apprentice question

Status
Not open for further replies.
D

ddubbs103

Member
Location
Massachusetts
Just wondering if anyone has an easy way to explain to a 2nd yr apprentice the theory behind why a 3phase motor does not need a neutral and single phase 2 pole (electric hot water heater) also. I was having a hard time explaining it to him. Any advice would be appreciated thanks.
 
I usually draw the schematic version of the coils and how connected and the transformer feeding it as well...

Like a wye system.....

wye_detail1.gif


Feeding a wye motor....
3ym01.gif


Same for delta, delta wye, etc.
 
Posted too soon - but anyway... I usually explain the point to point voltages of say a wye system i.e. 3 legs at 120 - phase to phase at 208, then describe the motor using only that balanced 208 phase to phase, and get them to answer what voltage if measured to ground will be in the middle of the wye drawing for the motor, and try to understand why...
 
ddubbs103 said:
Just wondering if anyone has an easy way to explain to a 2nd yr apprentice the theory behind why a 3phase motor does not need a neutral and single phase 2 pole (electric hot water heater) also. I was having a hard time explaining it to him. Any advice would be appreciated thanks.
Click to expand...
My understanding is that because the 3 windings of the motor are the same (identical) resistance so it's balanced, which means there is no current that needs a path back to the source. Same with the water heater. 2 resistive elements with same resistance so it's balanced and has no current looking to get back to source.
 
steelersman said:
.... Same with the water heater. 2 resistive elements with same resistance so it's balanced and has no current looking to get back to source.
Click to expand...

Not quite. There's two of them, yes, but the stat keeps them from operating at the same time. It's either one or the other that's on. Both are rated 240volts.
 
480sparky said:
Not quite. There's two of them, yes, but the stat keeps them from operating at the same time. It's either one or the other that's on. Both are rated 240volts.
Click to expand...
well ok then. So I was on the right track or I at least had the theory down. :)
 
steelersman said:
well ok then. So I was on the right track or I at least had the theory down. :)
Click to expand...

Yep... correct on the theory.... take two 120v incandescent bulbs of the same wattage, connect them in series, and apply 240v.... they will both light as usual.
 
480sparky said:
Yep... correct on the theory.... take two 120v incandescent bulbs of the same wattage, connect them in series, and apply 240v.... they will both light as usual.
Click to expand...
Then try it with a 60w and a 75w right next to them with a DMM in hand....
 
How about for the three phase, set three 9v batteries on a table with the connections up. Then arrange them in a triagle. Now pretend each battery is a phase. Nothing is needed in the center to get power.

For the water heater, set two 9v batteries side by side with a + next to a -.
Connect the + and -. Measure from the outside connectors and you get 18 volts. Measure from either outside connection to the inside center/"neutral" connections and you get 9 volts. Our fictional water heater needs 18 volts! No neutral connection needed.
 
ce2two

ce2two

check with an o'scope 20MHZ will do, 3 phase ,each phase is 120 degrees out of phase with respect to one an other ,i forgot what single phase is with respect to a and b phase (phase angle) i believe it' s 180 ??/???? i JUST LEARN SOME MUCH FROM THIS FORUM ?><XLNT ADVICE !!!!!!!!!!!!!!!!
 
480sparky said:
So they can read the voltage at each bulb. Then maybe they'll learn something about Ohms' Law.
Click to expand...
Pretty much... Because checking voltage between two 60w bulbs will offer an interesting result, and so would like wise checking the measured voltage between a 60w and a 75w to ground would be equally interesting.... Hey - if you're doing it in a building with a wye system - use 3 lamps... All equal, then all un-equal wattages....
 
ce2two said:
check with an o'scope 20MHZ will do, 3 phase ,each phase is 120 degrees out of phase with respect to one an other ,i forgot what single phase is with respect to a and b phase (phase angle) i believe it' s 180 ??/???? i JUST LEARN SOME MUCH FROM THIS FORUM ?><XLNT ADVICE !!!!!!!!!!!!!!!!
Click to expand...

180? ????
 
.... take two 120v incandescent bulbs of the same wattage, connect them in series, and apply 240v.... they will both light as usual.
__________________

That was what the old-timer I worked with 25 years ago used for a tester--put them across 120V and they lit dimly, across a 240V load they looked normal. Of course, you couldn't test much else...

(Not that these Old-Timer confections are all that great--Remember the guy in the World Trade Center in the 90's who had a home-made circuit finder consisting of a switch mounted in a handi-box with a length of so cord and plug ? When he plugged it in and flipped the switch it caused a direct short, thus "finding" the circuit ! He managed to shut down the computers on the floor he was working for half a day...)
 
one of these days we that remember the World Trade Center will be deemed to be old timers,.. My heart aches when I consider the loss ... Hopes, dreams , families,. forever changed .
 
Status
Not open for further replies.
Top