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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.

Operation of a Variable Frequency Drive (VFD)

A typical motor drive system is driven by a three-phase AC input which is fed to a drive section or power converter section.

The drive section has three main blocks:

  • A rectifier which converts AC-DC
  • A DC bus
  • A DC to AC inverter that converts the DC voltage into an AC signal (in most cases, a PWM waveform).

Although it’s not shown in this diagram, feedback loops and control logic monitor the motor load and adjust the drive system to control torque and speed. This enables the system to drive the motor under conditions ranging from no load to maximum load.

3-Phase Inverter Motor Drive Analysis Datasheet

Functional diagram of a variable frequency drive (VFD) – Fluke Corporation
Oscilloscope screen images showing a phasor diagram and 3-phase harmonics compared to IEEE-519 limits

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

Ripple Analysis

Ripple is defined as the residual or unwanted AC voltage on a constant DC component. It is typically measured on the DC bus. This measurement helps to understand how efficiently the signal is getting converted from AC-DC on the input side and the impact of unwanted components on the PWM signal on the output side.

A line ripple measurement gives the RMS value at the configured line frequency, and peak to peak of the time domain waveform for the configured phases and a Switching Ripple Measures RMS at the configured switching frequency, and peak to peak of the time domain waveform for the configured phases.

Ripple measurements on the DC bus of a variable frequency drive on an oscilloscope
Efficiency measurements on a variable frequency drive based on input power and PWM output power, using 2-wattmeter method on an 8-channel oscilloscope

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.

3-Phase VFD Troubleshooting and Characterization Reference System

An oscilloscope-based 3-phase test system enables system level measurements while observing VFD circuitry.  High sample rates and long record lengths provide detailed views from Hz to GHz.  There are many probe alternatives for this application, but here’s an example of an excellent system:

5 Series MSO
Recommended for its 8 channels and 12-bit ADCs

Option IMDA
Automates 3-phase measurements on the 5 Series MSO

THDP0200 x 3
High voltage differential voltage probes.  100 MHz and up to 1500 V

TCP003A x 3
30A AC/DC current probes

Watch a quick demo

A system for measurements on 3-phase variable frequency drives includes an 8-channel oscilloscope, analysis software, 3 high-voltage differential probes and 3 current probes