Q. How is motor efficiency impacted when the motor is controlled by a variable frequency drive (VFD)?
A. Typical motor losses for a 4-pole 15 horsepower (hp) motor are analyzed in Table 6.3. End users are typically aware of the nominal motor efficiency that is listed on the pump nameplate and represented in this table; however, motor efficiency does change as speed and torque change and is also affected by voltage harmonics present in the VFD output. The magnitude of losses due to voltage harmonics will vary with the
In most cases, these additional losses will be relatively small with a modern VFD and motor combination, and the additional losses within the motor at full speed and full power will be
less than 5 percent of the total motor losses. The distribution of losses in a motor is specific to a particular type of motor and power drive system.
Copper losses will reduce with load, while bearing and windage losses reduce with speed. In a cage induction motor, iron losses stay constant with constant flux, but can be reduced with more sophisticated flux control.
As mentioned previously, the efficiency of a motor will change when operated at reduced speed or torque. Figure 6.4 shows typical efficiency/speed curves for a VFD-driven, high-efficiency-type motor with a nominal efficiency of 95 percent at different percentage torque loads (T/Tn) and speeds. Figure 6.4 shows a reduced efficiency at reduced torque, independent of speed, and when speed is reduced efficiency is reduced additionally.
When VFDs are applied to control a pump, it is typically done to match the pump hydraulics to the system, which eliminates throttling of unnecessary head or bypassing of unnecessary pump flow. These improvements to overall system efficiency can greatly outweigh the reduction in motor efficiency.
For more information on applying VFDs to pump motors, refer to HI’s newly published Application Guideline for Variable Speed Pumping at www.pumps.org.
Q. I have heard the terms constant and variable torque pumping applications and that they impact applying variable speed pumping. What do these terms mean and how do they impact variable speed pumping?
A. One characteristic of a variable torque load is that the torque loading is a function of speed, and this type of load requires less torque at lower speeds than constant torque loads. Conversely, one characteristic of a constant torque load is that the torque loading is not dependent on speed and will require a higher torque at reduced speed than a variable torque load.
- Rotodynamic pumps are variable torque loads due to the nature of the pump affinity rules, whereby, head changes proportional to the square of the speed change.
- Positive displacement pumps are constant torque loads because the head they are capable of pumping against is independent of speed. Positive displacement pumps deliver a volume of fluid for every revolution of the shaft, so a reduced speed will directly impact the flow delivered, but it has no direct impact on the differential head or pressure the pump will deliver flow against.
When selecting a VFD for a rotodynamic pump application, many manufacturers recommend a variable torque or standard-duty-rated VFD. This labeling is commonly referenced for the overload capacity of the drive. An industry standard for a variable torque (VT) overload is 110 percent for 60 seconds. When selecting a VFD for a positive displacement pump application, the VFD manufacturer will recommend a drive specific to that application that is suitable for the constant torque (CT) load. An industry standard for a CT overload is 150 percent for 60 seconds.
For more information on applying VFDs in pumping applications refer to HI’s guidebook Variable Frequency Drives: Guidelines for Application, Installation and Troubleshooting at www.pumps.org.