for a 3 phase motor Answere the questions. 4. What is the HP rating ? a. What is
ID: 2990900 • Letter: F
Question
for a 3 phase motor Answere the questions.
4. What is the HP rating ?
a. What is horsepower?
b. Why are motors rated in horsepower?
c. What determines whether a motor will develop its full rated horsepower?
d. When sizing a motor for an application should I choose a higher HP than needed?
5. What is the voltage rating ?
a. What is the significance of dual voltage rating if you have one?
b. What would happen to torque, horsepower, speed, current, efficiency, power factor, motor life
expectancy, etc if a higher or lower voltage were applied?
c. Why does a voltage change affect these variables?
Explanation / Answer
4. What is the HP rating ?
Motors are rated in horsepower (HP) or fractions of horsepower (1/4, 1/3, 1/2, etc.) MThe term was adopted in the late 18th century by Scottish engineer James Watt to compare the output of steam engines with the power of draft horses.Still we are continuing his convention
a. What is horsepower?
echanically, one horsepower (1HP) is equal to 33,000 pounds being moved 1 foot in 1 minute (or 33,000ft-lb/min). One horsepower (1HP) is also equal to 746 watts of electrical power.
b. Why are motors rated in horsepower?
HP is non-SI units of power,
1 HP = 746 Watts.OR 1HP x 0.746 = kW
It was originally defined to allow the output of steam
engines to be measured and compared with the power output
of draft horses.
Practicaly all system measurements converts into Electrical
System for simplicity and easy valuable results
c. What determines whether a motor will develop its full rated horsepower?
Depends upon the input voltage and current.The voltage and current should be according to the rating given.Also the mechanical load should be more or equal to the rated power.Otherwise motor won't be in full load.
d. When sizing a motor for an application should I choose a higher HP than needed?
Yes,You should.During starting the torque required will be very high due to inertia and motor won't be able to develop that much high torque during intital stage.Hence a safety factor should be included and the motor power should be more than what is required.
5. What is the voltage rating ?
At that particular voltage,the motor will be able to deliver the rated torque at required speed.It is assumed that the current requirement is fulfilled
a. What is the significance of dual voltage rating if you have one?
There are two main electrical power network in the world.110V and 220 V,.Some motors are universal which can work on both the systems.It is achieved by different tapping of windings in the motor
b. What would happen to torque, horsepower, speed, current, efficiency, power factor, motor life
expectancy, etc if a higher or lower voltage were applied?
torque,=Less torque
horsepower,=Less power=Vi
speed,=It will be a function of speed torque characteristics of load.Normally less torque leads to less speed
current,=Also decreases in most cases.As the I=V/R.But in some constant torque cases,it increases to match the required power
efficiency,=Decreases,optimum efficiecy is achieved at rated voltage
power factor, =decreases,more inductuve effects
motor life expectancy=decreases drastically
c. Why does a voltage change affect these variables?
In the equivalent motor diagaram,it can be represnted as rotating transformer.So flux actually dpends upon the voltage.All other things depends upon the flux.Hence it affacts these variables.Also the current and power factor is affected by equivalent electric network
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