Motor overload tripping question 4

[Jeff_Cameron]

Hi all,
I am having an issue with some motors on a piece of industrial machinery. The problem I am having is that the motor overloads keep tripping for all of my coolant supply pumps. This is really a 2 part question.

Part 1: I have a 480v 3 phase motor with a nameplate FLA of 28.0A (at 480v). the overload is tripping up to 12 times a day. Full disclosure, I did not design THIS system. the motor overload is a Siemens overload with a max rating of 32A. I did notice that although the overload is rated up to 32A, the max horsepower rating for the overload is 20HP. The motor is rated at 25HP. The ambient air inside the panel is approx. 98 degrees F. The ambient air outside of the panel is 73-75 degrees F. Thermal imaging of the panel shows that the overload is reaching temps of up to 150 degrees F. The wires going to the overloads are around the same and the cables on the outside of the machine running to the motors are also significantly warm. the motors have cycle time of about 1.5 hours, during which there are valves opening and closing periodically. The motors are definitely not short-cycling. all wire terminations have been checked for sufficient tightness. Does anyone have any idea why this could be happening? is it just as simple as the overloads need to be sized for a higher HP to account for the heat? Also, the service factor is the standard 1.15.

Part 2: In general I understand that an electrical horsepower is equivalent to roughly 746 watts. How does the horsepower figure into the amperage sizing of a motor? for instance, this motor is 25 HP. If i take 25 x 746, i get 18,650w. i then divide that by 480 and get 38.85A. Do i then need to divide that by 1.732? doing so gives me 22.43A. even with that, where do they get 28 from? is that just because that's the full load condition? Its been a long time since I went to school for this so I may have forgotten a few things. thank you for your help I sincerely appreciate it.

 

[dpc]

Have you measured the current? Amps are amps. The motor FLA on the nameplate provides the information you need. Is this an electronic overload or electro-mechanical? If electronic, there will be an adjustment for maximum motor current. If electro-mechanical, there are heater elements that need to be sized to the motor FLA.

Motor HP rating is the OUTPUT power. To compute the current, you need the voltage, the power factor and the efficiency.

 

[Jeff_Cameron]

It is an electro-mechanical overload. the overload is sized for up to 32A, while the motor FLA is 28A. the weird thing is that the overload is rated for 20HP and the motor is 25HP. I don't understand how the overload can be rated up to 32A but only 20HP.

 

[itsmoked]

It is probably the same as most power stuff. A disconnect may be rated for 60A but you're free to install anything up to 60A. The overload is rated to 32A but someone installed a smaller value one.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[iceworm]

Look at 430.32.A.1 and 430.32.C
Overloads are set by the specific motor nameplate FLC, not from T430.250.
However 430.250 lists 27A FLC as typical for a 20 HP

As an example:
Using a 20 HP motor, 27A flc and 1.15 sf, the overload normal setting is 125% X FLA = 1.25 X 33.8A
And, per 430.3.C, the overload cold be set as high 1.4 X 27 = 37.8A

However, if he motor is a 1.0 sf, then the ratings are 115% X 27 = 31A
to a max of 130% X 27 = 35.1A

I'd say the starter is (was) build to absolute minimums. It is likely a size 2 and 25hp is the normal breakpoint. This being rated for 20HP is different - I don't know what you got.

Here is what we know:
The internal connections are clean and tight.
Internal cabinet temp is up to 150F
Overloads are tripping 12 times per day. (Unknown how many normal controlled re-starts)

First, is this something that has always been happening? Or has it just started?
Either way, measure the running current.

Twelve trips per day (even 24 hour days) is more than enough to burn up the motor. However if the control system is starting stopping 20X/day, the trips are not adding much heat.
Just curious: Does this have a history of burned up motors?

I suspect you already know all I have to say:
If this a recent problem, the motor and starter are fine - not a good design, but workable. If the system has changed to where the motor is now running in a overload condition, Fix what broke. Current comes down, starter cools, all is fine

If this is something that has been since startup:
The starter is too small. It should not be running that hot.
System re-design. Install a robust nema size 2 - and check the ratings.

As already mentioned, check the running current.
Check the motor service factor.
Is the running current within the service factor?
If it is, push up the overloads. A 1.15sf motor will be fine with 140% overloads. Although considering the trouble you are having, I suspect they already are pushed. Tripping 12x/day and re-starting 12X/day is a lot harder on the motor (read more heat) than running it hard, up in the sf.
28A flc and 1.15 sf, 140% = 39.2A

If the motor is running above sf, system re-design. More motor, less pump, less head

Note on overload settings:
The instructions are often convoluted beyond belief. Often they will give an FLC range and cross check to an overload number. No clues as to the actual trip level. Getting a number that corresponds to a 140% flc is difficult.

