I have a 1/2 HP pump motor (dayton, capacitor start) that I would like to add a run capacitor to. The motor is drawing about 105% of its FLA but below its SFA. I am hoping that a run cap would make it last longer and improve its efficiency. I am it the process of building up a thermostat control box for it that should have enough room to add a capacitor. My problem is that I don't know what size of run cap. I have seen rules like 10 MFD/HP to 15 MFD used in 1/2 HP well pumps. Any idea as to a good size to try ? Is too small OK but not optimum ? Too big I know could over heat the start windings.
If this motor is powering a centrifugal pump, it does not have enough head to reduce the gpm. to within the rated margins. Placing a restrictor plate or otherwise restricting the amount of flow on the output side of the pump will reduce the FLC. Sounds reverse of what you would expect and would be so with a positive displacement pump. However without enough head, a centrifugal pump simply moves too large a volume. IOW. The motor is being overworked.
The addition of a run capacitor is probably a bad idea. In a capacitor start split phase motor design, the windings are different (start is low inductance/high resistance and run is high inductance/low resistance). In a capacitor start run motor design, the start and run windings are intended to be identical. The centrifugal switch application would likely be designed for a different speed.
I've added start capacitors to plain split phase motors to increase the starting torque, and they have performed well. I wouldn't modify a capacitor start split phase in the way you're anticipating, for the above reasons. There might be a better way such as the restrictor, or some other suggestions, but I think I'd want a separate overload protection device in place for this application to save the motor while you're dialing in your modifications.
I had assumed that the motor windings in a capacitor start and a capacitor start/run were similar. But, as you state, they are quite different. They must be able to save some cost on a lighter gauge and fewer turns on the start winding. I guess that is why they make capacitor start motors. This is an above-ground pool pump and rather than use small diameter flex hose, I used 1.5" PVC. I have a ball valve on the output side but closing it down 1/2 of the way only drops the current from 8.4A to 7.7A. This also causes the plumbing to vibrate. This ball valve was for adjusting the flow to a solar panel. I have since hooked up the solar panel and it no longer functions very well as a flow restriction. The pump and filter set came with a 1.5 HP pump. But I came across a new 1/2 HP pump motor on ebay for $6. So I bought a new wet end for the 1/2 HP motor although it appears the impeller is slightly too big (smallest they have). For some reason the pool companies think that bigger is better when it comes to pumps. I may add a flow restriction in the strainer basket using some plexiglass. This would hopefully be better on the plumbing but it might cause cavitating. If that doesn't work I could try a ball valve between the pump and filter. But the ball valve costs more than I paid for the motor :-) Or I could buy a lathe :-) and take a 1/4 inch off the impeller. Its not grossly overloaded (FLA
8A/SFA 10A) so it should give a few years of life. While its alive it is saving me about $50/year on my power bill vs the 1.5 HP monster.
That reminds me about an article I saw a few years ago about some work done by a guy at a US Navy research lab.
It showed how much energy could be saved by adding a "run" capacitor to the existing fractional horsepower split phase motors used in home appliances and light industrial applications. (Must be many millions of those in daily use, 'eh?)
IIRC, he just put an appropriate sized motor capacitor right across the contacts of the centrifugal starting switch so that when the switch opened the motor operated in a "capacitor run" mode.
The energy savings shown in the article were impressive, but it's such a nuisance to get inside most motors to connect up the capacitor leads that I don't think his idea ever got anywhere.
I did this on a 4 HP capacitor-start 220-volt GE motor. It raised the power to enough to run my 2-stage air compressor which is rated at 5 HP with the pulley sizes (run speed) I used. Full load current didn't increase , probably because the cap driving the start winding just improves the power factor at full load -- which is all the time on a compressor. The motor does not overheat. The run cap doesn't supply nearly the current that the start cap does while starting.
I don't recall the value of the run cap, and it's been in there so long (15 yers or so) the numbers aren't visible anymore. I'd say just try it, using the 10 uF/HP rule, while monitoring line current. If you don't significantly exceed rated line current then I can't see how it could hurt anything.
Have you checked the voltage at the motor? If the voltage is low, the current will be higher. So if there is a long wire run to the motor or some other reason that causes the voltage to be low, you could correct that. To correct a long wire run, you could add a power factor correction cap across the line at the motor. That will drop the current thru the wire to the motor,but will not change the current drawn by the motor ( except by raising the voltage ).
Thanks Jeff. I googled it and wouldn't ya know its on line. Can from a letter to Eletronic Design (Apr 2001).
