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Thread: Again: Uprated, Fullrated Motors; Service Factor

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    Again: Uprated, Fullrated Motors; Service Factor

    I know this has been discussed before, but I wanted to clear something up in my mind. I understand the pump impeller loads the electric motor, so a 3/4hp impeller draws 3/4hp shaft hp. The pump's electric motor must provide the 3/4hp to the shaft, and on top of that, absorbs energy in terms of electrical and mechanical losses (electrical resistance, mechanical friction, etc). So, a 1hp name plate motor is drawing more than 1hp in electricity.

    Thinking "out loud" here:

    The electric motor has it's nameplate information: rated horsepower, service factor, service factor amps, volts, etc. My understanding of the service factor is this: it's a multiplier that states the motor may run at up to X times the name plate horsepower. So a 1hp motor with a service factor of 2 could provide up to 2 hp and would draw the rated service factor amps. The service factor is related to the internal construction of the motor, such as the gauge of copper wire in the motor, the motor wire insulation temperature handling capability, and other factors. Essentially, the service factor tells you how resistant to heat the motor is. I have read that when you operate a motor at it's full service factor hp, you decrease its lifespan. This makes sense: the higher power you provide to the pump, the more heat you generate. Heat is bad. I have read that running a motor at full service factor decreases it's life by half. I think people can debate this, but it's clear more motor heat will decrease the motor's life.

    I've also seen pool motor suppliers say that you should always replace a motor with a motor with a higher (hp x SF) value. What is sticking in my brain is that with pumping applications, there is not a lot of load variability or starting torque requirements. If a pump impeller is drawing 1hp, what is the harm in using an uprated and physically smaller 1hp motor? The smaller the motor, the more efficient it technically should be. Yes, you decrease your service factor a bit, but if you are drawing more than 1hp at the shaft there is something wrong mechanically with the motor. The impeller sets the shaft hp load.

    I guess the idea that the nameplate horsepower times the service factor equals the "brake horsepower" doesn't fit to what I understand the motor service factor to be.

    I would think using an uprated and physically smaller motor for dedicated spa jet applications would make sense since the smaller motor is more efficient and the motor only runs for small portions of its life. Using a full rated motor for filter pumps might make a lot of sense since they run all the time and will be exposed to a lot more heat in its lifetime.

    What do you all think?

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    Mod Squad jblizzle's Avatar
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    Re: Again: Uprated, Fullrated Motors; Service Factor

    There is absolutely no difference between an up-rated and full-rated motor if the HP*SF is the same.
    They are physically identical.
    It is all a marketing gimmick.
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    Re: Again: Uprated, Fullrated Motors; Service Factor

    Quote Originally Posted by robinasu View Post
    The service factor is related to the internal construction of the motor, such as the gauge of copper wire in the motor, the motor wire insulation temperature handling capability, and other factors. Essentially, the service factor tells you how resistant to heat the motor is. I have read that when you operate a motor at it's full service factor hp, you decrease its lifespan.
    The THP/SFHP (HP*SF) is the true motor rating and no, a motor that runs at it's THP should not decrease it's lifespan. If a motor is rated at a certain SFHP, then it can safely be operated at that level without any degradation to the service life. The service life of a pump motor is dictated more by the bearings than anything else and the thermal insulation and in most cases the winding will long outlive the bearings. A full rated motor with a 3/4 HP label rating and a 1.6 SF is exactly the same as an up rated motor with a label of 1 HP and a SF of 1.2. No difference in performance, life span or application.


    I've also seen pool motor suppliers say that you should always replace a motor with a motor with a higher (hp x SF) value.
    Most suppliers that I have seen say greater than or EQUAL to the SFHP. You can never have a smaller motor.


    I guess the idea that the nameplate horsepower times the service factor equals the "brake horsepower" doesn't fit to what I understand the motor service factor to be.
    Brake HP is NOT the rating of the motor although you will see many who use it that way. BHP is the load on the motor and for pool pumps, BHP varies over the head curve. The SFHP is the ONLY load rating for a motor (in addition to amp rating) and that simply specifies the maximum load on the motor and BHP should never exceed the SFHP but it can be less.


    I would think using an uprated and physically smaller motor for dedicated spa jet applications would make sense since the smaller motor is more efficient and the motor only runs for small portions of its life. Using a full rated motor for filter pumps might make a lot of sense since they run all the time and will be exposed to a lot more heat in its lifetime.
    Again, you need to review the definition of the up rated and full rated. They are exactly the same for the same THP/SFHP.

    Also, smaller motors (lower SFHP) usually are less efficient because the winding wire diameter is smaller since it drives a smaller load. So while it is not true that smaller motors are more efficient, it is true that smaller PUMPS are more efficient because they have less load on the motor and the motor will draw less power because of it.
    Mark
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    Re: Again: Uprated, Fullrated Motors; Service Factor

    Thank you all for the responses. I think I know where my problem is.

    If we assume a pump has a 1hp impeller, are you saying the rated nameplate horsepower on pool motors is just a marketing number? My understanding is NEMA specifies the nameplate horsepower as the rated horsepower of the motor and the service factor tells the user the motor is capable of a certain multiplier of overload.

    From what you are saying, with a 1hp pump impeller, I would need to find a motor with at least a 1hp THP; is that correct?

