Why single phase motors on 3 phase unit?

The HVAC units on the roof of our building are delivered 460 volt 3 phase power. The compressors and blower fan (evap) motors are 3 phase. The three
condenser fan motors in each unit are 460 volt single phase! They have a capacitor and two sets jumper leads to reverse direction. I assumed that these would be 3 phase, but when the tech replaced some bad motors, I saw the name plate on it with wiring diagram.
I wonder why they would use single phase motors where 460 volt 3 phase is only available?
These are 1984 vintage 12 ton Trame units. -S
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Fractional or low horsepower 3 phase motors are more expensive than single phase in my experience. Very typical from what I have seen in the industry. If the condenser fan motor gets above say 3 hp then they would have used 3 phase
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phase
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industry.
I wonder why this is so? It would seem the capacitor and extra connections (7 leads vs. three) would make the single phase motor more costly to manufacture and implement. -S
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<<
<<
Fractional or low horsepower 3 phase motors are more expensive than single phase in my experience. Very typical from what I have seen in the industry. If the condenser fan motor gets above say 3 hp then they would have used 3 phase
>>
I wonder why this is so? It would seem the capacitor and extra connections (7 leads vs. three) would make the single phase motor more costly to manufacture and implement.
>>
3-phase motors have several poles, and at least 3 power leads. Except for that, they are fairly simple and quite efficient.
1-phase capacitor-start motors have quadrature windings, which require at least 1 extra pole pair; also starting capacitors; also (usually) centrifugal switches to switch the starting capacitors in and out; also usually extra leads to reverse the rotation direction. The quadrature windings produce a rotating magnetic field, which starts the motor. Starting torque can be fairly high.
Shaded-pole motors run on 1 phase, but do not require starting windings and capacitors and all that goes with them; therefore such motors are relatively simple and inexpensive. The magnetic pole-pieces are bifurcated, with copper rings around diagonally opposite sub-pole-pieces. The alternating magnetic field induces current in the copper rings, and that current delays the magnetic field through those sub-pole-pieces, and thus creates a rotating magnetic field for starting the motor. For small motors that do not require much starting torque, as for fans, shaded-pole motors work well. The copper rings waste some power, but usually that is not significant for small motors.
I don't know why the subject condenser fan motors are 1-phase capacitor start, rather than 3 phase. Maybe replacement 1-phase motors are more readily available. What is their power rating?
Dick Alvarez alvarez at alumni dot caltech dot edu
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wrote

These are PSC (permanent split capacitor) motors. The capacitor is connected from the line to the start winding. However, it is not switched out once the motor comes to speed. The motor could not start without it, but if it was removed while the motor was turning, the motor would continue to turn, but current draw would increase. Some motors have a start and run cap in which the start cap is switched out of the circuit once rotational speed near running speed. This arrangement increases starting torque.
In this case, they are 460 volt 2 amp. with one capacitor.
-S
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I asked myself the same question years ago. Took me awhile to figure it out. It all has to do with phase rotation sequence:
Reciprocating compressors don't care which direction their crankshafts rotate; a fan does. By using a single-phase motor on the fan, the whole condensing unit will run properly no matter how the phases are sequenced. BTW, even Copland hermetic compressors - which have a gearotor type oil pump for forced lubrication - can run either way. The oil pump has intake ports that are milled into a disk that can rotate a few degrees by friction with the gearotor. That action will configure the pump intake and exhaust ports so flow is always in the right direction no matter which way the compressor rotates. Its pretty damn hard to see which way one of those is rotating. If they didn't design the oil pumps that way there would be allot of croaked compressors with seized bearings kicking around ;-)
That being said, scroll compressors want to rotate in one direction only. But if you hook one of those up backwards you'll know right away <plugging ears>.
Paul
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---------------------------
However, it is possible to reverse the direction of rotation of a single phase motor (note that this is implied in the original of this thread) so that is not the answer. The answer as someone gave before is that single phase motors in the 1 to 2 HP and down are cheaper than 3 phase motors in the same size simply because there are so many made.
--

Don Kelly @shawcross.ca
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You missed the point: every knows you can reverse a single phase motor; it's the fact that as wired from the factory, the fan will run the correct direction regardless of how the installer connects the condensing unit to the 3-PH supply.
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