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Topic Title: Motor control set up
Topic Summary: Advice please
Created On: 16 July 2017 02:18 PM
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 16 July 2017 02:18 PM
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Posts: 956
Joined: 24 March 2006

Advice please


I have a number (19) of small new, to be put into service DOL 3phase actuators/motors ranging from 0.2kw to 3.3kw.
The set up
The actuators operate for 30seconds only and are controlled via a limit switch via the travel of the actuator. The control circuit energises the actuator, the actuator moves hits the limit switch opens the control circuit, spring mechanism brings the actuator back to its closed position.
The overload relays in the control panel protecting the actuators are set less than the normal operating current of the motor (manufactures data) but the motor never reaches its critical point on its time tripping curve because the limit switch takes the control coil out before 30seconds travel time, so as long as the limit switch is hit within 30seconds travel, the underated overload relay does not operate and the process remains healthy.

The questions I have

Would this control set up be described as good practice, should the OLR be set to accommodate the operating current x1.5 ish at least in the control set up?,
would prolonged use of the process damage the underated OLR? reduce its operating life?

Your thoughts please

MrP coming home for good
 16 July 2017 03:13 PM
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Posts: 2538
Joined: 07 August 2007

In normal use, I would set overload relays to either the nominal FLC of the protected motor, or very slightly in excess, perhaps by 5%

However this particular application sounds atypical in that the motors only run briefly.
An overload relay setting of appreciably LESS than the FLC of the motor might well be appropriate.
If something in the driven apparatus breaks or jams, then a prompt tripping of the overload relay minimises the damage to the equipment.

I would not set overload relays appreciably in excess of the motor FLC, unless in truly exceptional cases.
The design of thermal overload relays and electronic replacements already takes account of starting currents, no uprating is required for starting currents.
 16 July 2017 11:31 PM
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Joined: 22 July 2004

A start-up is a start-up - the current drawn at the start, or indeed any time in the next 29 seconds, is not going to be affected by the fact the current is going to be cut 30 seconds into the run -the motor cant know in advance. Size protection in the normal way.
BUT, be aware that repeated starting is far more stressfull for a motor than steady running, so cooling needs to be good, or after a few sessions of start and stop, the motors may suffer overheating.

regards Mike

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