Wei Wang， Qiang Song,?， Yiting Li and Mukhtiar Ahmad

（1. School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China；2. Collaborative Innovation Center for Electric Vehicles, Beijing 100081, China）

Abstract: The traditional measurement method was inaccurate to evaluate the motor controller efficiency, which the measurement efficiency value could be more than 100% in practical testing experiments. To deal with this issue, an improved electrical measurement method for the motor controller efficiency is proposed in this paper, which is established by analyzing the power loss distribution and phase currents of the motor controller. It is demonstrated that the SiC MOSFET chips are the main power loss devices in the motor controller, accounting for more than 93.1% of the total power loss. The accuracy of the proposed method is compared with the traditional method in simulation. It shows that the test error of the efficiency obtained by the traditional method fluctuates on a large scale, which varied from 0.094% to 1.911%. Compared with the traditional method, the test error of the proposed method appears to be less than 0.083%, which provides significant guidance for the motor controller efficiency test and design.

Key words: electrical measurement method；motor controller efficiency；power loss distribution；SiC MOSFET

Motor controller efficiency is one of the most important indexes in the motor controller performance test. However, the motor controller efficiency obtained by the traditional electrical measurement method is inaccurate. Because of the high efficiency and high switching frequency of motor controller[1-3], a simple error in the current and voltage measurement process would lead to a grave error during the motor controller efficiency test. Therefore, a novel method is needed to accurately measure the efficiency of motor controller.

Since the motor controller efficiency test accuracy is not high by measuring the input and output power of the motor controller, an indirect measurement method is developed and the motor controller efficiency can be obtained by measuring its power loss. There are two types of methods for measuring the power loss of the electrical devices[4-5]. One is the calorimetric measurement method and the other is the electrical measurement method[6].

The calorimetric method[7-11]gets the power loss of electrical devices by measuring the temperature difference of the liquid or air at the input and output of the chamber, which is caused by the heat generated from the electrical devices power loss. In Ref.[5], a 250 kW solar inverter efficiency is measured by the calorimetric and electrical methods and the uncertainties of both methods are compared. In Ref.[10], a fully automated double-jacketed closed type calorimeter is introduced and the accuracy of the calorimeter is verified by the mobile loss. It can make the test results more accurate by the calorimetric measurement method. However, it only measures the power loss of electrical devices at the steady state, which is time consuming in the real measurement process[4]. In addition, the test equipment of calorimetric measurement method is very complicated as compared to the electrical measurement method[12].

The electrical measurement method is a common method to measure the power loss of electrical devices. The power loss can be obtained by measuring the input and output power of electrical devices. However, the test error by the traditional method fluctuates on a large scale because that the input and output power of motor controller are two nearly equal numbers for high-efficiency system. Compared to the traditional motor controller based on Insulated Gate Bipolar Transistors (IGBTs), motor controller based on Silicon Carbide Metal Oxide Semiconductor Field Effect Transistors (SiC MOSFETs)has a higher efficiency[13-14]. This would lead to a further reduction in the traditional method accuracy. In this situation, an improved electrical measurement method is proposed in this paper.

1 Establishing Simulation Model for Calculating the Losses of Each Components in Power Inverter

The motor controller power loss is mainly distributed in the laminated busbar, busbar capacitance, parasitic resistors and the power modules (including the SiC MOSFET chips and diodes). In order to establish the motor controller model, it is needed to extract these devices parameters by simulation software and experiment test.

In this paper, the motor controller adopts the SiC MOSFET half-bridge power modules rated at 300 A/1 200 V, which are produced by Cree Inc. The thickness of the laminated busbar is 1 mm and the gap is 0.05 mm, which is made of copper. The parasitic parameters of the power modules and the laminated busbar are extracted by the ANSYS Q3D software in Fig.1. It should be noted that the parasitic resistances of the motor controller varied with the frequency of the current are not taken into consideration in this paper. The on-state resistance of the power modules can be obtained by a double pulse test circuit.

Fig.1 Parasitic parameters extraction

Compared to electrolytic capacitor, the film capacitor has wider frequency response, lower equivalent series resistance(ESR) and higher voltage[15], which are adopted in the motor controller. The specific relevant parameters of busbar capacitance power module can be obtained from the datasheets provided by the manufactures. According to the extracted parameters, the simulation model can be established in Fig.2,which is used to obtain the motor controller power loss and its efficiency.

Fig.2 Simulation model of motor controller

The load motor used in the simulation model is a permanent magnet synchronous motor(PMSM), which is widely used in electric vehicles due to its high efficiency and power density. The detailed parameters of the PMSM are shown in Tab. 1.

