Electric-mechanical performance analysis of high speed motor for electric turbo charger

2.1 Electric performance analysis

Figure 2 shows flux density, winding topology and used materials of the designed PMSM using 2-D FEA (ANSYS Maxwell). It has surface permanent magnet type rotor and consists of 2 poles and 12 slots. This paper deals with analyzing parameters to reduce iron loss and eddy current loss which occurs prominently at high speed [1]. The electric performance analyses are validated according to current wave types (sine and pwm) and core material for high speed (POSCO, 15PNF1000, 20PNF1200, 20PNF1500 series). Table 2 shows the specification of the determined electric design parameters. Figure 3 shows eddy current loss and core loss distribution of the designed model.

Figure 4 shows avg. torque, torque ripple ratio and efficiency according to eccentricity. Eccentricity increases avg. torque and efficiency are slightly reduced but torque ripple ratio increase. Most of the electrical performance has not changed except torque ripple ratio. Generally, torque ripple ratio causes noise and vibration. In case of core losses in rotor and stator, the total losses are commonly expressed by Eq. (1) for sinusoidally varying magnetic flux density B with frequency f , and they are divided into hysteresis loss and eddy current loss. The each coefficients are obtained of measurement data of core samples.

P i = P h + P e = k h ⁢ B m n ⁢ f + σ e ⁢ ( B m ⁢ f ) 2(1)

where B m : maximum flux density, k h : hysteresis loss coefficient, n : Steinmetz coefficient in the range 1.5 to 2.5, f : frequency, σ e : eddy current loss coefficient.

Servo motor operates in a wide speed and torque range. When the speed increase, the iron loss increase. As the torque is loaded, the copper loss increase. Mechanical loss used by measurement. Efficiency by simulation is defined as below

η = P o / ( P o + P i + P c + P m )(2)

where P o : output power, P m : mechanical loss, P c : copper loss.

Generally, the shrink fitted rotor was used to compress PM due to the centrifugal force. In this paper, CFRP (carbon fiber reinforced plastic) is applied to substitute for sleeve. The structural stability of CFRP applied rotor is stable in spite of high speed as shown in Fig. 5. The CFRP applied rotor is better than shrink fitted rotor in terms of lightness. The reduction of rotor weight is very important to reduce turbo lag. The light materials such as aluminum impeller and CFRP sleeve should be used to be within 0.5 sec. Figure 5 shows response time (0.37 sec) of CFRP applied rotor to check turbo lag considering viscous friction of bearing, mass inertia moment and impeller load. To be within 0.5 sec, over-load rating of current by controller and power stack should be determined and controlled considering thermal stability. Rated point (3 kW@100,000 rpm) of the developed motor is designed at current density 6 (67 Arms). Intermittent point of motor considering over-load can operate because electric turbo charger usually use in a short time.

2.3 Modal testing

Figure 6 shows wound stator and CFRP applied PM rotor for 3 kW, 100,000 rpm. The end size of winding is involved in critical speed of rotor. The end size of winding has identified through the production. The rotordynamic analysis result is validated by modal testing. The fundamental natural frequency of rotor supported by free-free is about 4,452 Hz. The simulation result is well accorded with modal testing result, with a maximum error of 1.7%.

2.4 Rotordynamic analysis

The critical speed of the developed rotor considering rotation and gyroscopic effect should be above the operating speed, 100,000 rpm, and have a sufficient separation margin, 184%, as shown in Fig. 7. There is no critical speed in operating speed range. The critical speed of rotor should be above the operating speed (100,000 rpm). An appropriated separation margin is typically at least 20% considering API (American Petroleum Institute) standard 610 [2]. Bearing is applied to deep groove in consideration of the price. To spread the impact and dynamic load of the bearing is applied to metal mesh on the bearing. Next time, we are planning to apply to air foil bearing.

2.5 Acoustic analysis according to eccentricity

Generally motors have forced vibration and acoustic problems caused by electromagnetic. The eccentricity is definitely generated by machining tolerance and assembling cumulative tolerance effects. A torque ripple ratio increases according to increase of eccentricity.

Three cases, eccentricity (0 mm, 0.25 mm, 0.5 mm) between rotor and stator teeth, are considered in order to determine the effect of the eccentricity on the vibration and noise. To minimize eccentricity and avoid resonance between exciting frequency and natural frequency is a way to reduce noise. The first step, electromagnetic analysis is conducted in the three cases. The next step, excited electromagnetic forces are applied to teeth of stator. The forced vibration analysis is performed according to exciting frequency (100 ∼ 5,000 Hz) in Fig. 9. The last step, the generated surface velocity on motor and housing is mapping on contact surfaces of air model. Then the acoustic analysis is carried out according to exciting frequency of vibration velocity (100 ∼ 5,000 Hz) in Fig. 10. Figure 8a and b represents respectively forced vibration velocity and acoustic analysis result according to frequency (eccentricity: 0.5 mm). A-weighted sound pressure level (SPL) mainly increases around resonance between teeth passing frequency and natural frequency.

To develop low noise motor is to minimize eccentricity and avoid resonance. A-weighted SPL is rapidly varied below 2,500 Hz by way of unbalance magnetic force when eccentricity is considered as shown in Fig. 11. A-weighted SPL increase around operating speed and not change at near teeth passing frequency comparing no eccentricity result.