Mike Dolt Enterprises
Mike Dolt Enterprises
User: john d, Location: Florida
Asked on: Thursday, March 17, 2011 at 9:08 am EDT
Answered on: Tuesday, May 14, 2024 at 2:55 am EDT
Question: How do i calculate the generator requirments for a motor. I have an existing building with a existing 150kw 188kva label rated 520amp generator. They have an existing fire pump 30hp, 3phase, code F, S.F. 1.15, 208v motor. They want to continue to add to the generator. Do i use 430.7 and use LRA. That would be 167.7 kva leaving 20kva for other loads?
To calculate the generator requirements for a motor, you can use the following steps:1. Determine the motor's load
This is the amount of power that the motor will draw from the generator. For a fire pump, you can use the following formula:
In this case, the motor horsepower is 30 and the service factor is 1.15, so the load is:Load (kW) = Motor horsepower x Service factor
2. Load (kW) = 30 hp x 1.15 = 34.5 kW
Convert the load to kVA
To do this, you will need to multiply the load in kW by the power factor of the motor. For a 3-phase motor, the power factor is typically 0.8.
In this case, the load is:Load (kVA) = Load (kW) / Power factor
3. Load (kVA) = 34.5 kW / 0.8 = 43.1 kVA
Add the locked rotor current (LRA) to the load
The LRA is the current that the motor will draw when it is first started. This current can be several times higher than the full-load current. For a 3-phase motor, the LRA is typically 6 times the full-load current.
In this case, the full-load current is 520 amps, so the LRA is:LRA (amps) = Full-load current (amps) x 6
4. LRA (amps) = 520 amps x 6 = 3120 amps
Convert the LRA to kVA
To do this, you will need to multiply the LRA by the line voltage. For a 208V motor, the line voltage is 208V.
In this case, the LRA is:LRA (kVA) = LRA (amps) x Line voltage (V) / 1000
5. LRA (kVA) = 3120 amps x 208V / 1000 = 652.9 kVA
Add the LRA to the load to get the total generator requirement:
Total generator requirement (kVA) = Load (kVA) + LRA (kVA)Total generator requirement (kVA) = 43.1 kVA + 652.9 kVA = 696 kVA