Setting up instantaneous breaker and thermal over load

[ctaylo360860]

Hi I’m hoping to get some advice on setting up the instantaneous breaker and thermal overload for a drag motor on a grain bin system... the drag motor needs to stay on so that while they’re running leg and filling up the south bin it doesn’t plug the drag and spout back to the distributor. The motor is fed with #6 copper thhn. Thanks in advance all help and advice is much appreciated!

 

[JoeStillman]

The instantaneous-only breaker should be set for 800% (max) of the FLA. This is a dual voltage motor but your post doesn't say which. I'm going to assume it's 460 because somebody selected #6 CU so 45 FLA. 45 x 8 = 360 so the highest setting on the mag-only breaker would be 360A See NEC 430.52.

The overloads are covered in NEC 430.32. Because the S.F. is 1.15, the max overload setpoint is 125%. 45 x 1.25 = 56.25A, again, that is a maximum setting.

I'm assuming you want them set to max because you mention it "needs to stay on".

 

[ctaylo360860]

The instantaneous-only breaker should be set for 800% (max) of the FLA. This is a dual voltage motor but your post doesn't say which. I'm going to assume it's 460 because somebody selected #6 CU so 45 FLA. 45 x 8 = 360 so the highest setting on the mag-only breaker would be 360A See NEC 430.52.

The overloads are covered in NEC 430.32. Because the S.F. is 1.15, the max overload setpoint is 125%. 45 x 1.25 = 56.25A, again, that is a maximum setting.

I'm assuming you want them set to max because you mention it "needs to stay on".

So I messed up and posted the wrong picture at the beginning of the forum. the motor runs on a 480v system its a 9 lead motor and its wired up for high voltage. (4,7) (5,8) (6,9) (1,L1) (2,l2) (3,l3) the motor is code h,des.a, and class f. if you look at exception no. 1 under 430.55 it will allow you to go up to 1300% of the motor full load current on motors other than design B. if the motor were design b I should be able to go up to 1700% if needed. 48x11 is 528. when I set the it breaker to this value it instantly trips... 48x13=624/ I then set the Instant trip breaker at the next value which is 631. I got the breaker to hold, but I'm just worried once i run product it will trip, and i will have a much bigger issue... I found out the chain was loose so we tightened it, and i still have the same problem as before it trips instantly at the lower value of 528, but holds at 631... Checked the gear box and it was filled to the correct level... The only thing i haven't done is take the belts of the motor shaft so its free and see if it will hold at a lower value. the first picture of the name plate is a 40hp aeration fan. This fan is only about 45' away from the mcc, and has the exact same components and it also will trip out at the lower value of 528. i turned it up one value to 631, and it also will hold. at this point I'm at a loss and need to start running product. im going to set the overload at 50amps and it should trip if the motor starts to pull 125% of that value to protect it. i was just curious to why the old electrician had set everything to the max on the old existing identical system.. seems like he had no good reason other than ignorance... basically if that drag motor stops running it was plug the leg and distributor spout which is going to be pricey and a pain to fix as its all about 80feet in the air. I want it to stay on, but still be safe so that it doesn't catch fire.... thanks again everyone for all the help i really apricate it!

 

[topgone]

Your overcurrent setting should be set no greater than 125% of the motor's FLA. if you are using a thermal overload relay, most TOL relays incorporate a 115% of FLA on their setting ( if your FLA = 45A, setting the TOL at 100% will set the TOL to trip at currents beyond 51.75A (115% of 45).

The instantaneous setting will be dealing with the protection of the motor during short circuits at the motor terminals. You should compute for the available short-circuit current if a short occurs at the motor terminals. By exception 1 in the code, you are allowed to set the Inst trip at no greater than 1300% of FLA (for other than Design B, and no greater than 1700% for Design B motors). Or, you have to prove that your Instantaneous setting works, after an engineering evaluation.

 

[ctaylo360860]

Your overcurrent setting should be set no greater than 125% of the motor's FLA. if you are using a thermal overload relay, most TOL relays incorporate a 115% of FLA on their setting ( if your FLA = 45A, setting the TOL at 100% will set the TOL to trip at currents beyond 51.75A (115% of 45).

The instantaneous setting will be dealing with the protection of the motor during short circuits at the motor terminals. You should compute for the available short-circuit current if a short occurs at the motor terminals. By exception 1 in the code, you are allowed to set the Inst trip at no greater than 1300% of FLA (for other than Design B, and no greater than 1700% for Design B motors). Or, you have to prove that your Instantaneous setting works, after an engineering evaluation.

Correct I set the ol at 50. motor name plate is 48 amps. What I’m having trouble with is I have to go a little past the 1300% to get my design a motor to start...if I set it at 1100% which is 528 I get an instant trip if I go up to the next value on the breaker which is 631 I’m 7 amps over the 1300% which is 624amps. I had the motor still trip on me when trying to start up a couple times. I’m worried that the motor will trip out once we run product through it And plug everything... the motor doesn’t get product to it until it is completely started and running at full speed. I looked up the starting current of the motor and it’s only 347amps do you think the 40hp motor is undersized and not big enough to get everything moving inside the drag on start up? When I got the motor started it’s only pulling about 26 amps with no product. Do

 

[JoeStillman]

You're pulling more than the motor's nominal LRA. A Class H motor should draw 6.3-7.09 kVA/HP at locked rotor. So a 40 HP motor will draw between 252 and 284 kVA which at 460 V is 316 to 356 Amps. Why so much inrush current? I wonder if you have excessive voltage at the motor terminals?

Also, design A and B motors have lower starting torque than design C. Design A & B are for low-torque, centrifugal loads like fans and pumps while design C and D are for conveyors.

 

[ctaylo360860]

Thank you for the information. I will check voltage with and with out the motor running again and post the results. What I remember is voltage being like 512 volts phase to phase...

 

[Little Bill]

Duplicate to this...........

Setting up instantaneous breaker and thermal overload for drag motor..

Hi I’m hoping to get some advice on setting up the instantaneous breaker and thermal overload for a drag motor on a grain bin system... the drag motor needs to stay on so that while they’re running leg and filling up the south bin it doesn’t plug the drag and spout back to the distributor. The...

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