Measurements on Three-Phase Variable Frequency Drives (VFDs)

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.

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.

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.

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.

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.

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.

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 B MSO

Recommended for its 8 channels and 12-bit ADCs

Option IMDA

Automates 3-phase measurements on the 5 Series B MSO

THDP0200 x 3

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

TCP003A x 3

30A AC/DC current probes