Choosing the Right VFD

The electric motor represents a crucial part of any electrical drive system. Without the electrical motor, the whole system wouldn’t function. However, in order to work properly, even the most advanced motor needs some kind of controller. Usually, system designers and engineers take advantage of a VFD (Variable Frequency Drive). Choosing and purchasing the right VFD for your system can be a daunting task. You need to take into an account all the different variables and features of your chosen VFD to make it work properly. Luckily, over the last 30 years, VFDs have become cheaper and more reliable than ever, to the point that upwards of 40% of all newly-installed motors now include the technology.

To make your decision easier, we prepared a special buyer’s guide for the right variable frequency drive. We’re going to give you some general tips on what to look for when buying, in order to make the buying process a bit less time-consuming.

Allen-Bradley 1336-Series Adjustable Frequency Drives

How to Choose the Right VFD

  • Choose the Brand You Trust

This is one of the most important tips. If you’ve had previous experience with a particular brand, it’s best to go for that brand. There are many VFD manufacturers out there but our recommendation goes to Allen Bradley if you live in the USA. Allen Bradley VFDs are characterized by great build quality, as well as a huge array of power ranges. They offer low and medium voltage VFD, meaning that you can easily adapt them to your system. The key advantage of an Allen Bradley VFD is flexibility. Their drives allow you to seamlessly integrate them into your system, as well as control the torque for lifting applications. The PowerFlex DC Series is a perfect example of this versatility.

Many of their VFDs come with water cooling that will keep the temperature in check, which is crucial for a smooth and continuous workflow. Needless to say,  there are other trusted manufacturers and choosing the right one is largely a question of your experience. It’s all about fitting the corresponding VFD into your system and make it work as intended.

  • FLA (Full Load Amps)

Get to know more about the FLA. Aside from horsepower, this is the most important and reliable way to measure a VFD. To do this, you’ll need to compare the FLA of the motor with the amp rating of the VFD you’re considering. The key is to purchase a VFD with the amp rating slightly higher than that of the motor. The reason for this is that it will grant reliability over time, prolonging the life of the VFD. In cases of constant torque, this is especially important.

A VFD with a high FLA is more expensive but it’s also a great way to save money. By buying a VFD with slightly higher FLA, you ensure that all the power requirements are met, thus extending the lifetime of the VFD, and reducing the need for upgrades over time.

  • Motor’s Horsepower

Before you decide on your new VFD, it’s important to know your motor’s HP. It’s not obligatory but it will make the job a lot easier. Knowing your motor’s horsepower will also let you determine the power supply more easily. If the motor has a higher HP, you’ll need a stronger VFD. Allen Bradley, once again, offers a plethora of high-voltage and high-power VFDs, ranging from 2.3-10kV and motor current of 720A. Remember that undervolting your motor can drastically decrease its productivity and overvolting can lead to a power surge.

  • The Right Voltage

To choose the right voltage, you’ll need to match the voltage of the VFD and motor to the available voltage on your workplace. Let’s take the low voltage application for example. In the US, the recommendation is either 208 VAC, 230/240 VAC or 460/480 VAC. Since the low voltage application is the most common, you won’t have any problems with either of these three. But, for the medium voltage application, you should contact the manufacturer, seller, or an internal engineer to help you with the choice. This is if you work with the three-phase input.

However, the VFD can have a single-phase input. In this case, it’s a bit more complex. If you need a load of 3 HP or below (230 VAC, FLA below 10 amps), then you’re eligible for the single-phase input. Anything more than 3 HP indicates that you need three-phase input, which we explained above. Still, you’ll need to match the size of your VFD with the size of the motor. To do this, you’ll need to use a VFD with 2 times the FLA of the motor. If a motor has 5 HP and FLA of 28 amps, then you need a VFD with 56 amps, which translates to about 20 HP.

  • Type of Load

Think about the type of load you need. Is it a variable torque? Or perhaps a constant torque drive? This mostly depends on the type of motion you need. If you’re working with centrifugal movement, as seen in fans or a pump, the variable torque drive is the most suitable option. Variable torque drives are usually installed for energy savings in centrifugal applications. They’re much better at saving energy than constant torque drives, thanks to the ability to vary the torque, thus reducing the overall power consumption of the system.

A constant torque drive will keep the torque constant during any given period of time. For example, you might need to decrease the fan speed to 50%, which requires lower torque. In that case, a variable torque drive will do the job, whereas the constant drive will keep the torque at the exact same value.

Constant torque drives are great for belt conveyors, punch presses, extruders, and positive displacement pumps. All of these systems are using straight movement, making constant torque preferable to centrifugal, which requires variable torque drives.

  • Speed Range

Do you plan to run your motor at the full speed all the time? The general rule is that the motor shouldn’t be run at less than 20% of the specified maximum speed allowed. The motor can still run at its maximum/minimum capacity but if there isn’t any motor cooling, overheating is imminent. In case of overheating, the motor can stop working completely and in severe cases, the damage can be permanent. At slower speeds, you should incorporate auxiliary motor cooling.

You should keep your motor under its maximum limits, which is usually not more than 20% above the rated speed. Once again, the FLA plays a crucial role here. It will determine whether or not the desired VFD is suitable for your system.

  • Final Thoughts

These are only basic tips on choosing the right VFD. Different applications require different features and some of those features are harder to match with your needs. Take special care of FLA and overload ratings, especially if you’re working with heavy loads or with loads that are hard to start. The more effort you put into finding the right VFD, the more your stable your system will be the longer it will last.

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