Another 4th year Question

here is another 4th year question that you might all be interested in getting the answer to:
In the torque equation T=HP÷rpm×5252 . Where does the 5252 come from in the equation?
Well.....
since there are 550ft-lb per second per horsepower, and 60 seconds in a minute going from HP and RPM to foot pounds will involve using the 550 and 60, multiply that out and you get 33000.  Since we are talking angular motion (RPM) and not linear motion there is a conversion going on.  Remember that circumference = pi*D?  and since D=2r we can throw 2pi in to convert our angular speed to a linear speed at a given radius (in this case 1 foot). Put 33000 in your calculator and divide by 2pi (6.28 approximately) and see what you get.

for more info see http://en.wikipedia.org/wiki/Torque down near the bottom of the page.

Greg McMurphy
Instructor
LU112-NECA Electrical Training Center
Kennewick, WA 99336

4th year motor question

A question came in as a review question that might be of interest to the whole group.

Is the correct answer to q. 5 of lesson 11 A: 1/4 hp? All I could find in the text was that other than fractional HP motors required reduced starting applied to DC motors in order to prevent motor damage. pg.244 Motors Text. Does that mean .9 hp does not require reduced starting but 1 hp and above does making option B: 1 hp the wrong answer to q. 5?

My answer is, you are right in that "other than fractional" require reduced voltage.  So since 1/4 IS fractional and all the rest are NOT, and since 1 hp is the first size that is NOT fractional, I think b. is the answer.  I assume the last sentence in the student's question is worded backwards from what was intended.


Greg McMurphy
Instructor
LU112-NECA Electrical Training Center
Kennewick, WA 99336

Fire Alarm

Hey Gang,

You will be very happy if you remember to bring orange, green, and yellow highlighters for use in class on Monday of your next week of school.  We will be codeologizing the NFPA 72 2010.  We will also be tabbing the document as well.  Hope you are all doing well.  We are looking forward to seeing you in 3 to 4 weeks!!

Greg Greiner

LU112 NECA JATC

Instructor

greg.g@jatc112.org

Test Instrument Application Manual

4th year-
Hope all is going well with you, we ran across a tidbit of information that might prove helpful and we thought we'd share.
On a general note, the Test Instrument Application Manual is a companion to the Test Instruments text book, so if you find the application manual asking you questions that you don't understand how to answer take a look in the text.  One specific example of that issue is found in the work for this week of school.  You are asked to calculate voltage unbalance in a motor circuit and then say if that number is acceptable or not.  Take a look in the text, check the index for voltage unbalance calculations, and you will get the information you need.

The two motor tests deserve your full effort and attention.   If you have questions as you prepare for school please bring them up.
Greg M