Variable Frequency Drives- What are they and why do we use them with Waukesha PD Pumps?

Every Waukesha PD pump assembly has four main components- the pump, the motor, the base, and a variable frequency drive (VFD). In today’s post, we’re going to take a look at variable frequency drives, what they are and why we like to use them on our Waukesha PD pump assemblies.

To start, what is a variable frequency drive? Let’s take a high level, layman’s look. The gear drive motors we use are 3 phase motors, with typical North American service being 60 Hz and either 230 or 460V AC. Unlike DC motors, where we can adjust speed with a potentiometer, with an AC motor, we need to adjust the frequency. This is what the VFD does- it adjusts frequency (hertz). It does this by taking the input power, essentially converting it to DC so we can control it, and then tricking the motor into thinking it is AC. By adjusting the output frequency, we’re able to control the speed of the motor and the output of the pump.

Why would you need to control the speed of the pump, you might ask. Well, we may be running a variety of products at different flow rates. Maybe we’re using it as a CIP pump and need to boost the output speed to hit the required line velocities. Pump rotors also wear over time, so if we’re seeing slip in the pump, a workaround could be to boost speed. Whatever the reason, speed control is a great feature to have.

Modern VFDs also offer a number of features in addition to speed control. For one, they’re widely available in NEMA 4X enclosures, have become increasingly economical, and have features like integral disconnect switches, so they provide great option for simple, local, pump on/off. They also offer a number of inputs and outputs, so we can do things like remote (in addition to local) speed control. We can also use a VFD to turn off a PD pump in a high-pressure situation and even use it in batching setups (more on that later). We’ve even seen demos where the a NEMA 4X VFD was used to drive a pump to washdown the VFD!

What do you need to know to make sure you get the right VFD? Let’s start with the motor horsepower. VFDs are rated in HP and we’ll usually select a VFD that is the same HP as the motor or even a size larger. We’ll also need to know the input voltage the end user has available. This is commonly 230 or 460V, but smaller drives can also take 120V single phase in.

We also want to know a little about the environment the drive is going in. Drives can either be mounted locally on the pump base or remotely in a control cabinet. Most of the drives we do for our pump skids get mounted on the pump base frame so we’ll want them to be NEMA 4X or washdown rated.

Allen Bradley Powerflex Drive
NEMA 1X & NEMA 4X SMV Vector Drive

As far as brands go, there are a number out there. We typically like the use either Allen Bradley Powerflex drives or Lenze AC Tech SMV Vector drives. When you have 10, 15, or even 20 drives in a plant, you’ll come to appreciate using the same model so you only have to memorize one set of programming parameters! We like to use these brands because not only are they full featured, they’re readily available in NEMA 4X enclosures and economical at the motor sizes we use.

So when you know you’re going to need to run your Waukesha Sanitary Positive Displacement pump over a range of speeds, make sure you include provisions for a variable frequency drive. And, as always, If you have any questions about which drive to use or your Waukesha PD pump needs, contact a Triplex Sales Engineer today!

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