Locked-Rotor Current Purpose

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bphgravity

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What is the purpose of knowing locked-rotor current values of a motor?

For example: Lets say I have a 25 hp, 460V code A motor.

25 x 3.14 = 78.5 x 1000 / 460 / 1.732 = 98.5 amperes of locked-rotor current.

What does this tell me? Say I am using a typical inverse-time breaker. FLC is 34A x 250% = 85A breaker. Does this mean that this breaker may not hold on start-up?
 
Re: Locked-Rotor Current Purpose

This is all I could find:" Locked Rotor Current, LRC is the steady-state current taken from the power line with the rotor locked (stopped) and with the voltage applied. LRC and the resulting tourqe produced in the motor shaft ( in addition to load requirements ) determine whether the motor can be connected across the line or whether the current has to be reduced through a reduced-voltage starter".

[ January 26, 2006, 04:16 PM: Message edited by: Jhr ]
 
Re: Locked-Rotor Current Purpose

Locked rotor conditions imply high currents. My local prohibits motors from use in series calculations, and specifically asks that 'largest motor LRA / continuous A' be included on plans, as in state the compressor LRA for an HVAC installation. Their standard is that it be at least 3.5 times , to insure that the breaker would trip.
 
Re: Locked-Rotor Current Purpose

The current drawn by an induction motor is a function of the rotor speed. The applied voltage creates a rotating magnetic field. When the rotor is rotating at the same speed as this magnetic field, the input current is minimum.

When the rotor is stopped, the input current is a maximum. This is because the turning rotor produces a back EMF that reduces the input current. If the rotor is not turning, there is no back EMF, and the input current is the highest. This is refered to as the "locked rotor current" because it is the same current that would be drawn if the rotor were locked in place so it can't turn.

When a motor is first started, the rotor is stopped, so it draws the locked rotor current. The motor quickly starts turning, and the rotor current decreases. The current keeps droping until the rotor gets up to full speed. At that point the current is at minimum. (Full speed is slightly less than the speed of the rotating magnetic field, and depends on how much torque the motor must produce to turn the load).

In most cases, I think sizing the breaker at 250% of rated current is enough to prevent tripping on startup. It might not be enough if the motor load has a large amount of inertia, or if the motor has a particularly large locked rotor current.

Steve

[ January 26, 2006, 04:21 PM: Message edited by: steve66 ]
 
Re: Locked-Rotor Current Purpose

Hi Bryan :

let me ask you this question to make sure i understood this one,,

25 x 3.14 = 78.5 x 1000 / 460 / 1.732 = 98.5 amperes of locked-rotor current.
Click to expand...
correct me if i am wrong here 25 is HP motor rating par {times} 3.14 = ?? that what i try to figure out amp per Horsepower ? = 78.5 amps ? X 1000 / 460 volts/1.732 sq root of 3 = 98.5 Locked rotor current.

i did came up some figures i am not sure if i am reading the hp figures right but looking at my chart for motor hp rating for 25 hp running amp at 460 system 34 amps unless you got the figures for 230 system which it will show up 68 amps running amps.

that only question i try to figure out how you came up with this one.

Thanks for your time

Merci, Marc
 
Re: Locked-Rotor Current Purpose

You compare the LRA of a motor to the time current curve for a protective device to see if the motor will be allowed to start without causing a nusiance trip.

It should not be compared to the continuous current rating of an overcurrent device. It can be compared to the instantaneous rating of the OCPD.
 
Re: Locked-Rotor Current Purpose

Locked Rotor Current is a good approximation of the short circuit current that a motor will add to a short circuit on the power system. When a short occurs, the motor acts like an induction generator and delivers current to the fault until the energy stored in the motor's magnetic field and spinning rotor is dissipated. The current does not last long, but it must be accounted for in short circuit calculations.

Knowing a motor's locked rotor amps (LRA) and acceleration time helps in selecting upstream breakers. We make sure the circuit breaker or fuse can supply the locked rotor amps long enough to get the motor load up to speed and that it will trip if locked rotor amps continues beyond the allowable stall time.
 
Re: Locked-Rotor Current Purpose

i did came up some figures i am not sure if i am reading the hp figures right but looking at my chart for motor hp rating for 25 hp running amp at 460 system 34 amps unless you got the figures for 230 system which it will show up 68 amps running amps.
Click to expand...
Marc, for a 480V 3Phase 25 HP motor, my handy calculator gives a LRC of 186.29 Amps

For the same motor but using the FLA 34amps my handy calculator givec a LRC of 204 Amps. :cool: :cool:
 
Re: Locked-Rotor Current Purpose

You also need it when sizing generators for motor starting. Actually, you really don't NEED the LRA but instead there are rules of thumb you use to find starting kVA. But, in theory, if you know the LRA then you can calculate the actual sKVA. However the rules of thumb work out almost exactly.
 
Re: Locked-Rotor Current Purpose

Jhr - what's the formula for LRC? I've seen questions about LRC on tests - and if I remember right, it was asked pretty generally. Like - "How many times is LRC greater than FLA?" I seem to remember hearing that you can "rough" approximate it to six (6) times - but I'd really like to know the formula - as you mention your calculator is coming up with some values.

Thanks

Brett
 
Re: Locked-Rotor Current Purpose

One important point, do not believe the LRA is the maximum current the motor could draw.

It really represent the lock rotor current for the minimum spec'd operating voltage and is primarily for insuring the breaker does not exceed this value.

I have a 3.5 ton AC compressor with a 104A LRA spec. It also specs run voltage from 208 v to 250 v. At 240 vac it draws 120 amps rms at startup for 0.3 seconds. Its run current is 15.5 amps and is on a 40 A breaker.

For induction motors a lock rotor is like putting a short circuit on the secondary of a transformer.
 
Re: Locked-Rotor Current Purpose

Brett:

There is a table in the NEC for the range of LRA. That might be what the tests were refering to.

Steve
 
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