2.4. SUMMARY of INDUCTION MACHINES: AN INTRODUCTION

• The IM is an a.c. machine. It may be energized directly from a three phase a.c. or single phase a.c. power grid. Alternatively it may be energized through a PWM converter at variable voltage (V) and frequency (f).

• The IM is essentially a traveling field machine. It has an ideal no-load speed n₁ = f₁/p₁; p₁ is the number of traveling field periods per one revolution.

• The IM main parts are the stator and rotor slotted magnetic cores and windings. The magnetic cores are, in general, made of thin silicon steel sheets (laminations) to reduce the core losses to values such as 2 to 4 W/Kg at 60 Hz and 1 T.

• Three or two phase windings are placed in the primary (stator) slots. Windings are coil systems connected to produce a travelling mmf (amperturns) in the airgap between the stator and the rotor cores.

• The slot geometry depends on power (torque) level and performance constraints.

Starting torque and current, breakdown torque, rated efficiency, and power factor are typical constraints (specifications) for power grid directly energized IMs.

energized IMs.

• Two phase windings are used for capacitor IMs energised from a single phase a.c. supply to produce traveling field. Single phase a.c. supply is typical for home appliances.

• Cage windings made of solid bars in slots with end rings are used on most IM rotors. The rotor bar cross-section is tightly related to all starting and running performances. Deep-bar or double-cage windings are used for high starting torque, low starting current IMs fed from the power grid (constant V and f).

• Linear induction motors are obtained from rotary IMs by the cut-and-unroll process. Flat and tubular configurations are feasible with single sided or double sided primary. Either primary or secondary may be the mover in LIMs. Ladder or aluminum sheet or iron are typical for single sided LIM secondaries. Continuous thrust densities up to 2 to 2.5 N/cm² are feasible with air cooling LIMs.

• In general, the airgap g per pole pitch τ ratio is larger than for rotary IM and thus the power factor and efficiency are lower. However, the absence of mechanical transmission in linear motion applications leads to virtually maintenance-free propulsion systems. Urban transportation systems with LIM propulsion are now in use in a few cities from three continents.

• The principle of operation of IMs is related to torque production. By using the Maxwell stress tensor concept it has been shown that, with windings in slots, the torque (due to tangential forces) is exerted mainly on slot walls and not on the conductors themselves.

Stress analysis during severe transients should illustrate this reality. It may be demonstrated that the rotor winding in slots can be “mathematically” moved in the airgap and transformed into an equivalent infinitely thin current sheet. The same torque is now exerted directly on the rotor conductors in the airgap. The LIM with conductor sheet on iron naturally resembles this situation.

• Based on the J×B force principle, three operation modes of IM are easily identified.

Motoring: |U| < |U_s|; U and U_s either positive or negative

Generating: |U| > |U_s|; U and U_s either positive or negative

Braking: (U > 0 &U_s< 0) or (U < 0 & U_s> 0)

• For the motoring mode, the torque acts along the direction motion while, for the generator mode, it acts against it as it does during braking mode. However, during generating the IM returns some power to the grid, after covering the losses, while for braking it draws active power also from the power grid.

• Generating is energy−conversion advantageous while braking is energy intensive. Braking is recommended only at low frequency and speed, with variable V/f PWM converter supply, to stall the IM drive on load (like in overhead cranes). The energy consumption is moderate in such cases (as the frequency is small).