Motor starting 1

[kookypedia]

Guys,
Is there any way to tell how long it is going to take for an induction motor to start?
I know there are thermal and other curves for motors but I don't know if any of them tells the starting time of the motor ...

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[dpc]

If you are talking about how long it takes the motor to accelerate to normal running speed, then it is possible, if you have sufficient data and probably a transient motor starting analysis program. The accel time depends on the starting method, speed-torque curve of the motor and load and few other factors. You can't tell just by looking at the motor nameplate. Simplest method is to start the motor and time it.

 

[kookypedia]

Yes, I mean what is the typical way to get the acceleration time for a motor?
Is it just simulation or running it under load or data sheets or curves should have this information?

An investment in knowledge pays the best interest (Benjamin Franklin).

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[waross]

For low inertia loads such as many pumps, smaller conveyors, small fans and other low inertia loads, generally start the motor and time it.
For higher inertia loads you should use the motor torque curve and the inertia of the load to anticipate the starting time.
Motors have a limit as to the starting time.
You may have to use a reduced current starting method.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

I agree: the start time measuring is the accurate way.
However, you may calculate [approximate] the starting time if you know the rotor and the load inertia and the motor and the load torque vs. speed curves.
There are manufacturers guides how to calculate it-for instance ABB The Motor Guide [ GB 98-12/1999] ch.4.3 Starting motors. Or:
EPRI Power Plant Reference Series vol.6 Motors Ch. ACCELERATION.

 

[noside]

Try to contact the vendor, there no such enough details, its brand literature

 

[LionelHutz]

You have to simulate the start using the motor data plus the motor and load curves. Any single formulas that calculates the time in one step that I have seen can be inaccurate enough they should not be considered reliable.

The EPRI approach is actually more along the lines of how to calculate it. You could create a spreadsheet that does what they suggest and calculate the acceleration time for every 1% change of speed then add the times all together.

Entering the graphs into a specific program that calculates it would do the same without the work of creating the method.

 

[jraef]

Newton's second law, adapted to rotation: Accelerating Torque = Inertia * Angular Acceleration

The overall formula commonly used (in SI units) is:

t[sub]a[/sub] = Wk[sup]2[/sup] * Delta N / 308 * T[sub]a[/sub]
Where
t[sub]a[/sub] acceleration time (seconds);
Wk[sup]2[/sup] total connected inertia (lb * ft[sup]2[/sup]);
Delta N speed change during time t[sub]a[/sub] (rev/min);
T[sub]a[/sub] average accelerating torque (lb * ft) (average motor torque – average load torque)

The tricky part, as Bill mentioned, is getting the inertia and load torque curve data. You can get it fairly easy for the motor itself, usually the mechanical transmission components too, but getting it for the actual load can be challenging. For some machines, such as centrifugal pumps and fans, inertia at start-up is only the components themselves and the average load torque is usually something the pump manufacturer can provide because they needed that in order to select the pump. For a machine tool, it's also often fairly easy to attain this because it's consistent (as long as the tool is started unloaded). For a conveyor, hoist, or other material handling application it's going to depend on actual loading on the conveyor, which may vary greatly. Typically this would be calculated based on a "worst case scenario", for example after a power failure where the conveyor was fully loaded and dead still on re-start.


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