Wednesday, 10 September 2014

John Guthrie, Energy Efficiency Manager, ABB writes: So, you’ve identified an application that could benefit from some motor control. But do you know the type of load you’re dealing with?

It’s vital that you do if you are to select the right type of drive. They come in three different types:
  • Constant torque - This type applies when fixed volumes are being handled. So screw compressors, extruders, or straight line web handling are all constant torque.
  • Quadratic or Variable Torque – This is the most common load type. It covers the majority of centrifugal pump and fan applications.
  • Constant power - Used when a material is being rolled up, or unrolled - the diameter changes while the surface speed remains constant.
Be careful, as load types can be tricky, combining into different loads. You’ll need to select the correct load characteristic - opting for the wrong one could easily lead to under or oversizing.

Where the load is constant, say at half speed, a larger motor with more thermal capacity may be needed as well as the potential for forced motor cooling. Yet, a fan needs less torque when running at half speed, than when running at full speed, leading to a smaller motor frame selection.

What about current capacity? A constant torque drive should have an overload current capacity of 150 percent for one minute, especially where a good response is required to dynamic load changes. While variable frequency drives applied to variable torque loads need only an overload current capacity of 110 percent for one minute.

Get the load type right and you’re well on your way to getting the right drive and all the benefits it can bring to your process.

For expert advice call 07000 ABBAVP (07000 222287)

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