    I have a spa motor that is getting loud. That is where all of this is coming from. If I can select using THP, then I could use a Centurian B845 (THP 0.95) "1/2hp" full rated motor at 4.5 "service factor amps" over the "1hp" Centurian B2841V1 (THP 1.65) at 7.8 "service factor amps" to power my 1hp impeller. Is this correct?

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    Re: Again: Uprated, Fullrated Motors; Service Factor

    If we assume a pump has a 1hp impeller, are you saying the rated nameplate horsepower on pool motors is just a marketing number?
    The label HP is but THP is sized for the impeller.


    My understanding is NEMA specifies the nameplate horsepower as the rated horsepower of the motor and the service factor tells the user the motor is capable of a certain multiplier of overload.
    That is mainly for industrial motor applications that have frequent starts and stops or varying loads. For pool pumps, the motor is sized for the maximum impeller load (right side of head curve) AT the service factor rating. This fine because a pool pump motor runs at near constant loads and there are usually not frequent starts and stops.


    From what you are saying, with a 1hp pump impeller, I would need to find a motor with at least a 1hp THP; is that correct?
    No. There is no consistent definition for an impeller so to size the motor, you must look at the old motor that came off the pump. Some manufactures will call an impeller 1 HP but put it on a 1 HP up rated motor while others will use a 1 HP full rated motor. Even though two impellers are for 1 HP pumps, they may not have the same load. However, if the pump line has both up rated and full rated pumps, the manufacture will usually label the impeller for the fulled rated version. So you will normally see the same impeller part number for the full rated version and an up rated version that is 1/2 HP higher than the full rated version since the both have the same THP.


    I have a spa motor that is getting loud. That is where all of this is coming from. If I can select using THP, then I could use a Centurian B845 (THP 0.95) "1/2hp" full rated motor at 4.5 "service factor amps" over the "1hp" Centurian B2841V1 (THP 1.65) at 7.8 "service factor amps" to power my 1hp impeller. Is this correct?
    Assuming your old motor is 0.95 THP or less, then yes. If you old motor has a higher THP, then you will have to go with a larger motor. Do not go by the impeller, always go by the old motor. But if you have to guess, always guess to the larger motor (full rated).
    Mark
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    Re: Again: Uprated, Fullrated Motors; Service Factor

    The loud noise is almost certainly the bearings starting to go. One of my spa's pumps went recently and I found a friend who had a puller and a little knowledge of motors help me replace the bearings and the wet end seal. Saved me several hundred dollars and it is now running more quietly and smoothly than it has in years. There are youtube videos you can follow to see how this is done, if you are adventurous.
    19,000 vinyl pool, solar heat, .75 hp tristar, 48 sq ft de filter

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    Re: Again: Uprated, Fullrated Motors; Service Factor

    Quote Originally Posted by ChrisL View Post
    The loud noise is almost certainly the bearings starting to go. One of my spa's pumps went recently and I found a friend who had a puller and a little knowledge of motors help me replace the bearings and the wet end seal. Saved me several hundred dollars and it is now running more quietly and smoothly than it has in years. There are youtube videos you can follow to see how this is done, if you are adventurous.
    Thank you for the tip. The frame is rusted out, otherwise I would. Maybe I will get a motor cover so I can just change the bearings next time.

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    Re: Again: Uprated, Fullrated Motors; Service Factor

    It makes sense that pump manufacturers would need to select a motor that would survive at the pumps max GPM. In my spa application the head and flow are set with no operable ways to increase the flow, so perhaps there is an opportunity to select a lower THP motor if I find my operating point and power. Is the THP the hp electric, or hp at the shaft? I am assuming at the shaft. My idea is to run a physically smaller motor as close to THP as possible. I saw some generic charts showing for motors under 1hp the efficiency climbs with increased load, flattening off over about 1hp.

    I'm paying $0.35 for 3/4 of my kWh's. I see you are in CA, too, so you know what I'm saying. I used your tool to spec my two speed pump using the vacuum/pressure gauge method. Everything worked out pretty good using the old "1/2hp" impeller from the max-e-pro pump. On low speed I am hitting just around the min speed needed to activate the pentair intellichlor. On cold days it doesn't activate (thicker water), but it probably wouldn't activate due to low temperature, anyway. Thanks for making the chart available!

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    Re: Again: Uprated, Fullrated Motors; Service Factor

    THP is the power delivered to the motor shaft and there is really no way to know what that is to a high degree of accuracy.

    My idea is to run a physically smaller motor as close to THP as possible. I saw some generic charts showing for motors under 1hp the efficiency climbs with increased load, flattening off over about 1hp.
    The efficiency curve is very flat over most of the head curve. Plus smaller motors use thinner wire and tend to have lower overall efficiency than larger motors. So you will likely use more energy with a smaller motor than the correct sized motor. Plus I don't think your operating point BHP is much lower than the THP.
    Mark
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    Re: Again: Uprated, Fullrated Motors; Service Factor

    I was looking at Figure 1 on page 1:

    http://infohouse.p2ric.org/ref/40/39569.pdf

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    Re: Again: Uprated, Fullrated Motors; Service Factor

    As I was saying, that is a flat efficiency curve. Load doesn't drop below 80% which is in the flat range.
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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    Re: Again: Uprated, Fullrated Motors; Service Factor

    True, but overkill is underrated. : )

    Maybe I will pickup a PSC motor and just slap a triac speed control on it. Same for my booster pump. So much over pumping in this industry..

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