Generator sizing and fire pump motor starting load 1

[katwalatapan]

Hello,

I wanted to inquire about effects on generator sizing, if fire pump motor, soft-starting methods, and required manual electric/mechanical starting of the fire pump (as per NFPA 20) are taken into consideration.

A generator is required to be sized to back-up fire pump motor and controller loads in addition to regular building lighting, power, and motor loads. The fire pump motor is the largest load on the generator. The fire pump controller may have either a manual electric or manual mechanical starting (as required by NFPA 20), in addition to automatic starting. The fire pump controller also is configured for soft-starting the fire pump motor.

I've read that for fire pump application, the generator is permitted for a voltage drop upto 15%, but if the fire pump is operated manually by mechanical means, then the 15% voltage limit is not applicable. Even if the fire pump controller has a soft-starting method, if the fire pump is started mechanically, then it would be started across-the-line.

Considering the above scenario, I have the following queries:

1. What would be an acceptable voltage drop for the generator, when the fire pump is started via mechanical starting?

2. In a case, there the fire pump controller has a soft starting method, manual electric starting, and manual mechanical starting, would the generator be sized for fire pump motor locked rotor amperage calculated for soft-starting or the locked rotor amperage calculated for across-the-line starting. Would the generator size also be affected based on the increased threshold of voltage drop for manual mechanical starting?

Thank you.

 

[Parchie]

Please define what you meant by fire pump "started mechanically"?

 

[waross]

As the voltage drop exceeds 15% there is an increasing risk that a magnetic contactor will drop out due to low voltage. With no load the voltage will recover and the contactor will pull in again.
Repeat.
With a mechanically held starting device such as a power circuit breaker, the motor will stay connected. If the load draws down the frequency, the Under Frequency Roll Off feature UFRO) will become active. This will reduce the voltage in approximate proportion to the frequency reduction. This will maintain the volts per Hertz ratio to the motor. The action is similar to VFD starting. Starting current is reduced and the motor is started with less internal heat developed than if it were started DOL.
This method is only suitable for generators dedicated to the large motor. While the reduced voltage and frequency are suitable for friendly motor starting, other loads, particularly lighting will suffer.
The absolute minimum generator size for shared loads is the sum of the connected loads plus three times the current capability of the motor being started.
That said, for fire pump service with shared loads, it may be best to use generator sizing software to calculate the voltage drop. Such software was free on the Caterpillar web site.

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

 

[katwalatapan]

Thank you waross for your response.

Parchie: By mechanical starting, I mean the fire pump load is applied to the generator via a manually operated lever/handle.