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Hi just a question on setting the thermal overloads of a three phase motor. I have heard a few electricians state a few things regarding what it should be set at
1. Get operational current of motor and set it just above that reading
2. Set it at full load current of the motor
3.set it at 110% of full load current this motor is 85.8amps full load current so would be set at 94amps

I am asking this question as I don't know what is right are they all correct in there own ways? Plus some of the dials and numbers on a thermal overload relay are not so precise so I seen one motor that was set at just below 90amps basically in between 85 and 90 amps but the full load was 85.8amps. Would this thermal overload relay still be protected at this setting so motor doesn't cut out at full load but trips when gets slightly above full load current say 2 to 3 amps above? Was hard to too what the dial was at?
would be much appreciated if anyone would get back?
Thanks
 
What is the starting method for the motor?
If DOL - O/L should be set to the FLC of the motor unless you are de-rating because you have calculated motor duty and want to use the O/L as an early warning set up rather than a limit reached trip.
If Star/Delta then the location of the O/L can effect the setting required could be FLC if it monitors all the lines to the motor of at 58% of FLC if it only gives partial monitoring again these setting can be de-rated if you have calculated or measured current demand under full load.


If a motor is running at its FLC plated current and trips your O/L set at the same then it really needs looking into as the duty of the motor shouldn't really be 100% or its designed wrong or has running issues.
 
Thanks for your reply. Yeah it's DOL starter. I was just wondering if the overloads would still come into play and work if it is set above the full load current. So for example if the full load current was 85.8amps then if the setting was set in between 85amps and 90amps then the overload is still protecting it from overloaded current of the motor. As the setting had to be full load current. As the dials between 85amps and 90amps were pretty vague? reply would bench appreciated

thanks
 
Set at FLC plated or near as dammit that can be determined by the O/L dial ... a few amps won't be a big issue ...like I said if its running at 100% then you have issues somewhere either in design or operation... running a motor slightly over its plated current means it may start to getting heat build up faster than it can cool itself down and you end up with a runaway temp' rise but hopefully the O/L or temperature monitor cuts power 'if fitted' very early in this scenario.

In your case just set at 85amps if it trips at this find out why the motor is at its limit and resolve then you will have caught a problem before damage has been caused to the motor and the O/L has done what it was designed to do.
 
Set at FLC plated or near as dammit that can be determined by the O/L dial ... a few amps won't be a big issue ...like I said if its running at 100% then you have issues somewhere either in design or operation... running a motor slightly over its plated current means it may start to getting heat build up faster than it can cool itself down and you end up with a runaway temp' rise but hopefully the O/L or temperature monitor cuts power 'if fitted' very early in this scenario.

In your case just set at 85amps if it trips at this find out why the motor is at its limit and resolve then you will have caught a problem before damage has been caused to the motor and the O/L has done what it was designed to do.


Yeah so even if the motor is set in between 85 and 90 amps so it's sitting at say 87,88,89 a few amps higher won't make a difference to the affect to the motor. So once past this point I have set it at between the 85 and 90 it will trip the overload? As this was a previous job and can't get back to it?

thanks
 
There will be a bit of tolerance in the motor but don't rely on it in future just set to FLC unless star/delta then read the instructions you get with it on this occasion don't worry too much just rectify if ever back on site. The motors I deal with I set at 85-90% of FLC and due to the duty of the machines if they trip I increase to 100% and get machine running again and program a service of roller bearing etc ... or whatever is upping the current demand from normal, this can avoid longer down-times as I usually have a site maintenance engineer to adjust it for me and I pop in when I can to evaluate the rise in duty.
 
It’s amazing how the myth continues about setting overloads at100%+.

Under some circumstances you do set them higher than FLC, not many will work on such motors.
 
What about dash pots tony?

I would still set them to 100% unless the manufacturer’s destructions say otherwise.

The old motors that usually had dashpot O/L’s were built like tanks and would take prolonged abuse. The old crane duty motors would easily handle 125%+.
 
You can still get Allen West spares. After the company shut down Allen West (Brighton) started up to supply spares.

They are very helpful.
 
I would still set them to 100% unless the manufacturer’s destructions say otherwise.

The old motors that usually had dashpot O/L’s were built like tanks and would take prolonged abuse. The old crane duty motors would easily handle 125%+.

Hi tony thanks for getting back the overload settings were a bit vague so do you think if it has been set between or just below 90amps that the motor will still be protected for overload as full load current is 85.8amps? Replies will be much appreciated?

thanks
 
If a motor is running at its FLC plated current and trips your O/L set at the same then it really needs looking into as the duty of the motor shouldn't really be 100% or its designed wrong or has running issues.

I agree with all of what you posted but would like to comment on that last bit if I may.
Much of what I do is power electronics, particularly variable speed drives. One of the main reasons for using a VSD is energy saving, particularly on fans and pumps. One of the things we have to do is to provide guaranteed NNT efficiency figures - at the bid stage. A rock and a hard place. Too low, lose the job. Too high, lose profit - and credibility.

In short, the system efficiency can be serious money for me as a supplier and more so for the end user over the projected life of the plant. If you run a motor at off its design point the efficiency drops. Maybe not a lot. A few percent. But in terms of energy, that is cumulative.
 
Hi tony thanks for getting back the overload settings were a bit vague so do you think if it has been set between or just below 90amps that the motor will still be protected for overload as full load current is 85.8amps? Replies will be much appreciated?

thanks

106% if you can get the setting lower do so. To be honest thermal O/L aren’t particularly accurate so it could be above or below that setting.

It’s up to you to decide how critical the motor is and how well protected it needs to be. Any motor burnout is a PITA when you’re mauling you guts out changing it.
 
I agree with all of what you posted but would like to comment on that last bit if I may.
Much of what I do is power electronics, particularly variable speed drives. One of the main reasons for using a VSD is energy saving, particularly on fans and pumps. One of the things we have to do is to provide guaranteed NNT efficiency figures - at the bid stage. A rock and a hard place. Too low, lose the job. Too high, lose profit - and credibility.

In short, the system efficiency can be serious money for me as a supplier and more so for the end user over the projected life of the plant. If you run a motor at off its design point the efficiency drops. Maybe not a lot. A few percent. But in terms of energy, that is cumulative.

In general motors should be running approx 75% of full load current to achieve maximum efficiency, Ive had the green team before on our site with promises of energy savings etc on fitting VSD's and changing the lighting ETC .... on all points they put I drafted clear graphs and layouts exposing the flaws within their claims and only 1 motor would have benefited with efficiency but would have taken 3yrs to pay for itself and then before turning a better running costing albeit a small percentage difference ... with this one I just said the VSD market is excelling and prices falling year in year out so could wait 5yrs and get a much reduced costed VSD and cover all the benefits of fitting one now, It took a while to explain that because I was overspeeding some motors and giving a 10% increase in production their energy saving results were no longer reflective of the true situation. A 10% increase on production nearly generated a profit 2000 times that of the energy saved by fitting their energy efficient VSD .... I watched 2 sales reps, 2 company managers and 1 technician/engineer from this green company walk out like Id sucked the life out of them, they were so sure of their energy savings they had lost sight of reality and Im still sure a couple of them thought I was wrong even when their own technician agreed with my 10% production increase made their argument stale .... they had a reality check that day... anyone can be blinded by fancy talk and figures but I have to be having a real bad day to get this sales pitch past me.

Don't get me wrong in the right circumstances there is money that can be saved but what I hate is overselling and empty promises -
 

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