Power supply requirements for Variable Frequency Drives

This is the second article in a series of articles outlining the must-know factors when dimensioning a variable frequency drive for a drive system. This article reviews the role of the power supply. Read our other articles in this series on dimensioning for different load characteristics and motors.

The role of power systems

The high voltage national grid and it’s low voltage subsidiaries, provide us with electrical power and is fed by tens of thousands of turbo-alternators, wind turbines and the like. Due to the large inertias of these rotating machines, the industrial power utilities are usually inherently robust or ‘stiff’ and are generally considered compatible with starting and stopping large loads frequently without issues.

Public power supply networks are generally low-impedance, and can be capable of supplying heavy loads at start up with only small voltage fluctuations.

Rural settings

In rural areas, there may be longer overhead power lines which are needed to reach some remote locations. These longer lines increase the source impedance of the power supply, which makes the installation more vulnerable to voltage fluctuations – dips, sags and surges. The consequences of such must be taken into account when designing a VFD installation in such settings.

Higher power installations

During fault conditions in higher power installations, the possibility of potential short circuit current at the variable frequency drive input terminals must be considered. These fault currents may reach many thousands of amps in under one millisecond and can cause much damage to equipment. Additional impedance such as provided by standard line reactors can be added to the VFD power supply, which will slow down the rate of rise of such fault currents and allow circuit protection devices time to operate and clear such faults before damage occurs.

Important power supply considerations

Before you select a variable frequency drive and a motor, you will need to check the compatibility of the power supply voltage and frequency. If there are likely to be significant voltage variations from those specified by the VFD and motor manufacturers this should be recognised and those manufacturers consulted before equipment is selected.

Cable cross-sectional area and type must be correctly chosen for the complete VFD system and all protection and switching devices must be compatible with the electrical load and installation.

Specific power supply voltage, phase number and frequency, along with VFD rated input and output current will be specified in their user manuals so appropriate cable sizes and protection devices can be selected.

Key takeaways

• The power supply is a critical consideration as it must be to exceed the power of a drive system.
• Check the manuals of both VFDs and motors before taking a purchase decision to ensure compatibility.
• There are extra steps you may need to take for high power installations or installations in rural areas, for example, to protect against short circuit, voltage variations and other supply related eventualities.

For expert advice, Dalroad are here to help

Dalroad supports end users and electrical contractors of all kinds dimension their drive systems, offering them high quality inverter drives as well as best-in-class support.

We supply high-quality variable speed drives that can help you maximise the performance of your application, while adding in automated control functionality and boosting the energy efficiency of your project.

To learn more about our expert support service, get in touch with one of our variable frequency drive experts today.