Motor Thermal damage setting 1

[Deansharafi]

Hi all,
In setting an Overcurrent protection curve for a HV motor, how close can you design your O/C curve to the thermal damage curve?
Is there any written information that I can read?
I normally design my curve below the motor thermal damage curve, but I need to know what the limit is and how close can you get to thermal damage curve.
Thanks.

 

[digitrex]

Set the protection curve between the damage curve and the starting curve. i.e.

1. higher than the motor+load starting current curve

2. lower than the motor damage zone.

 

[electricpete]

IEEE620-1996 – IEEE Guide for the presentation of thermal limit curves for induction motors – section 4.3 (Margins) – “A margin between the locked rotor thermal limit curve and the current time curve for acceleration is desirable but may not be achievable in all applications. In such cases as large motors starting at low voltage and/or accelerating high inertia loads, a margin between the acceleration thermal limit curve and the time current curve should be the norm. If these conditions cannot be met, consultation between the manufacturer and the user is recommended."

Let’s dissect this paragraph. Why focus on locked rotor portion of the curve in first sentence? Because that is the part of the curve most likely to come close. It suggests that if you are coming close to the curve, the solution may be to provide separate locked rotor proteciton.

For the manufacturer’s part I believe they have captured what they consider worst case in their curve. For the user’s part they need to analyse starting performance under minimum voltage, normal voltage, and high voltage. Also they need to analyse uncertainty in relay setting. In my view the margin should be sufficient to account for uncertainty in the user’s calculations and relay settings.

Another relevant document would be IEEEC37.96-2000 – Motor Protection, as well as buff book (and maybe red and brown book).


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[mta2003]

Hi!

I think separate LRA protection is the best option, specially when starting current is very high and lasts for longer time due to high inertia e.g. large fans.
In such case if the thermal over load is set at higher value than the normal full load rated current then in case of a faulty bearing or similar fault, we may end up with a burnt motor.

God bless.
mta2003

 

[jbartos]

Comment on Deansharafi (Electrical) Apr 25, 2004 marked ///\\In setting an Overcurrent protection curve for a HV motor, how close can you design your O/C curve to the thermal damage curve?
///Why to ask a question that covers as close as possible. Naturally, provide separation as far as apart as possible.\\Is there any written information that I can read?
///It is understood that the separation is as far as possible, and protection of the motor during starting, acceleration curve proximity, as close as possible for as sensitive protection as possible.\\\
I normally design my curve below the motor thermal damage curve, but I need to know what the limit is and how close can you get to thermal damage curve.
///The limit is the actual damage curve. However, no designer will place curves as close as possible when it comes to damage curve. Normally, the various design and safety margins are a matter of criteria being set.

See Reference:
242™ IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems
IEEE Std 242-2001™ (Revision of IEEE Std 242-1986™)

for:
Figure 10-4—Time-current curve for a 480 V, 75 kW motor with a size 4 contactor, Class 20 overloads, and an instantaneous trip circuit breaker with a setting of 12 times full-load current
on page 355\\\

 

[Deansharafi]

Jbartos- Thanks for the comments. There will be instances that you may be obliged to go close to thermal damage to satisfy other system requirements such as co-ordination etc.

 

[Deansharafi]

Correction to my previous posting: such as "operational requirements" rather than "co-ordination"