Induction Motor Speed Control Using Power Line Communication C. Konate1, A. Kosonen2, J. Ahola2, M. Machmoum1, and J. F. Diouris1 1
Institut de Recherche en Electrotechnique et Electronique de Nantes Atlantique, CRTT 37, Bd de l’université, BP 406, 44602 Saint-Nazaire Cedex (France)
[email protected] 2 Lappeenranta University of Technology, Institute of Energy Technology, Department of Electrical Engineering, P.O. Box 20, 53851 Lappeenranta (Finland)
[email protected] Abstract—It is shown that an inverter-fed power line is a feasible medium for transmission of modulated information. This link is also a suitable alternative for the sensor cabling between motors and inverters in motor control and diagnostic applications. The use of the motor power cable in the speed feedback induction motor control application is presented in this paper. Keywords—Power line communication, control, induction motor, coupling circuit.
A
inverter,
motor
I. INTRODUCTION
S in on-line condition monitoring and control applications, physical wiring is needed between sensors installed at the motor end. Generally, in electric drives, the control station i.e. a frequency converter is located far (about 100 m) from the motor or the load. However, the cost of additional cabling installed in an industrial environment can be between $60 and $6000 per meter [1]. In addition, these cables can be unreliable because of the length, danger to damage, and external interferences depending on the method of signal transmission. At the same time, the motor is inevitably fed by an inverter through a power cable. An on-line winding temperature monitoring system for an inverter-fed induction machine using its power cable as a communication medium is described in [2]. The communication bandwidth is 9600 bps with the 3.5 MHz and 6.5 MHz FSK (Frequency Shift Keying) modulation frequencies used in the study. A motor cable is also used as a feedback channel for an encoder signal in the real-time control of a servosystem in [3]. The development of PLC (Power Line Communication) has led to a suitable way to combine power and broadband communication in the motor cable of inverterfed electric drives as is described in [4] and [5]. The practice takes advantage of reducing installation and maintenance costs. It simplifies also the system configuration. This method, This work was supported in part by ABB Ltd, the Finnish Funding Agency for Technology and Innovation (TEKES), the Finnish Graduate School of Electrical Engineering (GSEE), and the French Ministry of Higher Education.
introduced in [4] has been further considered in [6] in speed control of induction motors. This paper describes the communication channel and the use of the inverter-fed power cable to carry the speed sensor information to the speed controller in an induction motor speed control application. The structure of the paper is following. Section II describes the communication channel, its characteristics, and its frequency-domain model in the frequency band of 100 kHz−30 MHz. In Section III, data transmission in this disturbed environment is described. Section IV presents shortly the speed control of an induction motor. Section V deals with the laboratory experiments carried out in the Laboratory of Power Electronics at Lappeenranta University of Technology. Finally, the paper is summarized in the conclusion section. II. COMMUNICATION CHANNEL A. Channel Description The constructed test environment is built in the Laboratory of Power Electronics at Lappeenranta University of Technology. The drive under test comprised an Invensys 15 kW induction motor, a 90-meter-long Pirelli MCCMK 3x35+16 motor cable, an output filter (du/dt), an ABB ACS400 frequency converter, PLC modems, and coupling interfaces. The modems were connected differentially between two phases (L1, L2) of the power cable by capacitive coupling interfaces described in [4] and [5]. The data transmission concept is depicted in Fig. 1. The transmission characteristics of inverter-fed power line are completely different from those of traditional power lines that are presented in [7] and [8]. The inverter output voltage consists of pulses or square waves with variable frequency and duration [2]. An inverter-fed power line has completely different impedance characteristics than the normal power line because of the semiconductors at the output stage of an inverter. The inverter generates new frequencies distorting the output frequency. Thus, it requires much wider spectrum for power delivery. These make the inverter-fed power line
Low voltage grid
Field/process bus
Frequency converter
~
Electric motor
Motor cable
elements. This approach has been adopted to form a frequency-domain channel model in [5] and [13]. The relation between the input voltage Uin and the current Iin, and the output voltage Uout and the current Iout can be described (Fig. 2) as follows:
Load
M
~
Measurements
⎡U in ⎤ ⎡ A B ⎤ ⎡U out ⎤ ⎢ I ⎥ = ⎢ C D⎥ ⎢ I ⎥ , ⎦ ⎣ out ⎦ ⎣ in ⎦ ⎣
PLC modem Tx/R x
PLC modem Tx/R x
Fig. 1. Data transmission concept between an electric motor and a frequency converter.
