Dahlander pole changing motor
A Dahlander motor is a type of multispeed three-phase induction motor, in which the speed of the motor is varied by altering the number of poles; this is achieved by altering the wiring connections inside the motor. The motor may have fixed or variable torque depending on the stator winding. It is named after its inventor Robert Dahlander.
Dahlander motors are characterised by having six external connections to their stator windings. Although their use involves wye-delta switching, this should not be confused with wye-delta starting that is also commonly used with large three-phase induction motors.
Invention
Robert Dahlander, a Swedish engineer working for ASEA, discovered that switching the poles in a motor led to a reduction in the speed of the motor. In 1897 he invented an electrical configuration to switch between poles in a motor for which he was granted a patent along with his co-worker Karl Arvid Lindström. The new connection was named the "Dahlander connection" and a motor having such a configuration is commonly referred to as a "pole changing motor" or a "Dahlander motor".Operation
The primary factor in determining the speed of any induction motor is the number of poles, given by the formulawhere
A regular induction motor has an equal number of opposite poles; that is, at any instant, there are an equal number of North and South magnetic poles. Some smaller induction motors are connected so that all the poles are identical, causing the motor to act as though there is an equal number of opposite poles in between.
The Dahlander motor is based on a 'consequent pole' construction: the total number of poles of virtual poles expressed by the stator field is a combination of those with a winding directly around that pole, and the flux through the rest of the stator armature, giving rise to consequent poles inbetween them. With the Dahlander arrangement, the switching to the double wye connection reverses the direction of current flow in half of the windings. This is turn causes a halving of the number of poles expressed, as each pair of windings now acts as a single combined pole, rather than a pair of poles.
A Dahlander motor achieves different speeds by switching the configuration of the electrical windings, indirectly adding or removing poles and thus varying the rotor speed. The poles can be varied at a ratio of 1:2 and thus the speed can be varied at 2:1. Normally, the electrical configuration of windings is varied from a delta connection to a double star connection configuration in order to change the speed of the motor for constant torque applications, such as the hoists in cranes. Star connections varied to double star connections are used for quadratic torque applications, such as pumps.