Single Phase AC Motor Specs - Page 1
Forum section: electronics / general

author: just4user
author_rank: Contributor
timestamp: January 26, 2025, 08:07:57 am
content: Hello All,This is more of an electrical question that electronics, but I hope someone out there is knowledgable enough on the topic.I have a single phase dual-speed 220v AC motor (for an evaporative cooler) with the following specs written on its body:Power: .75HP /.25HPCurrent: 5.3A /2.6ARPM: 1425 /950PF: .78 /.67Now, first of all, what I'm interested to know is that why on the high setting, itfeels likeit's blowing 2-3 times more air, although its RPM is said to be only 50% more. If I were to naively look at the power difference, it could make more sense, but the power output is speed x torque, so the 3 times higher power is more about the increased torque (twice more to be precise). Secondly, do you think if the specs make sense at all?! If so, what could the internal config (star/delta/wye) be to match such specs?Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2025, 06:15:42 pm
content: Torque also increases with speed, so the required power goes up exponentially with increasing speed. It's similar to how a car with 200bhp will not have a top speed anywhere near double of a car with 100bhp.

author: TimFox
author_rank: Super Contributor
timestamp: January 26, 2025, 06:31:23 pm
content: To first order, torque is directly proportional to the motor current.Motor power is proportional to the product of torque times rpm.The nameplate values: current is twice, rpm is x1.5, so the power is thrice (as indicated).

author: themadhippy
author_rank: Super Contributor
timestamp: January 26, 2025, 06:32:19 pm
content: QuoteIf its a single phase motor none of the above,also wye is star.

author: just4user
author_rank: Contributor
timestamp: January 27, 2025, 06:10:44 am
content: Thanks all,themadhippy is right. I confused it with 3-phase motors. Its internal config must be 4-pole for fast, and 6-pole for slow. Given the slip, the mentioned RPMs sound about right for 50Hz. The current of slow being half of fast could make sense given the higher impedance (not just resistance).I'm still unsure though about why RPM alone doesn't match what I experience as the increased air output of the cooler.Btw, direct proportionality of torque to current is easy to grasp for a DC motor, but when the internal circuitry of a such AC motor changes, I don't think it can be prescribed as easily.P.S. Why do you think it has such low efficiency?! Shouldn't AC motors be at least 60% efficient?! Its efficiency is only 1/3 for slow and 1/2 for fast. The wires inside all looked thick and copper like.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 27, 2025, 07:44:05 am
content: Quote from: just4user on January 27, 2025, 06:10:44 amBecause fan blades and centrifugal pumps etc. are textbook examples of (roughly) quadratic loads. It comes to aerodynamics (or fluid dynamics). Try to see how much your fuel consumption increases if you drive at 200 km/h instead of 100 km/h. Or when swimming, slowly move your hand against the water. Double the speed and observe how much more force you need to apply now.There is a positive side to it. Loads like this are extremely easy to start up even with motor types with crappy starting torque, because the mechanical power requirement becomes significant only near the operating RPM.

author: just4user
author_rank: Contributor
timestamp: January 27, 2025, 10:35:10 am
content: After a bit of googling, I foundhttps://www.techpowerup.com/forums/threads/fans-static-pressure-vs-air-flow-vs-rpm.183804/The first two posts in there got me realize that I'm feeling the 2-3x airflow in the house, not right next to the blades, and since the air has to go through the ducts, it's probably the higher torque that's doing the extra magic. So the 950 RPM of slow setting is probably for zero-load, whereas in the complete system, it may be operating only at 600 RPM.