Yes, high internal temp can cause the overload to trip early. Some instructions will discuss cross ambient operation - not all.
Instructions for electronic overloads such as AB E300 are worse. They are amazingly convoluted to pick a number that corresponds with 140% flc.

RTFM until you absolutely understand how to set the relay to match the motor.

The overload rating could be the maximum FLC, not the overload setting. I don't know. Get the manual.


Harmless flakes working together can unleash an avalanche of destruction

 

[waross]

I don't understand how the overload can be rated up to 32A but only 20HP

I used to know all this stuff, but as I get older I hesitate to answer that question without seeing the actual rating and the fine print on the overload relay.
I will suggest that some contactors have an Ampere rating for resistive loads and an HP rating for motor loads.
Also, a 25 HP motor with a service factor of 1.15 is actually a 28.75 HP motor.
The HP, SF and FLA don't compute. With that service factor the current should be around 30 Amps.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Check the heater size - not the overload relay itself. These OL relays are effected by ambient temperature to some extent. Also, I strongly recommend actually measuring the motor current. And make sure that OL relay actually says 32 amps.

 

[jraef]

Something is wrong I think. Overload relays are not rated for horsepower, they are rated for current. STARTERS however are rated for horsepower. So my suspicion is that you have a STARTER or moe specifically the contactor portion of your starter, rated for 20HP that has an OL relay rated for up to 32A, which would not be at all unusual.

If this is the case, then someone has selected the wrong starter for your 25HP motor. IEC type starters such as those sold by Siemens are very narrowly rated for specific motor sizes; there is no room for error. Conversely had you used a NEMA type motor starter, it would be a Size 2 and rated for up to 25HP even if the motor was only 20HP. That’s the difference in design philosophy.

But be that as it may, tHe result might be that the contacts inside of the starter are over heating because they are too small for that motor and that heat is transferring directly to the OL heater elements, causing them to nuisance trip.

Continuing on like this runs a risk of failure of your contactor and possible equipment damage or a dangerous situation where the contacts weld and will not turn off your machine. You need to fix this immediately.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[che12345]

Dear Mr Jeff_Cameron
1. Motor nameplate states rated, 480V FL current 28A, service factor 1.15 are in order.
2. Inside temperature 36C, outside 23.8C are fine. The temperature at OL terminal 65C slightly on the high side.
3. OL having the maximum settable value of 32A is fine. Ignore the value of 20hp, it is irrelevant in this case.
4. If the OL is set at 28A and the motor [three line currents are balanced/equal] or lower that 28A, then it Shall NOT trip. The OL can take a short time (say within 5s) overload without altering the tripping characteristic.
5. In general applications, set the OL trip value = FLC . SF is irrelevant.
6. Monitor [all three phase currents] for say the duration of [two periods/trippings], i.e. say about four hours.
7. The OL would trip pre-maturely (i.e. at a lower current value):
a) if the [three line currents are unbalance], even though the line unbalanced currents are lower than the OL set value of 28A,
b) if the [load is pulsing] e.g. [exceeding 28A continuously or periodically] for duration exceeding say 3 min.
8. Replace the OL if the [three line currents are balanced] and runs [continuously not exceeding 28A].
Che Kuan Yau (Singapore)

 

[waross]

98 degrees inside the panel will make the situation worse.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr. waross (Electrical)7 Mar 20 09:00

1. "The ambient air inside the panel is approx. 98 degrees F. The ambient air outside of the panel is 73-75 degrees F. ..." see original post.
2. 98deg F is approximately = 36deg C, see my earlier post.
3. The OL [shall operate without any problem] with (ambient air inside the panel) temperature of 36deg C.
4. What do you mean that "98 degrees inside the panel will make the situation worse." ? Take note that it is 98deg F NOT 98deg C .
5. Please advise whether is there a [mistake in my deg F to deg C conversion]?
Che Kuan Yau (Singapore)

 

[waross]

Respectfully Mr. Che; How do you prefer to be addressed?

4. What do you mean that "98 degrees inside the panel will make the situation worse." ? Take note that it is 98deg F NOT 98deg C .
.....
2. Inside temperature 36C, outside 23.8C are fine.

