Are all Deltas the same ? 1

[DAVEZIMM]

I was trying to size a delta/wye starter and all the catalogs have HP ratings for the contactors less than DOL starters. In fact they suggest to size the overloads at .58 (root 3) of the FLA rating of the motor. I understand that in delta mode of a delta/wye motor that I am supplying each phase of the motor with separate contactor pairs, and that the individual winding current is .58 of FLA. My question is what if I hardwire the delta configuration and feed the motor with a single contactor rated for FLA and sized with the actual HP of the motor. Is there a difference in the total torque/current consumption of the motor ?
Please note I understand the reason for the wye configuration to reduce starting current... I am just trying to understand the differences in delta only if any.

 

[rbulsara]

1. You have answer to your first concern in your own post. The contactor see the phase (winding) currents of the 'delta' which is .58 times the line current, and hence its HP rating and the overload setting. Because the O/L relay also see the phase current only. Remember you use contactors to run one motor in star/delta starter, compared to one contactor in a DOL starter.

2. If I understand you correctly, you are comparing a motor controlled with a star/delta starter and a delta connected motor controlled by a DOL starter. Both configuration will have the same torque /speed/current/power characteristics in RUN condition that is when in delta mode.

 

[jbartos]

Suggestion to DAVEZIMM (Industrial) Dec 17, 2003 marked ///\\\
I was trying to size a delta/wye starter and all the catalogs have HP ratings for the contactors less than DOL starters. In fact they suggest to size the overloads at .58 (root 3) of the FLA rating of the motor. I understand that in delta mode of a delta/wye motor that I am supplying each phase of the motor with separate contactor pairs, and that the individual winding current is .58 of FLA.
///Within the delta winding only\\ My question is what if I hardwire the delta configuration and feed the motor with a single contactor rated for FLA and sized with the actual HP of the motor. Is there a difference in the total torque/current consumption of the motor ?
///Please, clarify the difference with respect to what.\\Please note I understand the reason for the wye configuration to reduce starting current... I am just trying to understand the differences in delta only if any.
///The delta winding configuration has each delta winding segment voltage Ea,b=Eb,c=Ec,a connected to sqrt3 x Ea,n=Ea,b=sqrt3 x Eb,n=Eb,c=sqrt3 x Ec,n=Ec,a, respectively. E.g. if Ea,n=Eb,n=Ec,n=120VAC, then Ea,b=Eb,c=Ec,a=sqrt3 x Ea,n=sqrt3 x Eb,n=Sqrt3 x Ec,n=208VAC. The torque and power will be 3 times higher for the delta winding connection than for the wye connection. This is available in basic ac machinery textbooks, e.g. M.G. Say "Alternating Current Machines."\\

 

[jbartos]

Suggestion: Visit

http://www.usmotors.com/products/ProFacts/1-120-7.htm

etc. for more info

 

[DAVEZIMM]

Thank you everyone for the responses to this question. The link to usmotors helped clarify the error in my thinking. I was almost going to fuse the L1, L2, and L3 connections feeding the delta contactors at .58*1.75% but realize now that the motor still draws the same current in delta, it's just split between the two contactors. Part of what was confusing me was a code requirement to only have overload protection on one contactor and I extrapolated that was the total draw on the motor.