3. When is the 87 Hz characteristic curve used in the inverter?

The 87Hz characteristic is used when constant torque is required at high speed.

Here's an example:

An electric motor with 1.5 KW, 230/ 400V/ D/Y, 50 Hz, 1430 rpm, 3.21 A in a star connection is switched to a delta connection. The coil voltage is then 3x 230 V, but is designed for 3 x 400 V. The three-phase frequency converter achieves this at 87 Hertz.

The motor data described above are multiplied by the factor √ 3 (1.73). They increase at 87 Hz to 2.6 KW, 400 V, 2474 rpm and 5.55 A at a constant torque.

This means that the frequency converter must be adapted to the higher amperage. For this application you will have to use at least a 1 or even 2 series larger frequency converter.

Important NOTE:

Use above 50 Hertz should only be for short periods of time. Because of the increased mechanical losses and the resulting higher thermal load, the use of the next larger three-phase motor is recommended.