Motor With High Starting Current And Long Starting Time

So long one is ready to spend money nothing is impossible; get a motor designed with primary mode forced hydrogen cooling and secondary mode water cooling coils to dissipate the heat for successful fulfillment of criteria laid in second paragraph of the post.

Asymmetric inrush current of the desired strength is fictitious since the impidance of the windings in a motor are theoretically balanced.

Since normal procedure is to cater abnormal conditions for a maximum of three seconds you have to select a cable in consideration of 50 seconds LRTW rather than FLA, same goes with the rest of the DOL elements which would be over sized but the back-up IDMT relay in the feeder panel will be depended on for protection.

The motor vendor tells us the motor starting time is one second although Allowable Run Up Time he has given are quite long

This gives me the impression that Starting time and ART are two different things which I am trying to figure out what the differance between them and how to coordinate both together and with the concerned overcurrent protection.

Appreciate ur comments if any

You see! When a motor manufacturer designs, manufactures & sells a motor to you, he does not know to what type of load you are going to couple it to. The starting time given by the motor vendor in the motor's data sheet is taking into account the moment of inertia of the motor only (this, for sure, the motor maufacturer would know). That is why this is normally very small, as an uncoupled motor can accelerate to its rated speed in quick time.

Whereas the Allowabls Run Up Time (ART) - takes due account of the moment of inertia of the coupled load too. And, that's why it will be more than the starting time of the motor alone - as some more weight (the coupled weight), the motor has to accelerate now.

And, when you set the overcurrent protection these should not matter. These parameters matter most in setting the Locked Rotor Element, the Stalling Protection Element, the prolonged start element ony. As for the motor over current protection goes, please make the current setting to the motor's rated full load current.

Dear Candlez,

What is the load on the motor? comperssor, pump, fan etc. I think you are considering direct coupling of load with motor shaft. Selection of Electrical accessories viz. contactor, O/L relay, Switchgear, cable etc. is already discussed. Alternatively, for high inertia load you may think of use of fluid coupling instead of solid /direct coupling. In that case motor will be started in no- load condition till it attains rated speed. In this case the starting time will be drastically reduced and solve your problem. Please remember while selecting fluid coupling the speed-torque characteristic of motor must be matched with that of fluid coupling.

Manindra.