am2 motor overload

[maddad2001]

Hi

has anyone recently done the am2? I just wanted a basic calculation for setting the overload (they give you motor rating in Kw) for the 3 phase dol motor. I have tried looking this up for last few nights, but keep getting inconsistent or over complicated findings.

and people at work say 'sorry mate it's been along-time since I worked that out, it should say on the motor plate'. But the assessor doesn't provide you a motor plate, just tells you the kw of starter/motor. I tried to find the answer myself, but I'm unsure and bit worried about this, mainly due to different interpretations. If anyone can help it would be hugely appreciated.

thanks for reading this.

 

[telectrix]

work out the phase current, taking p.f. into account, then set overload 10% over.

 

[Knobhead]

For a start you DO NOT set at +10%. You set at 100%. The O/L unit is designed to take start up in to account.

KW/3/(U/√3)/PF=A

So for 5KW
5000/3/230/.9=7.2A

If the PF isn’t given assume .9 (ask first if you need to take this in to account)

The +10% or even +25% settings went out with the ark.

 

[Knobhead]

work out the phase current, taking p.f. into account, then set overload 10% over.

Sorry Tel, that's rubish.

 

[maddad2001]

Thanks alot Tony

 

[Darkwood]

The Kw rating rating of a motor is the actual available turning power given in mechanical terms at the shaft .. note its not an electrical quantity, its a modern version of horsepower hence you problem finding a relative link ... the FLC (full load current) of a motor is what it will use to generate this mechanical power.

Any motor you connect direct on line will have a plate stating its FLC at a supplied voltage you are using and this is the setting of your O/L that Tony has quite rightly expressed, this is for direct on line starting only when other starting methods are used then the rules can change.... Im a little disturbed if the college is giving you only a KW rating and expecting you to know how to set the O/L .... I suspect you will find 2 values on the motor plate for each winding config' and you choose the one your using ...probably 400v in your case but ask someone who has done the exam in the training area.to confirm this as our many yrs of experience can give an over complicated answer to your query.

In tels defense the old ancient O/L pot were not as accurate as modern ones and the likes of the oil filled ones etc had to have a rule of thumb allowance applied hence his suggestion.. but he did see the birth of electricity :smilewinkgrin:

Just to add to the above the efficiency of the motor needs to be added to the calculations for accuracy too although its often skipped.

 

[Knobhead]

OK I should have included efficiency in the calculation which is the relationship of shaft KW to power input. That unfortunately isn’t a linear graph.
That information can only be obtained from the manufacturer or by calculation from the figures on the makers plate, which our OP won’t have access to.

Not knowing the precise requirements of the examination it’s hard to know where to stop.


DW
Dashpot O/L’s are old technology, but they’re still out there.

For motors you only set above 100% if the manufacturer recommends it. I’ve worked on dash pot O/L’s that have dated from the 1930’s to the 1970’s. They were all different.

It’s when they are linked to CT’s the fun and games really start because they are just marked in %, typically 70% to 130%. When the CT ratio is 2500/5 the sphincter starts twitching a bit.
Then there’s the minor problem of time delay, adjustable by the bleed through rate or oil viscosity. Silicon oil was available in 100, 200, 500 and 1000cS (Stokes are a measure of viscosity), blending it is an art in itself.

It’s one of the classic cases of RTFM. If you can find one.

 

[telectrix]

Sorry Tel, that's rubish.

rebuke accepted. :6:

 

[clematis432]

Becoming far too complicated, "Linear graphs" / "Motor efficiency" / "Bleed through rates" / "CT ratios" / etc,etc. With DOL motors(the vast majority are) set the overload at the stated nameplate FLC as long as supply voltage and motor voltage are the same. Thermal overloads are how accurate anyway ? You could argue all day about the precise setting of individual motors, can,t help thinking people are just showcasing their knowledge here, sorry.

 

[polo1]

Becoming far too complicated, "Linear graphs" / "Motor efficiency" / "Bleed through rates" / "CT ratios" / etc,etc. With DOL motors(the vast majority are) set the overload at the stated nameplate FLC as long as supply voltage and motor voltage are the same. Thermal overloads are how accurate anyway ? You could argue all day about the precise setting of individual motors, can,t help thinking people are just showcasing their knowledge here, sorry.

