Knowing and Understanding Your Variable Speed Drives (VSDs)

VFD or VSD - what do these terms mean?

Overview

All of these terms refer to the same type of equipment. The industry does not use one standard term, so AC Inverter Drive, Variable Frequency Drive (VFD), and Variable Speed Drive (VSD) are all commonly used to describe the same product.

We generally use AC Inverter Drive and VSD as our preferred terms.

Useful Knowledge

• “Inverter” on its own can refer to many different technologies.
• Inverter circuits are used in EVs, HVAC systems, solar systems, and many other products.

Recommendation

When searching, use full terms such as “AC Inverter Drive”, “Variable Speed Drive”, or “VSD” to make sure you find the correct equipment.

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What is a VSD?

Overview

A VSD is an industrial controller used to vary the speed of a three-phase motor. It allows accurate motor speed control instead of fixed-speed mains operation.

It is commonly used in place of DOL starters and soft starters where speed control or reduced start-up current is required.

Useful Knowledge

• A VSD has three main stages: Converter → DC Link → Inverter. The inverter output is PWM, not a pure sine wave.
• It is intended for use with a three-phase motor and should not be used as a general three-phase power supply.

Recommendation

Start by checking the motor nameplate, as the motor details are essential. It is also important to understand the load type, as this will affect both product selection and VSD setup.

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What controls do I need for my VSD?

Overview

At the most basic level, a VSD needs a run signal and a speed reference. When a run command is given, the VSD reads the speed reference and accelerates to that speed. When the drive is stopping or stopped, the speed reference is ignored.

Useful Knowledge

• The simplest setup is a two-way switch and a potentiometer. This is the default arrangement for many VSDs when remotely controlled.
• Any switches connected to the VSD must be volt-free. Applying mains voltage to the digital inputs will damage the unit.
• Many VSDs include Modbus RTU as standard, with other communication protocols available as options. This varies significantly between product ranges, so individual listings should be checked.

Recommendation

Choose the control method that best suits your application. If you are using a switch, make sure the contacts are volt-free to avoid damaging the unit. Always refer to the product manual for control wiring diagrams, as these vary between ranges.

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Can a VSD start automatically on power-up?

Overview

In most cases, a VSD can be set to start automatically when mains power is applied. However, this is generally not recommended and can be dangerous.

Useful Knowledge

• When the VSD is powered up, it takes time to initialise. Like a PC, it needs to boot up and check its status before it can operate.
• When a VSD is in standby mode, powered up but not running, it uses very little electricity and is ready to operate immediately.
• All VSDs have a limit on how many times they can be powered up each hour. Exceeding this limit can damage the unit.

Recommendation

Avoid automatic start-up wherever possible. In many cases, the VSD will need a setting change to operate this way, and it can introduce safety risks while also reducing the lifespan of the drive.

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What is dynamic braking?

Overview

Some VSDs are fitted with a braking switch that allows energy from the motor to be diverted to an external braking resistor. This is useful when slowing high-inertia loads in a short time.

Useful Knowledge

• Brake resistor selection is specific to the VSD and will vary between ranges. The product manual will state the minimum resistance and duty cycle allowed for the braking switch.
• In many cases, a parameter change is required before the braking switch will operate. The method varies between product ranges.

Recommendation

If you are unsure whether dynamic braking will be needed, select a VSD with a braking switch. This is usually straightforward on smaller VSDs below 30 kW and gives you the option to add a brake resistor later if required.

This is an interesting subject. For more information about dynamic braking, please see our Long Read: VSDs and Dynamic Braking.

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Can I use a single-phase VSD on a three-phase motor?

Overview

Yes, this is possible, but the motor must have a 230 Vac rating for full performance. This can be confirmed by checking the motor nameplate, where the 230 Vac details will be shown alongside the 400 Vac details.

Useful Knowledge

• A single-phase VSD converts the supply from single-phase to three-phase, but it does not change the voltage.
• Most 2-pole and 4-pole motors up to and including 4 kW are dual-rated for 230 Vac and 400 Vac.
• Motors are usually supplied set for 400 Vac operation. When using a single-phase VSD, the links in the motor terminal box normally need to be changed.

Recommendation

It is very important that the VSD voltage matches the motor voltage and that the motor links are set correctly. It is worth checking the links even on an existing installation, as the VSD may still run the motor but with very poor torque performance. Single-phase AC inverter drives are often blamed for poor torque, when in reality the issue is usually incorrect installation.

