# Calculating Motor Start Time To Rated Rpm

3 replies to this topic

### #1 OldGuy

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Posted 18 October 2010 - 05:00 PM

Hello folks,

I am new to the forum. I am trying to figure out how to calculate the time to rated RPM on a motor start for a DOL AC Induction Motor with a large inertial mass. Any help would be appreciated. Here is some of the information I have:

6000 Hp, 4500 KW, 4KV, 730 FLA, 6 pole, 3 Phase, 60 Hz, squirrel cage induction motor, DOL start, ~15% drop in supply voltage. Besides the rotor, pump shaft, and impellor, the motor has a 13,200 lb flywheel.

Load is a centrifugal pump that has enough suction head to not cavitate.

I have connected high resolution digital recorders for voltage, amperage, and RPM readings.

Line voltage initially drops to ~3500V at 1 sec, then recovers to ~ 4200V at 24 seconds, before fully recovering to 4186V at 25-27 seconds.
Amperage spikes, but after ~ 1 second it is stable at ~ 4900A then dropping to ~ 2800A at 24 seconds, before dropping off to FLA of 730 A at 25 - 27 seconds.

My question is whether there is some kind of mathmatical way of determining if these numbers are appropriate for this motor.

Any help would be appreciated, Thanks.

### #2 jOmega

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Posted 19 October 2010 - 03:08 PM

QUOTE (OldGuy @ Oct 18 2010, 12:00 PM) <{POST_SNAPBACK}>
Hello folks,

I am new to the forum. I am trying to figure out how to calculate the time to rated RPM on a motor start for a DOL AC Induction Motor with a large inertial mass. Any help would be appreciated. Here is some of the information I have:

6000 Hp, 4500 KW, 4KV, 730 FLA, 6 pole, 3 Phase, 60 Hz, squirrel cage induction motor, DOL start, ~15% drop in supply voltage. Besides the rotor, pump shaft, and impellor, the motor has a 13,200 lb flywheel.

Load is a centrifugal pump that has enough suction head to not cavitate.

I have connected high resolution digital recorders for voltage, amperage, and RPM readings.

Line voltage initially drops to ~3500V at 1 sec, then recovers to ~ 4200V at 24 seconds, before fully recovering to 4186V at 25-27 seconds.
Amperage spikes, but after ~ 1 second it is stable at ~ 4900A then dropping to ~ 2800A at 24 seconds, before dropping off to FLA of 730 A at 25 - 27 seconds.

My question is whether there is some kind of mathmatical way of determining if these numbers are appropriate for this motor.

Any help would be appreciated, Thanks.

.....from one 'ole guy' to another...

Yes, it is possible to calculate if you have all of the information necessary. Have a look at the following link where I've presented a methodology for calculating the starting time for a fan. The same can be done for your pump application with the addition of the requisite parametrical data.
Fan Accel Time Calc

The information you have provided, while giving us a glimpse of the application .... is insufficient for making such calculation.

Here are just a few of the datum that would additionally need to be made known:

• complete motor nameplate (rating plate) data.
• motor performance curves, incl. speed vs torque vs amperage
• motor locked rotor data
• motor inertia
• total inertia reflected to the motor shaft
• pump data - is it centrifugal or positive displacement type?
• pump curves w/system curve imposed thereon
• speed at which pump picks up pumping load
• voltage source data, including impedance and nominal loading at time of start

I'm sure there are more items to consider, but these are the ones that jump up, so to speak.

With sufficient data, the methodology shown in the above hyperlinked example for fan acceleration,
can be modified to accommodate your pump application.

There is one set of data that you provided that begs its questioning .... the voltage plot ...

It drops to 3500 then rises to 4200 before dropping to final value of 4186.
The rise to 4200 while still sourcing acceleration current is suspect when one considers that
at nominal steady state load, the voltage drops to 4186 at a much lesser value of load current.

Kind regards,

### #3 OldGuy

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Posted 19 October 2010 - 04:19 PM

Thank you for the response. I appreciate your time.

You are correct concerning the voltage data I provided. I did not do a good job proof-reading. Where is says 4200 V it should say 4000 V. The remainder is essentially correct.

You included :
• speed at which pump picks up pumping load
The system I am concerned with is a closed loop hydraulic system. When the pump is started there is no flow. Flowrate actually continues to increase for some time (up to a few minutes) after the pump is at full rated speed.

At what point would I say the pump has picked up the pumping load?

I am currently working on a separate project. It will take me a couple days to research and retrieve all the data you identified. In the interim, I will take a closer look at the link you provided and see what I come up with.

Thanks, again.

OldGuy

### #4 jOmega

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Posted 21 October 2010 - 01:02 PM

QUOTE (OldGuy @ Oct 19 2010, 10:19 AM) <{POST_SNAPBACK}>
Thank you for the response. I appreciate your time.

You included :
• speed at which pump picks up pumping load
The system I am concerned with is a closed loop hydraulic system. When the pump is started there is no flow. Flowrate actually continues to increase for some time (up to a few minutes) after the pump is at full rated speed.

At what point would I say the pump has picked up the pumping load?

Thanks, again.

OldGuy

In answer to your question, check the information/data you have for the pump; if there is a minimum operating RPM stated, then that is typically the point at which the pump picks up load.