When it comes to drives for high-speed applications, always
check if the motor you’re considering has a field-weakening
mode. Field-weakening mode manages the phase advance
between the current and the back electromotive force (BEMF)
of the motor in accordance with the power requirement.
Select the drive size that provides enough rated current (Io)
to the motor in order to generate the maximum torque needed.
To calculate this current, divide the desired torque by the torque
constant (Kt) of the motor. This is expressed in Nm/Arms.
Take note that as the size increases, the output frequency has
a nasty habit of decreasing. For this reason you should double-check the drive’s ability to provide sufficient frequency. Here,
you should calculate the minimum frequency requested by the
motor to reach maximum speed.
The result must be less than or equal to the maximum
output frequency of the drive. This is not to be confused with
the PWM frequency of the drive that corresponds to the
The next step only applies to synchronous motors with
magnets, largely because of their potential to reach high
voltages. During high-speed operations, the Back EMF (BEMF)
produced by the motor must be below the maximum DC
voltage of the drive’s power bus. Typical power bus values are
540 VDC for a 400 VAC power supply. As a general guide,
avoid exceeding 800 VDC. Check the voltage value of the
motor’s BEMF at maximum speed. This limit is given by another
motor constant, Ke, which is expressed in Vrms / 1000rpm:
The results of these calculations are best plotted in vector
diagrams. The diagrams shown here illustrate the motor running
in axe mode, field weakening mode below the maximum voltage
of the drive, and field weakening mode beyond the maximum
voltage of the drive.