I need help!

How do I decide what type of starter is required for an industrial motor control application?

They all seem to get the motor up to rated speed so why should I use an auto-transformer starter instead of a star/delta starter?

Thanks,
milliamp

It all depends on the result that you want. The simplest starter (and cheapest) is the Direct On Line (or across the line) starter. This comprises a contactor and thermal overload and the motor is switched directly to full voltage. The motor draws Locked Rotor Current (typically 6 - 9 x Rated current) and produces maximum torque.
The high overload current during start can cause voltage drop problems, and the torque may cause mechanical damage.
A solution to this, is to use a reduced voltage starter of which there are quite a number.
The simplest reduced voltage starter is the star / delta and this causes severe torque and current transients at switching to delta and causes bigger problems than DOL so should be avoided. The other most common reduced voltage starters are the primary resistance, using series resisters to limit the current, the auto transformer using a transformer to reduce the voltage, part winding using part of the winding only during start to reduce the flux in the iron, and the soft starter using SCRs to reduce the voltage during start.
If a reduced voltage starter (any kind) is used, it must be able to get the motor to full spedd before switching to full voltage or there is little or no gain.
The soft starter is the easiest to set up for effective starting at minimum current.
Best regards,

Thanks Marke,

You made some comments about the star/delta ie "causes severe torque and current transients at switching to delta and causes bigger problems than DOL so should be avoided". Could you explain this further please?

Thanks,
milliamp

Changing from a star to delta open transition causes the motor to be disconnected from the line for a moment before it is reconnected to the line as a delta. It is this disconnection that causes the worst of the problem. A closed transition with resistors or reactors is a better choice (adds cost $) so an autotransformer is still the better option since there is no disruption to the motor during transtion from start to run.

Changing from a star to delta open transition causes the motor to be disconnected from the line for a moment before it is reconnected to the line as a delta. It is this disconnection that causes the worst of the problem. A closed transition with resistors or reactors is a better choice (adds cost $) so an autotransformer is still the better option since there is no disruption to the motor during transtion from start to run. Also you usually have a couple of choices of taps (65 or 80 %) to tailor the starter to the load.

[Edited on 18/9/02 by Crowbar]

Yes this causes the transient, but you still have the problem of the motor running out of torque at part speed and having to change to delta at part speed. This causes DOL current to be drawn from the supply, so why use a star / delta in the first place??