KEYWORDS: Variable speed, stator switching, induction motors. ABSTRACT. The paper presents a simple low cost variable speed drive using stator neutral.
AN EFFICIENT VARlABLE SPEED DRIVE SCHEME FOR SQUIRREL CAGE lNDUCTlON MOTORS USING STATOR NEUTRAL SWITCHING
A.M. Sharaf**, M. Pothier, D.M. Luke The University of New Brunswick Box 4400, Fredericton, NB Canada **Currently a Visiting Professor at NTU - Nanyang Technological University, Singapore
KEYWORDS:
Variable speed, stator switching, induction motors compressors, etc. traditionally have low power/energy efficiency at the low to medium loading levels, Efficiency can be enhanced via construction methods, materials or through novel motor control strategies that provide for soft starting, adequate dynamics, speed control and higher operating efficiency under light and medium load conditions. Industry's push for variable speed motors was motivated mainly by the need to reduce the larger losses associated with niechanical constant speed drives such as throttling, use of dampers and mechanical gears. Both open loop and closed loop control techniques are implemented [3-81 and are coiiirnercially available. Solid state converters and power electronic interface schemes are utilized for variable voltage variable frequency control of the induction motor applied voltage and frequency. Most high performance induction motor drives utilize the voltage source inverter fed pulse width modulated converter interface scheme. Control techniques encompass trans vector control, field acceleration FAM control, field orientation and decoupled (d,q) control schemes. Trans-vector control 191 attempts to make the induction motor emulate the torque producing riiechanism of the dc motor, where the spatial orthogonality between the dc motor stator magnetic flux and rotor magnetomotive force is assumed.
ABSTRACT The paper presents a simple low cost variable speed drive using stator neutral switching of squirrel cage induction motors. The scheme is suitable for typical f i n , pump and compressor load applications. The scheme can be utilized as soft starter as well as speed control. The scheme can also be supplemented with on-line efficiency enhancement regulating loops to ensure electrical energy conservation.
INTRODUCTION AC induction motors are highly nonlinear, complex and highly coupled controlled processes [ 1,2]. It has been estimated that over 75% of the electrical energy consumed in industrial plants is used powering induction motors. Two groups of induction motors that split electrical energy usage are the (1-125 HP) and the over (1 25HP) ratings. Fortunately the larger size motors are designed to be highly efficient at rated loads. The smaller motors used in applications such as fans, pumps,
Paper APT 104-30-12 accepted for presentation at the IEEE/NTUA Athens Power Tech Conference: "Planning, Operation and Control of Todav's Electric Power Sy>tems",Athens, Greece, Sept. 5-8, f993.
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This paper presents another converter interface scheme which is economical, simple to iiiipleinent anti particularly suitable for the small size motor ratings. The proposed scheme is labelled the neutral point chopper drive and utilizes a six pulse diode rectifier and one Nosfet switch. It can be utilized as a soft starter for larger induction inotors or a variable speed drive. The same technique was originally utilized for rotor resistance speed control method [IO] and stator voltage and frequency control [ I 11. TIie paper is structured in the following order: 1) neutral point scheme and control strategies 2) efficiency enhancement technique 3) snitching limitations and regulation 4) conclusions and recommendations. The full drive system was digitally simulated using the Valid Software, Matlab and Tutsiin modelling packages.
rotating in the same direction as the fundamental 60 Hz rotating field. This generates additive torques. If the switching frequency (f,) is above 60 Hz (fundamental) the developed torques at fd counteract the fundamental torque causing braking action or subtractive torques. The proposed neutral point switching scheme produces modulation frequencies whose magiiitudes are always less than the fundamental GO I-Iz harmonic component. Figures 3-5 depict the resultant per unit magnitude of the voltages in terms of the funda~nental and the chopper duty cycle-ratio for chopping frequencies of 35 I-Iz, 85 I-Iz, and 155 Hz. The resultant voltage magnitudes at these modulation frequencies are a function of the duty cycleratio (a),defined as (%). I/f,
SWITCHING LIMITATI[ONS This norrnalizeil magnitude is independent of switching frequency but is a function of the duty cycle ratio. The 60Hz frequency component is illways greater than any of these modulating frequencies. This introduces the problem of high input currents Some filtering at low output torques. methods can be used in small-size motors. Other problems are introduced by the fact that other ripple frequencies may cause problems at small duty cycle-ratio, for example the 10 Hz and 45 Hz components have comparable level to that of the desired frequency of 25 Hz at l o w duty cycle-ratios of 10-20%, as shown in Figure 3. Digital siiaiulation employed the novel induction motor transient model presented in the Appendix.
NEUTRAL POINT SCHEME The neutral point chopper-scheme and the proposed controller are illustrated in Figure 1. The circuit has six diodes forming an uncontrolled rectifier with one or a set of hlosfets for switching the current on and off through the stator of the squirrel cage induction motor. The key to successful operation is the selection of the correct switching frequency (f,) to develop an MMF rotating at the desired frequency (fd). This desired frequency is determined by different equations depending upon whether the switching chopper frequency (f,) is below or above the fundamental frequency (60 Hz). Figure 2 depicts the resultant voltage drop across the stator phases for a switching frequency f, below 60 Nz. For an iaicluction motor the useful switching frequency f, is between 0 and 60 Hz (fundamental). This will ensure that the developed stator electromagnetic field of frequency (f,) is
REGULATION Since the voltage magnitude of the modulating MMFs of frequencies below the fundamental (60 Hz) is independent of the 827
switching fnequency, but only deper~kriton the duty cycle-I atio, tlieii a simple c ~ ~ i t i o l stiatcgy c a i be devised to ensure constant duty-cycle ratio over a iaiige of switching frequencies. Generally, both switching freciuemcy, f, and on time, T(,,, can be controlled to ensure both variable speed requirements under diffeient load torques. Figure 6 depicts the opcratioii at 40% duty cycle-ratio with v a r y i ~ ~ gthe ctloppii1g frequency. The motor is initially runnirag at a speed of 100 rattiaaas/s with a cor~stanit torque load of 20 I
