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Detection of Rotor Position of Permanent Magnet Synchronous Motor Faeka M. H. Khater1, Farouk I. Ahmed2 and Mohamed I. Abu El-Sebah1 1 Electronics Research Institute Cairo, Egypt Tel: (202)3310552 E-mail:
[email protected]
2 Cairo University Giza, Egypt Tel: (202)5738579 E-mail:
[email protected]
Abstract: This paper presents a new method for rotor position detection and speed estimation of permanent magnet synchronous machines. Information is obtained by monitoring controller signals to eliminate position and speed sensors. Drive system model is developed for free running machine and sensorless vector control operation then simulation is carried out to verify effectiveness of the proposed method. The method can reduce the system cost and improve the drive system reliability Keywords: Sensorless vector control, PMSM drive system, position detection, and speed estimation.
1.Introduction Recently, permanent magnet ( PM ) machines have occupied a wide range in high performance drive systems because of the simplicity in construction and maintenance. The complexity of permanent magnet control compared with dc motor, and higher cost compared with induction motor limit the permanent magnet applications. In the latest 30 years field orientation control ( FOC ) technique makes the control of the permanent magnet motor to emulate the dc machines in a simple manner. Also a great attention is given to cost reduction of permanent magnet material by producing a low cost magnetic materials 1-4). The Machine model of the drive system is presented in the stationary reference frame α,β and the control implemented in the rotating reference frame d,q. The rotating reference frame is the preferred coordinate system used for synchronous motors control via a d,q transformation. The control of current components requires the knowledge of the instantaneous rotor position (θ). A new proposed method can be implemented to detect isd and isq from the three phase currents ia, ib and ic. Traditionally speed and position sensing was obtained using transducer and interface components which are temperature and noise effected, such sensitivity produces a variations causing the system to need regular adjustment, as error increases, the reliability of the system decreases. In addition it has a difficulties in fixing and manipulating beside adding a high cost to the drive system and reducing the reliability. In high performance drive where constraints, efficiency, reliability, mechanical and cost are all very
important, it is not possible to use a speed, position or a torque sensor. In such situations, the necessary information can be derived via current and voltage sensors 5,6). In this paper, new position detection and speed estimation method has been introduced to eliminate position and speed sensor. It depends on monitoring the controlled output signal to the drive system. In such situations, the necessary information exists within the controller itself. This new method enables cost-effective design of intelligent controllers for permanent magnet synchronous motors, which can satis fy better operation, consisting of fewer system components, lower system cost and improved performance.
2. The proposed method The position detection and speed estimation method depends on the output control signal and the structure of the system controller without measurement. The stator voltage equation in the stator reference frame can be represented as following:
Vs = Ris + L
di s +e dt
(1)
Where Vs is the stator voltage and Is is the stator current. e is the back-EMF, which can be represented in the 2phase coordinates as following
diα dt diβ eβ = Vβ − Riβ − L dt
eα = Vα − Riα − L
(2) (3)
The right hand side of the equations is determined as follows: 1-The voltage waveform can be determined from the output control signal as
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(4) Vs = K ⋅ C where K is the power converter gain, and C is the output control signal 2-The current is assumed to be equal the reference value. 3-The rate of change of current can be determined from the change of the controller output. 4-The quadrature inductance (Lq ) and the direct inductance (Ld ) are equal and each has the value of L. The rotor position can be detected from the waveform of the resultant emf via a detection of zero crossing of the fundamental component. Fundamental component is obtained by using digital filters for the output control signals. In addition the speed estimation depends on the distance between each two zero crossing. Forming multi-waveform of resultant emf in 2 phase coordinates and 3-phase coordinates increase the position detection resolution. The novel method has been implemented during free acceleration of a permanent magnet synchronous motor fed from a pure sinusoidal source. Figure 1 shows the detected position and estimated speed with the novel technique, while Fig. 2 illustrates the actual position and speed.
Fig. 2 The rotor position and speed using the novel method The novel rotor position detection technique intends to simplify the hardware and software due to reduction of the steps in axis transformation and speed estimation.
3. Speed and Position Sensorless Controlled PMSM Drive In some applications where some constraints such as efficiency, reliability, and cost are all very important, it is not possible to use a speed or position sensor. In such situations, the necessary information can be derived from a simple dynamic model, which depends on the output control signal. This strategy has several advantages such as robustness and easy implementation. Let us consider a sinusoidal permanent magnet synchronous motor drive system uses the vector control method where the state variables are transformed to the rotating reference frame coordinates. In rotating reference frame coordinates the PMSM behaves like a separately exited DC motor. The exact value of the rotor speed and position is required to control the speed, and to transform the state variables. Figure 3 shows the proposed method applied to a sensorless vector controlled PMSM drive. Fig. 1 The actual rotor position and speed
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Fig. 3 Model of speed sensorless vector control scheme of a PMSM drive
4. Simulation Results The simulation is carried out to verify the proposed method with application to a vector control PMS drive. Fig 4 shows the estimated speed compared with the actual and the reference speed when sensing speed is used, while Fig 5 illustrates the estimated speed compared with the actual and the reference speed under speed sensorless operation. The simulation results shows that the proposed position detection and speed estimation method produces speed which is tracking the actual and reference value in both transient and steady state operation.
(a)
(b)
(c) Fig. 4 Speed Response (a) reference, (b) actual and (c) estimated
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6. References
(a)
(b)
(c) Fig. 5 Speed response (a) reference, (b) actual and (c) estimated
5. Conclusion This paper presents a new position detection and speed estimation method its simplicity leads to a much faster estimation routine for the PMSM. The method is verified via simulation for free acceleration and vector controlled cases. This approach is possible to reduce the overall system costs and to improve the reliability of the drive system.
[1] F. Blaschke, " The Principle of Field Orientation as Applied toThe New Transvector Closed-loop Control System for Rotating Field Machines, " Siemens Rev., Vol.34, No. 5, pp 217-220, May 1972. [2] I. Boldea and S.A. Nasar, Vector Control of AC Drives, Mexico: CRC PRESS, 1992. [3] C. French, and P. Acarnley, " Direct torque Control of Permanent magnet Drives," IEEE Trans. Ind. Appl., Vol. IA-32, No. 5, pp. 1080-1088, Sept/Oct 1996. [4] B.K. Bose, " A High-Performance Inverter-Fed Drive System of an Interior Permanent Magnet Synchronous Machine ," IEEE Trans. Ind. Appl., Vol. 24, No. 6, pp. 987-997, Nov/Dec. 1988. [5] A. Consoli, S. Musumeci, A. Raciti , and A.Tesla, " Sensorless Vector and Speed Control of Brushless Motor Drives," IEEE Trans. Ind. Elect., Vol. IA-41, No. 1, pp. 9196, Feb. 1994. [6] C. French, and P. Acarnley, " Control of Permanent magnet motor Drives Using a New Position Estimation Technique," IEEE Trans. Ind. Appl., Vol. IA-32, No. 5, pp. 10891097, Sept/Oct 1996.
