Thermal, Magnetic, and Overload Elements
Thermal, Magnetic, and Overload Elements
(OP)
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?
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?






RE: Thermal, Magnetic, and Overload Elements
"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: Eng-Tips.com Forum Policies
RE: Thermal, Magnetic, and Overload Elements
RE: Thermal, Magnetic, and Overload Elements
RE: Thermal, Magnetic, and Overload Elements
See here
http://www.tpub.com/gunners/127.htm
Google dash pot relays.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Thermal, Magnetic, and Overload Elements
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?
RE: Thermal, Magnetic, and Overload Elements
http:
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?
RE: Thermal, Magnetic, and Overload Elements
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
RE: Thermal, Magnetic, and Overload Elements
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
RE: Thermal, Magnetic, and Overload Elements
http:
RE: Thermal, Magnetic, and Overload Elements
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
RE: Thermal, Magnetic, and Overload Elements
RE: Thermal, Magnetic, and Overload Elements
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Thermal, Magnetic, and Overload Elements
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
RE: Thermal, Magnetic, and Overload Elements
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!
RE: Thermal, Magnetic, and Overload Elements
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: Eng-Tips.com Forum Policies
RE: Thermal, Magnetic, and Overload Elements
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?
RE: Thermal, Magnetic, and Overload Elements
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
RE: Thermal, Magnetic, and Overload Elements
RE: Thermal, Magnetic, and Overload Elements
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