»Connections of three-phase asynchronous motors

Generally circuits for the connections below are used:

• direct on-line start up (in star or delta connection),
• direct on-line start up with change of rotation direction,
• star-delta start up,
• star-delta start up with change of rotation direction,
• soft start up,
• frequency inverter start up.

When connecting the nominal voltage of the motor or winding has to be taken into consideration. With low-power motors (up to 3 kW) the winding of the motor is dimensioned for 230V voltage – that is why it can be connected to 400V power supply only in star connection; consequently the star-delta start up is not possible. Phase windings of high power motors are dimensioned for 400V voltage; star-delta start up is possible.

Figure 01: Organization of connection pins and connections to power supply in star connection (left) and delta connection (right).

There are 2 models of asynchronous motors with changeable number of poles:

• Asynchronous motor has two separate stator windings – each dimensioned for a different number of poles,
• Asynchronous motor has only one stator winding – by external switching of this winding the number of poles is changed (and consequently the number of rotations). The most well-known of these is Dahlander connection.

Figure 02: Organization of connection pins and connection to power supply of a motor with two separate windings – low speed (left), high speed (right).

Figure 03: Organization of connection pins and connection to power supply in Dahlander connection – low speed (left), high speed (right). This connection is as well called D/YY connection. In both cases the torque of the motor remains the same.

Figure 04: Organization of connection pins and connection to power supply in Dahlander connection – low speed (left), high speed (right). This connection is as well called Y/YY connection. This connection is used in connecting fans, as the motors have a square-law torque characteristic.

It is worth mentioning that the output line voltage of a 1-phase frequency inverter is 230V, whereas the output line voltage of a 3-phase frequency inverter of equivalent power is 400V. So the motor of which the winding is dimensioned for 230V is in the 1st example connected to delta connection and in the 2nd example to star connection.

The direction of motor rotation is changed simply by interchanging two input phases (see figure below).

Marjan Bitenc