It has been many years since I was dealing with overload relay settings and trips on a daily basis.
There used to be issues with the overload setting when the motor and the overload where at different ambient temperatures.
The O/L relays used interchangeable heaters to heat bi-metal strips or melt alloys.
Every starter had a selection chart. You looked up the motor FLA and selected the appropriate part number heater.
These devices were sensitive to the air inside the enclosure and the charts reflected this.
The contactor coil as well as I²R losses in the starter enclosure add internal heat that is rejected by the outer surfaces of the enclosure. The rate of heat rejection and the resulting internal temperature will be affected by the size of the enclosure.
Every enclosure had a specific chart.
A single starter in an enclosure just large enough for the starter.
A reversing starter in an enclosure just large enough for the starter.
A combination motor starter/disconnect.
A combination reversing motor starter/disconnect.
An MCC mounted starter.
An MCC mounted reversing starter.
An open starter intended for use in a fan cooled control cabinet.
Each chart was slightly different to account for the different rates of heat rejection from the different enclosures.
Our electrical code allowed O/L sizing to be adjusted in cases where the starter and the ambient where in different ambient temperatures.
36C above 23.8C is enough difference to have allowed an adjusting factor with the old heater style O/L heaters.
I spent a few years in operating plants where selecting the proper overload heaters and settings was part of my responsibility.

I worked in operating sawmills when sawmill electrification and design was a work in progress.
Motors tended to be undersized and tended to be badly abused.
The proper O/L settings were vital.
Mistakes led to motor burn-out.
There were lots of motors lost.
We learned and moved on.
Often motors were upsized after the second burn-out.
In my world, 98F inside the panel suggests a control panel with inadequate cooling.
I may be wrong but I would certainly be considering the panel temperature as a possible factor.
I would also consider the points raised by jraef.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr.Waross,
1. Respectfully Mr. Waross. Che is my sir name. I wish to be addressed as Che.
2. " It has been many years .... The O/L relays used (interchangeable heaters) to heat bi-metal strips or or melt alloys. Every starter had a selection chart. You looked up the motor FLA and selected the appropriate part number heater. These devices were (sensitive to the air inside the enclosure) and the charts reflected this. "
2.1 Your respected expertise are some 40-50 years old. Today, the OL on the market are different.
3. Mr.Jeff_Cameron stated that " the motor overload is a Siemens overload with ...."
3.1 This [present day] OL is not one with (interchangeable heaters)... (melt alloys)...
4. IEC reference ambient air temperature 40deg C for (not compensated type, and 20deg C for compensated type.
5. " In my world, 98F inside the panel suggests a control panel with inadequate cooling....
I would certainly be considering the panel temperature as a possible factor."
5.1 Panel inside ambient temperature is certainly a factor, agreed. But, with inside ambient temperature 98deg F which is approximately = 36deg C is certainly within IEC/ANSI/UL/NEC Standards. Internal ambient temperature of 36deg C [NOT 98deg C] is certainly fine, [NO additional] cooling is needed.
Che Kuan Yau (Singapore)

 

[RRaghunath]

Jeff Cameron, I just want to mention a couple of points for you to check out:
1) Electro-mechanical type oveload relays tend to change their characteristics over period of time due to heating-cooling cycles. Just check out how old is the relay and test the relay.
2) Hope the setting is 32A on the relay and not lower.
You mentioned 20HP, I guess the 20HP corresponds to 415V input power supply (and not 480V that you have) and probably, the relay insulation is rated 500V. Thus, the 20HP may not be much relevant for the issue. If relay model is mentioned, this can be verified from the catalog.
About your part-2 of the question, you didn't consider efficiency and power factor of the motor and that is the reason your result is lower than the name plate current rating 28A.

 

[che12345]

Dear Mr. RRaghunath (Electrical)8 Mar 20 04:06
1. Jeff Cameron, I just want to mention a couple of points for you to check out:
" 1) Electro-mechanical type overload relays tend to change their characteristics over period of time due to heating-cooling cycles. Just check out how old is the relay and test the relay". Agreed.
" 2) Hope the setting is 32A on the relay and not lower". Disagreed.
Please advise which IEC/ANSI/UL/NEMA/NEC Standards or manufacturers' instruction stating that the thermal OL [SHALL BE set at not lower than] 32A/28A=1.14 time the (motor name-plate current rating).
2. " If relay model is mentioned, this can be verified from the catalog".Agreed.
Che Kuan Yau (Singapore)

 

[RRaghunath]

Dear Mr. Che Kuan Yau, The service factor is 1.15 for the subject motor.
It means to me that the motor can be run at 115% of rating for extended period of time. Isn't it so!!

 

[che12345]

Dear Mr. RRaghunath
1. " The service factor is 1.15 for the subject motor.
It means to me that the motor can be run at 115% of rating for extended period of time. Isn't it so!! "
1.1 Disagreed. Motors are [NOT intended] to run at the SF ( be it 1.15 or higher) for (extended period of time).
2. Please advise which IEC/ANSI/UL/NEMA/NEC Standards or manufacturers' instruction stating that the thermal OL [SHALL BE set at not lower than] 32A/28A=1.14 time the (motor name-plate current rating)?
3. Reference: IEC 947-4-1 where the size of the connection wires, the minimum length of the connection wires and the tripping for different "Classes" the "gates" from "cold" and "hot" are specified at e.g. 1.0, 1.2, 1.5 and 7.2 times of current setting.
3.1 Note: a) The tripping characteristic curve is [NOT a single line curve] but (a band within the "gates").
b) There [SHALL be NO tripping] with current = 1 time the OL setting; under ("cold" or "hot" conditions).
Che Kuan Yau (Singapore)

