In this type of application, below base speed, torque required varies as speed2. It should also be noted that the horsepower required varies as speed3. Because of this relationship, this type of application is sometimes referred to as the cubed exponential application. Figure 2-41 shows the char- acteristics of this type of application.
This type of application is a prime candidate for energy savings using AC drives. As seen in the example, to obtain 50% flow rate, only 1/8 or 12.5% horsepower, is required (½ × ½ × ½). It should also be noted that the torque and horsepower curves normally end at the 100% spot (100% speed and 100% torque, flow, and horsepower). Because it is easy to over- speed a motor using a drive, users may have a tendency to increase speed above base, to obtain more CFM output. This is not normally recom- mended because of the mechanical limitations of the motor and the char- acteristics of the application.
Figure 2-40. Center driven winder (cross section of web roll)
Speed is Slow
Speed is Fast (Low Torque Required)
Speed is Slow (High Torque Required)
Chapter 2: Review of Basic Principles — Mechanical Principles 47
To obtain increased speed above base, a higher output frequency (Hz) must be supplied by the drive. With several Hz output above base, the torque and horsepower curves would follow the natural path indicated by the dashed lines. As shown in the figure, just a few hertz increase in out- put frequency would cause a much greater demand for motor torque and horsepower. However, flow would continue to increase in a somewhat “linear” manner.
This tremendous increase in torque and horsepower from the motor is now coupled with the fact that decreased motor torque is a natural result of above base speed operation. (Details on this characteristic will be dis- cussed in Chapter 3 – General Principles of Operation.)
The end result is a slight increase in drive output frequency, causing a very large requirement for torque and horsepower, at the same time motor torque is dropping rapidly. Nearly all of the fan systems engineered today take this phenomenon into account. It is rare to need to over-speed a fan, unless the above characteristics are addressed (duct work changes or over- sizing the motor, or both).
Because HVAC fan systems are engineered for below base speed operation, typical overload requirements are set at 10%. AC drives that are rated to NEMA standards, will allow 110% current for 1 minute as an overload capability.
As shown in the discussion above, this type of application has two main examples: centrifugal fans and pumps. Figure 2-42 shows a standard HVAC fan application.
In this example, we see that the centrifugal fan is coupled to the motor by means of a belt. The air inlet allows air to enter the fan assembly. Depend- ing on the status of the outlet damper (open, part open, closed), the fan
Figure 2-41. Variable torque application
Torque %Torque Flow & HP HP 50 Flow 12.5 100 100 110 50 % Speed Torque %Torque Flow & HP HP 50 Flow 12.5 100 100 110 50 % Speed
will blow the amount of air dictated by some control system, typically an actuator. The area in the dashed box indicates a three-phase start contac- tor and motor overloads to protect the motor, in case of an over-current condition. Also shown is the fact that what’s inside the dashed box can be replaced by an AC drive.
As can be imagined, the fan continues to run at full speed, 24 hours a day. This is required, regardless of the status of the outlet damper, because there is no device controlling the speed of the motor. The device that con- trols airflow has no effect on the speed of the motor and fan.
This method of variable flow control is quite inefficient. Figure 2-43 shows a graph of how the fan actually operates on the basis of the system shown in Figure 2-42. The fan curve indicates the fixed speed fan. The system curve indicates the status of the outlet damper and also the size and shape of the sheet metal duct work.
To reduce the airflow in this fixed-speed system, the actuator must close down the outlet damper, thereby restricting air output. If 50% airflow is desired, the position of the outlet damper must be modified by the actua- tor, which changes the system curve (indicated by the dashed curve). The air output is reduced; however, the pressure in the system duct work has increased, placing additional load on the motor. This additional load trans- lates to more horsepower required to operate with less airflow.
If the start contactor assembly were replaced by a variable-frequency AC drive, the outlet damper would be locked in full open position (indicated by the existing system curve). For every output frequency of the AC drive, a new fan curve occurs (indicated by the dashed curves).
As seen, where the new fan curves cross the fixed system curve, a different percentage of flow output is achieved. It can also be seen that the amount of pressure in the duct work never exceeds 100%. In essence, if there is
Figure 2-42. HVAC fan application M Replaced with an AC Drive
Air Inlet
Outlet Damper
Chapter 2: Review of Basic Principles — Mechanical Principles 49
less pressure, the motor doesn’t work as hard and energy is saved, com- pared with full-speed operation.
This same principle works for centrifugal pumps. Reduced speed is a defi- nite advantage when operating a pump, which leads to reduced energy use. However, when speed reduction is contemplated, the optimum effi- ciency of the pump must be taken into consideration. Pumps have “effi- ciency islands” around the crossing of the pump and system curves. Any energy savings gained can be lost if operation is done outside these effi- ciency islands because of greater pump inefficiency.