Overload relay for transformers HW question

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JasonCo

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Location
Houston, Texas
When current transformer-type overload relays are used, overload current in the motor branch circuit conductors causes ___ in the current to the overload relay.
a. a decrease
b. an increase
c. no change

I can't find anything on this in my book... I don't know enough to even guess what the answer is. If anyone knows, I was wondering how you came up with the answer, what is the logic behind it? I just have no idea, this one has stumped me
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
In a current transformer the output current is related to the input current by a fixed ratio based on the turn counts.
That means that when the primary current increases the secondary current will increase also.
It is very similar to a voltage transformer circuit, except the current is inversely related to the turn ratio rather than directly. The fewer turns in the secondary the higher the current.

Since an overload is an increase from the normal current, the output will be higher than normal too.

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Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
When current transformer-type overload relays are used, overload current in the motor branch circuit conductors causes ___ in the current to the overload relay.
a. a decrease
b. an increase
c. no change

I can't find anything on this in my book... I don't know enough to even guess what the answer is. If anyone knows, I was wondering how you came up with the answer, what is the logic behind it? I just have no idea, this one has stumped me
I don't think we should outright do your homewoork for you, because otherwise you are not learning to be an Electrician, you are just learning to be a management type by getting someone else to do the real work...

So what do you THINK might happen?
What is an OL relay doing?
Why would one use CTs in an OL relay?
How would that make a difference in how it operates?
 

JasonCo

Senior Member
Location
Houston, Texas
I don't think we should outright do your homewoork for you, because otherwise you are not learning to be an Electrician, you are just learning to be a management type by getting someone else to do the real work...
So what do you THINK might happen?
What is an OL relay doing?
Why would one use CTs in an OL relay?
How would that make a difference in how it operates?

Hey I really couldn't agree more! I want to actually learn the theory behind the question, there is nothing more important if I want to set myself apart from my peers.
What is an OL relay doing? Well, I believe it detects when there is too much current passing through the conductors and will trip the motor starter if there is an overload of current, thus killing the motor and saving it from getting damaged.
Why would one use CTs in an OL relay? When you say CTs, are you referring to Current Transformers? I couldn't answer the question though because I've never heard of current transformers or voltage transformers :/
How would that make a difference in how it operates? It would prevent the motor from getting damaged.


PS: Going off what GoldDigger is saying, the answer is C = No change. But, still I just don't get this stuff, I'd still love the help!

Edit: Well...Answer is defiantly not C... Just checked online
 
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Ingenieur

Senior Member
Location
Earth
It's a transformer to change current ratios, a current transformer or CT

How many windings in the primary? Hint motor lead
how many in the secondary? Hint, CT and it's more>1, say n
turn ratio 1:n
current goes up or down?

how big would the OL need to be to handle a few hundred amps?
practical?
or is there a better solution using a standard say 10 A OL?
how would you convert a few hundred amps to 10 A?

don't guess
or memorize
think
 

templdl

Senior Member
Location
Wisconsin
When current transformer-type overload relays are used, overload current in the motor branch circuit conductors causes ___ in the current to the overload relay.
a. a decrease
b. an increase
c. no change

I can't find anything on this in my book... I don't know enough to even guess what the answer is. If anyone knows, I was wondering how you came up with the answer, what is the logic behind it? I just have no idea, this one has stumped me
Doesn't this inquiry belong under the education topic? Uh
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
161206-0852 EST

JasonCo:

Any magnetically coupled transformer is a voltage or current transformer depending upon its application, meaning external circuitry, and how appropriate the transformer's specific parameters are to the application.

If there is very tight magnetic coupling between a primary and secondary, meaning a highly magnetic core material with the windings appropriately related, then the voltage ratio between the primary and secondary voltages is approximately the primary turns to secondary turns counts. For the current ratio it is the reciprocal of the voltage ratio.

There is no transformer where all the primary flux lines link with the secondary coil, but some transformers can come very close. To the extent that this flux linkage fails determines how accurate the transformation ratio is to the turns ratio.

When you have a rather light load on the secondary, then the transformer is close to a voltage transformer. When the load is very heavy, near a short circuit, then you are close to a current transformer.

CT means a current transformer.

A current transformer in an AC circuit is a means to use a reasonable measuring device, meter or other sensor, to measure higher current than the capability of the meter or sensor. In a DC circuit this done with a resistive shunt.

In an AC circuit you might use a 5 A full scale ammeter to measure 1000 A full scale with a current transformer with one primary turn, a wire passed thru the transformer core center once, and a secondary of 200 turns wound on the core. Or you could have 1000 turns on the secondary and pass the primary wire thru the core 5 times and still be a 1000 A full scale measuring system. But pass the primary wire thru the core once and it is a 5000 A full scale instrument.

A current transformer core for 60 Hz applications is typically made in the shape of a toroid made from a continuous strip of thin high quality magnetic material wound on top of itself. The continuous winding eliminates air gaps.

You can make your own rather good current transformer. I have a 75 W wattmeter and one day I wanted to measure a somewhat greater power level. So I took a big toroid I happen to have an wound a 5 turn secondary on it. The core was bigger than necessary, but did not introduce too much error for what I needed.

.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
161206-0959 EST

A magnetically coupled transformer is always simultaneously both a voltage and current transformer. So how you use it determines how you want to label it.

When current transformer-type overload relays are used, overload current in the motor branch circuit conductors causes ___ in the current to the overload relay.

Read this question very closely to get your answer. Current transformer has nothing to do with the answer. The CT is just excess fluff.

.
 
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