Measurements on Three-Phase  Variable Frequency Drives (VFDs)

Measurements and analysis on three-phase power systems are inherently more complex than on single-phase systems. Power converters based on Pulse Width Modulation (PWM), such as variable-frequency motor drives, further complicate measurements since filtering and triggering on PWM signals are challenging.

During debug and validation phases, oscilloscopes are the instrument of choice thanks to their versatility and speed.  They can precisely measure the performance of switching power converters and control circuitry.  With the right probes, they can measure with high bandwidth over wide ranges. 

Special 3-phase inverter motor drive analysis software enables fast, repeatable analysis.

3-Phase Line Measurements

In the lab, power quality measurements are used to understand the way in which equipment consumes energy supplied by the three-phase AC line.

For each phase, power quality measurements typically include:

• RMS and magnitude of voltage and current
• True power, reactive power, apparent power, phase angle, and power factor
• Frequency

In addition to numerical readouts of RMS voltage and current, phasor diagrams (shown at the left) can provide a quick way to see voltage and current relationships. Imbalances and phase shifts that impact power factor are immediately apparent.

Power factor is an important specification for any industrial equipment since it has direct impact on end-customers’ utility bills. Some drives include active circuitry to control power factor.

Harmonics can also impact the overall efficiency and even reliability of the end-customers’ system. Because of this, harmonic distortion is often subject to regulation. A harmonics bar-chart with IEEE-519 limits is shown at the left. User-defined limits may be used for margin testing.

Read about using phasor diagrams on an oscilloscope

Direct Quadrature Zero (DQ0)

Vector control systems use Clarke and Park transforms to simplify three-phase signals into D and Q control vectors. Being able to measure these vectors lets you confirm that the control system is working as expected.  Unfortunately, these important variables are often calculated in real time, deep within the control system, and are not brought out as external signals. 

DQ0 measurements (Opt. IMDA-DQ0) on Tektronix 5/6 Series oscilloscopes use signal processing to calculate and measure the D and Q vectors based on the drive’s output signals, so you can compare actual versus expected performance. Results are displayed as phasors, transformed waveforms, and scalar values.  

The DQ0 results are displayed as phasors, transformed waveforms, and scalar values.

Drive Output Measurements

Efficiency is one of the critical measurements of the motor drive system as an indicator of the overall performance of the system.

Efficiency measures the ratio of output power to input power. It computes and displays efficiency at each phase, and the total efficiency (average) of the system. Efficiency measurements use the 2V2I configuration (2-wattmeter method), on 8-channel oscilloscopes.

Mechanical Analysis

The IMDA mechanical analysis group (Option IMDA-MECH) supports Hall, resolver and quadrature encoder interface (QEI) sensors to track motor angle, speed, acceleration and direction. Measurements are configured using a few straightforward settings.

Measurements may be made with passive or differential analog probes. Hall or QEI sensors may also be measured with 8-channel TLP58 logic probes to save analog channels for use on other signals.

Speed measurements can be plotted to show the motor start-up sequence or deceleration over long records. Histogram plots provide insights into the jitter profile of the measured speed.

IMDA-MECH supports two methods for real-time torque measurements – torque sensors or load cells, and armature current. When using the armature current method, torque is calculated by applying a constant to the RMS motor current. The mechanical power of the system may be calculated and displayed using torque and speed values.

Electrical power can be determined on the input of the drive, using voltage and current measurements. Using torque and speed measurements on the output of the drive, the application can measure the ratio of mechanical output power to electrical input power, thus measuring overall system efficiency.