The locked rotor current is a function of the motor design, and doesn't depend on the load. Changing the load characteristics will quite probably modify the acceleration time, and thus change the period for which the current is high (approaching LRC).
Starting the motor with a pump attached and 50% discharge will not affect the instantaneous power at any point in the start if the motor is started direct on line (Full Voltage start). The variation in shaft load will change the accelleration torque available and will therefore affect the total starting time. Therefore, the total power consumed during start will be affected, same power but for a longer time will result in an increased total.
If you are looking at this from a point of supply/generator sizing, then the instantaneous power is independent of shaft load, the start duration is a function of start load and inertia.
If you use a reduced voltage starter to minimise the starting current, then the pump loading determines the lowest start current that can be achieved for that given motor. This will influence the sizing of supply or generator.
Best regards,
Suggestion: It is not posted in the original posting how is the pump being started. The 50% discharge may lead to the lower motor starting current if the motor is softly starter.
Starting characteristics are VERY MUCH dependent of the discharge opening of the Centrifugal pumps/blowers. Lower the discharge opening shorter will be the acceleration time. While in theory, the momentary kick in the current is independent of the load, what really matters it how long the high starting current is sustained before the motor comes to near full speed (acceleration time).
I believe jb is wrong: the starting current does not change with load regardless of start method. It is fixed at approximately locked rotor current.
I don't agree with rbulsura's characterization of starting current as momentary. Duration will vary with load and certainly may be several seconds or more for high inertia loads. There is a exponentially-decaying dc component that decays within a matter of cycles and may be viewed as momentary. The sustained starting current (approximately locked rotor current) may last much longer.
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Suggestion to the previous posting marked ///\\
I believe jb is wrong: the starting current does not change with load regardless of start method. It is fixed at approximately locked rotor current.
///Visit
The load torque of a pump during starting varies from 0-25% when the inlet/ outlet valve is fully closed, varies from
0-50% when the valve is app 50% open and 0-100% when the valve is totally open.
The starting characteristiscs of the motor does not change but the starting time will increase inversely to the difference in Motor torque and the load torque.
If VFD is used the system may accelerate without much trouble as the Pullout torque occurs at all speeds during acceleration.
If Soft starter is used the system may not accelerate within a reasonable amount of time.
If the motor is started on DOL the starting current 0f > 3 times will persists for a considerable amount of time. The motor may get overloaded & burnt. If sufficient cushion is available in the motor, the motor may start & survive.
It is ALWAYS better to close the valve/damper during starting of a pump/fan.
When the motor is first started you have zero flow, the flow from the pump will be dependent on speed, as the motor begins accelerating the pump, the flow will increase. Look at your pump curve and your motor starting torque curves, and you will see that the bhp requirement of the pump is dependent on speed. The motor will be able to provide 200-300% (or more)full load torque at start up and increase till it reaches the break over torque. If the motor is sized right, starting a centrifigul pump should never be a problem for the motor. We regularly start our centrifigul pumps will the valves fully opened, (of course we have a check valve in line to prevent backflow through the pump while it is off.) You also may need vacuum priming to insure you are pumping fluid and not air, and running a centrifigul with no flow is a no no.
You stated " The motor will be able to provide 200-300% (or more)full load torque "
What kind of motor? In USA the most common motor for pumps is a Design B in 2 or 4 poles, 10 HP motors and larger barely reach a locked rotor torque of 150%Tfl, 1 Hp, 4 poles has 175%Tfl locked rotor torque.
Comment: Wound Rotor Induction Motor, Squirrel-Cage Induction Motors Nema Design Letters C and D are capable of producing 200% to 300% of full-load torque at their start-ups.
jb - I stand by my previous comment. None of your links support your incorrect contention that loading affects starting current (or inrush current).
I agree with aolalde on the subject of starting current. Unless otherwise stated we should assume a centrifugal pump/fan will be driven by a NEMA design B squirrel cage motor. At 10hp per NEMA minimum locked rotor torque is 165% of full-load torque for 4-pole motor, even lower for other speed motors.
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Comment on the previous posting: Link no. 3 in my previous posting addresses impact of the soft starters on the motor inrush currents. Perhaps, an elaboration on the link content will clarify my posting better, namely:
"""Soft starters are ideal for meeting utility restrictions for limiting the inrush current while starting high horse
power motors. It provides a multitude of benefits for process & machine applications due to their smooth
output voltage ramp, torque limiting & overriding adjustable current limit feature. Typical motor loads that
are good candidates for soft starters include centrifugal pumps, fans, batch centrifuges, unloaded rock
crusher, unloaded compressors & lightly loaded compressors.
Most soft starters use voltage control to limit the motor starting current & torque by continuously ramping
the applied voltage when starting & stopping."""
Another situation exists when the motor is excessively loaded and remains in the locked-rotor condition. Then the inrush current and motor starting current become equal to the motor locked-rotor current.
Also, the current into L-R circuit exponentially rises until it levels off on V/R asymptote.
Visit
etc. for more info. There is not anything very difficult in the motor circuitry. It is essentially a combination of resistors and inductors (for lower frequencies); especially, when it comes to the locked-rotor conditions.