Have to say I think all the contributions, including yours, are valid. It's good to see the rationale/detailed explanation as well as the quick "do it this way & it'll be ok". And so a bit of "here's what I know" does no real harm:smilewinkgrin:

 

[telectrix]

right or wrong, i was taught the 10% over to allow for uncalculated losses, like not knowing the exact pf and efficiency figures. i defer to tony's greater knowledge esp. with bigger motors.

 

[Knobhead]

Becoming far too complicated, "Linear graphs" / "Motor efficiency" / "Bleed through rates" / "CT ratios" / etc,etc. With DOL motors(the vast majority are) set the overload at the stated nameplate FLC as long as supply voltage and motor voltage are the same. Thermal overloads are how accurate anyway ? You could argue all day about the precise setting of individual motors, can,t help thinking people are just showcasing their knowledge here, sorry.

I gave the simplest calculation in my first post, it should do for any examination. The examiners can’t ask for more without giving the full details of the motor. That would defeat the object of the examination.
Read the makers plate, set the O/L. It would be that easy.

Motors, O/L’s and control have been my bread and butter for 40 years. If anyone wants to learn then I’m more than willing to go in to detail on a one to one basis.

 

[plugsandsparks]

Tony, is the math correct ? I get around 8amps,

 

[clematis432]

I gave the simplest calculation in my first post, it should do for any examination. The examiners can’t ask for more without giving the full details of the motor. That would defeat the object of the examination.
Read the makers plate, set the O/L. It would be that easy.

Motors, O/L’s and control have been my bread and butter for 40 years. If anyone wants to learn then I’m more than willing to go in to detail on a one to one basis.

Read the maker,s plate set the o/l to FLC it IS that easy, in the vast majority of cases. I just feel this subject can be made too complex when really there is no need. I don,t disagree with the post either referring to a 110% setting, as the argument between nuisance trips and motor safety is another one where there is no definitive. Motors and control gear have also been my bread and butter for around 40yrs though I do not claim to have anything like the detailed knowledge you have and I mean that with the greatest of respect.

 

[davesparks]

Read the maker,s plate set the o/l to FLC it IS that easy, in the vast majority of cases. I just feel this subject can be made too complex when really there is no need. I don,t disagree with the post either referring to a 110% setting, as the argument between nuisance trips and motor safety is another one where there is no definitive. Motors and control gear have also been my bread and butter for around 40yrs though I do not claim to have anything like the detailed knowledge you have and I mean that with the greatest of respect.

The OP has stated that the data plate is not available, only the KW rating

 

[DanSwift1988]

Ask for the power factor of the motor. Then calculate as follows -

EXAMPLE!!!

1.5Kw Power of the motor (will be written on it)

over

1.732 (square root of 3, 3 phase) x 415 (voltage) x 0.8 (example of the power factor he might give you).

1500 / 1.732 X 415 X 0.8 (575.024) = 2.60

 

[Knobhead]

Tony, is the math correct ? I get around 8amps,

Ooopps, I use a spreadsheet and which gives three figures. Uncorrected, corrected for PF and corrected for PF & Eff. I glanced at the uncorrected figure.

 

[Darkwood]

I read up on the newer AM2 test the plate doesn't exist to take a value off because they don't have a motor it a box with terminals pretending to be a motor, also to note the harder sections of the AM2 have been dropped... no more steel conduit to a light fitting to bend which was the key area of failure in my day...I mean really dont understand why they bother doing it anymore its been simplified that much might as well give them a pass because they found the exam center!

 

[Bobster]

I read up on the newer AM2 test the plate doesn't exist to take a value off because they don't have a motor it a box with terminals pretending to be a motor, also to note the harder sections of the AM2 have been dropped... no more steel conduit to a light fitting to bend which was the key area of failure in my day...I mean really dont understand why they bother doing it anymore its been simplified that much might as well give them a pass because they found the exam center!

I visited a test centre last year for a look, the new AM2 is nowhere near at the standard it once was. In fact I thought it was a bit of a joke.

As for the motor, in the exam. It is now wired to terminals on a box. Efficiency doesn't come into play.

 

[Knobhead]

So what is required for this “test”?