# Thread: Motor HP vs Amps

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## 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. 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. 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)

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Yes I was talking PSC

5. 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

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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. This is a good read: http://www.leeson.com/TechnicalInformation/sphase.html

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.

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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. Originally Posted by captaincompressor
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.

10. Originally Posted by RACMT
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?

11. Originally Posted by mgenius33
33557 kWh is 33,557,000 watts per hour?
Kilo Watt per hour ---- thousand watt per hour so yes.

12. Originally Posted by mjohnson2981
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. 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.

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