 

[waross]

Old technology:
A plant could stock one relay and an assortment of replaceable heaters.
New technology;
A plant must stock an assortment of different sized relays with non-replaceable heaters.
No new technology but a marketing advance.
My post was meant to illustrate that thermal overload relays are sensitive to ambient temperature.
Even an ambient compensated relay may have issues if the relay ambient is higher than the motor ambient.
If the relay ambient is 10 Degrees hotter than the motor ambient, the relay will assume that the motor is 10 degrees hotter than it actually is.
This may lead to nuisance tripping in borderline applications.
But that is not the main problem here.

This is an excerpt from the code that I work to:
28-010 Special terminology
In this Section, the following definitions apply:
..............
Service factor — a multiplier that, when applied to the rated horsepower of an ac motor, to the rated armature
current of a dc motor, or to the rated output of a generator, indicates a permissible loading that may be carried
continuously at rated voltage and frequency.

This is an excerpt from the code that I work to:
28-306 Rating or trip selection of overload devices (see Appendix B)
(1) Overload devices responsive to motor current, if of the fixed type, shall be selected or rated or, if of the
adjustable type, shall be set to trip at not more than the following:
(a) 125% of the full load current rating of a motor having a marked service factor of 1.15 or greater;

For a motor FLA of 28 Amps you need a setting of 35 Amps.
This also explains why a 32 Amp relay is rated at 20 HP.
You need a higher rated overload relay.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr. Waross
1. " For a motor FLA of 28 Amps you need a setting of 35 Amps." Disagreed.
2. Reference:
2.1) NEMA: SF may be employed for several reasons....
It also states that: a) motor runs hotter - reduce insulation/bearing life,
b) IEC does not recognize the use of SF.
There was NO mention that: the OL [SHALL be set at] SF x Ie (e.g in this case 1.15 x 28A = 32.2A ). Not to mention at 35A which is 35A/28A=1.25, which is much higher than the rated SF 1.15.
2.2) Mr. E Cowen:
a) ".... SF 1.15 could provide a N hp x 1.15SF = 1.15 N hp for short time use. In general, it is not a good practice to size the motor to operate continuously above rated load in the service factor area.
taken note " ... for short time use ... " and " not a good practice ...to operate continuously above rated load ...."
b) There was no mention that: the OL SHALL be set not lower at SF x Ie (e.g in this case 1.15 x 28A = 32.2A ).
3. Please advise which IEC/ANSI/UL/NEMA/NEC Standards or manufacturers' instruction stating that the thermal OL [SHALL BE set at not lower than SF x Ie (i.e. in this case, 1.15 x 28A= 32.2A) the (motor name-plate current rating)
Che Kuan Yau (Singapore)

 

[waross]

Dear Mr. Che
This is an excerpt from the code that I work to:
THE CANADIAN ELECTRICAL CODE
Section 28 — Motors and generators
Scope
28-000 Scope
This Section supplements or amends the general requirements of this Code and applies to the installation, wiring
methods, conductors, protection, and control of electric motors and generators.
General
28-010 Special terminology
In this Section, the following definitions
..............
Service factor — a multiplier that, when applied to the rated horsepower of an ac motor, to the rated armature
current of a dc motor, or to the rated output of a generator, indicates a permissible loading that may be carried
continuously at rated voltage and frequency.

This is an excerpt from the code that I work to:
28-306 Rating or trip selection of overload devices (see Appendix B)
(1) Overload devices responsive to motor current, if of the fixed type, shall be selected or rated or, if of the
adjustable type, shall be set to trip at not more than the following:
(a) 125% of the full load current rating of a motor having a marked service factor of 1.15 or greater;

In Canada, the Authority Having Jurisdiction inspects electrical installations to this standard.
Installations not complying with this standard may be denied energization.

You may submit your disagreement, in writing, to the Appropriate Code Committee.
C6. Requests for amendments to the CE Code, Part I — General
C6.1
A request for an amendment to the CE Code, Part I, may be submitted to the Project Manager of the Committee
on Part I by any person, organization, or committee (see Annex B).
C6.2
A request for an amendment to the Code shall include a specifically worded proposal, reasons for the proposal,
and supporting data. The wording to be added, changed, or deleted shall be submitted in such a way that the
intent is clear. An unclear proposal may be returned to the submitter by the Project Manager after consultation
with the Section Chair and the Chair of the Committee on Part I.



Bill
--------------------
"Why not the best?"
Jimmy Carter