I need help selecting a three phase motor starting capacitor

[SixOn]

Motor specs are as below:
HP :1400
RPM :1487
Hertz :50
VOLTS :6900
AMPS :103
COS :0.88

What equations do I use to select the correct size?

What are the long/short term pros and cons of using capacitor for starting big three phase motors?

 

[jraef]

1) What is your goal in wanting to use it?
2) How are you starting the motor, i.e. vacuum contactor, circuit breaker etc.

In general though Capacitor Assisted Starting is not for the novice or the faint of heart. The consequences of getting it wrong are potentially devastating to sensitive electronics that are connected to the same system. The potential benefit is ONLY a slight reduction in cost compared to other more reliable and easy to implement starting strategies. Do a keyword search on this site for "capacitor assisted starting" for other discussions in this group in the past.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[dpc]

I'm not sure there are any pros. As jraef said, why do you think you might want to use starting capacitors in this application?

It takes a massive amount of capacitance to make much of an impact on anything when the motor is sitting at LRA with a pf of about 0.15.

The info you need is not in the specs you provide. You need Locked Rotor Amps and Locked Rotor power factor.

 

[waross]

And the capacitors must be switched out as the motor accelerates. I remember a paper on capacitor starting about 30 or 35 years ago. Haven't seen it in practice in a lot more years. I have investigated the method for a more recent application and came to the conclusion that the advantages were not worth the cost and complexity not to mention possible local system over-voltages should the capacitors fail to clear before the motor reaches speed.
Rough estimate;
FLA: 103 Amps
Estimated starting current: 650 Amps
Estimated starting KVA: 650 x 6900 x 1.73 = 7760 KVA
Estimated starting KVAr: 85% of 7760 KVA = 6600 KVAR
This is a very rough estimate.
Based on a rule of thumb for generator applications that the starting KW of a motor and the load on the prime mover is about 50% of the starting KVA which represents the thermal load on the alternator and influences the voltage dip caused by a starting motor.

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

 

[SixOn]

I will keep looking around the forum and see if I come across a thread that addresses my problem

 

[SixOn]

I went through the I think Im starting to confuse Capacitor assisted motor starting with power factor correction. I cant help thinking that the two principles should be somehow related. Should I use the same priciple in calculating the capacitor size required?

 

[waross]

About 6600 KVAR for motor starting.
About 600 KVAR for PF correction.
Much less for correction to 90%

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

 

[LionelHutz]

What you asked can't be determined from the info you gave. A full system study needs to be performed. This type of study is well outside of the realm of a "tips" website.

My gut feeling would be 3MVAR to 4MVAR might be within a realistic range.

My next thought is why not just use a VFD with a synchronous transfer to line power?

Sizing power factor caps and starting caps involves some of the same math but that's where the simularities end.

 

[SixOn]

Thanks guys for the response.

I'll come post again when I run into trouble.