3 Phase Motor Overcurrent Protection

i am surprised and a disappointed that Thermal Overload relays have not satisfied you. i have spent a couple of decades off and on on these, and rate them as the best value-for-money motor protection devices. Pardon my saying so, but mostly, the reason for their failure is usually wrong selection. If you give more details of the motor, i can assure you that a proper relay exists.

You can of course spend a fortune on Solid-state relay which will also do the job, at much higher cost.

Thanks for your valuable response.. But I was in search of an electronic relay..

Thermal overload relays failed to provide initial protection i.e in starting phase of motors..

We are using Siemens make 8.5 Kw 3-phase induction motors in a textile industry..

Solid state would normally be an overkill for a 8.5kW motor, but yours may be a special application.

Some questions :

• Squirrel cage or slip-ring?
• "failed to provide initial protection" means? Motor did not start rotating (locked rotor condition) and relay did not trip?
• When you have used Siemens motor, you would logically use Siemens starter with thermal o/l relay ?
• Is speed or torque control involved?
  

The motors are Squirrel cage.. "Failed to provide initial protection means" relay did not trip when either of phase fluctuates or it is not constant ..

Speed or torque control are not involved..

We want phase failure protection.

Motor overload devices will protect against the effects of phase failure. There are any number of manufacturers:

• Telemecanique
• GE
• ABB
• Allen Bradley
• etc.

(usual disclaimer)

The usual issue is that the overload setting does not correspond with the full load current. Naturally, these should be the same.

then watz the correct solution for the protection???

The information is very useful. Electronic relay is available but costlier.

i wish to present a paper.... can anyone give me a idea or innovative topic in electrical engineering??????? plzzzz???

GA .....kvsridhar, thank you for sharing your knowledge and doing it with class.

klearzen, thank you very much.

i am really enamoured of this thing which many may only see as a simple little device...i have gone into some depth into its design philosophy (done by some great people) and am still amazed at what $2 ex-factory cost can achieve. You may enjoy looking at the Patent by Cutler-Hammer on this device, way back when....4096465.pdf

Some images to help in case you don't have time to read that legalese

Thank you very much Mr. K.V. Sridhar for your kind attention and valuable response to my post. I found the information useful. If you don't mind I will bother you on my further queries..

Regards,

Vivek Jadhav

Glad to have been of help. You are welcome to contact me through the CR4 mail system any time. However, since i don't know a diode from diarrhea, i may not be of much help re electronic relays though

Cheers.

dear vivekjadhav,

kvsridhar already given you good points. but pardon me to go deeper. Please recheck the thermal protection relays you are using maybe it is oversize or not set properly.

hi dear,

so many overload protection relays have come to the market...

see the rating of your motor and use any OLP relay according to the rating.

Hi friends my question may seem a bit laymen and i accept it,

when selecting a Overload relay, say for 10HP Induction motor with full load of 15A, what would be the criteria for selection. I mean what is the thumb rule for selecting a overload relay based on the full load of motor,

Should it exactly equal to Full load?

Best would be to select:

1. A good reputed make

2. A relay with the the full load current in the middle of its setting range. Not exactly, but close.

3. Ensure that the relay has "Single-phasing protection" which also implies some unbalanced voltage protection...it may interest you that a 1% voltage unbalance can result in 6-8% current unbalance.

4. Make sure it is 'Ambient compensated'

5. Slightly more advanced - Is the motor a long-start type? you then have to choose Class 20, 30 or even 40 relay!

kvsridhar,

what you mean of long start motor? sorry this is the first time i heard this. i know a start delay for some application. what i experience once a motor had long start there a problem in the motor. I would like to hear more about this.

thank,

erlo

Some loads on motors are high-inertia type, and take a long time to accelerate.During this period, the motor will keep drawing almost the starting current, which could be 6-8 times the rated current.

The standard (Class 10) overload relay trips within 10 sec at 7.2 times the motor full load current. If the load is such that the motor has not yet accelerated to its rated speed within that time, the relay will trip unnecessrily. In such cases, one uses a Class 20 relay, which will trip within 20 sec at 7.2 seconds. Depends on the application, the motor also has to be specially designed.

@ KVSridhar,

This what happens to my case..

Motor draws initial higher current for starting torque, but after running, if it draws more current (may be due varying loads) relays fail to trip and this results in burnt out of motor winding..

i see. Interesting. Can't say i have come across this one before.

What is the make of the relay?

What is the motor FLA and the relay setting (i assume they are the same, just double-checking)

@ KV Sridhar Relay make is Siemens only..

FLA and relay setting is 16A.

Yes, it is natural that you would use Siemens relay and contactor. If it is 3UA series or later (Sirius) i see that they are single-phasing sensitive also. i am perplexed if it is not tripping on overload/phase unbalance.

This link gives some information on Sirius relays. 3UA seems to have been obsoleted.

One thing bothers me a little. The relay setting range ends at 16A. If your motor FLA is actually 16A, it may be prudent for you to see if a relay is available where 16A is somewhere near the middle of the setting range.

Good luck in your quest, as i said before, i will be most happy to help withing my area of experience.

dear kvsridhar,

thank you for the information. Nonetheless, i am not still convinced that vivekjadhav 8.5 kw motor which most probably at 1700 to 2400 rpm will be design to high starting torque machine. As i remember mostly, a high starting torque machine uses dc motors. like rod mill plant drives, mills etc.

vivekjadhav, what specific machine the motor drives? Becaue if the motor continues to draw more current, it probably undersize for the requirement or it has a insulation or winding problem.

just trying to troubleshoot the real problem.

erlo

From what has been discussed so far I understand that your motor drives a high inertia load at start and therefore, if you size the overload relay to suit the starting sequence, you will not have an adequate protection for the motor against the normal full speed runing...(?)

You are in a textile industry. I can guess that maybe you are running something like a Centrifuge Hydrauextractor to squeeze the water from some textiles... (?)

These high inertia machines have a long starting time to reach full speed. They normally have some special type of starter / Overload devices that work on measuring the energy being used against the time setting before changing from start to delta. When the motor goes to full speed (delta) , the starting process has been achieved, then the overload works as normal with the required tripping setting at that point.

Most makers have these high inertia motor starting systems or overloads.

If not available, use two O/Loads: One for the Starting process, rated higher than the normal motor current to permit a fail safe starting, then the 2nd O/Load rated for the correct nameplate rating for when the motor is running normally. There are more than one way to achieve this: Star / Delta if possible in your case, with two contactors for the Main contactor that will change over either at the delta sequence or after a delay in delta. This can be done by 1st closing the 2nd Main while the 1st is still ON, then releasing the 1st Main.

I hope this is clear.

You may also consider installing a variable speed drive

most models rated to handle 8.5kw would have the ability to monitor & set many different parameters. High inertia loads can also damage motors [& other equipment] during the shut down

Another thought occurs to me .. a very long shot, only a small chance.

Maybe the reason for motor burnout is not electrical at all ? Suppose it is internal-fan-cooled, and the air path is clogged due to fluff from your textile environment ? Then there will be no overcurrent, so no relay will sense and trip. Motor will just overheat and burn. i have seen some burnouts like this in the textile mills and tobacco plants here. Only a thermistor relay can sense the increased temperature and trip in such a case, but then the thermistors need to be built into the motor at manufacturing stage. Or some sort of sealed motor, impervious to ingress of dust. Too late for that in your case.

Please let me know if you have checked and ruled out this possibility, thanks.