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Aucom Emx3 - Inbuilt Overload Vs Mccb For Cable Protection


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G'Day All

We are undertaking the replacement of DOL conveyor starters with soft starters - 30kW 55A motors direct coupled to grain conveyors.

The starters are oversized AuCom EMX3s - nominal current 105A and thermally they have a nominal FLC of 60A at 50 degrees C at a duty of 4.5xFLC for 30s with 570s off time (105A:AC53b 4.5-30:570)

The technical risk I am trying to manage, and it comes up all the time in DOL to RVSS projects, is the potential for the soft starter to not be able to produce enough starting torque to get the loaded conveyor away.

I am not versed at all with conveyor calculations and I am not confident (without a very clear how-to) of doing conveyor calculations to determine the minimum breakaway torque required and relating this back to the motor's reduced voltage torque characteristic. Also the project does not have the budget for me to school myself on these calcs. Dont get me wrong I would love to learn how to do these calcs accurately but it just doesn't fit into the bounds of this project budget or timing.

So the current limit on this starter is 600% of FLA, but the starter has an instantaneous trip of 6x nominal rating = 600A. It seems the only thing I can do is to design this system to be able to do as close to a DOL start as I can - as I know the conveyors get away fine at the moment with a DOL start.

So, if I wanted to get the starter to do 700% or 800% FLA on a current limit start, 800% would be 440A which is under the starter's instantaneous trip level of 600A. However, this would require me to 'fake' the motor FLA setting in the soft starter, which would adversely affect the motor and cable protection. This could be fixed by installing an external overload relay.


So really the question I have to answer is can I justify the installation of an external overload relay on this starter.


Another option that has been suggested is to install a maintenance-only switch in the MCC which would activate secondary motor parameters sized to provide a DOL start - for clearing blockages.


Thanks all




Michael Belperio


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Hello Michael


So many questions interrelated!!


Firstly, to start a materials conveyor, you have a major variation in start torque requirements. An unloaded conveyor can often start at around 40% torque, but a loaded conveyor may need as much as 180% torque to start it.


The start current required to generate this torque, is dependent on the motor characteristics.

NB :

  • The motor design determines the torque developed by the motor under high slip conditions. At zero speed, the start torque is referred to as the Locked Rotor Torque. (LRT)
  • The full voltage start torque at zero shaft speed is called the Locked Rotor Current (LRC or LRI)
  • As you reduce the start voltage applied to the motor, you reduce the start current. (Ohms Law)
  • As you reduced the start current, you reduce the start torque by the square of the current reduction. Half start current results in a quarter of start torque.
  • High efficiency motors with a very high LRC can struggle to develop greater than 150% torque at less than 600% current, it depends on the LRC and the LRT.


I would always use a motor selected for a low LRC and a high LRT.

I would also ensure that the motor has a high start time constant (DOL start time rating). A double cage motor is ideal for this application.


I would select a current ramp start mode with the initial start current set up to produce around 40% torque, ramping up to the current required to produce say 180% current over a 10 second ramp time.


I would set the rotor start time for 40 seconds and the maximum start time protection to around 30 seconds.


If the motor has difficulty developing enough start torque to start under extreme loading conditions, you can activate the emergency start mode, or you could look at using Kick Start to break the conveyor away.


The secret is to get the best motor to start with.

A good means of selecting the motor is to take the LRT (%) divided by the LRC (%) for each motor you are considering and select the highest value of LRT/LRC.



Best regards,


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Kia Ora again Mark,


Unfortunately in this case the existing motors are going to be reused. I have the specs on them, and I have scoured the earth for data on the motors with respect to their starting torque performance - but I have not been able to find any information. It always seems to be the way, that old motors are simply forgotten by their manufacturers - who themselves are sometimes out of business.


Thanks again for your quick reply mate.

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