Setting overloads for three phase motors

[Buzz1980]

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

 

[Buzz1980]

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

 

[Besoeker]

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.

 

[Knobhead]

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.

 

[Darkwood]

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 -