Start Up Command for HT Motors


Thread Starter


For start command of HT Motors (e.g. 6.6 kv starting motor for Gas Turbine), what is the technical reason that minimum 15 minutes gap should be allowed between two start command ?

The technical reason is that a high-voltage AC induction motor draws a LOT of in-rush current during starting, especially when it's an "across-the-line" starter (directly connected to the voltage source, through a contactor--as opposed to a star-delta starter or a variable frequency device). This in-rush current causes a LOT of heating in both the stator and the rotor.

Therefore, the guideline for starting across-the-line high-voltage AC induction motors is two-and-one-half starts per hour. More than that and the motor windings can be overheated.

Also, for GE-design heavy duty gas turbines with high-voltage AC induction motor starters they quite frequently operate at 150-160% of rated nameplate current during purging and acceleration. So, if the unit goes through a purge and firing attempt and then trips, the motor has been started (with the associated high in-rush current) AND has been operated at 150-160% of rated nameplate current during purging. If the unit fired and was accelerating during starting and then tripped, again, the motor was started (with the associated high in-rush current), gone through a purge (at 150-160% of rated nameplate current), and was accelerated, again, usually at 150-160% of rated nameplate current. So, the stator and rotor will be warm. (That's one reason that after a successful start the starting motor is operated unloaded for several--a "cooldown" period--to circulate air over the stator and rotor to help cool the motor.)

Hope this helps! (By the way, the motor's insulation is rated for the overload conditions usually experienced during starting (the 150-160% of rated nameplate current.) Since it's not continuously operated at this level, it's okay. And as long as it's not abused--by trying to start more than two-and-a-half times per hour--then the motor will last decades.)

And, what is this "two-and-one-half" starts, you ask? Well, if the motor was at ambient temperature ("cold") during the first start attempt in a one-hour period, then it could, theoretically be started three times in one hour; but if the motor had been running in the previous hour or so before a start attempt and was hot (hotter than ambient), then the guideline says only two start attempts during this particular hour. So, "two-and-a-half" starts is the term for this dependency on the temperature of the motor prior to the first start in any hour.

If the unit is tripping during starting, the thing NOT to do is to keep trying to start it multiple times in the hope that it will eventually start.... The thing to do is to resolve the tripping problem after the first or second failed start attempt--not to just keep "pushing the START button" until it successfully starts. If it's not starting or not reaching FSNL, then something else is wrong, and abusing the starting motor is going to lead to a real availability problem down the road when the starting motor needs refurbishing.

Thank you very much for the technical reply. However please make it clear that, as you have mentioned above, what will be starting criteria if motor is star-delta starter or variable frequency device?
>Thank you very much for the technical reply CSA. However please
>make it clear that, as you have mentioned above, what will
>be starting criteria if motor is star-delta starter or
>variable frequency device?

Star-delta would make no difference, the heating effect is exactly the same. The starting current may appear lower (which is debatable) but will be taking longer to accelerate. The total energy going into the motor is the same.

Using a VFD could theoretically reduce the motor heating affect from starting and allow more starts per hour, as long as you limit the motor current to 100% FLA or maybe slightly more. But that generally means a long starting time and not all mechanical systems can allow that either.