It is possible to use a 400 Vac motor on a 230 Vac supply with reduced performance. For more information, please see our Long Read: Operating a 400 Vac motor from a 230 Vac supply.

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Can I save energy using an AC inverter drive?

Overview

Yes. VSDs can significantly reduce energy consumption on centrifugal loads such as fans and pumps, provided the load speed can be reduced.

Useful Knowledge

• On centrifugal loads, the relationship between speed and power is not linear; it is cubed. A 20% reduction in speed can reduce power consumption by up to 50%.
• At full speed, many VSDs also include an energy-saving function. This can help, but the saving is usually much smaller than the saving achieved by reducing speed.

Recommendation

Use VSDs for speed control on fan and centrifugal pump applications. Even a small reduction in speed can make a large difference. A 10% reduction in speed can reduce power consumption by up to 27% while reducing flow by only 10%.

This is a very interesting subject. For more information, please see our Long Read: VSDs on Centrifugal Loads.

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What is V/F control?

Overview

V/F is the most basic method used by a VSD to control an induction motor. Most VSDs are supplied with this as the default control mode.

Useful Knowledge

• In V/F control, low-speed torque performance can often be improved by increasing the boost parameter. However, too much boost can cause trips.
• V/F control must be used if the VSD is controlling more than one motor. In that case, the motor rated current parameter must be set to the combined current of all motors.

Recommendation

Use V/F control, with or without boost, wherever it is suitable. It is appropriate for many applications and is especially well suited to centrifugal loads such as fans and pumps.

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What is sensorless vector control?

Overview

Sensorless vector control is a more advanced form of motor control. It requires accurate motor data and an auto-tune procedure to deliver better low-speed torque performance.

Useful Knowledge

• For best results, the auto-tune should be carried out with the motor on a bare shaft if possible.
• Some VSDs will change the displayed speed from Hz to rpm when configured for sensorless vector control.
• Sensorless vector auto-tune may not be successful if the motor kW rating is much smaller than the rating of the VSD.

Recommendation

If the VSD is unable to start the motor in V/F control, even with boost applied, sensorless vector control should be considered. For the best results, decouple the load before carrying out the auto-tune and follow the product-specific instructions for the drive.

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What overload do I need?

Overview

Some VSDs have two power ratings. The rating you can use depends on the overload setting selected.

Useful Knowledge

• A VSD rated at 4 kW / 5.5 kW may provide 4 kW with 150% overload and 5.5 kW with 110% overload.
• Constant-torque applications, such as conveyors, normally require 150% overload.
• Variable-torque applications, such as centrifugal fans and pumps, normally require 110% overload.
• Some VSDs manage the kW rating internally and may not offer a visible overload parameter even though they are dual-rated.

Recommendation

When selecting a VSD, make sure you choose the correct unit based on both the kW rating and the overload required by the application. Also remember that the drive may not be set to the overload you want when it arrives.

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What is an EMC filter?

Overview

All VSDs produce RFI (radio frequency interference), which must be controlled. Many VSDs include an internal EMC filter to reduce RFI and meet the EMC requirements needed for UKCA and CE marking.

Useful Knowledge

• If a VSD does not include an internal EMC filter, an external EMC filter will normally be required for compliance.
• EMC filters use high-frequency capacitors to earth, which can cause nuisance tripping on some RCDs. In some cases, the earth connection for these capacitors can be removed.

Recommendation

If you are installing in the UK or Europe, select a VSD with an EMC filter, whether internal or external, to ensure the product remains compliant for UKCA or CE marking. If nuisance RCD tripping occurs, a Type B RCD may be required.

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VSD Trips - RCD

What is the difference between torque and power?

Overview

Torque and power are often treated as though they mean the same thing, but they do not. It is important to understand both the difference between them and how they are linked.

Useful Knowledge

• Torque is a force and is measured in Nm.
• Power is measured in W. A watt is a joule per second, and a joule is a unit of energy.
• Torque, power, and speed are all related. If you know two of them, the third is determined.

Recommendation

A simple way to think about it is to imagine pushing a box across the floor. You need force (torque) to move it. The amount of energy used depends on how fast you move it. That is the relationship between torque, power, and speed.

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Can I remove my gearbox when a VSD is fitted?

