watt-hour meter: finding revolutions per minute?

[Bradfoxthelegend]

This is the question from the test:

If a watt-hour meter has a disk constant of 3.6 is used with 600:5 ampere current transformers and 20:1 potential transformers, the primary Kh is?

The answers are in revolutions per minute. How do I get to revolutions per minute without knowing any of the output values?

[Skyehaven40]

This is the question from the test:

If a watt-hour meter has a disk constant of 3.6 is used with 600:5 ampere current transformers and 20:1 potential transformers, the primary Kh is?

The answers are in revolutions per minute. How do I get to revolutions per minute without knowing any of the output values?

=3.6*(600/5)*(20/1)
=8640

[jflan13]

=3.6*(600/5)*(20/1)
=8640

I have been looking for a way to answer that question for quite a bit. So simple and straight forward, Thank you

[mcmahanrf]

Simplify your current and voltage ratios:

Current 600:5 = 120:1
Voltage 20:1 = 20:1

Now assume a supply voltage and current, pick something that makes your math easy

Voltage:200V
Current:120A

Now calculate the watts for those current and voltage values for both sides of the instrument transformers:

Primary: 200V * 120A = 24,000 Watts*hr
Secondary: (200/20)V * (120/120)A = 10 Watts*hr

Now using your secondary disk constant that was given (3.6, this is the number of watt*hr per revolution of the disk) we can determine the number of turns per hr:

(10 watts*hr)/(3.6 watt*hr/turn) = 2.77777 turns (in one hour)

Now working backwards with that turn rate we can find the primary Kh:

(24,000 watts*hr)/(2.77777 turns) = 8640 watt*hr/turn or 8640 Kh



I have been looking for a way to answer that question for quite a bit. So simple and straight forward, Thank you