273 – Creating IPM Motor Efficiency Maps Accounting for AC Loss

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Model Data

Overview

IPM motors use rare-earth sintered permanent magnets with strong magnetic forces, and in addition to the magnet torque that occurs as a result of the magnetic fields of magnets as well as rotating magnetic fields, IPM motors are also capable of using reluctance torque generated from the difference between d-axis inductance and q-axis inductance. IPM motors are therefore in possession of wide operating regions, and are highly efficient. It is for these reasons that IPM motors are used for the likes of traction motors for electric vehicles.
Creating efficiency maps for both motor design and control design can be useful due to the fact that motor efficiency varies in accordance with rotation speed and load.
An example of creating efficiency maps is shown in JAC165. PWM iron loss is accounted for in JAC165 in pre-processing, but input is sinusoidal, and coil AC loss is ignored. Because power electronics are used for traction motors, ignoring the effects from PWM can lead to the possibility of overestimating motor performance.
In this example, an IPM motor efficiency map that accounts for AC loss from a PWM is created, and a comparison with an efficiency map that does not account for AC loss is performed.

Efficiency Map Not Accounting for AC Loss

The efficiency map not accounting for AC loss is shown in Fig. 1, and the operating points extracted to create an efficiency map that does account for AC loss are shown in Fig. 2.
In order to create the efficiency map accounting for AC loss from the PWM, an efficiency map not accounting for AC loss, similar to that of Fig. 1, is created first. Refer to JAC165 for more information on efficiency maps not accounting for AC loss.
It is from this efficiency map that operating points for the purpose of creating an efficiency map accounting for AC loss are extracted.
This document categorizes three regions consisting of a low speed region, a medium speed region, and a high speed region. Operating points are selected to include maximum torque points.

Modeling to Account for AC Loss

The control circuit is shown in Fig. 3.
The control circuit is created to account for the influence of the PWM. As so to additionally calculate AC loss, the coils model wire geometry similar to the schematic diagram.
A JMAG-RT model is used in the control circuit. Calculations of a transient state at the start of analysis are performed with the JMAG-RT model, and by performing FEA only with an electric angle period after reaching a steady state, analysis time can be reduced.

Efficiency Map Accounting for AC Loss

The efficiency map accounting for AC loss from the PWM is shown in Fig. 4. Additionally, as a comparison between accounting for AC loss and not accounting for AC loss, an efficiency difference map is shown in Fig. 5, and a copper loss comparison map is shown in Fig. 6.
From Fig. 5, it is understood that the difference between accounting for AC loss and not accounting for AC loss is approximately one point in low speed and medium speed regions. There is also a difference of two to five points or more in the high speed region. When there is a desire to reach high efficiency of 90% or higher like that of a traction motor, there exists the possiblity that this difference could have an effect on accuracy.
From Fig. 6, it is understood that whether AC loss is accounted for or not results in a large difference in copper loss in the high speed region. It is thought that the differences in efficiency shown in Fig. 5 occur as a result of this.

Loss Component Breakdown

Confirm the loss that occurs during low-speed, low-load and high-speed, low-load shown in Fig. 6.
A breakdown of loss components during low-speed, low-load is shown in Fig. 7, and the stator core eddy current loss frequency components when accounting for AC loss are shown in Fig. 8.
PWM carrier frequency is 6,000 Hz. From Fig. 7 and Fig. 8, it is understood that the PWM harmonic components have a large effect on eddy current loss at low-speed, low-load. For analysis not accounting for AC loss, PWM iron loss is obtained in post-processing. This reduces the difference from when accounting for AC loss.
A breakdown of loss components during high-speed, low-load is shown in Fig. 9, and flux lines and current density distribution frequency components when accounting for AC loss are shown in Fig. 10.
From Fig. 9, it is understood that the influence of eddy current loss from the PWM at high-speed, low-load is small, but also that whether AC loss is accounted for or not results in a large difference in copper loss. From Fig. 10, it can be considered that the difference in this copper loss is due to the eddy currents that occur as a result of the leakage flux within the slots.

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