different from the ordinary power supply. The cable that is the communication link in this application is characterized by its characteristic impedance Z0, attenuation coefficient α, and length L. The dielectric losses of the insulation material (normally PVC) dominate the signal attenuation in the cable. The characteristic impedances of low voltage power cables are typically in the range of 5−50 Ω depending mainly on the signal coupling, the insulation material, the cable cross-sectional structure, and the frequency. The characteristic impedance can be written as: Z0 =
r + jlω , g + jcω
(1)
where r, l, g, and c denote the per unit length resistance, inductance, conductance, and capacitance, respectively. According to [9], the attenuation coefficient can be estimated for PVC insulated low voltage motor cables as follows: ⎛ ⎝
α ( f ) = 0.5 ⋅ 10 − 6 ⋅ ⎜ f ⋅
1 ⎞ ⎟ Hz ⎠
0.6
1 , m
(2)
where f is the frequency. The motor is considered as the termination impedance of a motor power cable. The input impedances of low voltage electric motors are studied in [10] and [11] in the frequency band of 10 kHz−30 MHz and 10 Hz−10 MHz, respectively. The input impedance varies in the range of 1 Ω−10 kΩ depending on the type of the motor, the signal coupling, and the frequency. Thus, there exists impedance mismatch between the motor and the motor cable at the motor end. The impedance mismatch leads to signal reflections, and thereby multi-path propagation. The reflection coefficient ΓR at the cable end can be written: Z − Z0 , ΓR = L ZL + Z0
where A, B, C, and D are the frequency dependent coefficient matrices. The transfer function is then formed as the ratio of the output and the input voltage as follows: H=
B. Channel Model
All the elements in Fig. 1 are taken into account by the model of the channel. According to [9] and [12], all the components can be modelled as two-port transmission line
U out ZL . = Us AZ L + B + CZ L Z S + DZ S
(5)
The modelled elements are described by ABCD transmission matrices. These matrices can be combined by the chain rule to form the transmission matrix T: n
T=
∏T ,
(6)
i
i =1
where n is the number of network sections. The equivalent matrix T gives the relations between the input voltage and current, and output voltage and current of the communication channel (Fig. 2). The channel model is verified by measurement in the built test environment with a network analyzer Agilent 4395A with a S-parameter test set device Agilent 87511A. The network analyzer gives the scattering parameters of the channel. These parameters consist of the transmitted and reflected powers. S11 and S22 are the reflected powers, while S12 and S21 are the power attenuation through the channel for different directions. The modelled and measured transfer functions are shown in Fig. 3. The measurement equipment causes problems above 14 MHz frequencies. On the other hand, the individual component models that are combined by the chain rule (6) are verified by the measurement up to 30 MHz frequency in [5]. Thus, the modelled transfer function can be kept adequate. A complete channel model includes also the noise scenario. In the inverter-fed power line, the noise is also different from ordinary power line because of the high speed switchings of semiconductors, such as IGBTs (Insulated Gate Bipolar ZS
(3)
where ZL is the load impedance.
(4)
Us
~ Signal source
Iin +
Uin -
A
C
B
Iout
D
Two-port network
+ Uout -
ZL
Load
Fig. 2. Two-port network connected to a signal source and a load impedance. The signal source consists of a voltage source and a serially connected impedance.
