ENERGY SAVING MAINS-FED PM SYNCHRONOUS MOTOR WITH INTEGRATED SOLID STATE STARTER

Transcription

1 ENERGY SAVING MAINS-FED PM SYNCHRONOUS MOTOR WITH INTEGRATED SOLID STATE STARTER András Lelkes, Jens Krotsch ebm Werke GmbH & Co. KG P.O. Box 6, D-7467 Mulfingen http: // GERMANY Tel: ++49 (0)7938 / 8-590; Fax: ++49 (0)7938 / andras.lelkes@ebm-werke.de Abstract - A microprocessor controlled triac circuit starts a PM synchronous motor. As soon as the synchronous speed is reached, the motor works line commutated with constant speed. This starter is especially suitable for fan applications, because the high inertia of the impeller does not allow simple direct-on-line starting. A plastic bonded ferrite magnet synchronous motor with an integrated starter is a reasonably priced, highly efficient alternative to shaded-pole or permanent split capacitor type induction motors. At 0 W output power, for example, power consumption can be reduced by more than 60% compared to a shaded-pole motor. I. INTRODUCTION ebm is a manufacturer of motors and fans in a rated output power range from 5 W to.5 kw. For fan applications, mainly shaded-pole motors, permanent split capacitor (PSC) type induction motors and electronically commutated (EC) motors have been used. EC motors have some important technical advantages: wide speed range, easy speed controllability, and high efficiency. Low cost positioned and robust single phase induction motors have a significantly lower efficiency. A proposed energy saving, mains-fed, permanent-magnet synchronous motor presents a reasonably priced, highly efficient alternative to single phase induction motors. Using energy saving motors means economic and environmental advantages. Such high efficiency motors can be used in numerous applications. They are especially advantageous for refrigeration technologies, where fans have to work around the clock without interruption. Fig. illustrates the possible energy saving as a function of the motor s efficiency. For example, at 60% efficiency, the motor will consume only 38% of the energy which is consumed by a commonly-used shaded-pole motor with 0 W rated output power. A desirable efficiency of more than 60% cannot be achieved by single phase sub-fractional horsepower induction motors. On the other side, highly efficient EC motors cannot be used in many applications because of their higher price. The cost of a commutating electronics for small EC motors rises excessively, if the motor is supplied directly by mains. This commutating electronics becomes unnecessary, if the motor is operated line-commutated. However, in case of fan applications Relative Power Consumption 00% 90% 80% 70% 60% 50% 40% 30% 0% 0% 0% 0% 0% 0% 30% 40% 50% 60% 70% 80% 90% 00% Motor Efficiency 0 W rated output 5 W rated output Fig.: Power consumption of energy saving motors at 5 and 0 W rated output power (expressed as a percentage of shaded-pole motors power consumption) as a function of their efficiency PCIM 000 Nürnberg

2 direct-on-line starting is not applicable because of the high impeller inertia. This starting problem can be solved by proposed inexpensive electronics. II. SOLID STATE STARTER FOR MAINS- FED PM SYNCHRONOUS MOTORS The synchronous motor can be started by a microprocessor controlled triac circuit ( Fig. ). During the run-up phase the triac Tr is active. The polarity of the mains and the rotor flux are compared by the control unit. With an appropriate firing strategy, the rotor can be accelerated up to its synchronous speed. The motor current at acceleration is shown in Fig. 3. The control unit is in charge of avoiding demagnetization of the rotor during the time of acceleration and synchronization. The motor is permanently connected to the mains via the triac Tr, as soon as the synchronous speed is reached ( Fig.4 ). Once the rotor is synchronized, the motor works with constant speed and high efficiency. By means of the proposed method, an intelligent control can be realized even by a low cost microcon- Fig.3: Line voltage and motor current during acceleration (Tr is active) Tr Tr Fig.4: Line voltage and motor current at synchronous speed. The power consumption is reduced after switching to Tr. L Tr 3 µc Tr PM SM ~ N PE Fig.: Schematic diagram of the proposed single phase energy saving motor with solid state starter (: motor winding with tap, : Hall-effect sensor, 3: line voltage sensor) PCIM 000 Nürnberg

