On a typical application for a variable speed centrifugal pump, at which percentage speed setting will the VFD dissipate the most heat, in other words have the highest losses?

  • \$\begingroup\$ This is probably a good question to ask a mechanical engineer. Try asking on the Engineering Stackexchange... yes, there's a different Stackexchange for all engineers and engineering students, not just electricals. \$\endgroup\$ – user103380 Aug 15 '17 at 18:18
  • \$\begingroup\$ Welcome to EE.SE. Your question is 20% of the way to being closed as "unclear what you are asking". Edit your question to give some context, what your research has shown and where you are stuck. \$\endgroup\$ – Transistor Aug 15 '17 at 18:19
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    \$\begingroup\$ 11 ------------ \$\endgroup\$ – Olin Lathrop Aug 15 '17 at 18:27
  • \$\begingroup\$ 0 Hz is my guess. \$\endgroup\$ – Andy aka Aug 15 '17 at 18:42
  • \$\begingroup\$ heat loss is I²R, with10% no load excitation current, I ≈ Load Torque, while backEMF rises with RPM so V/f is a ramp for increasing RPM for no load constant current, so max heat depends on load profile vs speed , DCR of pump and drivers, applied Voltage. So if drivers are overheating on initial DC at some reasonable starting torque then driver ESR is too high. If it occurs later then ramp acceleration is too high. But if at full speed , rated V/f, then compare pump energy with RPM and pressure squared+? order with electrical energy and shoot for 50% efficiency. Got water cooling? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 15 '17 at 19:14

Since the torque required to drive a centrifugal pump is approximately proportional to to the square of speed and the power is approximately proportional to the cube of speed, the VFD will almost certainly dissipate the most heat at the maximum speed.

Most VFDs that are available today are essentially a 3-phase rectifier supplying a constant DC voltage input to a 3-phase PWM inverter.

The output current of a VFD is approximately proportional to motor torque. The losses due to forward voltage drop in the switching devices is proportional to current. The switching losses are proportional to speed unless the VFD has a design feature that changes the PWM strategy over the operating range. The input current of a VFD is proportional to output power. The losses due to forward drop in the input rectifier will therefore be proportional to output power.

The many VFD designs that are available or have been available in the past have different loss characteristics over their operating ranges, but it is very likely that all of them will experience the highest losses when driving a centrifugal pump at maximum speed as compared to any lower speed.


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