Over Load Setting vs Normal Load Amperes

The full load current will be less than the rated current of the motor rated HP.when loaded the machine beyond rated current of motor it has to be protected from burn out.

The full load current will be less than the rated current of the motor rated HP.

Where you get this from? The full load current (= rated current of the motor) is the maximum it can take continuously. Any extra mechanical load on it will cause it to draw more current, and eventually, depending on how high the extra current is, it will burn out if unprotected. Overcurrent can also happen due to more or less voltage being applied to the motor.

The IEC specifies the time in which the relay should trip at various overloads, please study IEC 60947-4.

Service factor is also used in the UK for "arduous duty" motors. Mainly foundry, mining or crane duty. Tough as old boots, I've watched one on fire and it was still running!

back to the basics and keeping it simple . . . here we go . . .

the current drawn by the motor at full load is indicated on the name plate of the motor.

the motor can draw an abnormal (higher and/or unbalanced) current (get thermally overloaded) due to a variety of causes - including but not necessarily limited to - single phasing or phase voltage unbalance at point of supply, unequal winding impedance, unequal contact or joint or cable impedance, improper working of cooling fan and a number of possible (internal or external) mechanical reasons causing the shaft to impress an abnormal load on the electrical system.

any "overload" can damage the motor - depending on the duration and the nature of overload. therefore it is expedient to have protection. so far so good.

in almost any machine the motor used is selected to be operated at a load lower than the name plate value. using a suitably under-rated motor gives us the advantage of improved life expectancy, lowers chances of failure due to temporary overloads and helps overcome the higher demands whilst starting.

different overload relays come with different characteristics to accommodate the starting behavior of the load and the motor. once selected then comes the procedure for setting the same.

if electrical protection is the only intent then setting it at the recommended FLC is sufficient.

however if the protection is backed by the thought of taking care of defects and faults in the loading of the motor and also knowing / preventing what is really going wrong in the process then a modified approach is needed. the motor is then "run at full load" and the overload relay setting is lowered slowly from the FLC setting to come closer the actual demand setting. this requires an intimate knowledge of the requirement of the process and also of the motor's starting behavior. i would allow the electrical contractor to set the overload at FLC. thereafter if required involve the process engineer and the machine builder to suggest the setting at lower than FLC - ie at the "best value" for the process.

t_{he motor can draw an abnormal (higher and/or unbalanced) current (get thermally overloaded) due to a variety of causes - including but not necessarily limited to - single phasing or phase voltage unbalance at point of supply, unequal winding impedance, unequal contact or joint or cable impedance, improper working of cooling fan and a number of possible (internal or external) mechanical reasons causing the shaft to impress an abnormal load on the electrical system.}

Many good points. GA to you.

However, an improper working of a cooling fan would certainly increase the heat, with no increase in the current. In such a case, would not a heat-sensing thermistor relay be more trustworthy than a current sensing one?

correct sridhar - thanks for blowing the whistle.

that condition/fault require temperature sensing.

Thanks a lot to all. Where this heat-sensing thermistor relay is installed ? In the feeder ? Or near the motor ?

Regards,

The best way, i am afraid, is to build the thermistors into the motor at the time of manufacture. Cannot be added later. There may now be some models which can be added later, you need to check the state of the art.

Thanks a lot Mr.Soebfatehi :

Quote : "thereafter if required involve the process engineer and the machine builder to suggest the setting at lower than FLC - ie at the "best value" for the process."

Unquote :

Can you correlate the over load setting Vs the normal load amperes in the exercises you have done? Broadly how many times the over load setting is higher than that of normal load amperes ? 110% or 120% so on ...?

Regards,

When the relay is set to any current, tripping must not happen at 1.05 times that current. and must happen at 1.20 times that current as per IEC. So, one does not set the tripping current, one sets the rated current on the relay, which then trips between 5% and 20% overload. A typical thermal relay curve, please look at the 3φ curve.

Thank You Mr.Sridhar,

Does this concept applicable to all the cases, as I have mentioned at starting of the thread ? Or only for LT Motors?

Regards,

i have commissioned a DOL starter for a normal 600HP, 6.6kV squirrel cage motor a long time ago. This would apply to that case. However, when you use these high power motors, you would also spend on more sophisticated motor protection relays, assisted starts etc. You have to take all this into account when setting the relay.

Dear experts,

Could you please explain about the protection relays. I need to protect both the equipment and the motor. .i.e., motor should trip, when the machine is operated abnormally.

We have HT motors run by 6.6 KV and the amperes are 17 amps in load and overload relay is set at 19 amps. I feel this is not sufficient.

Thanks & Regards,

_{"We have HT motors run by 6.6 KV and the amperes are 17 amps in load and overload relay is set at 19 amps. I feel this is not sufficient."}

What do you mean by "not sufficient"? What is the Full Load Current rating of the motor? If it is 19A, then that is the correct setting for the protection relay and you are running below that setting, which is a good thing. I don't see the problem here.

But if the motor Full Load Current rating is 16A, then you have TWO problems; the Overload setting is way too high and you are running the motor in an overloaded condition.

Please state all of the facts and be clear as to why you think this is insufficient.

JRaef has already answered.

Some additional stuff : This is an expensive motor. It is logical to use an expensive microprocessor-based electronic relay. Features like selectable trip class (10,20 or 30) and adjustable single-phasing sensitivity are available.

Furthermore, motor overheating need not be by electrical means only, it can also be due to clogged airways which will reduce cooling effect. Overcurrent relays will obviously not sense this. To prevent this, thermistor relays should be used, but they would need thermistors already embedded in the motor during its manufacture.

There are many relays in the market, you need to search. Right here on Globalspec would be a good start.

Good luck.

You have a number of great answers, so I'll just add briefly:

When your motor gets it nominal rated voltage, and;

it is operating at its rated load,

your "running amps" will be the FLA (Full load amps) of the nameplate data.

Overload current and above overload current is a region that the motor can handle for just short periods of time. That current is usually designated SFA on the nameplate data and if not designated or labeled, usually this indicates that it can tolerate 10% above nameplate FLA. Normal SFA rating is 15% above FLA (1.15).

Everything begins and ends with "running amps".

When you motor is in operation and your running amps is on or below FLA nameplate data, the world is good because this means its getting its voltage and it load ratings.

Too low and you can concern yourself but not usually.

To high!! and you need to concern yourself as the lifetime expectancy of the motor is reduced quantatively each minute that a motor operates higher than 10-15 percent above FLA.

Check incoming voltage, check load..... and all other advises listed previously..........Hope this helps...............Cy