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"Howdy Bob: I started out as an electrical power engineer at the University of Idaho. I have been working for the Navy for about 25 years and I learned analog engineering on the job, with a lot of help from your design articles. I did pass the P.E. exam a while back and haven't used it much, but I have kept an eye on the electric industry. We have been blessed with relatively cheap energy, until now.
Some of my friends here at China Lake studied ways to reduce energy consumption of fractional horsepower motors back in the Carter Administration energy crisis, but their solutions were ignored when the crisis passed. They published "Naval Weapons Center Technical Memorandum
4552" in July 1981. It showed their investigations and recommendations to retrofit fractional horsepower motors (1/3 through 3/4 hp) and turn them into two-phase motors by using an ac capacitor across the start (speed) switch.
This way, the start winding is used as a second-phase winding during running. At stop, the switch is closed and the capacitor is safely shorted. When the motor gets rolling, the switch opens and the capacitor phase shifts the line current to use the start winding to help push the rotor. It takes about 35 µF for a 1/3-hp motor, about 45 µF for 1/2 hp, and about 55 µF for a 3/4 hp. Again, these are ac caps with at least a 350-V ac rating. I got my caps from C & H Sales in Pasadena, Calif.
I'm going to retrofit my private well pressure pumps and my evaporative cooler motors so that the "wheel of fortune" doesn't gig me any more than necessary. Of course, the wiring has to be done safely, although the savings are on the order of 20%, if all is done properly. The motor pulleys need to be properly sized, and the minimum current point must be determined with a clamp-on ammeter, as the capacitor values are tried.
A second benefit is that the run winding with the start winding actually runs cooler than just the run winding alone. That should help the motor last longer. This TM 4552 isn't available in electronic form at this time, but I can send a copy to you with my added notes, if you're interested. (Note: this isn't a simple power-factor correction, but a conversion of the motor into a two-phase unit.) Ed Tipler via e-mail
Hello Ed. This seems kind of neat. It sounds like the sort of thing that is ignored. Yes, please send it to me.-RAP
All for now. / Comments invited! RAP / Robert A. Pease / Engineer snipped-for-privacy@galaxy.nsc.com-or:"
I was going to do that but at the time I thought of it I was standing on soggy ground after 6" of rain. The motor is on 70' of 12 GA cord. I am sure the volts probably drop a bit during starting so I would not do this to a compressor. But I would think that the voltage drop would be fairly small when drawing less than 10 Amps. The motor is only started once a day. I plan on pulling 10 GA in the conduit I buried after I get the deck up.
Thanks for verifying my memory. I should have thought of Bob Pease though. I still subscribe to Electronic Design magazine and usually open it to his column before reading anything else.
He's an alumni of the same joint I got my degree from. I've met him a couple of times, and can testify to his being a talented electronics guy as well as a true "renaissance man" who'd fit in well here at rcm if he he was so inclined.
Something else related to energy saving methods of motor applications was a device that was said to have been developed by NASA, that was marketed as Watt Mizer (I think). It was plugged into the wall, and the appliance was plugged into a receptacle on the magic box. This device had a couple of circuit boards and one or more ICs in it.
That might've been in the late 70s, but there have been other devices marketed that were claiming to produce the same energy savings. I haven't seen them lately, but they used to be on display at Lowes a number of years ago (7 maybe), that were different colors for different motor sizes.
Frank Nola of NASA. It actually did work somewhat but the thing was over-hyped. Still one guy claiming to make a product based on it.
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I don't think that adding a run cap is hype. It works for cap start/run motors and improves their efficiency. This paper has been reviewed by two PE's that think it works. I don't think the start winding and the run winding need to be identical to use a run cap on the start winding. It isn't like a three phase motor. Its more like the third phase of a three phase motor being run on single phase. So the start winding doesn't need to be as big as the run winding. That said I am not going to experiment with my pump motor. It was interesting learning a bit about motors. Seems that one of the California power companies did a study on pool pumps. They are recommending that all pumps be cap start/run and two speed motors. Most of the pool industry doesn't have a clue on sizing the pump and filter. I know people that are told to run their 1.5 HP pump 24 hours a day at a cost of $400/yr. My 1/2 HP will do the same job for $50/yr. They are sold a gas heater and pay $300/month to heat their pool. My pool is 86 deg and we haven't had a day over 80 all summer. The proper energy efficient pumps are out there just that the pool industry doesn't know how to market it. Bigger is not better.
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