Max Load Current Vs Frequency

[ajxxx]

Have a 15 HP motor whose full load current (@ 50 Hz - India norms) is calculated at around 21 Amps. Have added a VFD (Variable Frequency drive) and would like to always run the machine @ 90% of full load current for maximum efficiency.

Can anyone assist in helping me calculating full load current as frequency is changed? What would be the full load current @ 40 Hz, 35 Hz and 45 Hz?

Thanking everyone in advance. Appreciate the help, time and effort.

 

[jraef]

Full Load Current has to do with LOAD, not speed. Speed can effect how a motor responds to LOAD, but the only way to run a motor at 90% LOAD is to apply 90% LOAD.

You thinking about adding LOAD to increase efficiency percentage is inherently flawed by the way. You need to think about that a little motor. Try this analogy:

My car get 30MPG with one person in the car but it gets 20MPG if I have 4 people in the car. If I go alone to Grandma's house 100 miles away, that is 100 passenger miles and 3.3 gallons of gas, so 3.3 passenger miles per gallon. But if I force 3 more people into my car to go to my Grandma's house even though they don't want to or need to, I get 400 passenger miles and use 5 gallons, so I get 20 passenger miles per gallon. So my passenger MPG efficiency is greater when there are 4 people in the car correct? Does it thereby make sense to burn 1.7 more gallons of gas for no reason just because my passenger MPG efficiency is higher?

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Allowable full load current is determined by the ability of the motor to reject the I²R developed in the windings. If you add an external cooling fan the allowable full load current is mostly independent of the frequency. Without an auxiliary fan the allowable full load current is reduced as the motor cooling fan slows down with reduced frequency.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Strong]

Take the kids to Gramdma'a house!

Walt

 

[jraef]

You need to think about that a little more.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376