Published by Dranetz Technologies, Inc.
VFD motor controls have become very popular for both retrofits and in new applications. They offer increased energy efficiency, potential for less motor wear, lower startup currents and more. Potential disadvantageous include producing harmonics and damaged motors if they are not controlled properly.
VFD’s use a Pulse Width Modulated (PWM) voltage for more precise control of the motor to meet the dynamic needs of the application. The varying voltage frequency of the VFD presents problems for most PQ analyzers, but not the Dranetz HDPQ family.
Most PQ analyzers can only measure a stable 50/60Hz voltage source. The Dranetz HDPQ family can also synchronize to the current which is very beneficial for use in VFD and other applications.
PQ analyzer VFD testing applications include troubleshooting problems with controls, evaluating efficiency, harmonics, maintenance, and more. The Dranetz HDPQ family is ideally suited for VFD and many other related applications.
Configuring an HDPQ for VFD applications is simple – see application note (below).
Dranetz HDPQ Family VFD Output Measurements
INTRODUCTION
Most power monitoring instruments synchronize their measurements to a stable 50/60Hz voltage source. The reason is to get accurate readings between the voltage and current phases. This is fine for the majority of applications, including when measuring the input of a Variable Frequency Drive (VFD), but poses a problem when attempting to measure the output of a VFD – by design, the VFD output voltage frequency varies, making it difficult for the power monitor to synchronize.
Unlike most power monitors, the Dranetz HDPQ product family can also synchronize to the current which is very beneficial for use in VFD and other applications. The instruments default to synchronizing to the voltage, but can be easily changed to synchronize to the current. Doing so involves a simple setup change, and this Application Note describes the required setting.
WHY MEASURE THE POWER AND POWER OF A VFD?
Many VFD monitoring applications are at the VFD’s input to measure the power consumption, harmonics, and power line fluctuations. Being at the VFD input, a stable 50/60Hz voltage source is available for power monitor synchronization. However, when evaluating VFD efficiency, susceptibility to PQ issues, and troubleshooting issues with the controlled outputs, it is necessary to measure at the VFD’s output, which does not have a stable 50/60Hz voltage output. Significant errors will result when using most power monitors.
It turns out that the VFD output current has a stable 50/60Hz frequency. See the snapshots below – the top is the voltage and the bottom is the current.
DRANETZ HDPQ SETUP
The required current synchronization setting is located in the Nominal & Frequency tab of the Dranetz HDPQ Wizard setup. The Sync Channel setting defaults to Channel A Volts, but as shown below, when measuring the output of a VFD, select Channel A Amps as the sync channel instead. This is the only setting change that is required, and all other settings are as usual.
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