Motor Starting Power & Power Factor 1

[X49]

I am sizing a gas genset to start a 600V 45kW motor driving a centrifugal pump. Motor characteristics are as follows:

Starting Current: 405A
Rated Current: 54A
Total Moment of Inertia: 0.42 kgm2
Speed: 1775rpm
Load at pump operating point: 32kW

Motor starting power is the limiting factor for the genset in this application. My genset sizing software assumes a starting power factor of 0.36 and a running load (input) of 51.7kW, giving a starting load of 87.3kW. This assumes a "low inertia" load and NEMA Code H motor.

Is a starting power factor of 0.36 and starting load of 87.3kW reasonable for this application? I've heard that the rule of thumb for induction motors is a starting power factor of 0.2 (which would give a starting current lower than the running current in this case).

I may be over-analyzing things here but I would really like to know the relationship between starting power, starting power factor, and load characteristics.

 

[X49]

According to the genset software, both voltage dip and frequency dip will be limiting factors for the gas genset if the motor starting power factor is 0.36 and full voltage starting is used. This is with a maximum 30% voltage dip and 10% frequency dip.

If a solid state soft start is used, frequency dip (engine power) becomes the limiting factor. According to the software, starting power is not reduced by a significant amount by adding the soft starter (set at half full load starting current). The genset has a good three phase rectified regulator so I'm not concerned about the soft start harmonics.

I don't want to oversize the genset too much for motor starting or the engine will not run at a high enough load during regular use. As I mentioned, the running load is only 32kW plus a maximum of 10kW building loads, so a 125kW genset would be on the large side. A soft starter may be my best option to achieve this.

It doesn't look like I'll be able to get a calculation for starting power that is accurate enough. My best option may be to get a recommendation from the genset manufacturer.

 

[antigfk]

If you use a soft starter you will probably want to use one with a current ramp function. A VFD would probably be the best option technically speaking, but you'll have to work it out from a cost standpoint. You'll be able to dramatically reduce the generator size using a VFD to start your motor. Good thing about the VFD is that you can be sure that the motor will have enough torque to accelerate the load, plus current inrush & voltage drop won't be an issue.

 

[ScottyUK]

epete - You're probably right.

That was a good discussion. I've just re-read it.


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