Is there a rule of thumb on how long a motor takes to start?

[bdn2004]

My specific question is this:

We have a 350Hp motor on a fan that we know is lightly loaded. When its running it only pulls 250 Amps. Its FLA is 385 Amps. It has a Code Letter G. Based on the calculation this is about 2750 peak amps at starting.

How many cycles does this last? Is there a relationship between the load and the starting time? Is there a formula? Is there a conservative rule of thumb?

(BTW this is not a question on a college test! I'd just like to know quickly, you guys give great answers)

 

[rbulsara]

(BTW this is not a question on a college test! I'd just like to know quickly, you guys give great answers)

Do a web search on motor starting time formula. That will be even quicker!

Rafiq Bulsara

http://www.srengineersct.com

 

[bdn2004]

I have checked on line, entire on line courses on the subject.

I'm interested in a rule of thumb for talking purposes only. Is there not one?

Such as:

A typical squirrel cage motor, fully loaded, started across the line typically takes takes _____ cycles to start, the peak lasting for typically _____ cycles.

Same thing if it were only 2/3 loaded like the one of my question.

Yes/No....?

 

[dpc]

No.

David Castor

http://www.cvoes.com

 

[rbulsara]

dpc beat me to the punch.


Rafiq Bulsara

http://www.srengineersct.com

 

[bdn2004]

just knowing that is helpful...thanks

 

[electricpete]

The very first thing required to even get in the ballpark would be estimate of inertia of the fan... there can be pretty wide variation.

The more details available the better the estimate. The info to gather would be as many of the following as you have available:
load torque-speed characteristic (or we can assume roughly prop to speed^2)
motor inertia (or we can use NEMA thumbrule)
motor torque speed characteristic (or we can try to build it... requires more info like for starters full load efficiency or power factor... also if available current/p.f./eff at half-load and current at no-load).
anything that might be useful to judge any voltage deviation at the motor terminals... like length of cable, anything unusual about power system


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(2B)+(2B)' ?

 

[bdn2004]

what does that mean ... the inertia of a fan. I looked the word up? Is this a unit of force? what are the units?

law of inertia: Newton's first law of motion states that any object which is moving will continue moving at the same speed and in the same direction until a force acts on it. Similarly, the law states that any object which is at rest (still) will remain at rest until a force acts on it. This law is called the law of inertia.

 

[electricpete]

Rotating inertia. Sometimes called J, sometimes called W*k^2 or W*R^2 and G*D^2. In linear dynamics, mass plays the role of inertia F = M*A. In rotational dynamics, rotating inertia J is relevant: Torque = J * d/dt(w) where w is radian frequency.

Units of J in the SI system are kg*m^2



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(2B)+(2B)' ?

 

[electricpete]

I should clarify, W*k^2, W*R^2 and G*D^2 may be proportional to J but not exactly equal to J. There are some unit conversions and conventions to work through. SI J is my preference.

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(2B)+(2B)' ?

 

[bdn2004]

so you are saying for this fan there will be a specific value for J that should be known?
or can be found out?

can it be measured or metered?

 

[electricpete]

Yes and yes.

It may be in the fan drawings. It will normally be something that is obtained and utilized when selecting motor and motor protection.

You can estimate it from dimensions J = Integral (rho*r^2) dV

Also if you attach a known weight to the fan (or better yet fan plus motor... with everything tagged out for safety of course) at a known radius and watch the dynamics, you may be able to deduce the inertia.

=====================================
(2B)+(2B)' ?

 

[bdn2004]

Step...
1. Find the value of J in (kg * m^2):
a) Per the mfg drawings
b) By measurement.
2. Load torque speed characteristic. - I am assuming this is a graph? - Where do I find this?

 

[rbulsara]

With manufacturer of the equipment.

Rafiq Bulsara

http://www.srengineersct.com

 

[ozmosis]

Some useful formulae:

http://www.reliance.com/mtr/flaclcmn.htm

 

[ozmosis]

Also, why not just measure it? (the time to start I mean).
There are too many variables in existing equipment that would be covered by a SF in theoretical calculations. I would say the easiest is to monitor current and when your peak drops to running current, the motor is started and you have your time.
You seem to have the benefit of the actual application in your midst rather than on a drawing board, so it could be as simple as a stopwatch or connect a scope with a time function set to go.