Acceleration time 3

[esvhv]

Hello All

I have bit of a problem

I have two torque speed curves, one showing the motor torque the other load torque. But I need to find the Acceleration time; I have the true torque values along each line in n-m and the speed in rad/s. In addition I have the kW rating of the motor. But I do not have any other data.

Is it possible to find the Acceleration time, or an approximation for it.

Help would be much appreciated

 

[jbartos]

Suggestion: For the motor:
1. HP induction is known
2. Motor r/min is know
3. Motor Wk2
4. Load Wk2
5. graph of motor and load speed-torque characteristics

graphical approach might be used based on the simplified formula:
t(s)=[Wk**2(r/min1 - r/min2)x2xPI]/(60g x Tn), in seconds
or
t(s)=[Wk**2(r/min1 - r/min2)x2xPI]/(308 x Tn), in seconds
Wk**2 is inertia=(motor Wk2 + load Wk2)
R/min are RPM
Tn is the net average accelerating torque between rev/min1 and rev/min2

 

[Shortstub]

Jbartos,
This thread would have been unnecessary if WR^2 (GD^2) were known. Esvhv said he wanted to find t(s), but he needed other data. The other data needed was WR^2.

 

[esvhv]

I like Shortstub idea of using sets of performance data to find the inertia value,

But using this idea yields only the motor inertia, I require the load as well

Any suggestions???

In addition you quote the 3 second figure for a random motor but that is the range I am looking at do you have any other data

 

[Shortstub]

If it is a high speed compressor, then it won't materially affect starting time. Perhaps an additional 1-2 seconds. Of course, the assumption is that it is "unloaded" and there is no intervening gear-box.

If other than a compressor, than use the NEMA tables referenced by Electricpete!

 

[Shortstub]

Esvhv,

Earlier, Electricpete posted a NEMA MG-series standard that covers the calculation of "load" WR^2 values based on a known motor horsepower. Of course it will have to be converted to GD^2. But, it should provide you with the last bit of data you need. If the NEMA data-base is unavailable to you, let us know!

 

[electricpete]

Good point.

One note - If you use the NEMA maximum load inertia limit that I provided, you'll get an upper-bound for the inertia. That will give you an upper bound for the starting time. (Actual load inertia and therefore starting time may be much lower.)

 

[Shortstub]

Electricpete,

Your conclusion is valid only if the actual parameters are known. Remember my approach only provides Esvhv with a first-pass calculation... just to get him up to speed (excuse the pun). Furthermore, it assumes that the motor is designed per NEMA standards!

BTW, the data I collected came from a myriad of manufacturers (in alphabetical order so as to preclude preference) including ACEC, AEG, AEI, Allis-Chalmers, Ansaldo, ASEA & Brown-Boveri (now ABB), Electric-Machinery, GE, GEC, Louis-Allis, Siemens, Westinghouse, and other lesser known manufacturers from other parts of the world. (Pity, that many of disappeared!)

 

[esvhv]

Indeed the NEMA database is unavailable to me, and I would appreciate any help in obtaining it

Secondly how do I convert WR^2 values to GD^2,

 

[Shortstub]

Esvhv,
It gets curiouser and curiouser. How did you obtain the motor's rating, and speed-torque curves for both motor and load, but not have the PD^2 parameters? What is the application?

 

[esvhv]

O.k. let me phrase it this way. I know the motor rating from the name plate value, and I have the curves from an analysis completed by some one else.

If I obtain the max inertia value from the NEMA tables listed above, I would have to scale it down to a reasonable value. Would the amount of scaling depend upon the type of load i.e. a fan or pump.

Any other suggestion???