-10 Measured Modelled
-20 Transfer function (dB)
-30 -40 -50 -60 -70 -80 -90 -100
5
10
15 20 Frequency (MHz)
25
30
Fig. 3. Modelled and measured transfer function in the frequency band of 100 kHz−30 MHz. [5]
Transistors). In modern electric drives, the output voltage rise or fall times (du/dt) of IGBTs are in the range of 0.1−10 µs, and the switching frequencies vary between 2 and 20 kHz [14]. The inverter injects pulses with wide frequency spectrum and high energy content into the feeder cable of an electric motor as illustrated in Fig. 4. The pulses cause problems in several ways. For example, they generate electromagnetic emissions, damage the winding insulation of a motor, generate bearing currents and cause problems for the control system of inverter [15]. The undesired waves can be considered as aperiodic impulsive noise in the viewpoint of communication. III. DATA TRANSMISSION
Voltage (V)
Data is transmitted to the power line through differential coupling interfaces. The coupling circuit consists of a high frequency transformer T1, coupling capacitors C1 and C2, 1000
t r = 4 µs
500 f = 105 kHz
0 -500
0
0.05
0.1 Time (ms)
0.15
0.2
PSD (dBm/Hz)
50
inductances L1 and L2 that all together act as a high pass filter, and small signal diodes D1…D6 to clip the over voltage peaks as illustrated in Fig. 5. It is used to separate the circuit of a motor cable and the circuit employed in signalling from each other. These two circuits do not have the same voltage levels. The coupling interfaces are similar both at the receiver and the transmitter ends. As the inverter-fed power line is a harsh environment for communication, the choice of modulation techniques, error correcting codes, and applied frequencies are very important when designing reliable communication systems. On the other hand, the topology of the communication channel simple and it can be considered a type of point-to-point. Now, the communication is implemented with HomePlug 1.0 compliant modems [16]. The PLC modems forms an Ethernet link over the motor power cable. They encapsulate the Ethernet frames (IEEE 802.3) into their own protocol and transmit them to the power cable. The throughput and latency of the proposed method were measured. Extensive throughput and latency test results are presented in [17] and [4], and in [18] and [17], respectively. IV. SPEED CONTROL OF INDUCTION MOTOR A. Control
An induction motor has widely spread into industrial applications, because of its simple structure and low cost. These motors are usually fed by frequency converters to adjust and control the rotation speed in different applications. In variable-speed electric drives, the vector control methods, such as DTC (Direct Torque Control) [19] or current vector control [20], do not necessarily require measurements from the rotor as feedback information. However, in demanding control applications, especially at low rotation speeds, also feedback information is required because of errors in the induction motor model parameters of a speed estimator in sensorless control methods. Typical measurements provided as feedback information to a motor controller are the motor rotation speed or the rotor angle [6]. Generally, a feedback loop from the motor to the controller requires a wired connection. In this work, the power cable between the frequency converter and the induction motor is
0
C1 L1
-50 10
2
10
3
4
5
L1
6
10 10 10 Frequency (Hz) Fig. 4. Measured output voltage of an inverter between phase conductors (L1, L2) at the inverter end and its frequency content. The mains voltage is 500 V and the cable length 90 m. The switching of an inverter output stage generates steeply rising (tr = 4 µs) surge waves followed by the cable oscillation (f = 105 kHz). The energy content is at highest around the output and switching frequencies of an inverter, and also near the cable oscillation frequency. [4], [5]
PE
L2 L2
T1
Tx/Rx
D1
D4
D2
D5
D3
D6
R1 Tx/Rx
C2 Fig. 5. Capacitive coupling interface for the low voltage motor cable of a three-phase inverter drive. [4], [5]
400 V, 50 Hz
~
Tref
Motor cable MCCMK 3x35+16 90 m
Frequency converter
~
dSPACE DS1103 PPC
Induction motor 15 kW
Load generator
225
DC
220
M
du/dt filter
Pulse encoder
Digital I/O
RS232
PLC modem
PLC modem Ethernet UDP
Ethernet UDP PICDEM.net
PICDEM.net
Rotation speed (rpm)
Electrical grid
1m
Direct Motor cable Reference
215 210 205 200
1.5 m 7m
195
Fig. 6. Diagram of the test equipment constructed in the Laboratory of Power Electronics at Lappeenranta University of Technology. [6]
0
0.5
1
utilized as a feedback channel for speed information. B. Ethernet in Control Applications
V. LABORATORY EXPERIMENTS AND ANALYSIS A. Speed Control System
The control system and the necessary peripheral devices are described in detail in [18]. The diagram of the test system is illustrated in Fig. 6. The dSPACE equipment is used as a speed controller. The dSPACE sends the 16-bit motor speed information computed according to the received data from the pulse encoder to the PLC modems. The speed controller implemented in the dSPACE computes the torque reference according to the received speed information from the motor cable. In the case of the direct feedback the pulse encoder signal is used directly for this task without circulating it via the motor power cable. The frequency converter is driven by the torque reference Tref. B. Performance Tests
The performance of the PLC method was tested in the speed control application. First, the step response tests were carried out to ensure that the PI (Proportional-Integral) speed
2
2.5
3
Direct Motor cable
0.3 0.2 Torque p.u
The latency of Ethernet is non-deterministic due to the CSMA/CD (Carrier Sense Multiple Access with Collision Detection) bus reservation mechanism. This causes problems in real-time applications, which require deterministic response, such as in control applications. According to [21], in applications that require a time delay less than 1 ms, Ethernet is not a practical solution. The utilization of RTE (Real-Time Ethernet) in industrial applications has been studied e.g. in [22]–[24]. The utilization of Ethernet in distributed motion control is studied in [25], in which the rotor feedback speed information of a brushless DC motor is delivered between the speed controller and the frequency converter by using a 10 Mb/s Ethernet LAN (Local Area Network) and frame size 64 bytes.
