Variable Speed Drives Explained: How to Choose, Size and Use a Motor Inverter

A Variable Speed Drive (VSD)—also commonly called a motor inverter or Variable Frequency Drive (VFD)—is used to control the speed, torque, and direction of an electric motor. These drives are widely used across industrial and commercial applications to improve efficiency, reduce energy use, and give better control over pumps, fans, conveyors, compressors, and other motor-driven equipment. At Flowfit, we offer AC variable speed drives including IP20 drives, IP66 outdoor switched drives, IP66 outdoor non-switched drives, with single-phase and three-phase options.

What is a variable speed drive and how does it work?

A variable speed drive works by adjusting the frequency and voltage supplied to an electric motor. This means the motor does not have to run at full speed all the time. Instead, the speed can be matched to the actual demand of the application.

Inside the drive, incoming AC power is converted into DC and then electronically switched back into a controlled AC output. This process is why many people refer to a VSD as a motor inverter. For a wide choice of variable speed drives and motor inverters, Flowfit supplies solutions for a range of applications and environments.

Higher frequency = higher motor speed
Lower frequency = lower motor speed
Controlled ramp-up and ramp-down = smoother starts and stops

Quick takeaway

A variable speed drive gives you more control over an electric motor, helping reduce wear, improve efficiency, and fine-tune performance.

How do you size a variable speed drive for a motor?

Sizing a variable speed drive correctly is one of the most important parts of a successful installation. In most cases, the drive should be selected based on the motor’s full load current (amps), not just the motor power in kW or HP.

What to check when sizing a drive

Motor nameplate details: voltage, current, kW, frequency, and speed
Load type: pump, fan, conveyor, mixer, compressor, or other application
Duty level: normal duty or heavy duty
Overload requirements: whether the motor has to start under load or deal with shock loads

Factor	Why it matters
Motor current (amps)	Main sizing reference for the drive
Motor voltage	The drive output must match the motor voltage
Application type	Fans and pumps usually differ from conveyors and heavy loads
Overload demand	Heavy-duty loads may need a larger or higher-rated drive

Rule of thumb

Always size the drive to the motor’s amps and the application duty—not by power rating alone.

Can you use a variable speed drive with any electric motor?

A variable speed drive can be used with many three-phase induction motors, but not every motor is equally suitable. The motor type, age, insulation, cooling method, and operating duty all affect how well it will perform on a drive.

Common motor types used with variable speed drives

Standard induction motors: often suitable for general applications
Inverter-duty motors: designed specifically for variable speed drive operation
Older motors: may be more vulnerable to insulation stress and overheating

A standard motor may work perfectly well on a drive in many situations, especially at moderate speed ranges and lighter duty. However, where the motor will run for long periods at low speed, or in demanding applications, an inverter-duty motor is often the better choice.

What is an inverter-duty motor?

An inverter-duty motor is an electric motor specifically designed to run on a variable speed drive. It is not a completely different type of machine, but it is built to handle the electrical and thermal stresses that come with inverter operation.

Feature	Standard motor	Inverter-duty motor
Use with variable speed drive	Often suitable for lighter duty	Designed specifically for it
Insulation strength	Standard	Improved to handle voltage spikes
Low-speed running	Can overheat more easily	Better suited for continuous low-speed use
Durability on inverter supply	Can be limited in demanding use	Typically better long-term reliability

Quick takeaway

An inverter-duty motor is still an electric motor, but it is built to cope better with the switching, voltage stress, and low-speed operation associated with variable speed drives.

How do you wire a variable speed drive to a motor?

Correct wiring is essential for both safety and performance. The incoming supply is connected to the drive input, while the motor is connected to the drive output terminals.

Typical wiring layout

Input supply to drive terminals such as L1, L2, L3
Motor output from drive terminals such as U, V, W
Earth connection to both drive and motor
Control wiring for start/stop, speed reference, or PLC integration if needed

Important safety tip

Never place a switch or contactor between the drive and the motor while the system is running. Doing so can damage the variable speed drive.

What settings need to be configured on a variable speed drive?

Before a drive can run properly, it needs to be programmed with the correct motor and application parameters. The exact menu structure depends on the brand, but the core setup items are broadly the same.

Typical parameters to set

Motor voltage
Motor full load current
Motor rated frequency
Motor speed or RPM
Acceleration and deceleration time
Minimum and maximum speed limits
Control mode, such as V/f or vector control

Many modern drives also offer auto-tuning, which can help improve motor performance and make commissioning easier.

Why is my variable speed drive tripping?

Drive trips are one of the most common problems encountered during commissioning and operation. In many cases, the fault code shown on the drive is the best starting point for diagnosis.

