Dealing with variable torque loads

Sizing and rating the right motor for a pump application is not always straightforward, because when dealing with variable torque loads, things are not always as they seem and certainly not what you might expect. Edward Cowern of Baldor explains why.

A family of motor applications that tends to confuse people who are not regularly involved with them, is that of variable torque loads typical of pumps. These loads represent a high percentage of motor requirements, so it is desirable to have a little extra knowledge of their mysterious aspects.

There are two mysterious characteristics that these loads have. The first is the way they act when the speed is changed. The rules that cover these characteristics are called the affinity laws. Where the load is not changing (simplest case), the torque required to drive the pump goes up with the square of the speed. So changing the speed from 1160 to 1760 rpm, causes the required torque to go up by 2.3 times the original value. Also, since power is based on speed times torque, this goes up with the cube of the speed, almost 3.5 times the power required at the original speed.

The dramatic increases in the power required to drive these loads when speed increases is a little difficult to understand but it is very important. It is also important because small decreases can result in great energy savings. For example, decreasing the speed of a variable torque load by only 20 percent will result in a driving energy reduction of nearly 50 percent. This obviously has great importance when conservation is considered. It also accounts for the tremendous market that exists for variable frequency drives operating variable air volume (VAV) systems.

The second puzzling thing that occurs with variable torque loads is that the motor load actually decreases as the output or input to the blower or pump is blocked off or restricted. The reverse of this is that motor load increases dramatically as restrictions are removed.

Simple test

It defies good judgement to think that adding a restriction to the input of a pump would decrease the motor load. If you don't believe it, here's a simple test. Take a vacuum cleaner and listen to it carefully while you alternately open and close the suction. At first, you might think that the 'heavier' noise is the motor straining when the suction is greatest, but if you listen more carefully you will notice that the pitch of the motor goes up when the suction is closed. What this means is that the load is being reduced on the motor and it speeds up. If you still don't believe, you can do the same test, but with an ammeter on the motor. What you will find is that the amps drop as the suction level is increased.

The same is true of centrifugal pumps: closing down or restricting the output causes the pump to draw less mechanical power. Another way of looking at this is when the output of a centrifugal pump is closed off, the air or fluid inside the housing becomes a 'liquid flywheel'. It just spins around the vanes of the pump. Since there is no new fluid coming in to be accelerated, the only energy needed is what it takes to make up for the friction losses within the housing of the pump. It doesn't seem to make sense, but that's the way it is!

14 March 2006

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