Star Delta Question...

[csaw2121]

This is a question that gets asked sometimes in an Oral test I need to have for my Marine Engineer license... Any help would be appreciated as I'm a little stuck on this. I realize that the Star Delta configuration is for reduced voltage starting say 254V Vph from 440V supply (1/ root 3) (or is it line voltage?) and hence reduced torque and current for starting compared to DOL starting (6-8 times FLC) but...."What would be the problem with a star delta starter not changing over and staying in star configuration and what would you do if you had no spares and had to keep running in star?"I assume it would overheat and draw more current but I cannot put into words the reason why? Would it be because of the motor stalling slightly from reduced torque, slip increasing and hence drawing more current?Any help with this one would be much appreciated...

 

[Darkwood]

Get back into your notes..... star delta starting is for reduced current starting which is achieved by changing the winding config so you have 2 x 400v windings in series to 2 phases of the the supply as opposed to 1 x winding across any 2 phases in delta. Running permanently in star has a lot of factors to consider as to the consequences but usually most set up would overheat and damage your motor if it lacks thermal protection.... i have seen motor chug along for yrs before failing because they were connected up wrong as a DOL but again motor duty and characteristic plus protection all play key roles as to what will happen.

 

[snowhead]

Re what would you do without spares;

It's marine so presumably your never very far from the said motor (relatively speaking) so .....

 

[csaw2121]

Thanks for reply, So in essence the answer would be that the motor would run hot as star windings are designed for the reduced current and not for DOL current?? Possible earth fault/phase to phase fault if no protection?? I think the examiner is looking for a basic understanding as in reality we would get shoreside assistance, apart from Megger tests and bearing changes there's not much we are permitted to do onboard...

 

[Knobhead]

Torque will only be 30%

 

[Darkwood]

List of spares ... add 2 oars

 

[csaw2121]

Thanks Tony, Reduced current and reduced torque.
about 1/3 torque compared to DOL.

Ive just checked the question again and question states star delta starter of Synchronous Motor sticks in Star. Explain why overheating will occur, what damage to motor and overhaul of motor. Didn't mention the synchronous bit before sorry.

Overheating because of slip when in star as synchronous runs with no slip so will get large circulating currents in rotor??? Clutching at straws here as you can most likely tell...

 

[csaw2121]

If I replied two oars and some elbow grease I don't think examiner would be too happy

 

[Bobster]

Thanks Tony, Reduced current and reduced torque.
about 1/3 torque compared to DOL.

Ive just checked the question again and question states star delta starter of Synchronous Motor sticks in Star. Explain why overheating will occur, what damage to motor and overhaul of motor. Didn't mention the synchronous bit before sorry.

Overheating because of slip when in star as synchronous runs with no slip so will get large circulating currents in rotor??? Clutching at straws here as you can most likely tell...

No motor runs without a bit of slip.

 

[csaw2121]

Thanks Tony, comparatively little slip compared to induction motor. Any thoughts on the question?

 

[Knobhead]

Be interesting to know how you start a synchronous motor star delta?

 

[csaw2121]

Please put an A in front of synchronous....