Power Measurements by Angular Rotation in EV Motors

Introduction

As vehicle electrification gains popularity for its environmental benefits, precise measurement of various parameters is becoming essential. Among these include measurement of voltage, current, power, power factor, and frequency of traction motors for each rotation cycle. These measurements are equivalent to conventional engine characterizations including combustion efficiency, TDC and spark timing. When evaluating the rotor performance as it gradually increases its speed from a standstill, statistical measurements of each rotation cycle is important.

Challenges

Many power analyzers calculate measurement values at regular intervals or according to the cycle of a specific input signal. As a result, they may not always align with the rotation cycles of the motors. Motors typically have poles due to their structure, creating a relationship with the driving signal frequency that corresponds to the rotation cycles. This means that normal measurement methods cannot accurately capture data for each rotation cycle.
To address this issue, power analyzers need to obtain the rotor’s cycle signals using sensors. Rotation speed is commonly measured and calculated using the Z-phase signal from an incremental (ABZ) encoder. Additionally, devices such as Hall sensors, resolvers, and compressors, which cannot have sensors attached directly to their axes, require alternative methods for measurement.

Solutions

Synchronizing WT5000 Measurements to the Motor’s Rotation Cycle

• Setting Z-phase signal as a synchronization source
• Setting data update rate to Auto and data updating cycle at the minimum of 10 ms
• Synchronization source of motor evaluation function remains as voltage or current
• If the EXT Clock is set as the PLL source for harmonics measurement, then branch the Z-phase signal and input to the EXT Clock terminal

Synchronizing WT1800E Measurements to the Motor’s Rotation Cycle

• Setting Z-phase signal as a synchronization source
• Setting data update rate to Auto and data updating cycle at the minimum of 50 ms
• Branch Z-phase signal and input to the EXT Clock terminal
• Synchronization source of motor evaluation function remains as voltage or current
• If the EXT Clock is set as the PLL source for harmonics measurement, then branch the Z-phase signal and input to the EXT Clock terminal

Figure 1. Input of Z-phase signal to the EXT Clock terminal

Proposals by Each Model

Steps to Synchronize WT5000 Measurements to the Motor’s Rotation Cycle

To measure the rotation cycle using the WT5000 and an encoder’s Z-phase signal, follow these steps:

1. Set the data updating rate to Auto and the data updating cycle to 10 ms.
2. Set the Z-phase pulse as the synchronization source for each input element.
3. Optionally, branch the Z-phase signal output from an encoder at the D or H terminal of the motor evaluation function and input it to the EXT Clock.

However, note the following restrictions:

1. Even though data changes with every motor rotation, the same data will be output every 10 ms during constant input.
2. When the data updating rate is fixed at 10 ms, voltage and current measurements below 200 Hz cannot be performed. The same applies to frequency measurement. When the data updating rate is set to Auto, if the input frequency is high, the results will be the mean value of multiple cycles.
3. The input specification for the EXT Clock is limited to TTL 0 to 5 Vdc. Additionally, square-wave input with a 50% duty ratio is required.
4. When using the PLL source for harmonics, measurement can be performed by setting the EXT Clock input. However, a measurement gap occurs at the 10 ms setting, and the analysis resolution will be up to 10 degrees.
5. For parameters measured with the motor evaluation function like Synchronous speed, it is typically set to measure at every cycle of electric signals driving the motor, rather than at each rotation cycle. This involves computing for each short cycle, which is the product of the rotation cycle and half the number of motor poles.

The data measured with the WT5000 is stored and graphed using spreadsheet software (Figure 2). It shows that other parameters are plotted against rotational position. However, since measurement parameters (rotational speed, torque, mechanical power, etc.) in the motor evaluation function generate time differences with the frequency set at a rotation speed, it is set at the voltage waveform of the inverter output. Therefore, it is updated at twice the speed of the motor rotation cycle.
Following the notes above, the data measured by the WT5000 is stored and graphed using spreadsheet software to confirm its operation. Please refer to Figure 2. It shows that some parameters are updated each mechanical rotation. However, as mechanical measurement parameters (rotational speed, torque, mechanical power etc.) generate time differences with the electrical measurement (voltage, current, electrical power). Because a four pole motor was used (one electrical cycle per pole pair), the electrical measurement parameters appear to update at twice the speed of the mechanical parameters.

Figure 2. Measurement data for each rotation cycle using the WT5000

WT1800E Measurement Considerations

As data updating rate is at a maximum speed of 50 ms with the WT1800E, data will be updated at a cycle five times longer than the WT5000’s even under the same setting as the WT5000’s, resulting in a slight decrease in the time resolution. In addition, when the data updating rate is fixed, the minimum measurable frequency is 45 Hz. Additionally, under Auto setting with high input frequency, the result will be the mean value of multiple cycles. The measurement results are shown in Figure 3.

Figure 3. Measurement data for each rotation cycle with the WT1800E