Low voltage motors - star (wye) or delta?

[PaulDonnellan]

Hello all,

I have been looking at a specification for low voltage (LV)3-phase ac induction motors. It specifies that all LV motors shall be connected in star and not delta.

I can understand the reason for connecting HV machines in star (economics of providing kV/mm for the insulation systems etc.) but it is less clear to me for LV motors.

I would appreciate any thoughts.

Thanks.

P

 

[busbar]

Based on speculation—for smaller motors, wye windings with fatter magnet wire may reduce manufacturing costs, and better handle starting/overload thermal/mechanical conditions.

 

[bigamp]

It is also easy to detect loss of a phase winding inside the motor if it is star, there will be zero current in the corresponding phase conductor. LV Exe type hazardous area motors all seem to be star connected, I expect to allow for easy loss of phase detection.

In LV motors, smaller ones tend to be star. European practice (Siemens, ABB, Loher) seems to be that larger motors bring out both ends of the windings and allow connection in delta for 380V-415V operation and star for 690V operation.

 

[jbartos]

Suggestion: Only 3 leads may be brought into the motor terminal box. If the delta connection is to be implemented, then 6 leads would be in the motor terminal box and probably, they will be connected to the terminal board in the motor terminal box.

 

[GusD]

Hi Paul.
In NAmerica we normally bring 3 leads to the motor JB and the internal connection may be Star or Delta.There are motors that have up to 9 leads out to the JB with the other 3 leads hooked up Star internal.These were fairly common in Europe before the use of VFDs became in vogue.If the motor has dual voltages,dual speeds or a combination there of, it will require more than 3 leads out.
I don't know of any rule that requires a particular motor that due to its size or voltage rating, has to be Star or Delta.Internal characteristics and design will determine the connection.

GusD