DOL settings

[pitkin2020]

Hi all currently doing my AM2 and stuck with the settings for the DOL, I cant remember for the life of me how to work out what setting to use, been over my notes and nothing.
Any help, the motor is a 0.37kw motor 3 phase

Cheers

 

[telectrix]

do you mean the overload setting?

 

[pitkin2020]

yes sorry missed that bit out lol

 

[pitkin2020]

anyone???

 

[Lucien Nunes]

DOL is a method of starting (Direct on-line), there is not a thing called a 'DOL'. Rephrasing your question as 'How do I work out the setting for the overload relay in a DOL starter' then we will need to know the motor's line current as that is what the overload relay monitors. This could be found either by research or calculation. Where would you look for line current information for a particular motor? Or if you want to calculate, which parameters will you need and do you know what they are for this particular motor? Obviously the output power is 0.37kW but what others will you need?

 

[pitkin2020]

i really dont know, the only info i have is whats listed on the box (i.e the fakemotor), and i know its 3 phase.
been through all my college notes and the science book i have with me and nothing covers the overload

 

[pitkin2020]

IIRC you need to calculate the FLC A= KW X 1000/ 1.73 X V X PF, therefore 0.37 x 1000 / 1.73 x 230 = 0.9?? i think lol

 

[telectrix]

IIRC you need to calculate the FLC A= KW X 1000/ 1.73 X V X PF, therefore 0.37 x 1000 / 1.73 x 230 = 0.9?? i think lol

close, but it's a motor so should have a PF stamped on the rating plate. if in doubt, 0.8 is a reasonable assumption.

 

[Lucien Nunes]

Is the 0.37kW the electrical input power or the mechanical output power of the motor? If the output power, what other parameter would have to be allowed for in the calculation of line current?

BTW, if you have only been given the data '0.37kW, 3-phase' as you seem to be suggesting, then you can't answer the question. You are assuming the voltage but one should never assume...

 

[pitkin2020]

no rating plate, as i said its literally a box with a label that says "MOTOR 0.37KW" that is literally it lol

 

[pitkin2020]

Is the 0.37kW the electrical input power or the mechanical output power of the motor? If the output power, what other parameter would have to be allowed for in the calculation of line current?

BTW, if you have only been given the data '0.37kW, 3-phase' as you seem to be suggesting, then you can't answer the question. You are assuming the voltage but one should never assume...

Im not suggesting anything lol,the box says "Motor 0.37KW", nothing else other than 4 test points, one for each phase and an earth as its a simulated motor. Yes i have to assume the voltage as i have no way of testing voltage on a dead circuit, which can not and will not be livened up until this stage has been completed

 

[pitkin2020]

close, but it's a motor so should have a PF stamped on the rating plate. if in doubt, 0.8 is a reasonable assumption.

ty, i wasnt sure wether to use the 0.8 or not, give a figure of about 1.2 using that

 

[Lucien Nunes]

Assuming it's 400V, you've got to either calculate or look up the FLC for a 0.37kW motor on 400V. It's always best to check the rating of the actual motor in front of you - 99% of all motors will give the FLC on the rating plate. As your 'motor' doesn't have a plate, you can look up typical currents from motor data tables - there will be some variation from one maker to another but they will be close. Failing that, you can calculate. The 0.37kW is the output power (1/2 horsepower in old money) so to find the electrical input power you must include a term for the efficiency.

Line current = output power / (efficiency x power factor x line voltage x sqrt(3))

We don't know the efficiency and power factor so we will have to estimate them based on the size of the motor. For a modern motor, not knowing if it is 2-pole or 4-pole, I would say 70% efficiency and 0.8pf. Thus:

I=370/(0.7 x 0.8 x 400 x 1.732) = 0.95A FLC

Older or less efficient motors might a bit higher, perhaps 1.1-1.2A. Have a trawl through manufacturers data online and see how these compare to real motor catalogue figures.

 

[pitkin2020]

TY lucien your a star. There is a table printout in the AM2 notes but there was too much info to remember and i cant write anything down butwith your forumla i can atleast work it out now assuming they is some more info in the write up.
TY again

 

[netblindpaul]

Lucien is not wrong, but, you should NEVER in real life calculate motor current from the information you have been given.

If I was doing the AM2, I would be writing to tell them why they were wrong, and explaining why, and, should I fail because of it, then they would be facing my full wrath, which they would not be liking.

Thank goodness, I don't have to take this stupid exam.

 

[pitkin2020]

Lucien is not wrong, but, you should NEVER in real life calculate motor current from the information you have been given.

If I was doing the AM2, I would be writing to tell them why they were wrong, and explaining why, and, should I fail because of it, then they would be facing my full wrath, which they would not be liking.

Thank goodness, I don't have to take this stupid exam.

i appreciate that but i feel they wont care. I heard last week someone failed because they used 2.5mm singles instead of 1.5mm, whilst i understand it wasn't to spec seems a bit harsh for an outright fail, and the practical section is the most expensive to retake

 

[Lucien Nunes]

There is a difference - in this case based on what you've stated, the question is too vague. Exactly as Paul says, calculating FLC is a matter of last resort because a) you usually know it already from the motor data, and b) you usually don't know the other parameters accurately. Setting the O/L relay correctly is important for safety and reliability, so it should not be based on approximate or assumed figures.

 

[netblindpaul]

i appreciate that but i feel they wont care. I heard last week someone failed because they used 2.5mm singles instead of 1.5mm, whilst i understand it wasn't to spec seems a bit harsh for an outright fail, and the practical section is the most expensive to retake

Well yes they should fail if the spec says 1.5 & they have 1.5 available, but choose to use 2.5.

However, if the question is fundamentally technically incorrect/inadequate/flawed, then they would be getting a bullet off me, and trust me, they would not be dodging it.

I simply would be not doing that part of the exam and, getting the examiner to acknowledge that it was an unsound section.
My local AM2 centre would not question me, I know, but that is not the point.

In fact, if that is the situation, then I will be visiting my local AM2 centre again, I was there earlier on in the week, and it looks like I will have to return, and check this exam out, because if it is wrong, then it needs changing.

If what you have said is right, then the exam is wrong, and people could be failing due to the exam being wrong, and that is wrong.