Quote:
Originally Posted by Servicetech
Multispeed PSC motors don't save as much money on "low" and buyign a smaller single speed pump. Currently running a 1/4HP pump on my own pool (uses 250W) for (2) 4hr shifts per day.
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The motor the thread starter is talking about doesn't change the speed the same way as low/med/hi tapped PSC HVAC fans or shaded pole table top fan motors.
This motor's the real deal true sped with 2 pole/4 pole windings with industrial motor standard rating. A well made AC motor is quite efficient. The motor runs at constant RPM for each setting.
I'm looking in the catalog and I see these specs. You might find them to be strange specs, but fan law will tell you that cutting the fan to 1/2 the speed drops the GPH to 1/2 and (1/2)^3 =1/8 the horsepower requirements, hence 1:8 split.
0.75hp/3450RPM 230v 6.0A@1.67 SF
0.10hp/1725RPM 230v 1.0A @1.67 SF
Those amps are at maximum allowed overload conditions.
I guessed the PF, but I did it so on the conservative side. These motors are heavy and use copper generously, so they're not cheap.
1/6hp @ ~67% (PF at 0.9)
1 1/4 hp @ ~75% (PF at 0.9)
These are plug-to-shaft efficiencies. Not bad for something this size. It's at the 67% overload given in the specs, and they're more efficient when they're not overloaded, so you'll probably get closer to 80% to the shaft.
AC motors are very reliable, tough and they don't create harmonics. Adding a VFD will just make it much more susceptible to surge damage, reduce efficiency and induce high levels of harmonics into the line. You don't need controlled ramp or a bunch of in between speeds for a pool pump. You'll lose another 10% or so of kW going into it in conversion process in the VFD box.
Why the over load rating on motors? It's so you can size it for continuous use at 3/4hp, but have the ability to handle increased load from cleaning attachments and such without the need to actually oversize the motor. It will handle an overload up to 67% for quite some time without issues.
If you run a 3/4 hp VFD with and you overload to 1 1/4 hp load for more than a few seconds, it will let the smoke out.
Attached picture shows you how a common air handler fan is made multi-speed.