service sizing for variable speed motors

[2571]

Good morning,
We are installing two 200HP compressors in November to replace our exsisting old and tired compressors. the FLA of the new compressors is 240 amps. My first question is when calculating the circuit size, fused or instantaneous trip breakers do I apply the same NEC standards and math to a variable speed motor as I would to any other induction motor? Wire sizing I can under stand, but if I choose to fuse or uses breakers it just seams to be a waste of money if the inrush current isn't there any more. Question number two. How concerned should I be with 3rd, 5th and 7th harmonic on our ground wye system.

thank you

 

[swn1]

On the first question: you no longer have starting current to deal with. More precisely, you now have a lot of control over how much starting current you have. For a compressor application the motor can start unloaded so starting current at the line side of the drive will be a small fraction of maximum operating current.

The math and the rules don't change, the load characteristics do.

Keep in mind that even though you don't have 6x FLA for a second or two starting current, you do have inrush in the input stage of the drive itself. Much shorter duration but the magnitude can be very large.

On your second question: most new drives are delta connected and do not impress any current on the neutral, whether or not the power system has one. If your drive is wye connected you very much do need to be concerned about neutral currents.

In theory neutral current on a 3-phase system can be as high as three times the average phase current. In practice, I've seen it as high as 180% on a 480V feeder circuit. The cable jacket was uncomfortably warm.

Harmonics (and unbalanced phase currents) in the load can also stress the transformer. The drive and motor may work perfectly well with an open leg, failed diode, etc but the transformer will get very hot very quickly. If drive loads (and other nonlinear loads) add up to a significant fraction of the transformer's rated load you will want to monitor those effects at the transformer. Harmonics may add or cancel when loads are combined, just depends.

 

[jraef]

There are also new rules in the NEC that are specific to feeders for inverter fed motors. You should read them carefully, a lot of people failed to notice the changes and get burned by inspectors.

Also, unless the VFD is going to be right next to the motor please consider using the new shielded inverter lead cable that is available. It costs more but you will enjoy longer life and fewer problems with your motor.

http:/

http://www.jraef.com

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[2571]

Good afternoon,
I'm very new to the feild and I appreciate both of the tips I have recieved thank you for your time.

2571

 

[powerjunx]

Hi 2571,
You'll have a probable concern about your motor winding when it is not an INVERTER SPIKE resistance duty. I found a lot of old controls replace with VFD, the side effects is the motor winding is vulnerable to harmonics to the extent the insulation fails. It's worst!

 

[2571]

fbcybil,
the new compressor have variable speed drive rated motors, this is the lasted and greatest buzz word in the compressor industry. I may have misunderstood you but we aren't replacing the controls it the whole compressor. I'm curious to learn more!! My concern is the compressors are on or will be fed from the same buss as other piece of critical equipment, computers, plc, vsd etc... We have considered pulling a new curcuit to the compressor building but holy dollar signs. it's about 800 feet from our main distribution center. We have seen some effect of harmonic from some spot weld equipment on this bus. That was my first lesson in phase balancing. looking to learn more from all.

respectfully
2571

 

[Bestboy]

*"You'll have a probable concern about your motor winding when it is not an INVERTER SPIKE resistance duty".
*"Please consider using the new shielded inverter lead cable that is available".
This is new to me. I am currently doing two upgrades and I have surely not put these two factors into consideration, I wonder what else I have overlooked.

(1) Cooling tower upgrade.
This cooling tower uses two (15KW) motors each with 2 drive belts: - one motor for low speed (less cooling demand) and the 2nd motor for high speed (greater cooling need). I want to use one inverter-driven motor to cater for both scenarios. The reasoning behind this is less maintenance, better control and logical engineering decision. The mechanical guys don't agree with me because they say if the drive belts break while operating with one motor then we will sit without cooling until repairs are done. Currently if one set of belt breaks then the other set takes up the total load pending replacement of the faulty belts. My answer to this is that proper routine maintenance should be done to the mechanical parts of the cooling tower and 2ndly the inverter will be so gentle to the belts/mechanical parts therefore less vanulability to mechanical failure.
The motor is less than 10 meters away from the starters. Do I still need 'the new shielded inverter lead cable' There is no interference around.

(2) Vaporiser extractor fans
Hear I have 2 X 20KWs motors (one on-line the other spare) driving a vaporiser extractor fans. I want to replace the current old star/delta starters with two inverters. My plan is to re-use the motors (old but in good shape) and just renew the starter; this is until reading about 'INVERTER SPIKE resistance duty' in this thread.
Kindly give me some assistance hear. Any input will be appreciated.