3 troller. In contrast to conventional commutating electronics, the solid state starter operates without voluminous, temperature sensitive and life limiting DC link capacitors. So, the electronic starter can be integrated easily into the motor, which is a great advantage. III. EXTERNAL ROTOR MOTOR Like every commonly-used electric motor, external rotor motors consist of a stator wound with copper wire and a rotor. The rotor rotates externally around the stator. Electronically commutated (EC) external rotor motors have a permanent magnet in the rotor. The commutation is performed brushless and therefore without wear. The commutating electronics can be either integrated onto a PCB inside the motor or can form a separate unit. Fig. 5 shows an external rotor motor with integrated electronics. External rotor motors are especially advantageous for fan applications. The EC motor is very compact because of the short length of winding head and of its bearing system integrated into the stator s interior. This motor installed inside an impeller results in a fan unit requiring minimum space. The unique integration of the motor and the impeller permits precision balancing. This guarantees low loads to the bearing system. The motor is positioned directly in the air stream, so the very efficient cooling extends life expectancy. Fig. 6 illustrates a centrifugal blower with integrated external rotor motor. External rotor motors have a big inertia. This is no problem for EC fans without expectation of high dynamics. However, the inertia of the external rotor and the impeller make synchronization of a mains-fed, line-commutated motor more difficult. By means of the proposed method, the intelligent solid state starter can accelerate and synchronize external rotor motors, too. IV. STARTING AND SYNCHRONIZATION OF EXTERNAL ROTOR MOTOR Whereas direct-on-line starting of conventional line commutated synchronous motors is successfully used in various applications, this method is not applicable to external rotor motors or, generally, to systems with high inertia. Fig.5: Brushless external rotor motor with integrated electronics Fig.6: Centrifugal blower, forward-curved, single inlet, with external rotor motor PCIM 000 Nürnberg 3

4 During start-up, the mains frequency differs significantly from the motor's rotary frequency. Therefore, a rotor-position-dependent commutation method has to be applied which selectively switches only the appropriate halfwave or half-wave-part of the alternating line voltage on motor windings, in order to gain a maximum accelerating torque without risk of demagnetization of permanent magnets. This seems easily to be achieved by a logical comparison of line voltage polarity to the polarity of the induced voltage. The latter can be derived from a Hall-effect sensor which is placed in the external rotor's magnetic field. However, the use of static AC switches allows only a turn-on at a desired time, not a turn-off. If the triac is triggered briefly before a zerocrossing of induced voltage - as a result of equal polarity - the afterwards oppositely acting current produces a braking torque which lowers efficiency. In the worst case a nonreversible demagnetization of the permanent rotor magnets can occur. To avoid this, the time between a turn-on of the triac and the next expected zero-crossing of induced voltage has to be considered, too. An appropriate control strategy of triac Tr which is active during start-up and synchronization can be described as following: S = ( PV ) ( PV ) ( PVp ) ( PVp ) ( t > c ) VpZC S Gate-trigger signal of triac Tr P V Polarity of line voltage P Vp Polarity of induced voltage (Hall-sensor signal) P VTr Polarity of voltage across triac Tr t VpZC Time to next expected zero-crossing of induced voltage c Constant which defines the interval before the next expected zero-crossing of induced voltage, where a turn-on of the triac is suppressed () P V, P Vp and P VTr are digital input signals of the microcontroller. t Vpzc is an estimated value by observing the motor speed via P Vp. The triac is triggered, if line voltage and induced voltage are of the same polarity and the interval to the next expected zerocrossing of induced voltage is greater than a predefined limit c. Because of the alternating line-voltage, an additional requirement appears in equation (), which takes into account that the value of the line-voltage has to be higher than the value of the induced voltage to generate accelerating torque. This is done by detection of the terminal voltage of triac Tr. The described control strategy is not sufficient at high speed, especially if rotary frequency is close to line frequency. A higher torque can be generated, if triac Tr is triggered at a rotor-displacement angle greater than zero which means an earlier time in respect to the zero-crossing-point of induced voltage. The optimum angle is approximately π/4 according to ϑ opt = tan, R where the X d /R-ratio of proposed small-power motor is around. An advanced control strategy is given by equation (): S t VZC c = X d ( PV ) ( PV ) ( PVp ) ( PVp ) ( t VpZC > c ) ( P P ) ( P P ) t V VpZC < Vp t V ZC c V Time to next expected zero-crossing of linevoltage Defines the rotor-displacement angle for commutation (c = 4 ϑ = π/4) Vp () With this commutation principle, an acceleration up to a speed close to synchronism can be achieved, as it is shown in Fig. 7. A more serious problem is the process of safe synchronization due to the high inertia of external rotor motors. Two methods to perform this are discussed in the following. PCIM 000 Nürnberg 4

