What is a variable speed drive?

The basic function of a variable speed drive (VSD) is to control the flow of energy from the mains to the process. Variable speed drives sit between the electrical supply and the motor. Power from the electrical supply goes into a drive and the drive then regulates the power that is fed to the motor.

Inside the drive the input power is run through a rectifier, that converts the incoming AC power to DC power. The DC power is fed into the capacitors inside the drive. This is done to smooth out the electrical waveform which provides the clean power supply for the next step. Power then flows from the capacitors to the inverter, that changes the DC power to the output AC power that goes to the motor.

This step allows the drive to adjust the frequency and the voltage that fed into to the motor based on your current process demands. This means you run your AC motors at the speed or with the torque according to the demand needed. This is why you can save large amounts of money using the AC drives.

Besides saving energy, variable speed drives also help you reduce maintenance costs, waste and even ambient noise emissions. They are also a great way to help meet your environmental goals.

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What is the difference between a variable speed drive, a variable frequency drive and an AC drive?

Variable speed drives (VSD) are sometimes called Variable Frequency Drives (VFD) or AC drives.

Using variable speed drives to control motors can lead to big savings

Nearly 70 percent of all industrial electrical energy use goes to powering electric motors. These motors are the workhorses of business, from pumps moving fluids to fans moving air to compressors, conveyors, and every type of machine that depends on rotational force to get its job done.

And it’s these motors that are the key to help you reduce your energy use and CO2 emissions, or even find more energy efficient solutions for your customers. Pumps, fans and compressors are often run at constant speed, with control of flow rate and destination being done by manual opening and closing of valves in the pipelines. This usually makes the startup process very labor intensive as starting a pumping station, for instance, could take one person up to an hour. The labor costs add up with personnel having to sometimes travel long distances to remote sites. In some facilities this process is automated but consumption of energy is poor.

There is a more energy efficient alternative. Running these applications with variable speed drives results in instant savings in energy consumption while it also brings long-term competitive advantages. Any process that is using electric motors can get important monetary savings and operational benefits with the use of variable speed drives. You might want to take a good look around your own operations and processes, no matter how big or small, to see where targeted use of variable speed drives can help you get fast financial payback and long-term competitive advantages.

ABB Drives energy efficiency portal


How do variable speed drives regulate power to match the motor’s speed to what is required by the process?

1. Convert incoming AC to DC
Incoming 3-phase AC power is fed into a rectifier that converts it to DC power.

2. Smooth the DC wave
DC power is fed into capacitors, smoothing the wave and producing a clean DC supply.

3. Convert DC to variable AC
The variable speed drive calculates the motor’s required voltage and current. DC power is then fed into an inverter producing AC power at the precise voltage and current needed.

4. Calculate and repeat
The variable speed drive continuously calculates and adjusts the frequency and voltage providing only the power (speed and torque) the motor needs. This is how you can save large amounts of energy. 

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“What is a variable speed drive?”


How can you benefit from variable speed drives?

You can get savings in...

  1. Energy
  2. Operational costs
  3. Downtime
  4. Maintenance
  5. Raw materials

Why should you use a variable speed drive?

Electric motors play a very important role in our everyday businesses and lives. They move and run basically everything we need for business or pleasure.

All these motors run on electricity. In order to do their work of providing torque and speed, they need the corresponding amount of electric energy. All these motors consume electricity to provide torque or speed needed. If that torque or speed is too high or too low, mechanical controls will be used to slow down shift or control output. The result is inefficiency, with lots of wasted materials and energy. Motor’s speed should match exactly what is required by the process. Without the better way to control the speed, a lot of energy gets wasted and that’s not good for anyone’s business.

When using a variable speed drive, either torque or speed is controlled and we speak of “torque control” or “speed control”. When the variable speed drive operates in torque control mode, the speed is determined by the load. Likewise, when operated in speed control, the torque is determined by the load.

See how easy it is to match your drive with a motor

4 reasons to invest in variable speed drives to improve energy efficiency


Here’s why you should invest in variable speed drives (VSDs) to improve energy efficiency of your various applications, such as pump, fan, conveyor and compressor systems:

  1. Improved operational efficiency
    the need for manual labor is eliminated as the system controlled by drives can be automated, thus saving in man-hours and labor costs
  2. Savings on the electricity bill
    variable speed drives use only the needed amount of energy by running the motors at partial load, which can result up to 50% reduction in energy consumption of drive system
  3. Savings in capital expenditure
    immediate reduction in electrical consumption ensures fast financial payback for the VSD investment, in pump and fan applications even within months of installation
  4. Savings in maintenance and spare part costs
    with drives, the stress on the mechanical equipment during startup and operation can be minimized ensuring longer lifetime of the equipment

This type of valuable gains can be achieved not only in large industrial operations but also in smaller installations as well. You can estimate your potential energy and cost savings gained by low voltage AC variable speed drive control compared to traditional methods like throttles and valves by using our free EnergySave Calculator for compressors, fans and pumps.

Energy saving calculator

saneago-water-picture “One of our biggest overheads are our energy costs for pumping water. We knew we needed to lower electricity consumption and ABB suggested a solution that we found attractive, based on a combination of drives, motors and digital tools. We then worked together to systematically implement them.”

Osmar Qualhato Junior, Energy Management Supervisor at Saneago
Read the full case story »

ABB's variable speed drives

ABB's variable speed drives control the speed of electric motors to match the task in hand, saving power and improving performance in industrial plants, shops, offices and homes, across the world.

ABB is the world’s leading supplier of AC and DC low-voltage drives and high-power, medium-voltage drives, with a product portfolio of drive systems that extends across a large power range, from 100 watts to 100 megawatts.

ABB drives are used to improve energy efficiency in most industries and applications, from single-phase residential and commercial buildings to huge all-electric drive systems that power entire natural gas liquefaction plants and gigantic gearless mill drives that grind ore and minerals into smaller pieces at mines and processing plants.


ABB variable speed drives

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