HT Motors

See this

All about motor starting current

"HT motors operate on high voltage ( 6.6 or 11KV). IF we make delta connections than voltage experienced by windings will be line voltage, which will require more insulation of winding. In star connection voltage experienced by each winding is phase voltage hence less insulation is required. This is the reason HT motors always wound in star connection."

http://www.allinterview.com/showanswers/85678.html

http://powertips-elec.blogspot.com/2010/12/motor-starting-schemes.html

I don't follow "HT motors operate on high voltage ( 6.6 or 11KV). IF we make delta connections than voltage experienced by windings will be line voltage, which will require more insulation of winding. In star connection voltage experienced by each winding is phase voltage hence less insulation is required. This is the reason HT motors always wound in star connection." even if it is 32/0 in favour.

The insulation has to stand the voltage difference between the metal of the stator (effectively ground) and the volts in the winding. That voltage difference is line voltage/√3 whether the motor connection is star or delta. If that's wrong will somebody please explain why?

It may well be true that for some reason HT motors are always wound in star connection, I wouldn't know offhand.

In the star connection the winding insulation must withstand the phase to ground voltage. In the delta the winding must withstand the phase to phase voltage.

The starting current for a high voltage motor is much less that an equivalent low voltage motor, thus much less line disturbance..

I don't get that. The way I see it the voltage across the insulation is phase to ground in both cases. How does the "other" side of the insulation get to see the other phase voltage?

I know what star and delta connections look like, but I don't think the diagrams help to determine the voltage across the insulation, which is what we're discussing. If you could connect a voltmeter between a point on the outside of the winding (under the insulation) and a point on the stator metal (which is at ground or neutral potential) in contact with the outside of the insulation, I think the reading would be same in both cases, 230 volt. That's at the end near the line connection, it might vary at different points along the winding, I haven't thought that through.

I didn't see anything on your link that referred to it.

I thought we were talking about limiting start up current....This is from the previous link...

"It is, however, advisable to employ only HT motors, in large ratings. HT motors have little alternative to be switched other than a DOL or a soft starting. Y/D switching in an HT motor is neither advisable nor possible for its windings are normally wound in a star formation to reduce the winding's design voltage and economize on the cost of insulation. Auto transformer switching is possible, but not used, to avoid a condition of open transient during a changeover from one step to another and for economic reasons as noted above. Moreover, on a DOL in HT, the starting inrush current is not very high as a result of low full-load current. For example, a 300 hp LT motor having a rated full-load current (FLC) of 415 A on a DOL will have a starting inrush of approximately 2500 A, whereas a 3.3 kV motor will have an FLC of only 45 A and starting inrush on a DOL of only 275 A or so. An HT motor of 3.3, 6.6 or 11 kV will thus create no disturbance to the HT distribution network on DOL starting."

Just to add an autotransformer configured, as a Korndorfer starter doesn't have an open transition.

The down side, they are still expensive.

Giving whoever wrote that the benefit of the doubt and assuming he knows what he's talking about, fair enough, but I still don't see the logic of it.

I looked on a few websites for HV motors, and there wasn't much about star/delta connection. What there was did refer mostly to star, but delta was mentioned.

You have not mentioned the size of the motor, nor the type of the equipment coupled with the motor. Normally HT motors can be started direct on line unless they are large size motors. If the inertia of the driven equipment is high, the starting current will persist for longer duration, otherwise the starting current will fall within a short time.

In case of assisted starting using reactor/ autotransformer or electronic soft starters, starting current and torque will be less but starting time will be more.

If the cause is ' limit starting current ' (for large motors) then effect will be ' long duration starting ' using assisted starting. If the cause is ' develop more starting torque ', which assisted starting will not be able to provide then effect will be ' high starting current but of shorter duration ' using DOL starting. You have to adopt the right method of starting depending upon the application.