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

 

[Darkwood]

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.

 

[Buzz1980]

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

 

[Darkwood]

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.

 

[Buzz1980]

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

 

[Darkwood]

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.

 

[Knobhead]

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.

 

[Bobster]

What about dash pots tony?

 

[Robotstar5]

What about dash pots tony?

That was one of my jobs as an apprentice, going round checking/topping up dashpots, can't remember what liquid was used.

 

[Darkwood]

That was one of my jobs as an apprentice, going round checking/topping up dashpots, can't remember what liquid was used.

WD40 it does everything ...or vinegar ?

 

[Robotstar5]

WD40 it does everything ...or vinegar ?

It was a clear viscous liquid.

 

[Knobhead]

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%+.

 

[Knobhead]

That was one of my jobs as an apprentice, going round checking/topping up dashpots, can't remember what liquid was used.

Silicon fluid.

It came in several viscosities 100, 200, 500 and 1000 cS

Using a Stokes cup you could blend your own viscosity.

 

[Robotstar5]

Silicon fluid.

It came in several viscosities 100, 200, 500 and 1000 cS

Using a Stokes cup you could blend your own viscosity.

Thanks, I do remember the contactors & overloads were Allen West, you could even buy repair kits for them!

 

[Knobhead]

You can still get Allen West spares. After the company shut down Allen West (Brighton) started up to supply spares.

They are very helpful.