Fluke 360 AC Application Note

Fluke Clamp Meters Application Note - 360 AC.
Download
Page  of 3
4.9, 1927 votes
background image

Application Note

F r o m   t h e   F l u k e   D i g i t a l   L i b r a r y   @   w w w . f l u k e . c o m / l i b r a r y

“Let’s just oversize the motor and we can run it lightly loaded—that 
will save us some money and be easier on the motor.” This is a false 
belief among some who select and install motors. Properly sizing 
motors for a given load results in driving loads more efficiently, saving 
energy, and saving dollars. Motors typically are most efficient when 
they are 90 % to 95 % loaded. Just because a motor says “25-hp” 
on the nameplate does not mean the motor is producing 25 horse-
power as it operates. The motor may be producing quite a bit less 
depending on the load requirements. If the motor constantly runs at 
these reduced horsepower requirements money is being wasted and 
you should consider replacing it with a correctly sized motor. 

Additionally, you need to base 

the size of conductors and the 

fuses or circuit breaker supply-

ing that motor upon the full load 

current rating of the motor, how 

often it is expected to oper-

ate, and other factors. Installing 

larger conductors and break-

ers than needed is a wasted 

expense. It is also important to 

realize that even at low horse-

power requirements a motor still 

draws a relatively large amount 

of current. For example, a motor 

operating under no load at all 

still draws about 50 % of its 

rated current. 

When you replace a 

motor, match the motor 

to the job
When replacing motors it is 

important to match the motor 

to the job. In addition to select-

ing the proper voltage, phase 

(three phase or single phase), 

design letter, and code letter, be 

sure to select the proper horse-

power rating. If the motor has 

been replaced previously, or is 

running a pump, fan, or other 

equipment that was not sized as 

part of an entire system by the 

original equipment manufacturer 

(OEM), you may not be select-

ing the right size motor. Taking 

some basic voltage and current 

readings to estimate your own 

horsepower requirements will 

provide you with a more efficient 

system. 

Such information is valuable 

when conducting an energy 

study. If the motor load varies 

90 % or less from full load for 

extensive amounts of time, the 

application may be right for a 

variable speed drive and, thus, 

significant savings. For instance, 

if motor horsepower require-

ments can be reduced with a 

variable speed drive to decrease 

motor speed to 90 % of the 

motor’s full rated speed, then 

energy consumption is reduced 

to 73 % of what is required 

for full-speed operation. Just 

another reason to know the load 

requirements of your equipment. 

In some cases the motor may 

be overloaded, drawing more 

than its rated current. Whether 

it is bad bearings, a misaligned 

shaft or other maintenance 

issues, or just excessive load on 

By Randy Barnett

Determining load horse-

power, wiring, and 

breaker size for safe and 

efficient installations

Run a Fluke iFlex™ Fluke Current Probe around a single conductor. Or, 

you can center the jaws of a clamp meter around a single conductor.

the motor, one detrimental effect 

is occurring for sure: excessive 

heat is being produced in the 

windings. Heat deteriorates insu-

lation and is the leading cause 

of motor failure. While properly 

sized and installed overloads will 

trip the motor at typically 115 % 

to 125 % of the full load current 

value on the nameplate, the heat 

developed during this time is 

sure to shorten motor life.

0.575050s