, S3, S4 and SC are ports for several control func-
tions. When K1 is closed, the transducer operates nor-
mally. If there is a failure outside the transducer, K2 will
be closed, and the transducer will stop operation. In this
116 Air Compressor Control System for Energy Saving in Locomotive Service Plant
Figure 4. Connection circuit of the transducer
case, a signal can be detected in S3 port to protect the
whole system. However, if there is a failure in the trans-
ducer, MA and MC ports will be connected in it, by
which control system (PLC) can stop the whole system
operation. When the failure is removed, control system
makes K3 close, a signal inputs to the S4 port, which can
reset the transducer. M1 and M2 ports output running
state signal. Three-phrase AC power supply are linked to
R, S, and T ports on the transducer. Then, the variable
frequency AC power supply can be exported from U, V,
and W ports.
A PLC acts as a control unit in the system, from which
switches K1, K2 and K3 can output relative signals. Sig-
nals from ports M1, M2, MA and MC on the transducer
can also be input to the PLC.
2.2 Existing Problems and Solutions
Using the transducer would generate harmonic wave. The
external input industrial frequency power supply AC
(380V/50Hz) is rectified into DC by three-phrase bridge.
Finally, it is inverted into any frequency AC power sup-
ply by high power transistors after capacitor filtering. In
the rectifier circuit, the input current waveform is irregu-
lar rectangular wave. The wave is classified into basic
wave and harmonic wave based on the Fourier series.
The high-order harmonic would interfere with the power
supply system, and damages transforms, motors, capaci-
tors, switches and so on.
Solutions for the input side of transducer: 1) setting
reactance to increase rectifier impedance for improving
the rectifier overlap angle; 2) parallel using AC filter in
the power circuit to separate high-order harmonic at all
levels respectively from the power supply system.
Solutions for the output side of transducer: using high
frequency switch components, adding filter equipments
and adopting closed-loop control, using isolation, shield-
ing, grounding and reasonable routing to improve the
high-order harmonic interfere nce.
Besides, the starting torque required when the air com-
pressor starts up is large. However, using the conven-
tional method to control will bring damage to other rele-
vant equipments. This system adopts vector control tech-
nical to raise the starting torque of motor. Besides, more
accurate rotation speed can be gained to control the air
compressor. So, almost constant pressure of air can be
acquired from this system.
3. PLC Monitor and Control
3.1 Status Parameters Monitored
Cooling water and lubricating oil are the necessary sub-
stances for air compressor running normally. When the
air compressor operates, temperature rise of the com-
pressor body can be used to monitor if the compressor
runs normally, or not. So, there are three parameters to be
monitored in the system, which are pressure of cooling
water, pressure of lubricating oil and temperature of the
compressor bo dy.
The pressure of cooling water can be used to show
cooling system normal operation easily, which includes
pump, pipeline and valves. Any problem can cause the
pressure abnormal. So, a pressure sensor is mounted at
output of cooling water pump for monitoring the whole
cooling water system. In the same way, another pressure
sensor is used for the lubricating oil monitoring.
As the temperature of compressor body is one of syn-
thetic images to show all malfunctions . In order to meas-
ure temperature of the compressor body, a temperature
sensor is mounted at the output pipeline of cooling water
from the compressor. The temperature of cooling water
from the output pipeline is almost equal to the compres-
sor body temperatur e.
All these sensors are mounted on pipeline outside the
compressor, so it is convenient for maintenance and re-
3.2 Design of PLC Monitor and Control
In order to monitor and control the whole air compressor
system, a PLC with touch screen is used . The PLC is
composed of power module, CPU module, analog input
module, digital input module and digital output module.
The configuration of PLC system is shown in Figure 5.
A touch screen is connected to the CPU module, which
can display status of the transducer, pressure in the air
tank, pressures of cooling water and lubricating oil, and
so on. It also can accept many touch instructions instead
of mechanical buttons. The CPU module can receive all
kinds of data from analog and digital module by interior
bus. It also has memory for program and data, and a se-
rial bus RS232C for connecting with a compu ter.
The power module provides power supplies for every
module, which includes +5V and +12V voltages.
Copyright © 2009 SciRes JEMAA
Air Compressor Control System for Energy Saving in Locomotive Service Plant 117
Copyright © 2009 SciRes JEMAA
Table 1. Testing parameters 75KW motor
Supply 380 117 50 1450 69.5
320 80 42 1210 39.8
300 working days, the air compressor control system
will save energy in one year as fo llows.
Obviously, the new system can save lots of energy.
Besides, it has ot her advantag e s:
1) It reduces greatly noise, realizes soft start and soft
stop of the equipment and avoids the shock current of
power grid when the air compressor starts up.
Figure 5. Configuration of the PLC
The analog input module (0~+5V input) accepts volt-
age signals from the sensors to measure the compressed
air pressure, cooling water pressure, lubricating oil pres-
sure and temperature of the cooling water.
2) It has high degree of automation and overcomes the
disadvantages of manual adjustment.
The digital input module (12V input) accepts status
signal of the transducer from M1 and M2, failure signal
of the transducer from MA and MC, and some operation
signals from outside buttons .
High power air compressor is a kind of equipment widely
used in locomotive serv ice plant. In order to save electri-
cal energy and improve operation condition, it is neces-
sary to rebuild traditional control system of high power
motor with the transducer and PLC system. In fact the
system also has low cost, high reliability and efficiency.
It also reduces greatly noise of air compressor operation
and possibility of failure. Finally, this system has solved
the interference by the high-order harmonic and adopted
vector control technique to acquire high starting torque
and stabilize motor rotation speed. As the effect of this
system is obviously in energy saving, it should be widely
used in such a place as locomotive service plant.
The digital output module (relay output) sends out sev-
eral control signals, which are K1, K2 and K3, operation
status signal, and failure signal.
4. Experimental Results
We use a PLC and a transducer to rebuild one air com-
pressor control system in a locomotive service plant,
which use a high power motor of 75KW rating power. Its
other rating parameters are frequency of 50Hz, voltage of
380VAC and current of 150A. We have measured a set of
actual parameters of the motor and its power supply at
the same regular load condition before and after the re-
building, which are listed in Table 1. On the majority of
time, the motor can run normally at 42Hz frequency of
power supply to content air supply requirement of the
 X. Z. Deng, “Control of electrical and mechanical driv-
ing,” Wuhan: Publishing Company of Huazhong Univer-
sity of Science and Technology, pp. 153–178, 2002.
From the Table 1, it can be shown that current decrease
rate is  O. Ojo, Z. Q. Wu, G. Dong, and S. Asuri, “Variable fre-
quency control of an induction motor drive with reduced
switching devices,” IEEE International Conference on
Electric Machines and Drives, San Antonio, TX, United
States, pp. 1385–1391, May 2005.
δ = ( I1 – I2 ) / I1
= (117 – 80) / 117
Electricity energy saving rate is  B. Georges an d J. Aubin, “Application of PLC for on-line
monitoring of power transformers,” IEEE Power Engi-
neering Society Winter Meeting, Columbus, OH, United
States, Vol. 2, pp. 483–486, February 2001.
η = ( P1 – P2 ) / P1
= (69.5 – 39.8) / 69.5
As everyday has three shift working time and one year has  A. D Kurtz, et al., “High accuracy miniature pressure
transducer,” International Instrumentation Symposium,
Vol. 470, pp. 303–318, 2007.
W = ( P1 – P2 ) × 24 × 300
= (69.5 – 39.8) ×24 × 300
= 2.14×105 (KWh)