"Saving energy with variable speed drives?" Traditionally, throttling is used to regulate flows of air or fluid. While throttling reduces the flow, the motor is still running at full speed and works even harder as it has to work against a restriction. By reducing the speed of the motor, the variable speed drive ensures no more energy than necessary is used to achieve the required flow. This is because the torque needed to run a pump or fan is the square of the volume. For instance, reducing the pump speed to 80% only requires 64% of the torque (0.8x0.8) (Tµn2).
Furthermore, to produce 64% of the torque only requires 51% of the power (0.64x0.8) (Pµn3), as the power requirement is reduced in the same way.
The explanation for this lies in the pressure difference across the impeller. When less pressure is produced, less acceleration of air or fluid across the impeller is required. It is the simultaneous reduction of acceleration and pressure that multiplies the savings."
Does ABB really not understand pumps any better than this?
"Traditionally, throttling is used to regulate flows of air or fluid. While throttling reduces the flow, the motor is still running at full speed and works even harder as it has to work against a restriction."
Anybody who understands centrifugal pumps knows that throttling the discharge flow reduces the work or load on the motor. In many cases, throttling a pump with a valve will reduce the "work" or power consumption more than varying the speed. Restricting the flow from a centrifugal pump certainly does not make the pump work harder. Even restricting the flow from a fan does not make the motor work harder. This can easily be proven by putting you hand over the outlet of a blow dryer. You will notice that the motor actually speeds up, because restricting the air flow reduces the load on the motor.
"A centrifugal pump or fan running at half speed consumes only one-eighth of the energy compared to one running at full speed."
While this statement is true, what they are not saying is that reducing the speed of a pump to half speed, also reduces the head produced by 75%. Therefore, it is impossible to reduce the speed of a pump to 50%, and still produce enough head to function. Even reducing the speed of a pump by only 10%, reduces the head produced by 19%. Head or pressure is reduced by the square of the pump speed. Usually a reduction in speed of less than 10% is all that is possible, to still be able to produce enough head or pressure to function at all.
There are so many other good uses for variable speed drives on things like conveyor belts, escalators, even positive displacement pumps, that there should be no reason for such deception about energy savings with centrifugal pumps. Restricting the flow from a full speed pump does not make the motor work harder. Simply restricting the flow from a pump with a valve can naturally reduce the energy consumption by as much as 50%. The fact that it is only possible to reduce the speed of a pump by less than 10%, means that varying the speed can only reduce the energy consumption by 28% or less.
ABB is just one of many companies who make these type claims. Perpetuating the myth that VFD's save energy, is apparently very beneficial for the sales department. However, not understanding the facts can be very expensive for the end user.