|Electrical Machines - Basic Vocational Knowledge|
|5. Asynchronous motors|
|5.4. Circuit engineering|
Mode of operation
Where several separate electrical windings with varying numbers of pole pairs are required for the stator of the asynchronous motor or windings whose pole pair numbers can be varied by switching over the windings then the speed of the rotating field changes and, thus, also the rotor speed. Squirrel cage motors are used for this purpose because, as opposed to slip ring rotor motors, they are not bound by a specific pole pair. The pole-changing winding in the so-called Dahlander pole-changing circuit is thereby the most perceptible practical feature. This pole-changing winding permits a speed change in the ratio two to one. These circuits have been set out in fig. 70.
The coil groups are switched over from series to parallel connection where a smaller pole pair number is selected, that is to say a greater speed is selected.
(1) Delta connection for low speed (p = 2)
(2) Double star connection for higher speed (p = 1)
...U; ...V; ...W Partial windings
The speeds of asynchronous motors can be roughly stipulated by altering the pole pair number.
Where a stator has been executed with two separate windings which are both pole-reversible, then the speed may be established in four stages, for example, by means of the synchronous speeds of 500 - 750 - 1000 and 1500 rpm.
Motors with changeable pole pair numbers are frequently used for controlling machine tools where approximate setting is usually sufficient. Such motors are also used to drive pumps, ventilators, escalators etc.
The protection K1 that the series-switched coil halves of each stand of the stator winding are delta-connected.
The pole pair number p1 conditions the rotating field speed n1. The protections K2 and K3 ensure that the coil halves of each strand are parallel connected and the entire stator winding is star connected.
The new pole pair number p2 conditions the rotating field speed n2. Actuating S3 switches on K1 which retains itself by means of its closer in current path two. K3 and K2 are locked by the openers S3 in the current path five whilst K1 is locked in the current path three. Actuating S2 switches on K2 whose closer switches on K3 in current path five. K1 is locked by opener S2 whilst K3 and K2 are locked in current path one.