Where is the LRC used in a circuit?

ritelec

ritelec

Senior Member
Location
Jersey
Hi.
A single phase 250v motor has a code letter G. (5.6-6.29).
The motor is 2HP with a FLA of 8.3 amps.

At 2 x 6.29 x 1000 / 240 = 52.42 LRC

The starter will be a nema O rated 18 amps.
The circuit will be 12 amps (as per table 430.248 2HP) x 1.25 = 15 amps ( 15 or 20 amp circuit )

The starter is based on the HP and FLA of the motor, the breaker is based on the FLA.

Where does the LRC come into play and where is it used in the circuit lay out?

Thank you.
 
Locked Rotor Current is also called inrush current, the short duration maximum current that the motor will normally draw when it starts. The breaker, starter and wiring should be designed to handle this short duration current.

-Hal
 
hbiss said:
Locked Rotor Current is also called inrush current, the short duration maximum current that the motor will normally draw when it starts. The breaker, starter and wiring should be designed to handle this short duration current.

-Hal
Click to expand...
I always thought LRC, is just that. The rotor is stuck or locked and will not turn.

Inrush is when everything is in normal operation condition, at startup. It will be higher than LRC. The typical amp clamp will not catch that peak current.
 
Agreed, inrush current and locked rotor current are two different things, but in normal operation seen at the same time.

Locked rotor current is the steady state current flowing into a motor with a stationary rotor.

A normal across the line started motor starts with a stationary rotor, so when the motor starts you basically see the locked rotor impedance as a transient value, and you see current flows that are basically LRC as altered by transient effects.

The LRC is about 6x FLC (big range on that number), and decays away with motor acceleration.

On top of this you have inrush current. This is transient current related to magnetic saturation of the motor magnetic circuit (steel) when initially energized. This is pretty much transformer inrush ( a motor is a transformer where some of the coils rotate). This can be an even greater multiplier than LRC, but is very short duration.
 
Ok. Thank you. So that would be referring to the 10 or 20 kva interrupting rating of a breaker and I’m assuming the starter will also have this rating labeled on it?
 
Thank you.
My original post mentions the LRC at 52.5 amps.

A 15 breaker “may handle” 90 amps.

When they say the brk and starter need to handle the LRC shouldn’t there be a label saying “it will” handle 90 amps and not a guess of “it may”.

It sounds pretty specific that the want the device to be rated but there’s no nameplate rating on the device ???
 
ritelec said:
Thank you.
My original post mentions the LRC at 52.5 amps.

A 15 breaker “may handle” 90 amps.

When they say the brk and starter need to handle the LRC shouldn’t there be a label saying “it will” handle 90 amps and not a guess of “it may”.

It sounds pretty specific that the want the device to be rated but there’s no nameplate rating on the device ???
Click to expand...
Too much information to put on a label. But the information is found in the data for the breaker in question, and you can look that up.

For example this tells you how quickly an Eaton 'quicklag' breaker will trip:

A 15A 3 pole breaker carrying 90A will trip in something between 1 and 7 seconds.

-Jonathan
 
You must log in or register to reply here.
Top