What is the Difference Between VFD and Servo Drives?

Servo systems are now well-known for their use in CNC machining, robotics, and factory automation. The servo technology is further expanded with the manufacturing of servo drives, used for powering electric servomechanisms. Despite being an excellent option for industry use, we also have these VFDs (Variable-Frequency Drives) which tend to be used a lot as an alternative for servo drives, given their energy-saving potential.

Since both of these are widely used in industry and manufacturing processes, it’s important to point out some of the differences between them and in which situations is one better than the other.

To do that, we’ll first need to see what are VFDs and servo drives and then compare the crucial differences.

What is a Servo Drive?

A servo drive is an integral part of a servo system that consists of a motor, a controller, a feedback device, and of course – a servo drive. Simply put, a servo drive is a part of the servo system that receives a particular command from a control system, amplifies it, and transmits the current to a servo motor in the system. The servo motor produces a motion, tailored to the command it has received. The command signal usually represents some of the physical variables such as torque, velocity, and the desired position.

The servo motor is equipped with a sensor that reads the current position of the motor and reports it back to the servo drive. When the feedback is received, a servo drive compares the motor status and position with the commanded variables. If there’s a deviation from the given commands, the servo drive alters the frequency, voltage or any other variable to correct the deviations.

When first developed, servo drives were standalone components, completely separated from the servomotor. Nowadays, engineers and servo system manufacturers implemented servo drives directly into servo motors. These systems are called ’integrated motor-drive systems’ and they include both motor and drive, making it more compact and easy to use. The total amount of wires was also reduced, giving the system a cleaner look, smaller size, and easier setup.

What is a Variable-Frequency (VFD) Drive?

A Variable-Frequency Drive is also a motor controller but works in a different way than a servo drive. The VFD is a type of motor controller that works in conjunction with an electric motor by varying the variables such as voltage and frequency supplied to the electric motor. While a servo drive needs a command signal given to the motor in order to compare the real and desired position of the motor, a VFD directly controls the voltage and frequency supplied to the motor. VFDs are also known by other names such as AC drive, adjustable-frequency drive, inverter, and microdrive.

When it comes to the motor’s speed (RPM), a variable that is directly related is the frequency in Hz. The lower the frequency is, the lower the speed of the motor is, and vice versa. Sometimes, the electric motor doesn’t need to run at full speed and VFD can come in handy since it allows you to manipulate the frequency and voltage. If you don’t need full speed, you can decrease the frequency and voltage of the motor to suit your needs. This will also save the electric motor from potential malfunction and extend its life.

Generally, Variable-Frequency Drives are used for reducing energy consumption thus reducing the energy costs of an electric motor. These motor proved to be very good when it comes to tight process control and are used everywhere from small appliances to large electronic systems, for controlling electric motors.

Key Differences Between VFD and Servo Drives

The first main thing that differentiates Servo Drives from VFD is the encoder. While Servo Drives can’t work without an encoder for electronic commutation, a VFD can work perfectly fine without it. The Servo Drive is used for controlling permanent magnet motors, whereas VFDs are used for control of a squirrel-cage type motor. The main advantage of permanent magnet motors is the integration of rare Earth magnets in the rotor, which produce a high magnetic flux thus creating more torque in the rotor. This torque gives the motor low inertia for smooth acceleration and deceleration which is far greater than the squirrel-cage type motor.

On top of that, servo controllers are versatile in a sense that they can easily work with complex paths, and variable loads and speeds. Servo controllers are able to produce multi-axis moves and maintain the position across complex paths. Variable-Frequency Drives, working with encoder inputs, don’t have enough computing power to perof0rm this. Calculation of complex paths and varying speeds is almost impossible with VFD but since AC motors aren’t suited for this purpose, it’s not a gigantic downside.

The position-control capabilities of VFD are also inferior to Servo Drives. However, VFD is aimed at reducing energy consumption and extend the equipment life, which is superior to Servo Drives. Many electric-motor systems that use VFD usually see great improvements and energy savings. Given that 25% of the world’s electrical energy goes to industrial electrical motors, VFDs offer amazing benefits. But, as it stands now, servo systems are still more popular and more often implemented than VFD.

Conclusion

Can we precisely say which type of drive is better? Well, that’s a tough question to answer and it depends on your needs. While servo drives possess enormous computational speed and are able to swiftly accelerate and decelerate the motor, VFDs come with energy-saving capabilities that many electrical systems need. On top of that, VFDs are able to further extend the equipment life and provide additional protection from under-voltage, overvoltage, as well as phase protection.

To put things into perspective, Servo Drives are better at computations, maintaining a position along the complex path and varying speeds and loads, VFDs are much better at providing optimal performance and save some energy.

If your electrical system consumes a lot of energy, going with a VFD is your best bet. If you need additional computational power, Servo Drives are the go-to option.

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