DOL or Star Delta?

Normally we use DOL starting on motors ranging from 1HP to 15HP (+- 5% difference factor). Star Delta (16HP-30HP). Motors bigger than that are either soft started or VSD are used. Every one has had different experience with starting techniques I've told you mine.

Hope it Helps

Regards,

DG EE

My understanding so far is that ALL motors (squirrel cage) are capable of being started DOL. It is only the utility restrictions that make one use reduced voltage starting. i am open to correction by some motor experts on this.

You have a wound rotor motor. These being more expensive, are generally used where torque and/or speed control is required. If this is the case, you would need a slip-ring motor starter i think.

The motor will effectively be starting off load due to the clutch.

Go for DOL.

OK. It mean the motor can be started through DOL without any fear. But we have other machines in which even a 11KW Eddy current motor has star delta starter. I think their is not a RULE OF THUMB for such kind of applications; it depends upon many other parameters such as, start up time of motor, distribution board capacity, rating of cables etc.

Am I right?

You say you have a strong power supply but then call in to question the distribution board supply and cabling.

I'm afraid it's a case of you have the information, we don't, it's got to be your decision.

Please note that when you start a high capacity motor DOL, the system voltage drop does affect other loads connected to the same line/bus. However, there would be legal restriction by the Utility Company, only when you draw power from LT Lines from the Utility, because in such a case, the voltage drop caused by your DOL starting would affect your neighbour's electrical loads, thus making it a social problem. Please consult your local Utility Company for this limitation.

If you draw power from a HT Line, the voltage drop to your HT neighbour would be negligible under the same conditions. So, there would not be and there cannot be any legal restriction by the Utility in this case as to the maximum capacity of the motor that can be started DOL.

In this case, it is the user's electrical enginner who would decide the maximum capacity of the motor that can be started DOL, duly taking into account the effect of such starting on other loads connected to his LV Bus.

I have even commissioned may high capacity motors as high as 150 HP DOL on LV.

The "rule of thumb" is different for every installation and every jurisdiction, there is no universal rule that can be applied. Most likely your local utility may impose a rule, but even that can have exceptions; those rules typically are enforced only when nobody wants to take the time and effort to look at the starting profile in depth.

If you have S-D starters on motors as small as 11kW, that may be an indicator that your local utility has a rule like that. But it also may just mean that suppliers don't want to bother thinking about it and just supply S-D starters as a matter of course just in case.

Its not just about Voltage dip problem that one should be worried about while using DOL. What about the inrush current? What about increased maintenance? What about motor windings? These are some of the questions that one should be considering. I have seen DOL being used for even 100 HP+ motors - but NOT recommended at all. However for a 15 kW motor - it is just a borderline case. In fact Y-D would be a better bet. The problem with Y-D would be Transients, Jerking - but should be just about ok for this motor size.