Service factor HELP!

Something is wrong in your question.

Service Factor is a number that is applied to NEMA (North American) motors that states a factor by which the motor can be temporarily overloaded without significant damage or performance changes. There is no such thing as "service factor" when discussing IEC designed motors.

In your stated formula, you mention PF, which is POWER FACTOR. PF is something completely different and unrelated to Service Factor.

Your formula is close, but not very well thought out if you look closely.

Amps can be determined by looking at kW, but kW ALREADY means "W x 1000" so to go kW x 1000 means W x 1,000,000!

Now, even though you can correct that error, if your motor shows ONLY the voltage and service factor as you stated, not anything else, you cannot determine anything useful with that information. You need either the HP or kW rating of the motor. If you don't have that, you cannot determine the amps. Sorry.

JRaef is correct. Service is more about toughness while power factor is about quality of energy in your facility(capacitive vs Inductive). One note I wanted to make looking in my Ugly's electricians quick book is that KW means the short hand 4600 watt or 4.6 KW motor when calculating an IEC motor.

It's a little misleading but common among electrician's use of formulas. Otherwise find your HP load as mentioned.

Actually... the "kW x 1000" part is accurate.

Given a 10 HP motor, we can say that the formula to solve for 3 phase amps is...

A = 7460 w / E x 1.73 x EF x PF

or...

A = 7.46 kW x 1000 / E x 1.73 x EF x PF

Huh?

1 HP is equivalent to 746 watts.

(kW X 1000) has no place in the calculation for FLA -- you skipped a step or two . . .

If it's a motor with a nameplate on it which shows "service factor," then it ought to also show FLA and/or HP.

HP X 746 / (Volts X P. F. X 1.73) = Amps (or designed FLA)

A S. F. = 1.1 means the motor can withstand 10% higher current, but that doesn't translate to higher designed HP or FLA ratings.

Let me try this, another way.

I need to purchase six one horse power motors. The name plate ratings are 460 volt, 1.9 amps 1.15 service factor.

Every time I purchase my heaters they end up too small. When I question what went wrong with my calculations I am told that I didn't figure in the service factor of 1.15 or in rare cases 1.

When all of the plant is running operating voltage is 480 volts. One day of the week two of the biggest machines are down for maintenance and on those days the operating voltage is 499 volts. When the whole plant is down, voltage is @ 506. We also have less than 1% unbalance phase ratio in the plant.

So I have to use 499 volts to compute my loads.

EASA has a graph of THE EFFECTS OF VOLTAGE VARIATION ON INDUCTION MOTOR CHARACTERISTIS. It is proof of my dilemma, with no mathematical solution.

No mater witch ELECTRICAL FORMULAS FOR CALCULATING, I use, one part of them needs to have current to compute. ie; % Efficiency, KW, KVA.

Service Factor for example, 10 HP motor with a SF 1.15 can handle heat of the load of a 11.5 HP motor.

How do I compute the FLA for MORCP?

I'm a little confused . . .

Where are the heaters? On the main contactor? On each motor contactor?

Also, what's "MORCP?"

Hearter are Motor Over Running Current Protection.

And thanks for your help.

It took awhile, but I finally found some tables. Check out http://www.cooperbussmann.com/2/MotorProtection.html

Hope this helps.

You should not be recalculating FLA based on voltage fluctuations at all. NEMA design motors are to be protected by the FLA value at nominal voltage of the nameplate, not something you calculate. The only thing you may do is to add Service Factor IF you absolutely need to use it, but not if you want your motors to last a long time. Running into the service factor WILL shorten your motor life. The quick and dirty rule of thumb is a loss of 50% of the motor life for every 10deg C that the motor runs over winding design temperature, and running into the SF will definitely increase the motor winding temperature. Read the instructions for your heater element charts and don't deviate unless you absolutely must.

Your motors are designed for 460V, +- 10%. 460 x 1.1 = 506V so you are not outside of design voltage range ad your motors are not in trouble. If they are tripping on overload continually, the problem is likely that they are undersized for the load. If the motors were selected by an OEM, I'd be cautious about their sizing. Sometimes an OEM will decide to use the SF rather than go one size larger on the motor. The problem with that is that they don't care if, in the long run, it cuts the motor life in half, as long as it outlasts their warranty. Only you as the user will care.

JRaef

Thank you

Every thing you are saying is true.

These 1HP motors draw 2.5 amps at 499 volts. This is more than 125% of the name plate rating.

The motor manufacture says this is correct amperage for this voltage. And to my conversations with these engineers, it sounds as if they come up with these values on a test bench. Not any of the motor manufactures have ever given me a calculation that they used to come up with this value.

And as far as the motor being undersized it is the correct size for the application. The no load amps are 2.3 A at 499 volts.

Wait a minute here:

These 1HP motors draw 2.5 amps at 499 volts. This is more than 125% of the name plate rating.

The motor manufacture says this is correct amperage for this voltage. And to my conversations with these engineers, it sounds as if they come up with these values on a test bench. Not any of the motor manufactures have ever given me a calculation that they used to come up with this value.

And as far as the motor being undersized it is the correct size for the application. The no load amps are 2.3 A at 499 volts.

If the motor is rated at 1.9 amps, and you are draw 2.5 amp, then the motor is undersize for your application, espically at 499 volts. Your 1hp motor is designed to RUN CONTINEIOUS at rated nameplate current of 1.9 amps, not 2.5 amps.

I'm confuse about this being the correct size for the application. If the nameplate on the motor is rated for 1.9 amps, how can this be correct when the no load amp is 2.3??

Change out the 1 hp motor to a 1.5 hp motor for safe operation. The service factor is only used in case of emergency, not continueious usage.

MidniteFighter

EASA Has a nice graph showing the relationship between voltage and induction motors.

Obviously... that is why I used 7460 watts for a 10 HP motor.