5 0 π (b) (a) step is favorable for minimizing the amplitude of acoustic noise. Flux-reduction is suitably performed at an instant, when the time-rate of rotor-displacement angle change is equal or near zero for the first time during start-up. The result of the proposed method is shown in Fig. 8. -π t [ms] Fig.7: Motor current (a) and rotor-displacement angle (b) at approximately 90% of synchronous speed One possibility is to enlarge the range of the permitted rotor-displacement angle by c and c of equation (), as soon as the synchronous speed has been reached. Thus, the motor is 'naturally' pulled in to synchronism, as long as ϑ opt is not exceeded. However, this causes a low-frequency oscillation of torque, damped only by static load, which results in acoustic noise. To maximize efficiency, the high magnetic flux essential for the start-up has to be reduced, for instance by increasing the number of turns via triac Tr. In opposite to conventional internal rotor motors, the moment of the adjustment is of great importance for achieving synchronism. Furthermore, this Another more advantageous method for synchronizing external rotor motors in order to minimize the acoustic noise and the risk of demagnetization is the following: If the motor speed is close to the synchronous speed, the rotor-displacement angle is continuously measured by electronics and led to a reference value by a digitally implemented phaselocked-loop (PLL) algorithm. Assuming a constant mechanical load, the angle depends only on the winding voltage, which is varied by the PLL via phase control of triac Tr. When the rotor-displacement angle is locked to a predefined reference value, the flux is adjusted to maximize the motor's efficiency. To avoid oscillation of torque caused by flux reduction, the reference value of the PLL is defined equal to the rotor-displacement angle which would be expected afterwards. Fig. 9 confirms that the proposed method allows safe synchronization of high-inertia external rotor motors. Because of the relatively high resistance of the windings, this sub-fractional motor works with an unusually small rotordisplacement angle at nominal load in opposite to high-power synchronous motors. π synchronism has been reached, triac Tr is activated π synchronism has been reached triac Tr is activated 0 0 PLL active -π start-up via triac Tr -π start-up via triac Tr t [s] t [s] Fig.8: Rotor-displacement angle before and after flux-reduction Fig.9: Rotor-displacement angle by use of PLLsynchronization method PCIM 000 Nürnberg 5

6 Efficiency V. EXPERIMENTAL RESULTS Parameters of the single phase, mains-fed, permanent-magnet, external rotor motor with an integrated electronic starter: Type: M4Y055BD-A Voltage: 30 V, 50 Hz Rated speed: 500 rpm Rated output: 0 W Torque: 6,4 Ncm cos ϕ: 0,98 Consumption: 6 W Efficiency: 63 % This motor is compared with a shaded-pole induction motor of the same output power. The speed of the 4-pole synchronous motor is constant 500 rpm, the speed of an induction motor depends on the load. 00% Power Consumption [W]. 90% 80% 70% 60% 50% 40% 30% 0% 0% 0% Mechanical Load [W] synchronous motor shaded-pole motor Fig.0: Comparison of efficiency shaded-pole motor synchronous motor Fig.: Comparison of power consumption Fig. 0 and Fig. show the measured efficiency and the power consumption of both motors as a function of their output power. At 0 W motor output power the reduction of electrical consumption using the proposed energy saving synchronous motor is 6 W or 6%. Within a year, 7 kwh of energy per motor can be saved at 00% duty factor. This is an important energy reduction with regards to an estimated world-wide annual production volume of more then 00 million of shaded-pole motors. VI. REFERENCES [] Alvaro, N.; Acquaviva, S.: A device for controlling a synchronous electric motor with a permanent magnet rotor. Patent application EP A, European Patent Office 998. [] Erbetta, C.; Oljaca, M.: Electronic device for starting and controlling a permanent-magnet single-phase synchronous motor. Patent application EP A, European Patent Office 995. [3] Klein, H.-W.; Altenbernd, G.; Wähner, L.; Bass, M.: Device for driving a single phase synchronous motor, especially for driving a pump in a household appliance. Patentschrift EP B, European Patent Office 997. [4] Kunz, W.: Elektronische Anlauf und Betriebssteuerung für einen Einphasen-Synchronmotor. Offenlegungsschrift DE A, Deutsches Patentamt 998. [5] Marioni, E.: Electronic device for starting a synchronous motor with permanent-magnet rotor. Patent specification EP B, European Patent Office 998. [6] Mühlegger, W.: Verfahren und Vorrichtung zum Starten einer einphasigen Synchronmaschine. Patentanmeldung EP A, European Patent Office 995. PCIM 000 Nürnberg 6