 

[jbartos]

Suggestion to Shortstub (Electrical) Apr 21, 2003 marked ///\\Jbartos,
This thread would have been unnecessary if WR^2 (GD^2) were known.
///Not quite. The needed data identification according to industry standards would be needed. As you very well know, there are myriad of various data available with very little explanations about their context.\\ Esvhv said he wanted to find t(s), but he needed other data.
///See my comment above.\\ The other data needed was WR^2.
///This is what I presented in general from IEEE Std 399. This standard assumes that the motor and load manufacturers will be providing these data since they are the correct and accurate source to provide them.\\\

 

[jbartos]

Suggestion to esvhv (Electrical) Apr 17, 2003
I have two torque speed curves, one showing the motor torque the other load torque. But I need to find the Acceleration time; I have the true torque values along each line in n-m and the speed in rad/s. In addition I have the kW rating of the motor.
///The idea about the kW size would be helpful since from responsibility standpoint. Small and inexpensive motor related to inexpensive process could be set by some approximations and empirical information from this Forum. However, larger motor and more responsible application would be much better to do with the motor and load manufacturer tech support. In fact, this is almost requirement in many responsible larger motor applications.\\ But I do not have any other data.
///This would imply that you are dealing with some smaller motor sizes that may have their data estimated or approximated.\\Is it possible to find the Acceleration time, or an approximation for it.
///It is usually being done over industry standards and catalog/spec data sheets for motors and loads.\\Help would be much appreciated
///Nameplate data would have been helpful.\\\

 

[esvhv]

I think Jbartos was correct This thread would have been unnecessary if WR^2 (GD^2) were known.

The integration of 1/MT - LT has been completed and all I need is a means of obtaining the WR^2 value, or a totally different means of finding the acceleration time without using the inertia values

 

[electricpete]

Here is a NEMA document which summarizes the requirements oF NEMA MG-1.

http://www.nema.org/index_nema.cfm/1427/8672E77E-D3BC-494F-A0D0CE6E55B99A74/

Table 45 gives those MAXimum load inertia's by motor speed and horsepower.

Estimated Motor Rotor Inertia for Dynamic Braking is not given in the above document, it is in NEMA MG-1-2002 Section 14.46. (This standard costs money). But section 14.46 only contains a formula (the one above), not a table.

 

[electricpete]

I should point out this estimate is for NEMA "medium" induction motors, which is roughly between 1-250hp (depending on speed).

 

[Shortstub]

Esvhv,

Here is the rest of the puzzle useful in your mathematical pilgrimage to determine acceleration time for the 10MW (13,600 Hp), 2-pole motor:

Load Wk^2 presented in NEMA Std MG 1-20.42, covering only 60-Hz machines, is:

Load Wk^2 = AxB-CxD, lb(f)-ft^2 units, where,

B = [(Hp^0.95)/(rpm/1000)^2.4]

C = [(Hp^1.80]/(rpm/1000)^1.8]

To convert the above Wk^2, in lb(f)-ft^2 units, to GD^2, in kg(f)-mt^2 units, multiply by the factor K, where,

K = 4.41E-02

The NEMA caveat is that this calculation may not be applicable for all Hp's not shown in their table.

Now you have all the parameters, Hp, Tl, Tm, motor GD^2, and, load GD^2. This procedure will provide you with a "first-pass" determination of acceleration time, ta.

 

[Shortstub]

Esvhv,

The 'A' & 'D' constants missed the final cut. However, Electricpete had given 'A'. Repeating here:

A = 24 for 300 to 1800 rpm machines, and 27 for 3600 rpm.

The 'D' constant is 6.85E-02.

 

[GusD]

Hello esvhv.

Please help me.I must have missed something that everybody seems to have seen.
You didn't mention the type of starting for this motor.Or the type of motor for that matter.Is it across the line start?VFD application,constant torque or variable torque?
The torque rating of the motor is normally specified taking the load torque in consideration.If this particular motor develops sufficient torque to accelerate the load without tripping and do it often, than its accelerating torque and time are of no concern.Caveat "Most times"
By the way,moment of inertia is usually stamped on most motors nameplate.The load Inertia might be a problem.
I believe all the good information provided by everybody should help you run this motor and I might add it help me as well.

Thank you all

GusD

 

[jbartos]

Suggestion to the previous posting: Many postings are coming in here without nameplate data. Had they been posted, many of them would have been resolved smoothly.