Servo Drives and VFDs are both used in machines to drive motors and control motion. They seem like they do the same thing, so why choose a servo drive vs a VFD?
Some Background On Servo Drives and VFDs
Traditionally, servo drives work with permanent magnet motors and are used in applications that need precise velocity or position control. A defining characteristic of servo systems is that they use feedback to achieve precision and accuracy. Servo drive systems are capable of quick acceleration and deceleration and can follow exact motion paths whether point to point or with defined contouring.
VFDs on the other hand are paired with induction motors and are used in applications that need velocity control. A defining characteristic of VFD systems is that they can control velocity by adjusting the frequency of the voltage that they send to the motor. This is where they get their name, by the way - VFD stands for Variable Frequency Drive. Another important point is they don't use feedback on the motor so the velocity control is open loop. This means VFDs won't compensate if there's a stall or if the load changes, resulting in velocity control that isn't as precise as with servos. VFDs can be set to ramp up to a given velocity and then drive at that same speed for long durations.
A typical Servo Drive
A typical Variable Frequency Drive
As with many engineering decisions, there are no hard and fast rules and there are plenty of examples where servo drives and VFDs have capabilities beyond their traditional roles. For example, due to improvements in technologies and the ever present need to provide more features, nowadays it's not hard to find servo drives that can power induction motors - both with and without feedback. In the same vein, there are plenty of VFD's that can also power induction motors with feedback (an induction motor with feedback is commonly referred to as a Closed Loop Vector motor or CLV).
The result is there are areas where the capabilities overlap between servo drives and VFDs.
In applications that require precise motion control, servo drives are far superior to VFDs.
When to Choose a Servo Drive vs a VFD
In most cases the decision is obvious for which one to choose based on the needs of the application but the choice can become uncertain when both can do the job. We'll first cover the easy situations then go over what happens when both are suitable.
Servo drives are definitely the choice in applications like robot arms when coordinated motion is needed between multiple axes. Or when rapid acceleration and deceleration is needed, for example with pick-and-place gantries. Or when precise velocity control is needed to grow a silicon ingot, or when exact sub-micrometer positioning is needed for semiconductor applications.
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VFDs are the obvious choice for conveyor belts when the velocity needs to be set to a certain speed. Or with hydraulic pumps and air blowers. Or with some electric vehicles where precise control is not a requirement.
The middle ground is when both can do the job. For example velocity mode and position mode applications where the precision would be considered somewhat loose for a servo, but within the capability of a VFD.
In a conveyor system, a VFD is a better choice than a servo drive as long as it's not constantly starting/stopping or changing directions.
Conveyor systems are a good example. On the one hand a simple conveyor application could just need to turn on in the morning then run all day at the same speed. A VFD would be an excellent choice for this. On the other hand a more demanding conveyor system that needed to frequently start, stop, go forward, backward, match speed with another conveyor and more would be a better choice for a servo system.
In the grand scheme of things there's a whole spectrum of conveyor systems with different requirements, some of which fall within the overlapping capabilities of both servos and VFDs. When the choice isn't clear then the analysis comes down to performance, features and price. Although to be fair, all engineering analyses come down to performance, features and price, but in general...
Performance and Features
Choosing a system can come down to weighing the lower cost of a VFD system against the superior features and performance of a servo drive system.
Consider systems that have the features that you need or really want. What motion does the system need to accomplish and what features will make the final product better or more convenient?
Cost
Once you've narrowed down your selection of candidates based on performance, then take a look at the costs. Servo systems generally cost more than VFD systems because a lot of the cost comes from the servo motor. Servo motors use permanent magnets which add to the material and manufacturing costs vs induction motors. Additionally the servo drives oftentimes cost more than VFDs because they have more features. Once you're at this point it's literally a tradeoff between cost and performance.
Another example is electric vehicles and electric mobility. In this case there are a few more considerations we haven't talked about yet: efficiency, size and power. Electric vehicles and electric mobility applications can cover anything from order fulfillment where robots pull products from a warehouse and prepare them for shipping, to service robots in hospitals, to automated forklifts, to factory transporters that move engines and cars, to very large machines that move loads that weigh multiple tons. Efficiency, size and power become deciding factors for many of these types of applications in addition to the velocity and position control capabilities discussed earlier.
Efficiency
In mobile applications, efficiency is especially important for preserving battery life so they can run longer between recharges. This is where servo drives have an advantage over VFDs.
Many mobile applications rely on batteries for their power since they can't be plugged into the wall. When batteries are the power source, then efficiency becomes a top concern for system designers. This is because increasing the efficiency allows machines to run for longer periods between charges thus increasing the up-time of the system.
Recall that servos use permanent magnet motors while VFDs use induction motors. It turns out permanent magnet motors are much more efficient than induction motors, so when efficiency is needed, servo systems have a clear advantage.
Servo Drives can be far more compact and more tightly integrated than VFDs.
Size
For smaller mobile applications the size of the components starts to become an important consideration for two reasons. First smaller parts make it easier for system designers to integrate the components within their design. Second smaller components weigh less which reduces the overall weight of the machine. With less weight there is less mass to push around which means better acceleration performance and longer battery life. As with efficiency there's a definite advantage when it comes to size with servo drives vs VFDs. For the same amount of power, a servo motor will be smaller than an induction motor. Also the latest servo designs have been miniaturized and optimized for mobile applications. For these reasons servos are the winner when smaller sizes are needed.
AC induction motors can be built much larger than servo motors can, so VFDs are the choice by default for very high power systems.
Power
Power is important for very large machines. Servo systems can be sized up to a few hundred kilowatts, but induction systems can get up to the megawatts.
As the power requirements go up, servo systems eventually fall out of contention leaving induction motors and VFDs, although this transition takes place at a much higher power level than most applications require.
Servo Drive vs VFD Quick Reference
| Category | Servo Drive System | VFD System | Comments |
|---|---|---|---|
| Cost | ✓ | Induction motors cost less to make than servo motors, therefore VFD systems are typically less expensive than servo systems | |
| Power | ✓ | VFDs can operate at much higher power ranges - into the megawatt size | |
| Velocity Control | ✓ | ✓ | Both servos and VFDs have great velocity control depending on the needs. Servo drives are superior because of feedback, but VFDs have their place in many applications |
| Position Control | ✓ | Servo drives have superior position control due to their use of feedback. They are specifically adapted to positioning applications | |
| Motion Control | ✓ | Servo drives are the only way to go for coordinated motion and rapid motion profiles | |
| Features | ✓ | Servo systems have more features since they are used in a broader range of industries and applications | |
| Small Size | ✓ | Servos are available that have been optimized for small applications | |
| Efficiency | ✓ | The permanent magnets on servo motors make servo systems more efficient than VFDs |
Conclusion
For the most part choosing between a servo drive and a VFD is pretty straight forward. Servo drives have the advantage when it comes to performance and features, while VFDs have an advantage with cost. Sometimes the choice falls in an area of overlap where either can work, at this point it takes a little more consideration to find the best system for your needs.
by René Ymzon, Marketing Manager
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