Why is it almost all 3-phase formulas I see for HP (horsepower) give the 746Watts equivalency or equated value when the applied voltage for that value can always vary? (E.g. 110V, 230V, 440V, 480V, etc...) Or is it synonymous to all voltages and varied in amperage accordingly?
Motor load calculations
- Thread starter ebinguam
- Start date
- Status
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I don't believe voltage has anything to do with Hp and watts. The amperage may change when the voltage changes but watts and hp remain the same
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Both HP and watts are measures of power. Electrical watts are equal to the product of voltage and current (for a power factor of 1).
Unless there is some reason about the motor design that changes the power factor, the design current will decrease as the design voltage increases.
Just as you suspected might be the case.
Unless there is some reason about the motor design that changes the power factor, the design current will decrease as the design voltage increases.
Just as you suspected might be the case.
kwired
Electron manager
- Location
- NE Nebraska
Watts is watts, divide by voltage and you get amps. If you know you need a 10 horsepower motor to drive a specific load it still takes same watts to drive that load regardless of voltage.
What you are missing if you go with watts only though is extra current that may be (will be) present because of efficiency and power factor.
That is part of why NEC wants us to use the tables at the end of art 430 to select motor current for motor calculations, those values in the table factor in for the worst case scenario you would ever find for efficiency and power factor and will always yield a sufficiently sized supply conductor, other then situations where voltage drop needs consideration.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
Nicely said.:thumbsup:
HP='Horse' power is mechanical power, whereas 'watt" is electrical power. The motor mechanical power output is related to its electrical power input through its efficiency of conversion of electrical power input to mechanical power output:
Mechanical power output=Electrical power input*Efficiency
But Electrical power (input) is a function of voltage. So it appears that motor mechanical output may be changed by changing the voltage with efficiency kept constant.
However if, for a given motor, you increase the voltage beyond its rating to have higher mechanical power output, there is risk of insulation damage. Hence the need for an appropriately designed motor with same efficiency but with higher voltage and mechanical output!
Mechanical power output=Electrical power input*Efficiency
But Electrical power (input) is a function of voltage. So it appears that motor mechanical output may be changed by changing the voltage with efficiency kept constant.
However if, for a given motor, you increase the voltage beyond its rating to have higher mechanical power output, there is risk of insulation damage. Hence the need for an appropriately designed motor with same efficiency but with higher voltage and mechanical output!
Last edited:
kwired
Electron manager
- Location
- NE Nebraska
Power is still power. Horsepower and watts are just part of two different units of measures standards and you can convert directly from one to another. (I believe there is more then one standard that uses horsepower as a unit, but one hp in one standard may not be one hp in another standard.) Just like you can directly convert from inches to meters in linear measurements.
Typical North American made electric motors are rated in horsepower - that rating is for the output shaft, not the input power.
The input power is almost never rated in watts but rather volts and amps. - from that one can easily calculate VA input, but one still needs to know efficiency and power factor to be able calculate input watts.
Also keep in mind all those nameplate ratings are for when you are driving rated full output load, and when input voltage is at rated nameplate voltage and frequency.
JoeStillman
Senior Member
- Location
- West Chester, PA
Converting HP to Watts is just like converting inches to meters or gallons to liters. They are all just different units of measure.
Sent from my SM-G900V using Tapatalk
Sent from my SM-G900V using Tapatalk
Unfortunately, I am still not able to transfer the enlightenment I received in this topic as above to clear any misunderstanding as below.:happysad: ..........
JoeStillman
Senior Member
- Location
- West Chester, PA
For engineering calculations, we always use the "nominal" HP, Watts, Amps, etc. Of course they are all interdependent, but for power system designs, what we really want to know is "How bad can it be?".
If the design task is choosing a motor, you can't count on being able to get more than nominal HP to the shaft unless you are using a VFD. An even then, there is risk of overheating the motor. It's best to pick a motor with a higher nominal HP than required by the driven machinery. The motor doesn't produce constant HP. It delivers just enough to drive the machine, until you meet its nameplate rating. Above that, slip increases and torque decreases until it stalls.
If the design task is choosing a motor, you can't count on being able to get more than nominal HP to the shaft unless you are using a VFD. An even then, there is risk of overheating the motor. It's best to pick a motor with a higher nominal HP than required by the driven machinery. The motor doesn't produce constant HP. It delivers just enough to drive the machine, until you meet its nameplate rating. Above that, slip increases and torque decreases until it stalls.
It is not that my intention is to derive more mechanical output from a given motor by increasing the voltage but trying to keep other parameters such as efficiency, power factor and current for the same motor as constants. But a general concept of motor for its mechanical output relationship with voltage increase, when its other parameters such as efficiency, power factor and current are kept constant.
kwired
Electron manager
- Location
- NE Nebraska
You don't change efficiency or power factor without redesigning the motor.
Current varies depending on input voltage and output load. Efficiency and power factor will also change when voltage and load changes. Efficiency and power factor marked on motor nameplate are only valid if nameplate voltage is applied and rated full load is being output.
Good design usually means the motor is at least next standardized size larger then what the driven load needs for input power, and you will seldom have a motor that is loaded exactly to it's output rating.
You also don't want to change input voltage other then to stay within about 10% of the nameplate voltage, or you will invite other problems. Exception is to change voltage and frequency in same proportion, but output power will also change accordingly.
I've kept that idea for a long time and guess what? Changes in the operations caught up with motor size selection and always demand upsizing from a carefully designed motor capacity! Take the case of a pumping service where the pump operating point shifts somewhere near the vicinity of the pump's end of curve and require a larger hp!
Methinks, a bigger motor drive is desirable if you consider expansion as inevitable in the very near future.
Exactly.
The question is if it is possible to change the output of a conceptual motor by changing its voltage, keeping other parameters as constants .
Last edited:
kwired
Electron manager
- Location
- NE Nebraska
But efficiency and power factor do vary if the voltage varies as well if the driven load varies. Nameplate values only are valid if the input voltage,frequency, and output load match what is on the nameplate.
460 volt rated motor is going to have slightly different current, efficiency, and power factor if actual input voltage is 495, but still takes same amount of power to drive same load.
- Status