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  1. #1
    Join Date
    Sep 2010
    Location
    Indiana
    Posts
    365

    Motor HP vs Amps

    I understand the importance of replacing motors by FLA rating. But when doing this, can you change HP as well?

    Example:

    Century D1076 - 3/4 HP, 208/230 V, 1075 RPM, 4.0 FLA
    GE 3588 - 1/2 HP, 208/230 V, 1075 RPM, 4.3 FLA

    Which is the stronger motor?

    Say I am replacing a 3/4 blower with a 4.5 FLA rating - Which motor would be the better fit?

  2. #2
    Join Date
    Feb 2013
    Location
    Florida
    Posts
    69
    The FLA rating doesn't mean much on motors of differing horsepower, since the tested full load of a 1/2 hp motor isn't going to be the same as the full load of the larger one. The smaller hp motor may have too much slip under that load and lose rpms, as well as having a high current draw. Manufacturers typically don't oversize anything, so if they engineered it to use a 3/4 hp motor you should probably stick to that.

  3. #3
    Join Date
    Oct 2013
    Location
    Alberta Canada
    Posts
    414
    FLA is just a rating the manufacturer puts on the motor that you shouldn't be over. If you put a half horse in where a 3/4 was you will surely be over your fla. (This is assuming you were close to fla on the 3/4 hp) if you were way under your fla, you have restricted duct work. (Also assuming we are talking psc motors)

  4. #4
    Join Date
    Sep 2010
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    Indiana
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    365
    Yes I was talking PSC

  5. #5
    Join Date
    Jun 2013
    Location
    Northern Ontario, Canada
    Posts
    636
    You are looking at it backwards. At that amperage, voltage and rpm the amount of work that the motor can achieve is expressed in the following equation. The amount of work they achieve is then expressed in Kwh for how much energy it used to achieve that work. A motor can be used from 50 to 100% of load, any less than 50% the magnetism starts to degrade significantly. A motor is most efficient at 75% load. Motors will also have a service factor on the name plate, usually 1.15 which indicates that the motor can sustain 115% load for short periods of time with out being damaged.

    1hp = 33,000 ft lbs / min

    .75hp = 24,750 ft lbs / min x 60 min = 1,485,000 ft lbs / hr / 44.25372896w = 33557Kwh

    .5hp = 16,500 ft lbs / min x 60 min = 990,000 ft lbs / hr / 44.25372896w = 22371Kwh

    So with that being said, with your data, the answer to your question of what one can do more work in an hour: 3/4hp

  6. #6
    Join Date
    Sep 2010
    Location
    Indiana
    Posts
    365
    I understand that 3/4 hp can do more work in an hour than 1/2 hp can. On paper, the 3/4 should have more torque at 23 ft. lb. than the 1/2 at 15.3 ft. lb.

    Assuming you do not switch motor types, just PSC, wouldn't energy in correlate to energy out?

    It seems like the PSC motors I've looked at have an efficiency of 63% to 72% with most being listed at 65%

    I have 2 motors in hand, both are 1/2hp, 1075rpm, 115v. One is listed as 6.0 amp, the other at 9.8 amp.

    I would expect that if I pulled a 9.8 amp motor out of the furnace and replaced it with a 6 amp version, it would probably overamp and go out on overload. What if I put a 8.5a 3/4hp motor in place of the 9.8a 1/2hp? Because it is a 3/4 would it be underloaded? It seems like if it would cut 1 amp draw from the furnace, the manufacturer would have installed a 3/4 originally and possibly achieved a higher SEER.

    What I would love to do is set up a test station with a tachometer and clamp meter and test this in a real world, but I don't have to means to do it right now.

    I have read the Fasco Facts, which is what has me thinking. Is there another good reference for an education on PSC motors?

  7. #7
    Join Date
    Jun 2013
    Location
    Northern Ontario, Canada
    Posts
    636
    This is a good read: http://www.leeson.com/TechnicalInformation/sphase.html

    This is also a good read: http://www.shoemakerindustrial.com/D...ing%20Load.pdf

    If you under load a motor, it will draw less amps than it was designed to run at while having 208v @60hz applied. There will be increased slip and less magnetism so to do the same amount of work at the desire RPM it will have to use a lot more energy to keep up which will kill your electrical bill.

  8. #8
    Join Date
    Jul 2012
    Posts
    1,194
    I have had calls on old units when the warehouse or my truck was out of 1/3 hp, I would go up to 1/2 hp. No problem.

  9. #9
    Join Date
    Oct 2007
    Location
    Hallandale Beach, FL.
    Posts
    436
    Quote Originally Posted by captaincompressor View Post
    I have had calls on old units when the warehouse or my truck was out of 1/3 hp, I would go up to 1/2 hp. No problem.
    The problem is when another tech goes there and replaces your 1/2 hp with a 3/4 hp because he didn't have a 1/2 hp on the truck.
    I miss you mom and dad.

  10. #10
    Join Date
    Sep 2008
    Location
    VA
    Posts
    1,791
    Quote Originally Posted by RACMT View Post
    You are looking at it backwards. At that amperage, voltage and rpm the amount of work that the motor can achieve is expressed in the following equation. The amount of work they achieve is then expressed in Kwh for how much energy it used to achieve that work. A motor can be used from 50 to 100% of load, any less than 50% the magnetism starts to degrade significantly. A motor is most efficient at 75% load. Motors will also have a service factor on the name plate, usually 1.15 which indicates that the motor can sustain 115% load for short periods of time with out being damaged.

    1hp = 33,000 ft lbs / min

    .75hp = 24,750 ft lbs / min x 60 min = 1,485,000 ft lbs / hr / 44.25372896w = 33557Kwh

    .5hp = 16,500 ft lbs / min x 60 min = 990,000 ft lbs / hr / 44.25372896w = 22371Kwh

    So with that being said, with your data, the answer to your question of what one can do more work in an hour: 3/4hp
    33557 kWh is 33,557,000 watts per hour?
    "The only true wisdom is in knowing you know nothing" Socrates

  11. #11
    Join Date
    Jun 2013
    Location
    Northern Ontario, Canada
    Posts
    636
    Quote Originally Posted by mgenius33 View Post
    33557 kWh is 33,557,000 watts per hour?
    Kilo Watt per hour ---- thousand watt per hour so yes.

  12. #12
    Join Date
    Sep 2002
    Location
    Virginia
    Posts
    3,228
    Quote Originally Posted by mjohnson2981 View Post
    I understand the importance of replacing motors by FLA rating. But when doing this, can you change HP as well?

    Example:

    Century D1076 - 3/4 HP, 208/230 V, 1075 RPM, 4.0 FLA
    GE 3588 - 1/2 HP, 208/230 V, 1075 RPM, 4.3 FLA

    Which is the stronger motor?

    Say I am replacing a 3/4 blower with a 4.5 FLA rating - Which motor would be the better fit?
    3/4 hp is stronger , duh , but its designed to be fitted with a bigger fan , more aggressive pitch

    There are TONS of posts here about swapping condenser fan motors , and what Not to do

    You will encounter overheating if you mismatch too much

  13. #13
    Join Date
    Jul 2009
    Location
    South Carolina
    Posts
    3,261
    I always go by HP when replacing a motor but the real test is matching the new motors amp draw while under load. The problem is you can't test it until you install it. Assuming both manufacturers labeled their motors correctly then the 3/4 should be more powerful even though it has a lower amp rating. Some motors can be more efficient and use less current.
    Gary
    -----------
    http://www.oceanhvac.com
    An engineer designs what he would never work on.
    A technician works on what he would never design.

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