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Variable Frequency Drives: What You Need to Know

Posted on by Kurz Industrial Solutions

When it comes to variable torque loads, we need to understand two things:

  1. The way the loads act when the speed is changed
  2. The output or input of the motor load – it surprisingly decreases as the output/input to the variable torque drive (blower, pump, etc.) is blocked or restricted.

The Cowern Papers provides us with a great example of this counter-intuitive behavior:

“I once had a call from a motor user who had burned out a motor driving a blower on a heating system. The motor was driving a blower that drew air through a filter and fed it to a ducted distribution system. When I asked if there had been any changes in the system he said, ‘Well, we extended the ducts into another room and cut the end off to let the air flow, but that would have made it easier for the motor not more difficult.’ When I told him that the opposite was true he couldn’t believe it. It defies good judgment to think that adding a restriction to the output of the blower would decrease the motor load.”

The different load types

We’re focusing on constant torque and variable torque (although constant horsepower is also a type of load). To understand the difference, it just comes down to whether or not the load is speed-dependent.

Constant torque: defines a load where the amount of torque needed to drive the machine remains constant – agnostic to the speed at which it is driven.

Variable torque: require low torque at low speeds – and increasing torque as the speed increases. Variable torque loads comprise a large percentage of motor requirements, making it smart to know more about these loads vs. constant torque loads (which are much more straight forward).

Types of variable torque loads include:

  • fans
  • blowers
  • centrifugal pumps

Obviously, fans and blowers are primarily moving air. Centrifugal pumps, however, can move different types of cliques like oil, coolants, water, etc.

Remember: the motor speeds up when the load is being reduced.

The Cowern Papers gives us another way to look at it: “when the output of a centrifugal pump or a squirrel cage blower is flossed off the or fluid inside the housing becomes a ‘liquid flywheel.'” That means it just spins and spins around the vanes of blower or pump. The only energy needed is what it takes to make up for the friction losses because there is no new fluid entering that needs to be accelerated.

If you have questions about variable torque loads and how they apply to your business, reach out to our experts.

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