0.4
1.5 Time (s) (a)
0.1 0 -0.1 -0.2 -0.3 -0.4
0
0.5
1
1.5 2 2.5 3 Time (s) (b) Fig. 7. Step response test. (a) Rotation speed as a function of time. (b) Torque reference given by the controller during the test. [6]
controllers are tuned to be comparable as shown in [6]. The identical maximum overshooting from the reference value was allowed in the tests. Next, the ramp tests were carried out to test the tracking capability of control. In this test, the speed of the rotor was accelerated from zero speed to the nominal speed of the induction motor (1455 rpm) in about 0.8 s. Finally, the loading tests were carried out to reveal the control’s incompleteness or its weakness. In this test, the motor was driven at a constant speed of 750 rpm and in a random time instant a 58 % (of the motor nominal torque value) stepwise loading change was caused. C. Results and Analysis
The step response results obtained from the tests are illustrated in Fig. 7. The ramp results obtained from the tests are illustrated in Fig. 8. The loading results obtained from the tests are illustrated in Fig. 9.
760
1400
750
1200
740 Rotation speed (rpm)
Rotation speed (rpm)
1600
1000 800 600 400
720 710 700
Direct Motor cable Reference
200 0
730
0
0.5
1
1.5
2 2.5 Time (s) (a)
3
3.5
Direct Motor cable Reference
690 680
4
0
0.5
1
1.5
2 2.5 Time (s) (a)
3
3.5
4
1
1
Direct Motor cable
0.8
Direct Motor cable 0.8
Torque p.u
Torque p.u
0.6 0.4
0.6
0.4
0.2 0.2
0 -0.2
0
0.5
1
1.5
2 2.5 3 3.5 4 Time (s) (b) Fig. 8. Ramp test. (a) Rotation speed as a function of time. (b) Torque reference given by the controller during the test. [6]
The main difference between the use of the direct feedback link and the motor cable communication is the latency caused by PLC modems. The latency consists of different components, which are studied in [18]. Indeed, the latency component is time-variant and its average is 8.0 ms. The effect of the latency is more visible when the torque load is increased. It deteriorates the performance of the speed control system. Thus, the motor cable communication link cannot be suitable in all control applications. This method could be employed for non-time critical applications or applications, where system time constant is essentially longer than the feedback delay. On the other hand, the performance of the applied control method could be improved by using modern and sophisticated control methods, such as predictors and compensators, or by optimizing the communication methods for the control application. According to [26], a disturbance observer in the control loop increases significantly the stiffness to load disturbances in the applied control system.
0
0
0.5
1
1.5
2 2.5 3 3.5 4 Time (s) (b) Fig. 9. Loading test. (a) Rotation speed as a function of time. (b) Torque reference given by the controller during the test. [6]
VI. CONCLUSION This paper presents a control application that uses the power cable of an induction motor as a feedback loop in an inverter-fed electric drive. The communication channel made up by the power cable is modelled and verified by the laboratory measurements. The PLC modems cause time delay in communication, and hence set some limitations to the induction motor control system. The applied control method fits well for non-time critical control applications or systems that have time constant essentially longer than the feedback delay. The system can be improved by optimizing the PLC modems to work in specific control applications. This can be done by reducing the latency and jitter between modems with suitable communication methods.
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