TOL Sizing

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mityeltu

mityeltu

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Tennessee
I have been asked to do something I do not know how to do.

Sizing a TOL for a motor is no big deal, but I have been asked to deterine if the TOL has enough margin after it has been in use to restart the motor after a stop.

Here's the scenario: Motor is running, plant receives accident signal, load sheeding that includes the motor under consideration, motor reloaded to system after 300 seconds.

The question is, will the TOL isolate the motor when it is required to restart, and what, if any, is the margin available assuming it does not isolate.

I understand the idea here: TOL heats up as the motor is running to some value. Load is shed and TOL begins to cool. 300 seconds pass and load is required to restart. The inrush current over the period of starting will heat the TOL. Will the restart cause a trip? If it will NOT cause a trip, what is the margin available?

I just have no idea where to find this sort of information. Has anyone ever come across this before? Does anyone have any suggestions on where to start?

I know you will ask, so here is the existing motor data: 3-phase, 460V, 3HP, FLI=3.9A, LRI=31A, SF=1.15, starting time=6 sec., TOL=Siemens K33

I know the starting time is long, but it's what I was told to assume. This motor is for a 6.9kV shutdown board room chiller package oil pump.
 
Thermal motor overload relays come in 3 different time/current curves

Class 10 will trip in 10 sec with a 6X load - this is the most common for IEC style starters and other 'self contained' bi-metallic styles.
Class 20 will trip in 20 sec with a 6x load - this is the most common for NEMA style starters and other 'replaceable element' bi-metallic styles.
Class 30 will trip in 30 sec with a 6x load - this is not a common.
 
Motor thermal OL relay / heater curves are designed to match the motor thermal damage curve as closely as possible. To that extent, you don't WANT the TOL to allow restarting of a hot motor to where the motor insulation will be damaged, that's the point. So is there a "margin"? Not supposed to be, but if there is, it's only by accident / tolerance error.
 
A lot of motors, especially larger ones, have specs on how often they can be started in an hour to protect them from overheating.

You might well be surprised by how few starts per hour are allowed.

It is not unheard of to have timers built into control circuits to prevent trying to restart a motor too soon.
 
Jraef, I understand the point, but if the motor hasn't tripped, then there is margin. The question is just an exercise in thermo, but I don't know where to get this kind of information.

The TOL closely matches the thermal damage curve for the motor - agreed; BUT, there must be some kind of margin between heating and damage or the motor would not be allowed to start at all. The question is, how much margin is there between where the heating is right now and where it will be when the motor restarts? Will there be enough thermal capacity in the windings/TOL to permit the motor to restart without damage? I need a way to quantify this.
 
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When the motor stops turning it continues to heat up because it looses it cooling fan. Thermal overload relays are not very good at mimicking this heating overshoot as they begin to cool almost immediately. The difference in the heating and cooling time constants of motors is one reason that the restarting of 'hot' motors is usually more restrictive than the starting of 'cold' ones.

If you are this concerned over a 3hp motor, you should be looking at the time current curves for your specific relay. However, based on your relatively reasonable starting characteristics, I would not anticipate a problem.
 
jim dungar said:
If you are this concerned over a 3hp motor, you should be looking at the time current curves for your specific relay. However, based on your relatively reasonable starting characteristics, I would not anticipate a problem.
Click to expand...

I am looking at them. The problem is, I don't know how to quantify the time that will be available for the motor to restart based on preheating of the TOL.

This is a non-1E application, but it is an adjunct to 1E devices and therefore is required to be operational from an SAR/TS point of view. The question came up because someone decided that during a LOOP these motors might trip due to restarting in spite of the 300 second cooling prior to reloading the motors onto the diesel generators.

It has fallen to me to evaluate the past operability of the shutdown boards based on this and I'm having trouble because I have no idea how to quantify the margin on these TOLs. I can see how long these motors should run based on FLI and how long they should run based on LRI, but to say that after preheating for x-amount of time, the TOL has this much margin, and then to restart the motor after 300s cooling there will still be so much margin.... I'm kinda lost.

I'm looking for a way to calculate this.
 
jim dungar said:
When the motor stops turning it continues to heat up because it looses it cooling fan. Thermal overload relays are not very good at mimicking this heating overshoot as they begin to cool almost immediately. The difference in the heating and cooling time constants of motors is one reason that the restarting of 'hot' motors is usually more restrictive than the starting of 'cold' ones.

If you are this concerned over a 3hp motor, you should be looking at the time current curves for your specific relay. However, based on your relatively reasonable starting characteristics, I would not anticipate a problem.
Click to expand...
Jim has hit on a very important point regarding the heating and cooling time constants. Old fashioned thermal overloads, bi-metal and eutectic alloy type, are not set up to "monitor" anything, they are just designed to mimic the thermal damage curve of a typical motor.

For that reason, and in your specific case, I strongly recommend using a very good Solid State Overload Relay. There are cheap ones, and there are good ones, but there are no cheap good ones. The better ones will use a good "Thermal Model" in software that is much closer to being real and will know the difference between a hot and a cold motor. Plus, it WILL allow you to actually view the Thermal Model Usage and cooling time and even adjust the cooling time constant (if you can get it from the motor mfr, which is unlikely on a 3HP motor). I work for Rockwell, we provide this with the E3 Plus SSOLs, but you must have it connected to a hand held programmer or a network of some sort to view the information. I used to work for Siemens, they have the SIMOCODE Pro that does that as well, but also requires network connectivity to view anything (no handheld option). I was on the development team of a really nice SSOL from Motortronics a few years ago called the RX Series; it has a built-in display and will allow you to view the remaining thermal capacity at any time. Then if it trips it allows you to view the reset time, which is based on the motor cooling constant. That is a nice relay for the money (even though they are now a competitor and no longer pay me to care). GE / Multilin has a new low-end SSOL product called the MM200 that says it provides thermal model based protection, but it doesn't specifically say if it allows you to monitor it. I've just had to put one in on a project, it has yet to be started up so I can't say anything about it yet, other than the fact that they are really proud of that thing ($$). Those are the ones I know, but there are others. Of those, the Motortronics RX Series is likely the best bet for you if you are not using a communications network for any sort.
 
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