Finding an optimum field weakening point
Motors supplied from variable frequency drives (VFD) have the freedom to set the field weakening point almost arbitrarily. By doing that you can optimize the drive system. The message of this article is how to find an optimum field weakening point.
Let’s start with a definition. What is a field weakening point (FWP)? As the name indicates, it is a speed point above which the magnetic field inside the machine is being reduced.
A quick reminder how a VFD works (not going into details to stay focused on FWP topic): The inverter produces variable voltage at variable frequency. Voltage source inverters do it either by chopping the (almost) constant dc link voltage or by alternating the polarity of cells in a multi-cell or MMC topology. As we know, the magnetic flux is proportional to the ratio of voltage and frequency.
Therefore, at lower machine speed (lower inverter frequency) the machine voltage needs to be reduced as well to avoid saturation of the magnetic circuit.
While you cannot reduce the frequency without reducing the voltage (avoid overfluxing of the machine), you can increase the frequency while keeping the voltage at constant level. This is what happens when operating the motor beyond the field weakening speed.
How to find an optimum field weakening point
Having refreshed the theory we come to the actual topic: How to find an optimum field weakening point?
The question is what we optimize for. Selection of the field weakening point affects the motor itself and also the inverter part of the VFD. Certain optimization may lead to reudced cost of a motor while the inverter cost increases and vice versa.
As described in [1], the relevant quantity for motor design is torque rather than power. Therefore, high torque motors require more material and are more costly. Shifting the field weakening point to lower speed makes the machine design more expensive. At the same time, the inverter can get cheaper as the inverter current for the same speed reduces.
Applications where optimization of field weakening point is especially interesting:
- loads with constant torque
- high overload at lower speeds
- applications with wide field weakening range
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References
[1] Power, speed and torque, https://mb-drive-services.com/power-speed-and-torque/
[2] Load types and characteristics, https://mb-drive-services.com/load-types-and-characteristics/