Tab. 1 Basial parameters of the PMSM

In order to obtain the motor controller power loss and design the motor controller efficiency test method, some assumptions are made in this paper, as followed: ① The current and voltage sensors are ideal sensors with no limitations of the measurement range and their parasitic parameters are not taken into consideration;② The DC power supply is an ideal voltage source, which provides a stable voltage; ③ The power modules are the same, and its parasitic inductance is not considered. Because the parasitic inductance has several orders of magnitude smaller than that of the motor, and its power loss to be zero in one period due to producing reactive power; ④ The current and voltage signals in the circuits are acquired in real time, and the time delay of signal acquisition is not taken into consideration.

2 Power Loss Distribution

The motor controller power loss mainly distributes in the SiC MOSFET chips, freewheeling diodes, laminated busbar, busbar capacitance and the parasitic resistors. The power loss of each component can be achieved by

Then, the motor controller power loss can be got by adding these devices power loss together,which can be obtained by

where Plosses, u(t) and i(t) represent the device power loss, the transient voltage and current of the device, respectively. t1and t2represent the beginning and end of the fundamental period of the phase current, respectively. T represents the fundamental period. Pmc, Pdio, Pmos, Ppar, Pcapand Pbusrepresent the power loss of the motor controller, diodes, SiC MOSFET chips, parasitic resistors, busbar capacitance, and laminated busbar, respectively.

The motor controller power loss is mainly influenced by the switching frequency, junction temperature and the operating condition of the motor[16]. The values of these factors are shown in Tab. 2. It should be noted that other factors remain unchanged when analyzing the influence of the single factor on the motor controller power loss in this paper.

Tab. 2 Parameters setting for simulation

Because the power loss of the laminated busbar, bus capacitance and the parasitic resistors are all the parasitic loss in the circuit, they can be added together when analyzing the power loss distribution of the motor controller. Then, the proportion of the power loss of each component can be calculated by

Fig.3 Influence of main factors on the power loss distribution

where Kmosand Kdiorepresent the proportion of the power loss of the MOSFET chips and diodes,respectively. Kothrepresents the proportion of total power loss of the laminated busbar, the bus capacitance and the parasitic resistors.

The influences of these factors on the power loss distribution of the motor controller are shown in Fig.3.

Fig.3 shows that the proportion of the SiC MOSFET chips power loss is the largest accounted for more than 93.1%. The proportion of the SiC MOSFET chips power loss increases with the increase of the switching frequency, which is accounted for about 93.1% to 96.5% in Fig.3a. The influences of the junction temperature on the proportion of the SiC MOSFET chips power loss have the same trend, which is accounted for about 94.7% to 95.8% in Fig.3b. However, the proportion of the SiC MOSFET chips power loss decreases with the operating power of the motor,which is accounted for about 96.9% to 4.9% in Fig.3c. In Fig.3d, the power loss of the SiC MOSFET chips accounts for more than 93.4% in the motor speed ranges. Compared to the SiC MOSFET chips power loss, the proportion of other devices power loss are very small.

3 Improved Efficiency Test Method

3.1 Traditional test method

As shown in Fig.4, the motor controller efficiency can be obtained by measuring the current and the voltage at the input and output of the motor controller. Then, the input and output power of motor controller can be obtained by the power analyzer. Finally, the motor controller efficiency can be calculated by the input and output power of the motor controller, as follows

where ηtmcrepresents the motor controller efficiency obtained by the traditional method. Pinand Poutrepresent the input and output power of motor controller, respectively.

Fig.4 Traditional test method.

3.2 Improved test method

From the analysis of the power loss distribution of the motor controller in the previous section, it can be found that the motor controller power loss is mainly distributed in the SiC MOSFET chips. Since the other devices power loss are very little in the motor controller, the SiC MOSFET chips power loss can be approximately regarded as the motor controller power loss.

In Fig.5, the green rectangle, yellow rectangle and blue rectangle represent the SiC halfbridge power modules, respectively. Taking the phase A for example, the voltage (UVT1m) of the SiC MOSFET chips on the upper-bridge arm can be obtained by measuring the voltage between the positive pole of the motor controller input and the phase A of the motor controller output,as shown in Fig.5a. Accordingly, the voltage(UVT2m) of the SiC MOSFET chips on the lowerbridge arm can be got by the same method, as illustrated in Fig.5b. Similarly, the voltage of the SiC MOSFET chips on the other bridge arms can be obtained by this method.

Fig.5 Voltage measurement of the SiC MOSFET chips on the upper- and lower-bridge arms

SiC half-bridge power module consists of the upper and lower bridge arms, in which the turnon and turn-off of the SiC MOSFET chips determine the corresponding phase current at the motor controller output. In order to analyze the relationship between the phase current and the current of SiC MOSFET chips, it will take the phase A current for example, displayed in Fig.6.

Fig.6 Current of phase A and the corresponding SiC MOSFET on the upper- and lower-bridge arms

In Fig.6, IAis the phase A current in the motor controller output. IVT1mand IVT2mrepresent the current of the SiC MOSFET chips on the upper-bridge arm and lower-bridge arm, respectively. It can be found that the phase A current is formed by the current of the SiC MOSFET chips on the upper and lower bridge arms. Therefore,the current of the SiC MOSFET chips can be separated from the phase current.

