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Overload Relay Setting Vs Rated Current, Set OLR, how to compute if I know the rated current |
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  Apr 6 2009, 04:02 PM
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#1
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Junior Member  Group: Members Posts: 3 Joined: 6-April 09 Member No.: 5,293  |
How to compute for the OLR setting if the rated current is given already?
Show me the computation if the motor control starter is Across the Line, Wye Delta and Auto transformer. Can we see a basis of this compuattion in the International electrical code? |
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Apr 30 2009, 01:06 PM
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#2
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Junior Member Group: Members Posts: 3 Joined: 6-April 09 Member No.: 5,293 |
QUOTE (Froilan @ Apr 6 2009, 08:02 AM)  How to compute for the OLR setting if the rated current is given already? Show me the computation if the motor control starter is Across the Line, Wye Delta and Auto transformer. Can we see a basis of this compuattion in the International electrical code? The OLR setting should not exceed the rated current of the motor which can be seen in the motor nameplate. OLR Setting =< Rated Current of Motor for Across the line Motor Starter OLR Setting = < 58% of Rated Current of Motor for Wye Delta or Auto Transformer Motor Starter What CP 15 International Electrical Code Stated is only the OLR setting should not exceed the Rated current of motor. Other members please help me to enlighten to enlighten up on this issue. Thanks anyway 
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Apr 30 2009, 06:52 PM
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#3
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Posting Freak Group: Moderator Posts: 2,383 Joined: 24-April 02 From: Christchurch, New Zealand Member No.: 1 |
Hello Froilan
The overload setting must not exceed the rating of the motor. In the case of a star delta starter, the overload can be installed in series with the supply, in which case the settting must be lees than or equal to the motor rating, or it can be installed in series with the windings (most common) in which case it must be set to less than or equal to 58% of the motor ratings because the winding current is equal to 1/root(3) of the line current. The overload setting for an auto transformer starter must not exceed the rating of the motor. (it does not measure winding 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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May 4 2009, 11:23 AM
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#4
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Junior Member Group: Members Posts: 3 Joined: 6-April 09 Member No.: 5,293 |
QUOTE (marke @ Apr 30 2009, 10:52 AM) Hello Froilan The overload setting must not exceed the rating of the motor. In the case of a star delta starter, the overload can be installed in series with the supply, in which case the settting must be lees than or equal to the motor rating, or it can be installed in series with the windings (most common) in which case it must be set to less than or equal to 58% of the motor ratings because the winding current is equal to 1/root(3) of the line current. The overload setting for an auto transformer starter must not exceed the rating of the motor. (it does not measure winding current) Best regards, Mark. The reason why I'm bringing this issue up because I just transfer my Job and it's very hard for me to implement the usual "right" practice in setting this Over Load Relay because they insist the 115% safe value of motors which I want them to be convinced that OLR setting should not exceed the rated current otherwise motor is not proteced already by this OLR or tends to shorten the life span of the Motor because of operating more than the rated capacity. What I understand on the 15% extra is the tolerance of the motor before it burn out but not to reach up to this setting. By right during the design stage the motor should be choosen 20% higher than the computed load. Please help me to have more inputs on the topic so I can correct the wrong practice in my present work. More Power!
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May 4 2009, 06:40 PM
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#5
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Posting Freak Group: Moderator Posts: 2,383 Joined: 24-April 02 From: Christchurch, New Zealand Member No.: 1 |
Hello Froilan
There is a 15% safety margin in the ratings of the motors to allow for tolerances in the calibration of the protection devices. Theoretically, a protection device must not trip at less than rated current and must trip at 115% rated current. Typically, with good protection relays, the average trip point is in the order of 105% rating, but commonly there are 3% - 5% tolerances in CTs etc. If you set the trip point at 115%, you can expect the motors to trip at 120% - 125% on average. This is enough to shorten the life of the motor. If the protection operates late, the motor can operate at a higher temperatures and not necessarily fail immediately. Every ten degrees C hotter you run the motor, you halve it's life, so you may not see the effects of a higher overload setting for some time. 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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