What Is A Variable Speed Drive And How Does It Work?

Motor speed control on electric drives improves the relationship between speed and power. Variable speed drives (VSDs) can match their speed precisely to any given load, offering an energy-efficient and economical solution to power consumption.

A VSD works by converting the fixed inputs of an AC motor to variable outputs to control its speed and torque. Frequency and voltage coming in at a steady rate are converted to voltage and frequency output at a rate that varies according to the motor’s demand. A VSD provides only as much power as the load requires, greatly improving overall system performance.

Why Use A Variable Speed Drive?

As well as motor efficiency and system optimisation, VSD motor control can help achieve considerable energy savings. Automated systems running on electric motors lose on average more than 50% of their input energy. Even though individual motors may be extremely efficient, their effect on the entire system is often much less so. If the output requirements of a process vary by 30% or more, it makes sense to use a VSD to reduce energy waste.

You can estimate how much energy a VSD will save when it’s used for a constant or variable torque load. To do this, you’ll need to determine how long the equipment will be running in various load conditions with a standard motor. Then, work out how much power you’ve used and compare that with a VSD running under comparable loads. Manufacturers of motor drive systems may also specify efficiency values for various load situations.

Energy Efficiency

The UK government’s Energy Technology List contains a database of over 4,000 electric motors and VSDs, plus a similar quantity of other energy-efficient products. It provides information to compare different manufacturers, notable features, conformity assessment certification and product performance. All electric motors are required to carry a CE or UKCA marking, whichever may currently be applicable.

European regulations have revised their requirements for motor energy efficiency as part of the EU eco-design programme. Since July 2023, electric motors have had to conform to stricter requirements. Motors sold in the EU rated from 75–200kW must be energy efficient to IE4 level.

You can derive considerable advantages from incorporating VSDs into existing motors, especially when integrated as part of an IIoT system. As well as cutting the motor’s maximum power demand, the system components will suffer less stress and last longer. You’ll find a VSD puts out less noise and heat, so your workplace will be safer and more comfortable. You can control variables like temperature, flow and pressure more accurately, so it pays to integrate intelligent VSD control into larger systems like pumping facilities or building management systems (BMS).

Integrating an electronic VSD into an in-service AC motor will give you effective speed control technology. You’ll need to evaluate your motor and load requirements to make sure they’re compatible, but you shouldn’t need to invest in a new motor. If you’re running a system on a DC motor, it might be worth exchanging. Maintaining an AC system will cost less — and modern AC motors with VSDs offer comparable performance.

Variable Frequency Drives (VFDs)

Today’s IIoT-networked VSDs are often called variable frequency drives (VFDs) because they vary the AC electrical input frequency to control drive speed. They’re particularly popular for processes relying on AC induction motors, as they can vary motor speeds over a very wide range and are capable of phase conversion.

If your production processes don’t require precision motor speed control, then a VFD isn’t going to benefit you. The same applies if your processes run at a constant speed, or if slowing equipment down would cause operating problems.

Sizing Your Motors

For maximum efficiency, choose the correct motor for the work you want it to do. Assess how much power you’ll need to start your machinery, its running load and whether it can adapt to load changes. Also factor in the driven equipment’s torque and speed requirements.

Bear in mind that electric motors are typically less efficient if you run them for loads of less than 40% of their rated output. Their best efficiency is achieved with loads of 70%-80% of rated output. If you install a motor that’s too powerful for the job, it’ll use more energy and be less efficient, as well as costing more to install and operate. If, on the other hand, your motor is not powerful enough, it’ll have to work harder to get the desired results. This may cause overheating and reduce the motor’s efficiency and working life.

When sizing up motors and VSDs, you’ll need to evaluate the required load, so that you don’t end up replacing them with similar items. Always base your sizing on actual loads rather than the rated motor capacity.

Advanced Manufacturing With Variable Speed Control

VSDs are usually offered as autonomous devices, but you can also obtain them as integrated VSD and drive packages. Choosing this option has several advantages, such as:

• Reducing costs
• Reducing required floor space by eliminating separate enclosures
• Getting rid of long cable runs connecting motors and drives
• Making variable speed control available to other motor systems

The latest generation of VSDs are IIoT enabled so that a single connection point is capable of handling multiple sensors and data points. This means you can program them for predictive maintenance schedules and rapid performance analysis, which you can monitor remotely. VSD software also includes the capability to program maximum energy savings.

Energy-efficient electric motors and VSDs are an excellent alternative to the more resource-draining manufacturing processes, which rely on compressed air or fuel combustion. As we move further into Industry 4.0, IIoT-enabled electric motor systems will represent a crucial step in advanced manufacturing strategies.

Originally published at https://www.rowse-automation.co.uk.