Sizing thermal OLs on motor starters with CTs vs. starters with no CTs 1

[electromike]

I have encountered a situation with GE motor starters where the OL selection chart for a size 5 starter with CTs and a 168 Fla at 150 hp calls for almost a double size heater element than what would be expected with a 5 amp secondary current on the CT. The heaters recommended by the chart would be for about a 8 to 9 fla. I would expect about a 4 or 5 not 8 or 9. Does anyone have a explanation for this?

 

[dpc]

Make sure you are looking at the correct voltage (230 V vs 460 V).

What is the actual CT ratio?

 

[waross]

After looking at heater selection for direct and CT heaters, I suspect that the CTs are 200:10Amp.
You can't assume 5 Amp secondary CTs.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

I have the following opinion for your consideration
1. Based on 150hp 168 FLA , the voltage would be around 440-460V.
2. Agreed, for a CT rated 200/5A , the heater size 4-5A would be appropriate for (IEC) Tripping category Class 10.
3. Info: very often for (IEC) Tripping category Class 30 for long starting time , the manufacturer proposes the over-load relay matches with their CT, which is deliberately designed to saturate. This result to lower current flowing through the heater during starting, achieved a longer tripping time. That is to say, if a Protection Class CT say 200/5A would have full starting current through the OL relay, which would not achieve the IEC Class 30. Therefore a high heater is needed.
4. Attention: a) A Protection Class CT which does NOT saturate needs a higher heater fine, but it des NOT offer correct protection for over-load during running.
b) A Metering Class CT does saturate during starting (not very effectively though) would prolong the tripping time say < Class 30, but would protect the motor over-load during running with the setting 4..5A, NOT 8..9A.
5. It would be informative to hear the reason/clarification from the OEM.
Che Kuan Yau (Singapore)

 

[che12345]

CT with x/5A, or x/1A are easily available on the market in IEC or IEEE world, without exception. A x/10A CT ? May be readily available in Canada market, I don't know ?
Che Kuan Yau (Singapore)

 

[waross]

GE my not buy CTs on the open market for there starters.
GE is in the business of manufacturing electrical components.
They may easily build whatever ratio and rating CT that they want.
Special build 200:10 Amp CTs would give GE a lock on the parts replacement market.
Both the numbers and the business model may favour the use of special build 200:10 CTs.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[che12345]

1. A x/1 A CT with a load resistance of 1 Ohm, the burden is 1 VA. A x/10 A CT with the same resistance is 100 VA !. Similarly, a x/5 A CT with a load resistance of 1 Ohm, the burden is 25 VA. A x/10 A CT with the same resistance is 100 VA !.
2. In LV, both in Protection/metering class CT are usually designed say <30VA. Otherwise the iron core would be of very large size; in order to avoid saturation, maintaining the ratio error (metering CT) and ALF (on protection CT).
3. In general, there is NO reason/advantage to manufacture any "special" x/10 A CT, which deviates from the norm in the IEC or IEEE world; without exception.
Che Kuan Yau (Singapore)

 

[waross]

An exception;
GE is in business to make a profit.
The profit margin on replacement MCC parts is enormous.
Case in point, a quotation for an additional MCC section.
The manufacturer assumed that this was to add to an existing MCC.
As captive market the section was originally priced as replacement parts at about $20,000.
We informed the manufacturer that for that price we would install the section as a stand-alone, single section MCC and would be going for competitive quotes.
The price dropped to about $5,000.
With that kind of anticipated profit margin, the cost to someone else to waste 75 VA is unimportant.
Possibly 500% profit versus 20% profit.
Or, offered at $5000 or offered at $5000 plus an additional $15,000 profit.

Does GE manufacture special ratio CTs for large motor starters?
I don't know.
Is the suggestion that GE may manufacture special ratio CTs for large motor starters reasonable.
I think so.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[electromike]

x/10 ct's! Never seen this but it would explain this difference.

 

[electromike]

It is 460v/60hz

 

[electromike]

I just had to replace a tie section of a main tie main. This was sporty. This all came about because of "master electricians."