Application Catalog

90 - Analysis of the Effect of PWM on the Iron Loss of an IPM Motor


Module: DP, LS



Current vector controls are generally used in interior permanent magnet synchronous motors (hereinafter referred to as IPMs), and among them PWM inverters are widely utilized to create a command current. It is vital to get a good understanding of iron losses in order to raise the efficiency of an IPM motor. However, iron losses increase when power is converted by the PWM inverter because the carrier harmonic created by the PWM becomes superimposed on the current and the magnetic flux density waveform in the IPM motor's core.
There are two methods for obtaining iron loss that considers the PWM's carrier harmonics: Couple a control/circuit simulator that contains the PWM inverter with a magnetic field analysis by inputting the current waveform obtained from the simulation into the analysis, or input the actual measurements of a current into a magnetic field analysis.
This Application Note demonstrates an analysis in which a coupled analysis between a separate JMAG-RT model and a control/circuit simulator is carried out, and the effects of a carrier harmonic against an IPM motor's iron loss are displayed by inputting the current waveform calculated from the analysis.

Iron Loss

Table1 Iron loss results

A comparison of the iron losses, hysteresis losses, and joule losses when the IPM motor is driven by a PWM inverter and a sinusoidal current are shown in table 1.
The iron losses in both the rotor core and the stator core increase when the IPM motor is run using a PWM inverter. The joule losses increase the most severely, increasing 70% in the rotor core, and 20% in the stator core.

Joule Loss/ Joule Loss Density Distribution

The joule loss frequency components are shown in Fig. 1 and Fig. 2, and the joule loss density distribution is shown in Fig. 3 and Fig. 4.
The fundamental frequencies (360 Hz in the rotor core and 60 Hz in the stator core) and harmonic components have almost the same amounts of iron loss in the rotor core and stator core whether a sinusoidal current drive or a PWM inverter drive is used. However, iron loss is only produced during PWM inverter drive for the PWM's fundamental frequency (10 kHz) and its harmonic components. (These values are indicated inside the green square in the fig. 1.)
The increased joule losses when the motor is run using a PWM inverter are caused by the carrier harmonic.
The effects of a carrier harmonic need to be evaluated to analysis the characteristics of an IPM motor in detail.

Fig. 1. Joule Loss Frequency Component of Rotor Core

Fig. 2. Joule Loss Frequency Component of Stator Core

Fig. 3. Joule Loss Density Distribution of Rotor Core

Fig. 4. Joule Loss Density Distribution of Stator Core