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Auto Transformer Star Point |
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 Dec 25 2008, 07:23 PM
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#1
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 Group: Members Posts: 1 Joined: 25-December 08 Member No.: 5,045  |
I am in the Electrical training field and motors are run at no load. I have noticed that whether the three phase auto transformer star is open or closed makes no difference. Can somebody please give me the reason why the star point must be closed during start up?
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Dec 25 2008, 07:59 PM
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#2
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Posting Freak Group: Moderator Posts: 2,336 Joined: 24-April 02 From: Christchurch, New Zealand Member No.: 1 |
Hello Jakes
Welcome to the forum When the star point is closed, the transformer supplies a reduced voltage to the motor where the voltage is determined by the transformer ratio. When the star point is open, the transformer behaves as a primary reactor and the voltage applied to the motor is determined by the ratio of the motor impedance to the total impedance of the motor plus the reactance of the transformer. There will be a difference in start current between the star point closed and the star point open with lower start current in the line for the star point closed. Best regards, Mark. -------------------- Mark Empson administrator
Skype Contact = markempson | phone +64 274 363 067 LMPForum | LMP Tech Warehouse | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | LMP Software | Mark Empson Website |
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Apr 28 2009, 09:04 AM
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#3
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Junior Member Group: Members Posts: 3 Joined: 28-April 09 From: Perth Member No.: 5,336 |
As a follow-up to this question, I assume that for a standard 50/65/80% autotransformer, only 64% of the motor full load current rating will suffice for the current rating of the transformer contactor, as this covers the "worst-case scenario" of the 80% tap being selected for use (please correct me if I am wrong there).
What I would like to know though is what rating is recommended for the star contactor? The current through it will be negligible, right? |
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Apr 28 2009, 10:09 AM
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#4
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Posting Freak Group: Moderator Posts: 2,336 Joined: 24-April 02 From: Christchurch, New Zealand Member No.: 1 |
Hello Marco
The current flowing to the motor is directly proportional to the voltage reduction. The current flowing in the supply is proportional to the square of the voltage reduction. Therefore the curent from the 80% tap is 80%, not 64%. Best regards, Mark. -------------------- Mark Empson administrator
Skype Contact = markempson | phone +64 274 363 067 LMPForum | LMP Tech Warehouse | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | LMP Software | Mark Empson Website |
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Apr 28 2009, 11:57 PM
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#5
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Junior Member Group: Members Posts: 3 Joined: 28-April 09 From: Perth Member No.: 5,336 |
QUOTE (marke @ Apr 28 2009, 06:09 PM)  The current flowing to the motor is directly proportional to the voltage reduction. The current flowing in the supply is proportional to the square of the voltage reduction. Therefore the curent from the 80% tap is 80%, not 64%. Mark, Yes, but the transformer contactor is installed on the primary side of the transformer, so it would see only 64% current flowing through it, right? |
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Apr 29 2009, 07:04 PM
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#6
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Posting Freak Group: Moderator Posts: 2,336 Joined: 24-April 02 From: Christchurch, New Zealand Member No.: 1 |
Hello Marco
Yes, that is correct provided that you are using a single stage Korndorffer auto transformer starter and that the contactor is bypassed when full voltage is applied. In some installations, that contactor is used as a main contactor and remains in circuit while the motor is running. This makes it easy to fit the overloads etc, otherwise you do not have a common path for the overloads to be connected and so they are out of circuit during start. Best regards, -------------------- Mark Empson administrator
Skype Contact = markempson | phone +64 274 363 067 LMPForum | LMP Tech Warehouse | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | LMP Software | Mark Empson Website |
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Apr 30 2009, 12:57 AM
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#7
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Junior Member Group: Members Posts: 3 Joined: 28-April 09 From: Perth Member No.: 5,336 |
Yes, fair enough.
Back to my original query then ... is there a standard "rule of thumb" used for sizing the star contactor? For commonality of equipment I would probably just use the same rating as the transformer contactor, but I am also thinking that you could potentially make it a lot smaller to save on space & cost. It is really just a voltage reference point, with negligible current flowing through it - is that correct? |
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Apr 30 2009, 06:46 PM
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#8
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Posting Freak Group: Moderator Posts: 2,336 Joined: 24-April 02 From: Christchurch, New Zealand Member No.: 1 |
Hello Marco
The star contactor carries the motor current plus the transformer current so would need to be at least the size of the motor current. Best regards, Mark. -------------------- Mark Empson administrator
Skype Contact = markempson | phone +64 274 363 067 LMPForum | LMP Tech Warehouse | L M Photonics Ltd | Empson family | Advanced Motor Control Ltd | LMP Software | Mark Empson Website |
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