• Starting methods are related to IMs fed from the industrial power grid.
• With direct starting and stiff local power grids, the starting current is in the interval of 580 to 650% (even higher for high efficiency motors).
• For direct starting at no mechanical load, the rotor winding energy losses during machine acceleration equals the rotor-attached kinetic energy at no load.
• For direct frequent starting at no load, and same stator, rotors with higher rotor resistance lead to lower total energy input for acceleration.
• To avoid notable voltage sags during direct starting of a newly installed IM, the local transformer KVA has to be oversized accordingly.
• For light load, starting voltage reduction methods are used, as they reduce the line currents. However, the torque/current ratio is also reduced.
• Voltage reduction is performed through an autotransformer, wye/delta connection, or through softstarters.
Softstarters are now built to about 1MW motors at reasonable costs as they are made with thyristors. Input current harmonics and motor additional losses are the main drawbacks of softstarters. However, they
• are continually being improved and are expected to be common practice in light load starting applications (pumps, fans) where speed control is not required but soft (low current), slow but controlled, starting are required.
• Rotor resistive starting of wound rotor IMs is traditional. The method produces up to peak torque at start, but at the expense of very large additional losses.
• Self adjustable resistance-reactance paralleled pairs may also be used for the scope to cut the cost of controls.
• Speed control in cage rotor IMs may be approached by voltage amplitude control, for a very limited range (up to 10 to 15%, in general).
• Pole changing windings in the stator can produce two speed motors and are used in some applications, especially for low power levels.
• Dual stator windings and cage rotors can also produce two speed operations efficiently if the power level for one speed is much smaller than for the other.
• Coordinated frequency/voltage speed control represents the modern solution to adjustable speed drives.
• V/f scalar control is characterized by an almost linear dependence of voltage amplitude on frequency; a voltage boost V0 = (V)f1 = 0 is required to produce sufficient torque at lowest frequency f1 ≈ 3 Hz. Slip feedforward compensation is added. Still slow transient response is obtained. For pumps, fans, etc., such a method is adequate. This explains its important market share.
• Rotor flux vector control keeps the rotor flux amplitude constant and requires the motor to produce two stator current components: a flux current IM and a torque current IT, 900 apart. These two components are decoupled to be controlled separately. Fast dynamics, quick torque availability, stable, high performance drives are built and sold based on rotor flux vector control or on other related forms of decoupled flux and torque control.
• Linear torque/speed curves, ideal for control, are obtained.
• Flux, torque coordination through various optimization criteria could be applied to cut energy losses or widen the torque-speed range.
• For constant rotor flux, the IM behaves like a high saliency reluctance synchronous motor. However, the apparent saliency is produced by the rotor currents phase- shifted by 900 with respect to rotor flux for cage rotors. As for the reluctance synchronous motor, a maximum power factor for the loss, less motor can be defined as
cosϕmax = 11+− LLscsc / L/ Lss ;L / Ls sc >15÷ 20 (8.72)
Ls – no-load inductance; Lsc – short-circuit inductance.
• Wound rotor speed control is to be approached through frequency converters connected to the rotor brushes. The rotor converter rating is low. Considerable converter cost saving is obtained with limited speed control so characteristic to such drives. With adequate frequency converter, motoring and generating over or under conventional synchronous speed (n1 = f1/p1) is possible. High and very high power motor/generator systems are main applications as separated active and reactive power control for limited speed variation at constant frequency is feasible. In fact, the largest electric motor with no-load starting has been built for such purposes (for pump storage power plants).
• Details on control of electric drives with IMs are to be found in the rich literature dedicated to this very dynamic field of engineering.
8.7 REFERENCES
1. W. Leonhard, Control of electric drives, 2nd edition, Springer Verlag, 1995.
2. I.Boldea and S. A. Nasar, Electric drives, CRC Press, 1998.
3. F. Blaabjerg et. al, Can Softstarters Save Energy, IEEE – IA Magazine, Sept/Oct.1997, pp. 56 – 66.
4. F. Blaschke, The Principle of Field Orientation As Applied to the New Transvector Closed Loop Control System For Rotating Field Machines, Siemens Review, vol. 34, 1972, pp. 217 – 220 (in German).
5. D. W. Novotny and T. A. Lipo, Vector control and dynamics of a.c. drives, Clarendon Press, Oxford, 1996.
6. Boldea and S. A. Nasar, Vector Control of A.C. Drives, CRC Press, 1992.
7. Masenoudi and M. Poloujadoff, Steady State Operation of Doubly Fed Synchronous Machines Under Voltage and Current Control, EMPS, vol.27, 1999.
8. L. Schreir, M. Chomat, J. Bendl, Working Regions of Adjustable Speeds Unit With Doubly Fed Machines, Record of IEEE – IEMDC – 99, Seattle, USA, pp. 457 – 459.
9. L. Schreir, M. Chomat, J. Bendl, Analysis of Working Regions of Doubly Fed Generators, Record of ICEM – 1998, vol. 3, pp. 1892 – 1897.