Overview

If a gearbox or other gearing is fitted to your system, it multiplies the motor torque. A VSD reduces speed, but it does not multiply torque, so existing gearing should normally be retained when fitting a VSD.

Useful Knowledge

• An induction motor controlled by a VSD behaves as a constant-torque machine up to its base speed, which is normally 50 Hz.
• As speed is reduced by the VSD, available power also reduces. Applications that require constant power rather than constant torque, such as centre winders, need more careful consideration.

Recommendation

If you are adding a VSD to an existing system, do not remove mechanical components that are needed to increase motor torque.

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Can I use a braked motor with a VSD?

Overview

A braked motor can be used with a VSD, but the brake must be powered from a separate supply and not from the motor terminals.

Useful Knowledge

• The output voltage of a VSD starts at around 0 V and rises with speed demand. At start-up, there is therefore not enough voltage available from the VSD output to release the brake.
• If the VSD is started while the brake is still applied, the drive is likely to trip on overcurrent because it cannot overcome the holding brake.

Recommendation

Make sure the brake has its own separate power supply and is not connected to the VSD output. A digital output from the VSD can be used to control the brake supply so that the brake is released at the correct time. Many VSDs include a built-in brake control function.

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What is STO?

Overview

STO stands for Safe Torque Off. It is a safety feature built into many VSDs. When STO is active, the VSD cannot operate. ‘Torque Off’ means the VSD cannot apply torque and therefore cannot hold or control the motor in any way.

Useful Knowledge

• STO is intended for use when the motor is stationary, preventing it from turning without switching off the mains supply. This is especially useful on machinery during tasks such as tool changes.
• STO is not intended to stop the motor. When STO is activated, the VSD stops controlling the motor and the motor coasts to a stop in an uncontrolled manner.
• STO can be used in conjunction with a mechanical brake, but a safety assessment must first be carried out to confirm suitability.
• If a motor must be stopped within a fixed time for safety reasons, a Safe Stop (SS) function should be used. STO should then be used once the motor has stopped to prevent restart.

Recommendation

Before using any VSD safety feature, the machine must be assessed by qualified personnel to determine which safety functions are required and how they should be applied. Always follow the safety instructions provided in the VSD product manual.

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What is the slowest speed I can run at?

Overview

As a general guide, a suitable turndown is around 10:1 in V/F control and around 100:1 in sensorless vector control. These are general figures, and manufacturers may state different limits.

Useful Knowledge

• Motor cooling becomes important at low speeds because the cooling fan is mounted on the NDE of the motor and becomes much less effective as motor speed falls.
• If the motor is to run below half speed, depending on the application, forced ventilation may be needed to maintain adequate cooling.

Recommendation

Make sure the chosen control mode, whether V/F or sensorless vector, is suitable for the turndown required. If the motor will operate below half speed, consider adding forced ventilation to prevent overheating.

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Are long motor cables a problem?

Overview

They can be. The maximum cable length for a VSD will be stated in the product manual and varies by product. For screened cable, a typical limit is between 30 m and 50 m.

Useful Knowledge

• Long motor cables can have high capacitance, depending on the cable type. This allows additional current to flow to earth, increasing the load on the VSD output and potentially causing overcurrent trips.
• Adding an inductor to the VSD output can help offset cable capacitance. On very long cable runs, a more active filter such as a dv/dt filter may be needed.
• The maximum cable length is the total motor cable length. On multi-motor systems, all motor cable lengths must be added together.

Recommendation

Calculate the total motor cable length for the installation. An output inductor, sometimes called an output motor choke, is often required once the cable exceeds 50 m and may remain effective up to around 150 m. Beyond that, a more active filter is likely to be needed. Any choke used must be rated for the output current of the VSD.

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Can I connect more than one motor to my VSD?

Overview

Yes, but the combined current of all motors must not exceed the output current rating of the VSD. Do not total the kW rating – total the motor currents instead.

Useful Knowledge

• If more than one motor is connected to a single VSD, the drive must be set to V/F control. Sensorless vector control will not work in this arrangement.
• Each motor must have its own overload protection. The VSD can only monitor the combined current and cannot individually protect each motor.

Recommendation

Add together the full-load currents of all motors to determine the required VSD rating. Make sure each motor has its own overload protection, as individual protection cannot be provided by the VSD in a multi-motor setup. V/F control will be required.

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