robustness, service life and maintenance-free operation in induction motor are .... The motor voltage is controlled by the two half the duty cycle bridges involving ...
IEEE PEDS 2005
Single Phase PWM Controlled Voltage Converter for Pumps and Fans without DC Link Electrolytic Capacitors M. Reisinger Linz Center of Competence in Mechatronics GmbH Linz, Austria martin.reisinger(lcm.at Abstract-Single-phase induction motors are widely-used to drive pumps and fans up to several kilowatts. To satisfy the increased requirements in various application areas like modern air conditioning systems, speed-variable drives have become inevitable. Furthermore installations in homes, shops and offices make high demands on the running smoothness of the drive, With basic concepts like a phase-controlled single-phase AC
voltage controller it is difficult to settle these claims. Moreover the indicated application areas are very cost sensitive. Advanced solutions including frequency converters often do not have a competitive position. This paper shows a novel concept to cope with these difficulties. The introduced converter is controlled by a high frequency pulsewidth modulation (PWM) signal. This allows an almost sinusoidal motor current with small distortions
and therefore low noise emissions of the drive. The proposed circuit topology introduces low harmonics in the line current. This circumstance permits to comply with the relevant regulations without the need of bulky inductors or an active front end. The converter has a high efficiency even at high PWM frequencies. The absence of DC link electrolytic capacitors allows a space-saving design without drawbacks concerning reliability and lifetime at higher ambient temperatures. Keywords- single-phase voltage converter, PWM controlled, drivefor pumps andfans, acoustic noise
I. INTRODUCTION The combination of beneficial technical attributes like robustness, service life and maintenance-free operation in conjunction with a competitive price caused ahigh acceptance of induction motors as drives for pumps and fans. Induction motors up to a rated power of several kilowatts, which are designed forbeing supplied from the single-phase mains, have been produced in large quantities. This development has been motivated by the general availability of the single-phase mains in residential, commercial and light industrial environment. Several implementations are well-known to establish a rotating field for torque generation in the case of single-phase operation. An overview is given in [1] and [2]. Fig. 1 shows the equivalent circuit of the motor utilized to measure the electrical converter characteristics and the vibrations of the drive, which
0-7803-9296-5/05/$20.00 © 2005 IiEEE
W. Amrhein, S. Silber Department of Electrical Drives and Power Electronics Johannes Kepler University Linz, Austria
auxiliary winding WH, which is 90 electrical degrees displaced from the main winding WA, is placed in the stator to give the motor a starting torque. The auxiliary winding is permanently connected in series to a capacitor, what results in the required phase shift of the coil currents in WA and WH. The configuration in Fig. 1 is a socalled permanent-split capacitor motor. Single-phase induction motors are manufactured with increased rotor resistance to allow speed control by means of a voltage converter with a fixed frequency. Cost-effective and rugged concepts like fixedcfred single-phase aC rugge concers are phase-controlled single-phase AC voltage controllers are appropriate for this purpose. Speed control by voltage variation suffers from the fact, that decreasing rotational speed, i.e. increasing slip, diminishes the efficiency of the motor. In applications involving pumps and fans this characterized problem is of subsidiary importance because of the square interrelationship between torque and rotational speed. The required torque decreases disproportionately with the rotational speed. This statement is in an analogous manner true for the low starting torque. are presented in this paper. An
Problems
concerning variable speed drives for pumps and
fans with phase-controlled single-phase AC voltage controllers result from the deviation of the motor current from the desired sinusoidal shape. With decreasing effective motor voltage the distortion of the motor current increases. Harmonics in the motor current cause alternating torques, which are superposed
to the desired fundamental torque. Vibrations initiated in the induction motor are transferred to mechanically linked parts
like fan blades, protective gratings, pipework and the
enclosure. This generates an unpleasant acoustic noise in the
audible range. Fig. 2 and Fig. 3 show the results of a measurement carried out with a fan driven by a phasecontrolled single-phase AC voltage controller. The fan with a rated power of 2.2 kW is operated at 30 % of its rated speed (375 rpm). This represents a critical operating point, because of regarding speeds up to this level the acoustic noise due to the air flow is marginal and the motor induced noise appears exceptionally troublesome. Fig. 3 shows the spectrum of the acceleration level, which is directly measured at the stator of
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