Selection of Starters For Motors

It depends on the supply and if the drive can handle DOL starting. If the supply is weak then S/D doesn't put as much strain on it, the same can be said for the drive, can it handle the high torque of DOL starting?

Different type of starters/starting methods are available. The selection depends on how rigid or weak the supply system is. Common starting methods are:

a. DOL

b. Auto-transformer starter

c. Star Delta starter

d. V/F controlled starter (VFD/VSD)

There selection is based on hand calculations or simulation thru' s/w wherein voltage drop during DOL starting is observed. If the same is within permissible limit, DOL is obvious choice. If not, other starting methods in b, c, d above can be resorted, to reduce starting current and hence voltage drop. Lot of factors goes into selection of b/c/d such as economics, reliability, load characteristics, O&M issues etc.

While selecting b/c, note that, as they are nice in reducing the starting current but consequently also pull down the starting torque of motor (by factor V²). Make sure the selected starting method, gives sufficient torque to the motor to take on the load.

The original poster also needs to take the supply tariff into account when selecting. If the tariff is a peak-demand one, then replacement of DOL starters with more sophisticated ones, beginning with the biggest motors across the facility, could show very quick payback times.

If it were me, S-D is never an acceptable choice, I think they cause more problems than they solve. I would use solid state and if it's under 10HP, you might even look into a VFD because they are now so inexpensive.

But as mentioned, the decision on using DOL or some other sort of Rediced Voltage starting is typically based on an evaluation of the available supply stiffness and/or the mechanical issues. Quite honestly at 10HP it's pretty difficult to justify anything other than DOL, unless perhaps variable speed offers other benefits.

All depend on what you need.

You need lowest cost, DOL is the choice, but High start current is consideration.

You need good deal between cost and performance Star-Delta is the choice, but high current start is still consideration.

You have little more budget, Soft starter or transformer start system is the choice.

You have more budget, VSD is the best choice, Low current start, high performance(depend on VSD type), low electricity use.

One more advantage of DOL starter is,

A Star-delta starter can only be used if in the motor name plate it is stated as delta connected for the rated line voltage. For example, if in name plate it is stated as Star connected, and if you use star/delta starter, you will burn the motor.

If your supply system is strong then go for DOL otherwsie Star/Delta

for 10HP, DOL would suffice.

for more than 10HP wheathere to use star delta or vfd