By Edward Cowern, P.E. – Baldor/ABB
A family of motor applications that tend to confuse people who are not regularly involved with them, is that of Variable Torque Loads. These loads represent a high percentage of motor requirements, so it is desirable to have a little extra knowledge of the mysterious aspects of these loads. First, Variable Torque Loads are fans, blowers, and centrifugal pumps. In general fans and blowers are moving air but centrifugal pumps can be moving many kinds of liquids including water, petroleum products, coolants, etc.
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”. In order to simplify we will discuss only the performance of these loads when they are applied to systems where the load is not changing. For example, we can discuss a pump arrangement as shown in Figure 1. This is a pump circulating chilled or hot water through a closed system. What we find is that the torque required to drive the pump goes up as a squared function of speed (Speed2). Thus, increasing the speed causes the torque required by the pump to go up, not directly with speed, but in proportion to the change of speed squared. For example, if we change the speed from 1,160 to 1760 RPM the torque required will go up by the ratio of (1760 ÷ 1160)2. This would mean that the torque required would go up by 2.3 times to 230% of the original value. Also, since horsepower (HP) is based on speed times torque, and the speed has increased by 52%, the new value of HP would be 2.30 x 1.52 or almost 350% of the HP required at the original speed.
The dramatic increases in the horsepower 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% will result in a driving energy reduction of nearly 50%. This, obviously, has big importance when conservation is considered. It also accounts for the tremendous market that exists for variable frequency drives operating Variable Air Volume (VAV) systems used in heating, ventilating, air conditioning and variable speed pumping used in similar 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. This would be the situation in Figure 1 as the valve is closed. The reverse of this is that motor load increases dramatically as restrictions are removed. As an example of this, 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 could not believe it. It defies good judgement to think that adding a restriction to the output
of the blower 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 the 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 or a squirrel cage blower is closed off the air or fluid inside the housing becomes a “liquid flywheel”. It just spins around with the vanes of the pump or blower. 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 or blower. It doesn’t seem to make sense, but that’s the way it is!
As another example, think of fans applied to dust collection systems, the maximum load occurs when everything is as clean as can be. As the filter bags get coated with dust, the back pressure increases and the load on the blower and motor is reduced.
The amount of overloading or underloading that occurs as a result of changes in the “back pressure” on the pump or blower will depend on the specific design of the impeller used. Some types of pumps and blowers are designed to be non-overloading. But in most cases the worst case loading occurs at the open discharge condition.
When dealing with Variable Torque loads things are not always as they would seem. If there is some question about how this equipment performs, it is best to contact the equipment manufacturer and discuss the matter.