starting high inertia load

[Guest]

Hi

I have an application in our sawmill for a soft starter on a high inertia load (a re-saw). I am concerned about start time, starting torque and the implications it will have thermally on the motor and soft starter. I have all the performance curves for the motor (including thermal) but don't know how to calculate the start time for a given current limit. Can you advise if there is a formula for this? I have the total load inertia but would imagine that I need to convert this to a torque profile so I can compare it to the motor torque profile. Is this correct? How do you convert a load inertia figure to torque? Is a saw like this a flat torque profile? Am I better off using a VSD for this (it only runs at a fixed speed)?

 

Thanks for your help.

[marke]

Hello

 

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Starting high inertia loads is potentially a problem for the starter and the motor.

Firstly, the full speed kinetic energy of the driven load is dissipated in the rotor of the motor during start. You need to ensure that the motor is capable of starting that load inertia. The thermal capacity of the rotor is usually specified either as maximum Locked Rotor time or maximum load inertia. - Both refer to the same thing, just expressed in a different manner.

There are major differences in the thermal capacity of motors and it is extremely important with high inertia loads that the rotor is designed for that level of inertia. Using a rotor that does not meet this requirement will not necessarily result in an imediate failure, rather it will shorten the life of the rotor. When checking on the inertia, it is the effective inertia as seen by the motor shaft. If the motor and load are at different speeds, you need to multiply the actual load inertia by the speed ratio squared to get the effective shaft inertia seen by the motor.

 

The soft starter must also be rated for the required starting duty. Some manufacturers rate their equipment for light duty operations only and in this case, you will have a shortened life. Other manufactures design for light duty but give alternative ratings for medium and heavy duty. There are some manufacturers who rate for heavy duty as standard. You still need to asscertain the required start current and time and ensure that the starter is capable of those figures. Some manufactures call a medium duty start heavy duty etc. Check the actual figures rather than the description.

 

There are formula for calculating the start time for different start currents, but there is a need to do a number of calculations, not just one calculation.

The easiest way to get the figures, is to download the Electrical Calculations software from http://www.LMPhotonics.com/busbar32.zip and go into the "induction motor" "start current curves" and then add the data for your motor and load. You can then go to the "induction motor" "acceleration" page and select your motor and your load and select the starter of your choice. This will graph out the time taken to reach full speed.

 

I would expect that it would be much cheaper to use a soft starter than a VSD for this application but there are of course a number of things to consider.

 

• The purchase cost of the soft starter will be lower.

• The running efficiency of the soft starter will be lower.

• The start current with the soft starter will be higher

   

  If you post or email the data to me, I can give you the answers.

   

  Best regards,

[Guest]

Hello Marke

 

Many thanks for your very prompt reply to my questions. I have tried your calculations software and can see that this will help me. However I still have one question for you if you don’t mind. I have the load inertia figure but need to convert this to a load torque in order to enter the values in your software. My understanding of how to do this is to use the formula T = change in angular distance/time (T = rad/sec2/sec). However one of the values I am trying to establish is how long it will take for the soft starter to accelerate the load, yet this formula asks for time. It appears to be a “circular reference”. Am I missing something here?

[marke]

Hi anonymous

 

You do not need to convert the inertia into torque.

In the load information, you enter the load inertia. Be carful of the units though. You can use the conversions pages to convert the inertia units if they are incorrect.

 

Enter the load curves - torque against speed. This is the work torque. If the load is purely inertial, you can set all the torques to 0. If the load has work which changes with speed, i.e. a fan, then you enter all the toques at different speeds (ignoring the inertia). Many loads have a speed squared function so you can approximate the load torque using this curve. The software uses this torque and the torque developed by the motor to calculate the acceleration torque. (shaft torque minus work torque at that speed)

 

Does this answer your question??

Best regards,

[marke]

For more information, have a look at http://www.LMPhotonics.com/hi_inert.htm This gives some guide lines for designing high inertia starting solutions.

 

Best regards,

[Guest]

Hello Marke

 

Once again thanks for clarifying things further for me. I've followed your suggestions now that I understand the inertia/torque matters. I was getting confused on your use of work and torque. Although a resaw can be classified as purely intertial, I have applied a small portion on work torque to account for frictional loss etc. I'll now see how we go and let my electrician purchase a soft starter.

 

I trust this hasn't been too comercial for this forum!

[marke]

Hi unknown

 

No problem about the discussion, can't say I saw anything commercial in this thread at all. This is what the forum is for, to discuss and learn. I have a philosophy of engineering things correctly at the beginning and preventing problems further down the track. To many decisions are based on price and not enough work is put into engineering a solution that will work.

This forumis used by key personel from a number of competing companies internationally and I am pleased to say it has remained a neutral ground which is how I want it to stay. I believe that at an engineering level we can all learn together and work to prevent the "oops" situations caused by price driven sales.

If you need further advice that you do not wish to post on this forum, you can contact me directly. Contact details are on http://www.LMPhotonics.com

 

Best regards,