Leistungsmessung bei Elektrofahrzeugen

Power Measurement on Electric Vehicles

When evaluating technical systems, the distribution of energy at a certain point in time is critically important. By relating the energy to a unit of time, you can obtain the power as a characteristic variable of the system state.

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The efficiency of an electric motor is one of the decisive variables in optimising the achievable range of an electric vehicle for a given capacity of the battery system. The efficiency results from the ratio of the mechanical output power to the effective electrical input power. To determine the efficiency of an electric motor both the electrical input and the mechanical output must be captured.

One of the typical measurement tasks in the development of electric vehicles is to determine the power of the electric drive train -- i.e. the battery, converter and electric motor. There are many aspects to properly determine the effective power in a three-phase system between the converter and the electric motor.  You must pay close attention to which measurement hardware is used, the measurement setup, the sampling rate and data evaluation in order to ensure accurate results. On the one hand, the DC supply of the battery system as a single-phase DC link with superimposed ripple and, on the other hand, the three-phase system between converter and motor where the motor currents are determined by the rpm speed along with harmonics resulting from the PWM (abbr. pulse width modulation).

Active power can be converted into other forms of power (mechanical, thermal, etc.). Reactive power imposes additional stress upon the circuit and apparent power is the geometric/vector sum of the two.

Here's an overview of these calculations:

 

 

Effective Power (P):

Effective power, also known as real power or active power, represents the actual power consumed or supplied by a load. It is calculated by multiplying the instantaneous voltage and current values and taking the average over a specific time period.  

 

Apparent Power (S):

Apparent power represents the total power flowing in a circuit, including both effective power and reactive power. It is calculated by multiplying the instantaneous voltage and current values without considering their phase relationship.  

 

Reactive Power (Q):

Reactive power represents the non-working component of power that oscillates between sources and loads due to inductive or capacitive elements in the circuit. It is calculated by multiplying the instantaneous voltage and current values and taking the average over a specific time period while considering their phase relationship.  

 

For three-phase systems, it involves summing up the powers of each phase. 

Power Factor (PF):

Power factor represents the ratio of effective power to apparent power and indicates how effectively a load utilizes electrical energy. It is calculated by dividing effective power by apparent power. PF = P / S. Power factor is typically expressed as a decimal or a percentage.

 

Normally, current and voltage contain higher frequency harmonics: In the case of non-sinusoidal periodic quantities of current and voltage, it is helpful to represent periodic signals according to Fourier. This can be used to show which frequencies in current and voltage contribute to the effective power and thus determine the required bandwidth or sampling rate of the power analyser. 

It is important to ensure that power measurement devices used in EVs are properly calibrated and have sufficient accuracy and resolution to meet the requirements of the application. Regular maintenance and calibration of the power analyser and the power quality analyser are essential to ensure accurate readings and optimal performance of the EV's electrical system.  

HV power measurement devices allow direct measurement of voltages up to 1000 V RMS. In addition, there are inputs for current probes, Rogowski coils and measuring shunts. Performing power calculations for 1, 2, or 3 phases online in a measuring system is a common practice in electrical power measurement and analysis. The power measurement equipment typically includes various sensors, transducers, and algorithms to accurately calculate different power parameters such as effective power, apparent power, reactive power, and power factor. All this can be done online with our power analyser and power quality analyser. 

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