AN2388 datasheet(6/54 Pages) STMICROELECTRONICS

6 / 54 page

Background

AN2388

6/54

causes the rotor to slow down (or slip increasing). On the contrary, a decreasing of the load

causes the rotor to speed up (or slip decreasing). The slip is expressed as a percentage and

can be determined with the following formula:

Figure 2.

Rotor structure

Slots in the inner periphery of the stator accommodate 3-phase winding a, b, c. The turns in

each winding produces an approximately sinusoidally-distributed flux density around the

periphery of the air gap. When three currents that are sinusoidally varying in time, but

displaced in phase by 120° from each other, flow through the three symmetrically placed

windings, a radially directed air gap flux

density is produced that is also sinusoidally

distributed around the gap and rotates at an angular velocity equal to the angular frequency

ωs of the stator currents.

The flux produced by the stator current is a sinusoidally-distributed wave. This flux revolves

and collides with the rotor bars, generating rotor current in the short-circuited rotor bars.

Because of the low resistance of these shorted bars, only a small relative angular velocity ωr

between the angular velocity ωs of the flux wave and the mechanical angular velocity ω of

the two pole rotor is required to produce the necessary rotor current.

The relative angular velocity ωr is called the slip velocity. The interaction of the sinusoidally

distributed air gap flux density and induced rotor currents produces a torque on the rotor.

1.2

Three-phase induction motor and classical AC drives

Three-phase AC induction motor are widely used in many fields. They are classified in two

categories:

●

Squirrel cage motor

●

Wound-rotor motor

90% of the three-phase AC Induction motors are squirrel cage motors because of their lower

cost and the possibility of starting heavier loads with respect to wound-rotor motors. The

range of power ratings goes from one-third to hundred horsepower.

The wound-rotor motor is a variation of the squirrel cage induction motor. While the stator is

the same as that the squirrel cage, it has a set of windings on the rotor which are not short-

where;

Ns = synchronous speed in RPM

Nr = rotor speed in RPM

100

−

Slip

Rings

Conductors

Skewed slots

Shaft

AN2388 Datasheet(PDF) 6 Page - STMicroelectronics

AN2388 Datasheet(HTML) 6 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

Datasheet Download

Link URL