VSD Series Part 1: Variable Speed Drive (VSD) – what is it?

Updated: May 24

This blog post is part of the Variable Speed Drives (VSDs) series, focusing on understanding VSDs.


Until recently, VSDs were relatively expensive and using them requires special conditions and expertise. Today, their affordability and ease of use make them available in nearly every industry – in the mining, manufacturing, commercials, and even domestic sectors, to mention a few.


With the high cost of electricity and the need to reduce carbon footprint and control processes to minimise waste, businesses increasingly use VSDs in various operations. In this blog post, we will look at the basics of VSDs, their advantages, and how to select the right VSDs for your application.


What is a VSD?


In simple terms, a VSD is an electronic power control device that controls the speed of an induction motor. The VSD accepts control commands from the control system and regulates the speed or torque in response to the speed reference signal.


The components of a VSD include:

  • Input rectifier - This section contains components that take energy from the supply and convert it from AC into DC.

  • Inverter - The DC voltage is converted back to AC in accordance with the requirements of the process. The inverter produces a pulse width modulated voltage waveform which controls the speed of the induction motor.

An example of VSD components of a seawater pump.

A VSD is sometimes referred to as Variable Frequency Drives (VFD), AFD (Adjustable Frequency Drive), Adjustable Speed Drive (ASD), AC drive, and inverter.


How does a VSD work?


A VSD contains an advanced digital signal processor that adjusts the output voltage and frequency to achieve precise motor shaft speed control without additional physical sensors.


Some of the key features that distinguish them from other types of motion control devices include:

  • Constant torque

  • Precise speed control

  • Precise regenerative braking

One of the key benefits of utilising the VSD is its ability to supply the motor's full rated torque starting from zero speed to full speed with minimal starting surge issues to the power grid. An example is a conveyor belt overloaded with rocks heading uphill that can be brought back to full speed in a precise and smooth acceleration profile. Starting from a standstill, the VSD can hold the load without letting it roll back and then apply full rated torque till the belt is operating at full speed, all without causing breakers to trip from overload. The speed control allows motors to be oversized to meet extreme requirements without wasting energy during low demand. In a traditional electric motor, the motor always runs at full speed. A mechanical device, such as a belt, gearbox or fluid coupling, is used to modify the motor's output speed and/or assist in starting the load.


An example of a conveyor belt on a mine site.

Why Do You Need a Variable Speed Drive (VSD) System?


The benefits of a variable speed drive are numerous, but the main reasons to use a VSD are to:

  • Save energy

  • Reduce maintenance costs

  • Improve process control

Motors use substantial energy and can incur high running costs, meaning the great potential for savings. As the demand for energy changes, the speed of the motor can change to match the demand. This will save energy since you are not running the motor at full power all of the time; you won't be wasting as much money on extra electricity consumed by using your equipment at total capacity when you don't necessarily have to. Besides, it will also reduce stress on machinery components and extend their life, so the maintenance costs are reduced. Also, as the speed of machinery can be controlled automatically, it reduces the need for manual intervention which otherwise increases the risk of accidents and injuries. By matching power demands exactly, your motors will be more efficient. Better still, VSDs come with an intelligent sensor system that monitors the supply, motor current, voltage, and speed, and gives you accurate data for condition monitoring and adjusting processes in real-time, so you can make changes on the fly without having to shut down your equipment or waste time waiting for parts to be made or installed.


Where VSDs are used in the industries


VSDs are particularly useful in the mining industry. In mining, VSDs are used in many applications that help transfer and process ore around the site, including conveyor belts, ventilation fans, elevators, and heavy load lifting. While most traditional mining companies have been able to control their processes only through manual means, VSDs have allowed them to improve their operational efficiency and reduce waste. This is especially important since the mining industry consumes about 10% of the global energy.


In the wastewater management industry, VSD controls the speed of pumps and motors that transfer sewage, ensuring that they run no faster than they need to. This helps to prevent overloading and saves energy.


In commercials, VSDs are used to control lights, fans, motors and other appliances.


How to select the right VSD?


When it comes to appropriately and effectively selecting the right VSD, here are six key items to consider:


High Voltage or Low Voltage

First and foremost, you need to determine whether you'll be using a low-voltage or high-voltage drive. If you're dealing with over 700KW of power, it's probably best to use a high voltage VSD.


Correct motor selection based on load type

Think about the type of speed and torque response your load requires. Is it constant torque like a lift? Or is it quadratic like a centrifugal water pump?


Starting torque requirements

Check if your load has very high starting torque requirements - if so, you'll need an inverter rated for "Heavy Duty" operation (i.e., 150% inverter output for 30 seconds every 3 minutes).


Temperature rating

Make sure that the VSD housing has appropriate cooling requirements. Up to 4% of the VSD output power can be lost as heat in the VSD enclosure.


Cable Length

Check How long those cables from your VSD to your motor will be. In general,, keep them under 160 meters. Extra filtering requirements are needed if exceeding the maximum length.


Motor braking

Lastly, it would help if you determine whether you're going to perform braking. If so, how much braking do you want? Do you want to put the energy back into the grid, dump it into a load bank or burn it off in the motor's stator? These questions have economic and performance effects; they can be used in combination with one another (or not), depending on your needs. If you don't need braking, then you'll be looking at what kind of speed control you want.


VSD for water pumping

We hope the information above helps!


We can provide you with more information on selecting the right VSD specific to your application; See how we can assist you.


Part two of the series will focus on the power quality analysis of VSD. See you in our next blog post!


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