Motor Calculated FLA 2

[nightfox1925]

I have a 75HP motor with power factor 0.85,efficiency of 0.95 and load factor of 0.98. The nominal voltage is 600V and the motor utilization voltage is 575V

I have a spread sheet showing a FLA calculation using the motor utilization voltage which would end up to be 69.57A.

Using nominal voltage, it would result to 66A.

My question is: for purpose of preliminary load calculation, which one should be used, nominal or utilization? Will it make sense if I will include the load factor as well in calculating the load?

 

[waross]

What is the nameplate current?
600 Volts is the nominal transformer voltage.
575 Volts is the nominal motor voltage. This allows for about 4% voltage drop in the feeders and circuit conductors.
Put another way, the motor rated voltage is based on the assumption that the 600 Volts supplied by the transformer will have dropped to 575 Volts at the motor terminals. If you want a more precise value than assuming 575 Volts at he motor terminals, measure or estimate the actual voltage at the motor terminals. It may be less than 575 Volts.
The limiting factor is the nameplate current. This is how much current the motor may draw safely without overheating.
It is rare to have a load sized so close to the motor size that a few percent voltage difference is significant.
The values to use in a preliminary survey often depend on the purpose of the survey. For conductor sizing and equipment sizing the Canadian code requires the nameplate values to be used.
For running cost estimates or gen set loading, you may wish to use the actual loads for the survey.

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

 

[Kiribanda]

nightfox1925,

You have to get the rated full load current of the motor. For that you have to use Rated motor voltage 575 V, Rated F.L. PF=0.98 and Rated F.L.EFF=0.95.Then the Rated FLA of the 75 HP motor = 69.47 A.

Use this value for protection settings.
Use 98% of this for oad flow/ demand calculations.

 

[LionelHutz]

69.57 is the calculated rated current.

Do you mean what value of current you need to use to determine the system required to support the motor?

68.2A would be the absolute minimum used for the steady state load. However, a 75hp is commonly "recognized" as having a nominal current of 77A in North America. I would tend to use 77A because motors do get changed and the next 75hp might have a higher rated current. I would only examine each motor to determine if any require more that the commonly listed current, not less.

Here is an example of a motor current table, the first one that showed up in Google. This table shows up in many CSA and UL documents.

http://www.thepanelshop.com/motor_current_ratings.htm

You also have to look at the motor start to ensure the system is OK during that time. The starting current will likely be around 400A to 500A and last at least a few seconds.

 

[waross]

If you are bound by electrical codes, you have to do it the way the code specifies.
In Canada you must use the motor nameplate data as a minimum value.
In the US you must use the value given in the appropriate table in the National Electric Code. (I understand that this is so that if a high efficiency motor is replaced with a standard motor at a later date, the conductor ampacity will be adequate.)

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