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Thermal overload relay selection

Created by: jeet2907 at: 7/3/2013 1:05 PM (22 Replies)

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7/5/2013 11:15 AM	Rate (2)
DILIP_1111 Posts: 43 Rating: (9)	Hi dear, Below explanation may be helpful for your query. 1. Size of Over Load Relay For a star-delta starter there is a possibility to place the overload protection in two positions, in thelineor in thewindings. OVERLOAD RELAY IN LINE: In the line is the same as just putting the overload before the motor as with a DOL starter. The rating of Overload (In Line) = FLC of Motor. Disadvantage:If the overload is set to FLC, then it is not protecting the motor while it is in delta (setting is x1.732 too high). OVERLOAD RELAY IN WINDING: In the windings means that the overload is placed after the point where the wiring to the contactors are split into main and delta. The overload then always measures the current inside the windings. The setting of Overload Relay (In Winding) =0.58 X FLC (line current). Disadvantage: We must use separate short circuit and overload protections.
DILIP_1111 Posts: 43 Rating: (9)
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7/5/2013 12:19 PM	Rate (1)
@nkur Posts: 202 Rating: (16)	Hi Ste39 It means i have to go for that OLR which is 50% to that of the motor
@nkur Posts: 202 Rating: (16)
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7/5/2013 2:54 PM	Rate (0)
Ste39 Platinum Member Joined: 1/21/2013 Last visit: 10/23/2024 Posts: 4620 Rating: (708)	Hi @nkur, To put the thermal protection to 50% you have to put in the winding which exchanges a delta (not below the line), but you must protect at least upstream of the contactors using fuses.

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7/15/2013 12:08 PM	Rate (0)
Jason85 Posts: 39 Rating: (9)	Selection of overload relay in Y-D starter is a bit tricky.. and one needs to understand Y-D starting properly. Anyways, the overload relay should be connected within Delta Circuit only and its setting range must correspond to the load current in the motor windings themselves, i.e. 1/ root 3 times the line current drawn by the delta connected motor. The overload relay is therefore set to 0.58 times the operational full load current of the motor and provides overload protectionin both star and delta modes. Refer the circuit diagram to understand the wiring and position of mounting etc. Attachment Y_D wiring.jpg (780 Downloads)
Jason85 Posts: 39 Rating: (9)
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7/16/2013 10:58 AM	Rate (0)
king_1985 Posts: 109 Rating: (11)	Valli Since, In delta the current reduces to 1/1.732 = 57.7% that of star contactor we acheive reduction in starting current. In delta current reduces than star?????? Are you sure??
king_1985 Posts: 109 Rating: (11)
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7/16/2013 11:25 AM	Rate (0)
Ste39 Platinum Member Joined: 1/21/2013 Last visit: 10/23/2024 Posts: 4620 Rating: (708)	Hi king_1985, I think Valli saying that when you switch to the delta, current is reduced because half goes in the line contactor and half passes into the delta contactor. That's just my idea.

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7/16/2013 12:34 PM

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Jason85

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Star-Connected Balanced Load

Delta-Connected Balanced Load

Phase current=Line current:

Phase voltage=Line voltage

7/16/2013 6:39 PM	Rate (0)
king_1985 Posts: 109 Rating: (11)	Ok.. I think he talked about phase current and i was talking about line current But we should take consideration of Line current because OLR is to be mounted in line current, right???
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7/17/2013 7:13 AM	Rate (0)
jeet2907 Posts: 235 Rating: (52)	king_1985 Ok.. I think he talked about phase current and i was talking about line current But we should take consideration of Line current because OLR is to be mounted in line current, right??? No dear in star delta we mounting OLR in phase current(mounting OLR exectly below to the main contactor),we can also put OLR in line current but we cant do that bcoz the main reason of money saving.If we put OLR in with line current (or say above the main contactor )we require relay range similar to the FLC so its higher rating and its cost is more than halved rating.Its a main reason to put OLR in phase current. For your refrence please find the attach sheet of star-delta wiring drg and control too. Attachment stra-delta scheme.pdf (742 Downloads)
jeet2907 Posts: 235 Rating: (52)	Last edited by: jeet2907 at: 7/17/2013 7:13 AM
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7/17/2013 10:46 AM	Rate (0)
@nkur Posts: 202 Rating: (16)	Hi there Does the Line current or Phase effects the mounting or rating of OLR??? please explain....
@nkur Posts: 202 Rating: (16)
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