According to the definition of the turn-on and turn-off time of SiC MOSFET, it can approximately estimate the operating times of the SiC MOSFET chips by judging whether the measured voltage between the input and the corresponding output of the motor controller is less than 90% of the voltage (UDC) of the direct current power supply or not. Hence, the phase current can be separated into the current of the SiC MOSFET chips on the upper and lower bridge arms by the measured voltage, as follows

where IVTimand UVTimrepresent the current and the voltage of the SiC MOSFET chips on the upper and lower bridge arms, respectively, and i=1,2, 3, 4, 5, 6, k=A, B, C. Here is to explain that if i=1, 2, then k=A; else if i=3, 4, then k=B; else if i=5, 6, then k=C, which is also useful to the following formulas.

Finally, the SiC MOSFET chips power loss can be calculated by

The motor controller efficiency can be obtained by

where ηimcrepresents the motor controller efficiency obtained by the improved method.

According to the above analysis, the motor controller efficiency can be obtained indirectly by measuring the motor controller power loss. The schematic diagram of the improved measurement method proposed in this paper can be established in Fig.7.

Fig.7 Improved test method

4 Analysis of Test Error

In order to verify the accuracy of the proposed method, the reference motor controller efficiency should be obtained by simulation, which can be calculated by

where ηrmcrepresents the reference motor controller efficiency. Then, the accuracy of the proposed method can be verified by comparing the test error of the motor controller efficiency obtained by the traditional and improved methods,which can be calculated by

where Δtmcand Δtmcrepresent the test errors of the motor controller efficiency by the traditional method and improved method, respectively.

4.1 Influence of switching frequency

The influence of the switching frequency on the motor controller efficiency and the test error obtained by the both methods are shown in Fig.8.

As shown in Fig.8, the reference motor controller efficiency decreases with the increase of the switching frequency, which is varied from 99.66% to 97.78%. This is due to the switching loss of the SiC MOSFET chips increased with the switching frequency. However, the motor controller efficiency obtained by the traditional method is almost invariable in the switching frequency range, and the test error is increased with the switching frequency varied from 0.094% to 1.911%. Compared to the traditional method, the accuracy of the proposed method is improved,and the test error is less than 0.036% in the switching frequency range.

Fig.8 Influence of the switching frequency

4.2 Influence of junction temperature

The influence of the junction temperature on the motor controller efficiency and the test error obtained by the both methods are shown in Fig.9.

Fig.9 Influence of the junction temperature

In Fig.9, the reference motor controller efficiency is decreased with the increase of the junction temperature, which is varied from 99.37% to 99.20%, because the conduction loss of the SiC MOSFET chips are increased with the junction temperature. The motor controller efficiency got by the traditional method are more than the reference efficiency. The test accuracy is increased with the junction temperature and the test error is varied from 0.405% to 0.360%. The test accuracy of the proposed method is improved, as compared to the traditional method and the test error is varied less than 0.030% in the junction temperature range.

4.3 Influence of the motor operating power

The efficiency and the test errors of the motor controller obtained by both methods are affected by the operating power of motor drawn in Fig.10.

Fig.10 Influence of the operating power

As illustrated in Fig.10, the reference motor controller efficiency is decreased with the increase of the operating power of PMSM, which is varied from 99.55% to 99.34%. The test accuracy of the traditional method is almost invariable in the range of operating power of PMSM and the test error is about 0.398%. Although the test accuracy of the proposed method is increased with the operating power of PMSM, it is improved in comparison with the traditional method and the test error is varied from 0.001%to 0.025%.

4.4 Influence of the motor speed

The influence of the motor speed on the motor controller efficiency and the test error obtained by both methods are shown in Fig.11.

Fig.11 Influence of the motor speed

As shown in Fig.11, when the motor speed is less than 4 000 r/min, the reference motor controller efficiency is increased with the increase of the motor speed, which is varied from 98.87% to 99.75%. It is opposite to the one as the motor speed is more than 4 000 r/min, and the efficiency is varied from 99.75% to 98.51%. The reason is that as the motor operates under the same power, the motor controller power loss is decreased with the increase of the motor speed until the rated speed and it is opposite after the rated speed. The test error of the traditional method is varied from 0.114% to 0.72%. Compared to the traditional method, the test accuracy of the proposed method is improved and the test error is varied from 0.018% to 0.083%.

5 Conclusion

In this paper, the power loss distribution of motor controller is analyzed. Therefore the SiC MOSFET chips are verified to be the main power loss devices in the motor controller, which accounts for more than 93.1% of the total power loss. By analyzing the topology structure and phase current of the motor controller, the improved method is proposed to measure the motor controller efficiency. The test error by the traditional method fluctuates greatly from 0.094% to 1.911%. Compared with the traditional method, the test accuracy of the proposed method is improved, which is less than 0.083%.Therefore, the proposed method has an obvious improvement on the test accuracy of the motor controller efficiency.