VFD problem or.....

[GlennH]

I wasn't on site but a coworker (hvac tech) called me about a problem. Cooling tower with 2 fans on VFDs, not sure on size ,around 20HP, 208 volts. Been running fine for about 2 plus years.

While there he was asked to look and see why the breaker for the 1 fan was tripping. He found that both breakers were "too hot to touch"

The setup: Breakers in panel in basement, VFDs right beside panel, load to cooling tower on roof are each run in separate conduits- about 50', there are no other wires in conduit to roof

He says everything about this setup is HOT, pipes to roof, conductors in pipe, disconnects on roof, sealtite, and the motors.

He checked amps with meter, below FLA and the vfd amps agreed with his meter.

VFDs are not showing any faults, I think they are Yaskawa

Only the 1 breaker has tripped several times, the facility maint. cleaned the vfd filters, apparently they were quite dirty.

Coworker told them they needed to get their electrician involved so they called the control contractor :?:?

He said they are having quite a bit of major electrical work being done at the facility right now.

Addendum.. Site electricians changed both breakers, did not make any difference ,still very hot


sorry for second hand information, but any ideas what could be causing this??

 

[Jraef]

Are there line reactors in front of the VFDs? If not, that's the first thing I would do. Yaskawa (and most Japanese) drives do not come with DC bus chokes so the harmonic current distortion can easily be 80% THD on a 6 pulse drive. That is current that flows at higher frequencies than the fundamental and eventually causes heating of line side components, conductors, transformer windings etc.. A simple 3% line reactor will knock that down to 35-40% I-THD and help protect the VFD. Better yet might be a Passive harmonic Filter, get it down to around 7% I-THD.

 

[GoldDigger]

Just out of curiosity, what are the main reasons that harmonics, particularly high ones, cause heat issues?
Magnetic induction? Skin effect increasing conductor resistance?
Something else?
I know that triplen harmonics can specifically affect a neutral, but this appears to be effecting the ungrounded conductors.

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[GlennH]

thanks Jraef, I will pass it on

 

[mike_kilroy]

... load to cooling tower on roof are each run in separate conduits- about 50', there are no other wires in conduit to roof

...He says everything about this setup is HOT, pipes to roof, conductors in pipe...

Did they run EACH INDIVIDUAL MOTOR PHASE in separate conduits??


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[GlennH]

NO 2 vfds, 2 conduits to roof, loads to motors in their own conduit

 

[Jraef]

Just out of curiosity, what are the main reasons that harmonics, particularly high ones, cause heat issues?
Magnetic induction? Skin effect increasing conductor resistance?
Something else?
I know that triplen harmonics can specifically affect a neutral, but this appears to be effecting the ungrounded conductors.

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I²R losses. Skin effect mostly, because as frequency increases, the current flows closer to the surface of the conductor due to unequal flux linkages. But on top of that, there is the "proximity effect" that is basically irrelevant at the fundamental frequency, but increases at higher frequencies. Combined, these tend to increase the overall AC resistance (R) of the cable to be above the R_DC, which is what most conductor VD calcs are based on, because again, it's assumed that the frequency is 50/60Hz, not 300, 420 or 660Hz.

Irms = √[(I₁)² + (I₂)² + (I₃)² +…..+ (I_n)²]

where:
I₁ = fundamental current
I₂ = 2nd harmonic current
I₃ = 3rd harmonic current
I_n = nth harmonic current

In a 3 phase system the orders divisible by 2 or 3 will cancel each other out, but the non-triplen odd order harmonics will not, so their I²R effect is additive. The amplitude of each of the higher order harmonics goes down as the order goes up, but the combined effect, like I said, can be 80% I-THD (or more) for an un-mitigated 6 pulse inverter, and if the conductors were sized too close to the bone, the result is over heating. That's the main reason why the NEC added what became 430.122, requiring that feeder conductors be sized at 125% of the VFD input current rating, not the motor connected to it.