Hello again! I was wondering if anyone was able to help explain how to calculate simple current for dual voltage delta connections? I really couldn't draw this in multisim, so please envision a Wye and Delta connections with two 10 ohm windings per phase.


Wye connected, 480v line voltage to phase = 480/1.732, or 277 volts per phase.

High voltage connections mean series impedence, then 10 Ω + 10 Ω = 20Ω. 277v / 20Ω = I =13.85A

Low voltage connections mean parallel impedences, so 277v / 5 Ω = 55.4A

Now Delta line voltage = Delta phase voltage, so does high voltage vs low voltage connections both use 480v? and the impedences are like the above series / parallel amounts above? I'm confused because of the relationship of line current =1.732 ( phase current) in delta connections.

Thanks for the help!

 

A balanced 3-phase 10Ω delta connection at 480V line voltage would draw 48A in each 10Ω branch. The line current draw would be 48/√3 or 27.7A. If each of the 10Ω branch resistors was reduced to 5Ω then the current values would double.

 

Thanks for the reply, If I read the low voltage delta connections correctly, the two coils are in parallel.... ?? so if the voltage is now taken at the parallel point, wouldn't it become 240? (a voltage divider) I guess I don't see how they call it low voltage? 480/20 ohms = 24 amps? and 480v /10 ohms (1.732) = 83.1A?

 

Yes, in the low delta configuration there are a pair of parallel windings between each line (L1,2,3). So between L1 & L2 windings 1/4 and 7/10 are connected in parallel. The actual voltage across the parallel pair(s) would depend on the applied line voltage. Motors are configured in this way to allow connection to a range of line voltages without loss of performance. So perhaps in this mode one would expect to have a low line voltage condition - say 240V line voltage rather than 480V. One would have to look more closely at the motor specs to ascertain the allowable range of operating line voltages.

Keep in mind you can't directly translate analysis of 3 phase passive component connections to a 3 phase motor. There are considerations of mechanical load, efficiency and air gap magnetization (etc.) that must be considered - which makes it rather more complicated.

 

Thank-you for helping, we were discussing the benefits of a soft start Wye connection, and was asked to calculate the expected current(s), and efficiency. All was going well until I hit the low voltage delta...

 

Just to clarify on your question about the voltage divider effect. Resistors (impedances) in parallel don't form a voltage divider - they form a current divider. So the parallel combination of the line-to-line motor windings won't form a voltage divider - rather they will allow a higher input current drive to the motor at a lower line-to-line voltage without exceeding the individual motor winding current ratings.

 

I guess I never thought about the reactive (inductive) coil... I saw the ohms and thought only about voltage.... good point, thanks! Perhaps Monday will bring some new information in class. Thanks again!