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kazengg
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Join Date: Apr 2011
Posts: 43

Selection of Thermal Overload

05/15/2013 3:38 PM

Hi

i have only following specification from my client, and has to buy thermal overload,

0.75 kw, 380 v, 3-phase, 50 Hz, geared induction motor

I=750/(380*1.732*.8(pf)*.8(eff))=1.78*1.25(25%)=2.25 Amp so overload 2-3 amp

i have taken these calculation from internet, practically this overload will be fine?
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Fredski
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Join Date: Feb 2011
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#1

Re: selection of thermal overload

05/15/2013 3:41 PM

I think your client needs to do a bit better job in doing their due diligence in selecting who they use as outside contractors
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jack of all trades
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#4
In reply to #1

Re: selection of thermal overload

05/15/2013 4:05 PM

He could be the consultant not the contractor.

Jack - Oh I could tell you stories about consultants.

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Fredski
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#7
In reply to #4

Re: selection of thermal overload

05/15/2013 8:49 PM

good point
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lyn
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#2

Re: selection of thermal overload

05/15/2013 3:54 PM

Are you charging your client for your services?

If so, please see #1.

The overload will be fine, till it blows.
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jack of all trades
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#3

Re: selection of thermal overload

05/15/2013 4:03 PM

Best you go back and check the motor's actual and starting current (and other real-world characteristics) also.

Due diligence and all.

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redfred
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#5

Re: selection of thermal overload

05/15/2013 4:06 PM

Toy Boat,

Toy Boat,

Toy Boat,

Toy Boat,

Toy Boat,

Toy Boat,

Toy Boat,

Toy Boat,

Is this a homework question or a demonstration of your competence?

Toy Boat,

Toy Boat,

Toy Boat,

Toy Boat.

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lyn
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#6
In reply to #5

Re: selection of thermal overload

05/15/2013 4:14 PM

In defense of this person who so casually includes "engg" in their name, I can say with certainty that they demonstrate complete incompetence in many fields.

A review of their stimulating historical questions proves this conclusively.
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kazengg
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Join Date: Apr 2011
Posts: 43
#8
In reply to #6

Re: selection of thermal overload

05/15/2013 11:14 PM

Thanks Mr. Guru for ur comments. If u please pin point the questions from where u judged my incompetence. Incompetence can never be judged by questions.
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leveles
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#9

Re: Selection of Thermal Overload

05/16/2013 9:54 AM

Come again: when and how did thermal overload requirements get translated into overcurrent?!? The last time I looked, those are physically different breakers for different purposes. They look different, and placed different.

The trip points:

one in Celsius

the other in Amperes

How does one gets converted into the other?
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JRaef
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#10
In reply to #9

Re: Selection of Thermal Overload

05/16/2013 10:50 AM

Thermal overload relays do exactly that. They are current sensing devices pre-calibrated to mimic the thermal damage curves of motor winding insulation based on the amount of current flowing to the motor. That concept goes back about 100 years now. Keep up.

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leveles
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#12
In reply to #10

Re: Selection of Thermal Overload

05/16/2013 12:17 PM

I DO keep up.

Over many years I found myself in agreement with you, and learned a lot of high power and Variable Frequency Drives.

This time I was talking from experience, trying OP to open up. Many motors, I dealt with (in the few and sub KVA range) have a fuselike device attached to the winding, or a bimetallic resettable switch (frequently inadequate quality), singularly for overtemp. sensing, and tripping.

Having a long life in electronics and lower power apps. R&D, mostly, I think you misstepped on this one.
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JRaef
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#13
In reply to #12

Re: Selection of Thermal Overload

05/16/2013 4:27 PM

Sorry, when posting from my iPad, I don't have access to the "smileys". That should have had one of these... because I was messing with you.

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leveles
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#14
In reply to #13

Re: Selection of Thermal Overload

05/16/2013 4:34 PM

Fine, have a good day.
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jack of all trades
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#15
In reply to #13

Re: Selection of Thermal Overload

05/16/2013 7:08 PM

You know back in my day we didn't have "smileys" but we made do.

;)

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JRaef
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#16
In reply to #15

Re: Selection of Thermal Overload

05/16/2013 8:20 PM

But we didn't have internet forums either.

We talked face to face, where winks and nudges were more obvious.

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JRaef
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#11

Re: Selection of Thermal Overload

05/16/2013 10:57 AM

Despite the abuse that you may or may not deserve, you need to state your problem a little better. Overload settings MUST by definition be based upon the motor NAMEPLATE FULL LOAD AMPS, not a calculation, because your calculation has to assume facts not in evidence.

However if you GOAL is to pre-select and appropriate RANGE of Overload Relay adjustment because you do not yet HAVE the motor nameplate data, then you have achieved your goal, but "your mileage may vary" as they say in car commercial disclaimers. Motor efficiency and base power factor have a lot to do with the FLA results and with a small adjustment range comes a risk of being off by just an RCH.

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Users who posted comments:

Fredski (2); jack of all trades (3); JRaef (4); kazengg (1); leveles (3); lyn (2); redfred (1)

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