Common variable speed drive faults

Overcurrent: motor overload, jammed load, or incorrect parameter settings
Overvoltage: rapid deceleration or regenerative energy
Overtemperature: poor cooling, high ambient temperature, or excessive load
Earth fault: insulation breakdown or wiring problems
Undervoltage: supply problems or unstable incoming power

Troubleshooting tip

Start with the fault code, then check the motor load, drive settings, cooling conditions, and wiring. This usually gives the quickest route to the cause.

Do you need a braking resistor on a variable speed drive?

A braking resistor is used when a motor needs to stop quickly or when the driven load feeds energy back into the drive during deceleration. This is common on higher inertia applications.

When a braking resistor may be needed

Fast stopping requirements
High inertia loads
Applications such as conveyors, hoists, and some machine tools
Repeated deceleration causing overvoltage trips

Without adequate braking, the drive may trip on overvoltage when the motor decelerates too aggressively.

Can a variable speed drive save energy?

Yes, variable speed drives can provide major energy savings, especially in applications where the motor does not need to run at full speed all the time. Pumps and fans are among the best examples.

Reduced power consumption by matching motor speed to demand
Less mechanical wear due to smoother starting and stopping
Improved process control and more consistent operation
Lower maintenance costs over time

Key benefit

The biggest savings usually come from slowing pumps and fans to match actual demand rather than running them at full speed continuously.

What’s the difference between a soft starter and a variable speed drive?

A soft starter reduces the mechanical and electrical shock of starting a motor, but it does not provide full speed control. A variable speed drive both starts the motor smoothly and controls its operating speed.

Feature	Soft starter	Variable speed drive
Controls start-up	Yes	Yes
Controls running speed	No	Yes
Energy saving potential	Limited	Higher
Cost	Usually lower	Usually higher
Best for	Basic motor starting	Full motor control

What information do I need to choose a variable speed drive?

To select the right drive, it helps to have a clear picture of the motor, the supply, and the application. If you are comparing IP20 drives, IP66 outdoor switched drives, IP66 outdoor non-switched drives, single-phase drives, or three-phase drives, start with the application environment as well as the motor details.

Category	Details
Motor details	Voltage, current, kW, frequency, speed, number of phases
Application	Pump, fan, conveyor, mixer, compressor, or other load
Power supply	Single-phase or three-phase input available
Environment	Ambient temperature, dust, moisture, ventilation
Control needs	Local keypad, remote signal, sensor input, or PLC communication

Selection tip

Indoor panel installations often suit IP20 drives, while harsher environments may need IP66 outdoor drives. Switched and non-switched versions are available depending on how you want the drive isolated and controlled.

Frequently asked questions about variable speed drives

What is the difference between a VSD, VFD and inverter?

In most industrial contexts, these terms are used to describe the same type of product. In the UK, VSD or variable speed drive is often the preferred term, while VFD is also common in technical use. Inverter or motor inverter is another widely used term because the drive electronically creates a controlled output to the motor.

Can I run a 3 phase motor on a single-phase supply using a variable speed drive?

Yes, in many cases you can use a suitable drive with a single-phase input and three-phase output to run a three-phase motor where only single-phase power is available.

What is an inverter-duty motor?

An inverter-duty motor is an electric motor designed specifically for operation on a variable speed drive. It usually has stronger insulation and is better suited to continuous variable speed operation.

Can a variable speed drive damage a motor?

Not when the system is selected, installed, and configured correctly. Problems generally come from incorrect settings, unsuitable motors, poor grounding, or wiring faults.

Do variable speed drives work on single-phase motors?

Most variable speed drives are intended for three-phase motors. Some specialist solutions exist, but in most cases a three-phase motor and matching drive provide the best performance and reliability.

What is the difference between IP20 and IP66 variable speed drives?

IP20 drives are generally intended for cleaner indoor environments such as electrical panels or control cabinets. IP66 drives offer a higher level of environmental protection, making them more suitable for outdoor use or harsher industrial conditions. You can view Flowfit’s range of IP20 and IP66 AC variable speed drives here.

Do I need a qualified electrician to install a motor inverter?

Yes. Installation should be carried out by a qualified electrician or competent engineer to ensure safe wiring, correct protection, and proper commissioning.

Why buy variable speed drives from Flowfit?

Expert support: Guidance on drive sizing, motor compatibility, and application selection.
Wide product choice: IP20, IP66 outdoor switched, IP66 outdoor non-switched, single-phase, and three-phase variable speed drives.
Reliable products: Quality solutions for industrial and commercial motor control.
Wide application range: Suitable for pumps, fans, conveyors, and more.
UK-based service: Fast delivery and helpful technical support.