EFFECTIVE MOTOR CONTROL
FOR HIGH-EFFICIENCY MOTORS
Tony Foster, Regional Sales Manager, Intelligent Energy Management Systems, Power Parameters Pty Ltd
Modern high-efficiency motors have the drawback of higher inrush and starting currents, which means protection equipment must be specified differently.
O
ther than for legacy installations, or special cases, it is generally mandatory to install high-efficiency induction motors in new installations. The current high-efficiency IE3 standard is being adopted globally but is not yet an enforceable standard in Australia. However, it will not indefinitely ‘stay away’ and the current-MEPS imposed top limit of 185 kW in Australia can also be expected to broaden. As for the idea that variable speed drives improve efficiency regardless of the efficiency of the motor, an argument can be made that there may well be an excessive use of this form of control, and that for many applications there is both an engineering case — as well as overall efficiency case — to be made for DOL or star-delta control. As will be explained in more detail, the challenge of providing effective control without spurious interruption because of initial high current (inrush and locked rotor current) must be met with appropriate control gear.
High-efficiency motors There are many techniques employed in the physical construction of high-efficiency motors, but the overall aim is to reduce stator iron and copper losses, windage losses and rotor copper losses. In our CO2-conscious world, the aim is worthy, although in terms of many practical applications it can be argued that notwithstanding improved efficiency of a motor, the effect of the attached drive
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chain may well bring to nought the efficiency dividend of the prime mover. One often sees this reasoning used as a counter argument. On a practical note, manufacturers supplying international markets are increasingly locked into IE3 standards, and these motors are making their way into the Australian market and the even higher efficiency IE4 motors are, so to speak, just around the corner. As is the case in so many other fields of engineering, there are no silver bullet solutions — and high-efficiency motors are no exception. Thus engineering considerations have to take into account lowered starting torques and high inrush currents that can pose problems for adequate motor protection. The performance characteristics in terms of starting current (locked rotor) and inrush current relative to IE1 motors are shown in Figure 1.
Back to school A basic understanding of induction motor physics is helpful in understanding how the requirement of higher efficiency has its effect on motor operational parameters. In Figure 2, the basic equivalent circuit of an induction motor is presented. The circuit is shown on a per-phase basis. In essence an induction motor can be considered a rotary transformer, and under locked rotor conditions it is in fact a transformer with a shorted secondary (the squirrel cage). In the case of a wound-rotor induction motor, external
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