Korndorfer-method 3

[Trine]

I’m a student who recently got a sparetime job in a big company. My new boss has given me a task about an asynchronous motor that during start-up uses the Korndorfer-method.
My job is to calculate the time of each stage; The stage of reduced voltage and the stage of reactor start.
I have searched everywhere for information about the Korndorfer method – both in books an on the Internet, without any success. I found a small amount of theory but no equations.

Does anyone know if there exist any equations about the time intervals?

 

[itsmoked]

You saw these threads?

http://www.eng-tips.com/viewthread.cfm?qid=101028

http://www.eng-tips.com/viewthread.cfm?qid=72012

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[itsmoked]

Normally it is set by someone with experience who makes an educated guess. Then a start is done and the results watched closely. Adjustment is made if required and after noting the motor's starting duty cycle limits the adjusted values are tested.

You will likely NOT find some number or equation anywhere as every single application has it's own peculiar requirements. If the motor is starting some large heavy inertial load the time between levels would be longer than if the starting load was light.

Essentially the motor starts and ramps up to some level where it will not be able to accelerate the load further. At this point the next or last step is made. You do not want to hesitate at this point AT ALL. So that's what you watch during testing. You may never actually need to know the time as you just "speed it up" or "slow it down" until it's correct or you get the effect you need.

Typical numbers are likely 1.5 to 4 seconds with 2 probably being pretty common. Others may have different estimates of "typical". But of course the more detail we have on the motor size and the specific type of load the better the estimates.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

One additional piece of info that might help; quit looking specifically for "Korndorfer" starting. This is likely a red herring thrown at you by your boss to check your resourcefulness. The Korndorfer Method is just an adaptation of Reduced Voltage Auto-Transformer starting (RVAT) that provides a form of closed transition. Today, virtually all RVAT starters use the Korndorfer Method so to make reference to it is essentially a redundancy. Just search for information on Autotransformer starting transition timing and you should do better.

 

[aolalde]

There is a general formula:

t = [Wk^2 X (Delta rpm)]/[308xTacc]


t= time in seconds
Wk^2 = constant of the total system moment of inertia (Lb*Ft^2) (load+motor+coupling)
Delta rpm = Final speed – Initial speed (rpm)
Tacc = Motor torque – Load Torque = Net accelerating torque.

Your problem: These parameters are changing with every speed change. Then you need the Torque curves for the motor and the load and make an integral for time or divide the full speed range in steps of speed and make an average calculation for each step for a graphic solution. Normally you will need a computer program.

 

[Trine]

Thank you for your input and advices. Now I have something I can go on with.
In fact it was me who found out about the name Korndorfer after the presentation of the task.
But I’m sure that my boss has given me the task to challenge me.

I might return to you with some further questions.