MOTOR WATTAGE
- Thread starter dkrumnow
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- Lockport, IL
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- Retired Electrical Engineer
Re: MOTOR WATTAGE
First, please turn off the ALL CAPS feature. It makes the post more difficult to read. It is also considered impolite, as though you were SHOUTING at us.
"Horsepower" is a unit of power, and so it watts. Converting HP to Watts is similar to converting feet to inches. One foot is equal to 12 inches. 746 watts is equal to one HP.
However, a simple "unit conversion" would not take into account the mechanical efficiency, electrical efficiency, or power factor of the motor.
So to find the right number to use for calculation purposes, I would start with Table 430.250. A motor rated at 1.5 HP would draw 3 amps at 460 volts. Multiply 3 amps times 460 volts times 1.732 (the square root of three, a term that comes into play for three phase motors), and you get 2390 VA. That is the load for your motor, for the purposes of a load calculation.
Please note that the largest motor must be accounted for by taking 125% of its VA load. So if this is the largest motor, you need to add another 598 VA.
First, please turn off the ALL CAPS feature. It makes the post more difficult to read. It is also considered impolite, as though you were SHOUTING at us.
"Horsepower" is a unit of power, and so it watts. Converting HP to Watts is similar to converting feet to inches. One foot is equal to 12 inches. 746 watts is equal to one HP.
However, a simple "unit conversion" would not take into account the mechanical efficiency, electrical efficiency, or power factor of the motor.
So to find the right number to use for calculation purposes, I would start with Table 430.250. A motor rated at 1.5 HP would draw 3 amps at 460 volts. Multiply 3 amps times 460 volts times 1.732 (the square root of three, a term that comes into play for three phase motors), and you get 2390 VA. That is the load for your motor, for the purposes of a load calculation.
Please note that the largest motor must be accounted for by taking 125% of its VA load. So if this is the largest motor, you need to add another 598 VA.
john m. caloggero
Senior Member
Re: MOTOR WATTAGE
The horsepower rating of a motor is the actual power the motor can deliver at the shaft. One HP is 746 watts. Therefore, 1.5 HP will use 746 x 1.5 = 1,119 watts plus electrical and mechanical loses.
The horsepower rating of a motor is the actual power the motor can deliver at the shaft. One HP is 746 watts. Therefore, 1.5 HP will use 746 x 1.5 = 1,119 watts plus electrical and mechanical loses.
P
Physis 2
Guest
Re: MOTOR WATTAGE
Let me add a question.
Why do the NEC tables usually come up with values that imply somewhere around twice 746 per HP?
I, not working with motors a lot, have been disturbed by this for some time.
Let me add a question.
Why do the NEC tables usually come up with values that imply somewhere around twice 746 per HP?
I, not working with motors a lot, have been disturbed by this for some time.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: MOTOR WATTAGE
Motors are rated in mechanical OUTPUT power not electical INPUT power.
The values in the NEC are worst case possible inputs(in fact no NEMA motor should ever have these values), so that an old motor can be exchanged with a new one without having to change the conductors.
Motors are rated in mechanical OUTPUT power not electical INPUT power.
The values in the NEC are worst case possible inputs(in fact no NEMA motor should ever have these values), so that an old motor can be exchanged with a new one without having to change the conductors.
P
Physis 2
Guest
Re: MOTOR WATTAGE
Is it partly because voltage drop could increase current? Is the efficiency of older motors, like, way the hell out there?
Is it partly because voltage drop could increase current? Is the efficiency of older motors, like, way the hell out there?
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Re: MOTOR WATTAGE
Then Sam we have the other side of the spectrum that manufactures use that says there air compressor is a 5hp but it runs fine on a 15 amp circuit. I have just never found the formula they use to get this?
Then Sam we have the other side of the spectrum that manufactures use that says there air compressor is a 5hp but it runs fine on a 15 amp circuit. I have just never found the formula they use to get this?
P
Physis 2
Guest
Re: MOTOR WATTAGE
Soooooooo, then I'm not the only guy who's noticed this?
Soooooooo, then I'm not the only guy who's noticed this?
Re: MOTOR WATTAGE
factory rep explained to me that at 0 pounds of pressure, that fabulous compressor motor is actually putting out 5 hp of work,,,
as the pressure builds the hp delivered decreases... good sales pitch anywho..
does lead to a suprise the 1st time a customer asks you wire for a 5hp (1 ph) compressor and then rolls in with a critter with a 20 amp plug on it.
factory rep explained to me that at 0 pounds of pressure, that fabulous compressor motor is actually putting out 5 hp of work,,,as the pressure builds the hp delivered decreases... good sales pitch anywho..
does lead to a suprise the 1st time a customer asks you wire for a 5hp (1 ph) compressor and then rolls in with a critter with a 20 amp plug on it.
Re: MOTOR WATTAGE
This very topic was the recent focus of a lawsuit - details at:
http://www.aircompressorsettlement.com/faq.php3
This very topic was the recent focus of a lawsuit - details at:
http://www.aircompressorsettlement.com/faq.php3
Re: MOTOR WATTAGE
I thought CH was a bit more professional than that.
Augie, at zero pressure, power is zero. At full pressure, flow is zero, and power is zero. The max falls somewhere between. Of course you knew that.Originally posted by augie47:
as the pressure builds the hp delivered decreases... good sales pitch anywho..
I thought CH was a bit more professional than that.
Re: MOTOR WATTAGE
"Augie, at zero pressure, power is zero"
REPLY
I agree if the motor is stopped. BUT at startup at 0 psi on compressor discharge you are telling me the motor and compressor are running for free.
Send me a bunch of those please.
At full pressure, flow is zero, and power is zero.
REPLY true if the motor is stopped.
The max falls somewhere between.
REPLY Maximum run current is at max output pressure.
FOR EXAMPLE Ingersoll Rand 2 HP (if I recall correct) twin cylinder
5 amps at 0 psi
12 amps at 140 psi.
"Augie, at zero pressure, power is zero"
REPLY
I agree if the motor is stopped. BUT at startup at 0 psi on compressor discharge you are telling me the motor and compressor are running for free.
Send me a bunch of those please.
At full pressure, flow is zero, and power is zero.
REPLY true if the motor is stopped.
The max falls somewhere between.
REPLY Maximum run current is at max output pressure.
FOR EXAMPLE Ingersoll Rand 2 HP (if I recall correct) twin cylinder
5 amps at 0 psi
12 amps at 140 psi.
Re: MOTOR WATTAGE
Dan, Let me qualify my statements. I am assuming no flow at maximum pressure. In an ideal situation, the reed valves would not open, then you would deliver no power.
The electrical analog is high current, 0 volts and no current, full voltage. No power in either case.
I am also neglecting any losses, frictional or otherwise.
All that aside though, you have demonstrated that max power is not delivered at low pressure.
Dan, Let me qualify my statements. I am assuming no flow at maximum pressure. In an ideal situation, the reed valves would not open, then you would deliver no power.
The electrical analog is high current, 0 volts and no current, full voltage. No power in either case.
I am also neglecting any losses, frictional or otherwise.
All that aside though, you have demonstrated that max power is not delivered at low pressure.
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