Here at Marshall Wolf Automation, we receive calls on a daily basis asking which VFD is best suited for a particular pump or fan motor application.
While our tech support team is more than happy to help our customers find the right VFD, this simply isn’t a ‘right or wrong’ type of question. In order to best match your application needs, we generally start off by finding out what type of pump you have and what features are desired from the VFD.
First, let’s start with the absolute basics; what is torque exactly?
Torque is defined as a twisting force that tends to cause rotation. For those of you who may be wondering why we’re reviewing high school physics, stick with me here.
The first step we take to determine which VFD will work best is to find out what the application parameters are.
Most of the specifications our tech support team will start out with can be found on the information plate on your motor.
Pumps can be divided into constant torque and variable torque applications. One of the more common applications we end up working with is the variable torque variety, mainly involving prototypical centrifugal types of pumps. The easiest way to distinguish variable torque and constant torque is to focus on the name.
VTL (variable torque load) drives are designed to deal with alternating torque demands and can range their speed/torque from low to high. Think about HVAC fans and sump pumps. Though the drive may need to run the motor at intermittent torque and speed, the application will not be at full capacity 100% of the time. Many times these drives are designed with less overload capability compared to their counterparts due to their variability.
Conversely, constant torque loads are not dependent on speed and maintain the same level of torque regardless of demands. Applications such as conveyors, compressors or hoists incorporate these types of drives due to their higher heat and overload ratings. Unlike VTLs, constant torque drives are generally used in torque-consistent applications and may only hit 150% overload current for 30-60 second intervals (usually at start-up or when another load of dirt is added to the conveyer).
So what types of pumps and fans are available?
The other type of pump we will discuss is the positive displacement pumps. These fall under the constant torque load category due to their ability to pump independently of speed and pressure. This is due to the construction of the internal compartments in which the fluid is moved.
When considering a centrifugal pump or fan, the Affinity Laws are our best friend.
By referencing these constants we can better understand how pressure (pump head) variation is a result of the square of the change in speed and that power is a function of the cube of speed.
Using a VFD on variable torque loads allows us to take advantage of these affinity laws. As the speed of a centrifugal load decreases, the horsepower requirement will decrease with the cube of the speed. Head pressure will decrease with the square of the speed, while flow is proportional to speed.
|Brand||Model Series||Centrifugal Pump Type||Positive Displacement Pump Type|
|Delta Ind. Automation||EL series, MS300||X||X (MS300 only)|
|Delta Ind. Automation||C P series||X|
|Delta Ind. Automation||C Series||X|
|FUJI VFD||ACE, MEGA||X||X|
|FUJI VFD||MINI, ECO||X|
|LSIS||H100, H100 Bypass||X|
*Oversized by at least one HP increment.