Thermal, Magnetic, and Overload Elements

[timm333]

The IEC combination motor starters have three trip elements: the magnetic trip, the overload trip, and the thermal trip. (Overload trip element and thermal trip element is NOT the same thing).

I understand that magnetic trip is for short circuit protection and overload trip is for motor overloads protection. What dose the third element (thermal trip) do?

 

[jraef]

I think you are mistaken. The overload and thermal trips ARE the same thing. Who told you they were not?


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[timm333]

Ok, are the "thermal trip overload" and "magnetic trip overload" also the same thing?

 

[sibeen]

YES

 

[waross]

I haven't seen a magnetic trip overload for maybe 30 years. It was very old then. I'd forgotten about them. Are they coming back?
See here

http://www.tpub.com/gunners/127.htm

Google dash pot relays.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[timm333]

Let me put this way. I have attached the IEC symbol for overload. It has two parts. The top part is the traditional overload symbol, it looks like this: ]

But I am not able to understand the bottom part which looks like this: I>

What is this symbol (I>), and how to show it on the motor schematics?

 

[timm333]

Let me put this way...Please see the IEC symbol for overload at page 13 of this link:

http://literature.rockwellautomation.com/idc/groups/literature/documents/wd/100-wd001_-en-p.pdf

It has two parts. The top part is the traditional thermal

symbol, it looks like this: ]

But I am not able to understand the bottom part which looks like this: I>

What is this symbol (I>), and how to show it on the motor schematics?

 

[waross]

The IEC combination motor starters have three trip elements:

From your link;

Manual Starter

Many NEMA manual starters must be protected by a breaker of fuses upstream.
SOME IEC manual starters have a construction similar to a thermal magnetic circuit breaker to give short circuit protection. This provides better coordination when group fusing is allowed.
We recently had a discussion in another forum concerning RTFM. Part of that is knowing what page you are on.
The symbols for combination motor starters are on page 12.
If you are using a manual starter, do what most other people do and ignore the symbol for the integral magnetic trip. On most other pages, a thermal magnetic trip is show the same as on page 12.
You went to enough trouble to google some diagrams, and I respect that, but you must also read them carefully.
RTFM

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

See page 5 for symbol explanation.

Square bracket ] or [ symbol is for thermal element or overload.

I> is for Magnetic or instantaneous element for IEC/Europe.



Rafiq Bulsara

http://www.srengineersct.com

 

[timm333]

I understand about the RTFM. But the problem is that if you see the page 5 of the link below, it says that the symbol I> is part of overload relay. In other words this symbol has nothing to do with the instantaneous trip of the upstream breaker or fuse.

http://literature.rockwellautomation.com/idc/groups/literature/documents/wd/100-wd001_-en-p.pdf

 

[waross]

Are you asking about combination starters as you asked, or about manual starters as you implied.
Combination starters often provide only overload protection via a thermal trip. The over current (short circuit) protection is provided by an upstream circuit breaker with magnetic trips or by upstream fuses.
Manual motor starters may have a magnetic trip added to facilitate group fusing and reduced wire sizing to the motor.
Manual motor starters with a magnetic function are similar but not identical to thermal magnetic breakers.
Some differences;
A Manual starter typically has a fixed magnetic trip and an adjustable thermal trip.
A breaker typically has an adjustable magnetic trip and a fixed thermal trip.
A manual starter may have a differential trip.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[timm333]

I am talking about combination starter. I just need an IEC equivalent of the traditional NEMA 2 starter for an industrial motor. Should I eliminate the I> part in the symbol on the single line diagram?

 

[waross]

Yes. See page 10.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

You need to speak to one of the IEC starter supplier/manufacturer and ask as if they can provide what you want or learn what do they offer and choose from it.

It is not very clear from your posts as to what you actually understand and what you need to understand.

Rafiq Bulsara

http://www.srengineersct.com

 

[ScottyUK]

The I> indicates that it is trip element which operates when current is greater than some threshold. In some circumstances the symbol I>> is sued, which is a mag trip.

Sibeen,

A thermal element and a magnetic element are not the same thing. Or were you answering a different question?


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[jraef]

Just because Allen Bradley says something does not make it so... You are right, A-B placed the symbol description in a category they called "Overload Relay" on page 5 of that document. But that is an incorrect reference point. As much of a shock as this will be to some people, they messed up!

But if you read further, you will see that the only place they actually show it in a schematic power diagram is on page 13 as part of a Manual Motor Starter.

But they are not the end-all / be-all for the electrical world, especially when it comes to things IEC. That chart is essentially attempting to rationalize a comparison of JIC / NEMA symbols to IEC symbols; useful, but by no means definitive.

Outside of North America, what we would call a "Manual Motor Starter" is just a manual disconnect device with a thermal trip element. Not so for IEC. They can use it as a Motor Protection Switch because the IEC device has all three elements necessary in a motor protection circuit: a Disconnecting means, Motor Overload relay (thermal element) and Short Circuit Protective Device (SCPD), the magnetic trip element. So when people use them in "combination motor starters", they need only add the contactor.

In traditional NEMA controls, no such all-in-one device exists, so we have a separate Disconnect, SCPD (fuses) and Overload Relay (thermal) attached to the contactor. The only variance is that the SCPD can be a Circuit Breaker, which also acts as the disconnect device, and can come with Magnetic-Only trip elements (often called an MCP Breaker) or a regular Thermal-Magnetic Circuit Breaker. When we use a T-M CB in a combination motor starter, the Thermal element in the breaker becomes redundant to the Overload Relay and is generally ignored as it will by design need to be selected at a size too high to protect the motor anyway. Even a Manual Motor Starter in NEMA parlance still needs the SCPD (CB or fuses) ahead of it.

So the reason I went to this length is to point out that you can't really make a direct comparison because it really depends upon how you are planning to put together your IEC motor starter: Disconnect / SCPD + Contactor + OL, or IEC Motor Protective Switch + Contactor. Either way is valid in IEC designs, although those Motor Protective Switches have limited sizes available.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[timm333]

Thanks Jraef for the great explanation. I still have the question about "symbol”.

1. If I use “SCPD+Contact+OL” then can I use the IEC symbol at page 13 with a contactor added at the point where it says U-V-W?

2. If I use the “motor protective switch + contactor” then can I use the IEC symbol at page 13 with a contactor added at the point where it says U-V-W?

 

[waross]

I'm starting to wonder if you have ever seen a combination starter or if you have any idea what it is.
You are trying to convert a NEMA drawing to an IEC drawing. Despite several good explanations, you persist in asking for permission to use an IEC symbol for a device that doesn't exist in the NEMA world.
Have you been called to task for using an improper symbol and are now trying desperately for justification?
Wait, I have a sudden inspiration.
Why not use the IEC drawing for a combination starter to depict a combination starter.
That would be on page 12. You have permission to change the symbol for a fused switch to the symbol for a breaker if need be.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[timm333]

Well why not you admit that you also don't know what you are saying ... If I replace the fuse symbol with circuit breaker symbol on page 12, there will be "two" overloads in the same one line, because the IEC symbol for a circuit breaker already has one overload built in it. Does not make any sense to me to use two overloads in the same one line.

 

[waross]

Yes that would be the symbol for the overload element in the breaker and the symbol for the overload element in the starter.
Every time I count them I get two.
But, every time I look at the IEC symbol for a breaker on page 2, I don't see any overload element shown.
What's the back story to this.
You asked a question and don't like the answer.
There's got to be a back story.

Bill
--------------------
"Why not the best?"
Jimmy Carter