Power Cable Hum at Motor Startup - 3 Phase, 40 Hp, Line Start

[elenasdad]

Motor runs fan for cooling tower on roof of 15 story apartment building.
A loud humming sound is generated for about 2 secconds at startup that is heard throughout the building. It sounds similar to a 120 Hz tranformer noise. The noise appears to be generated by the vibration of the power feed cable within its conduit and not does not appear to be transmitted mechanical vibration from the motor itself.

Tests show the startup current to be normal, the phases to be resonably well balanced and the voltage drop across the feed cables to be marginally acceptable.

Has anyone seen this before?
If the three phases are balanced and the power cables are twisted how is a net magnetic force being generated to vibrate the cables ?
Does the power feed cable necessarily need to be replaced (very $$$) or could the problem be the motor ?
Would reducing the startup current using the a Partial Winding Start or VFD be likely to reduce or eliminate the problem ?

DETAILS
Motor:
US Electrical Motors 40 Hp, 3 Ph, 60 Hz, 200 V 108A, 1775 RPM
INS F, Sign B, Code G, SF 1.15, PF 86.5., KVAR 93.6
3 Wire Y Connected, Across Line Start (is PWS capable)

Power Feed Cable:
3 Wire Copper AWG #2 within single ~ 2" OD Metal Conduit
Approximately 250' run with several 90 deg. turns

Measurements Taken:
Power Feed Cable insulation tested OK (not performed personally/
do not have detailed results)
In Rush Current @ Startup 538A +/- 3A All Lines
Run Current 84A +/- 2A All Lines
_______________________________L1_________L2_________L3
Run Voltage @ Panel___________121.0 V ____121.4 V ___121.1 V
Run Voltage to @ Motor_________118.0 V ____118.8 V ___118.1 V
Off Voltage @ Panel ___________121.9 V ____122.6 V ___122.4 V

____________________________ L1 to L2_____L1 to L3____L2 to L3
Run Voltage @ Panel __________210.2 V _____210.3 V____212.0 V
Run Voltage @ Motor__________205.4 V_____205.2 V_____206.9 V
Off Voltage @ Panel___________212.2 V_____212.2 V_____212.2 V

 

[davidbeach]

You mention twisted cables, how are they twisted? Are they bound into place? How are the cables anchored in the vertical run? Your conductors are a couple of sizes too small, they should be 1/0 per the NEC for a 40HP, 200V motor.

Motor starting is not necessarily a balanced condition. My hunch is that long vertical runs of undersized conductors are bouncing around in the conduits during inrush.

 

[electricpete]

2 seconds sounds like a possible duration of starting current.

If current in two phases were hypothetically in-phase, they would attract to each other.

If current in two phases were hypothetically exactly 180 degrees out of phase, they would repel each other.

With a phase like 120 degrees, they will attract for portion of the cycle and repel for portion of the cycle. This might amount to the vibration that you see.

I suspect whatever the cause, if the noise is in fact coming from the cable/conduit, you can expect a reduction using reduced voltage start. Reduction in current means reduction in flux.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[jraef]

I agree with davidbeach.
I have a "cheat sheet" in front of me on recommended conductor sizes for motor leads that takes voltage drop over distance and starting conditions into account. It does not show 200V, but for a 230V 40HP at 200ft, it shows 2/0 cable. At 300ft it shows 3/0. You are in between those figures, but at 10% lower voltage, which means higher current, I would have gone with 3/0 cable. You cables were ized only for the running current at best, without taking distance into account and oversizing for inrush. Having too small of a cable means more voltage drop on start up. That may be fine for you, but it will mean even higher cable stress which is undoubtedly what you are hearing. I would recommed increasing those cable sizes to avoid eventual insulation breakdown and a ground fault in your future.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[itsmoked]

Why not the reduced voltage starter? Since this is *only* a fan it makes the perfect candidate for taking a long, l o n g, lonnnnnnnnnng, time to start up. This would allow continued skirting of the wire size issue not to mention the fact that bigger wire may still hum unacceptably for a living space on startup.

 

[BigWallyB]

All correct answers, that's why I love this site. I second (third?) pete and itsmoked on softstart. Right cheap fix. Not the "perfect one" but acceptable. If there is still vibration you haven't lost that much and may still be able to quiet it cheaply (mechanically, short piece of foam?) With the severe undersizing of the wires I would consider the conduit placement to make sure a heat source is not going to make jraef seem extraordinarily prescient.

Life is what happens while we're making other plans.

Wally

 

[electricpete]

The original poster acknowledged voltage drop marginally acceptable evaluated through some sort of test.

So, just to understand the points raised by respondents, are you guys saying undersized cables can be related to the hum? Or just giving general advice unrelated to the hum.

I would think if anything undersized cables would reduce the current and associated hum (although lengthened in duration).

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[bigbillnky]

Hello elenasdad,
I have to agree with electricpete when it comes to cable size, voltage drop, and hum. A few things come to mind as possible causes.

First, in the post you state the conduit size is 2" OD. If this is in the US, that does not correlate to a standard size. Maybe 1 7/8" OD? The reason I mention this is the possibiltiy the conduit being oversized is causing some of the problem, i.e. more space for the conductors to move around in.

Second, the NEC requires the conductors serving the motor in your application to be supported at a maximum of every 100'. Vertical conductors of smaller sizes tend to be more prone to movement when installed in vertical applications owing to the fact they do not have the weight and mass to resist movement.
Good luck to you.

Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[aolalde]

See thread238-68697 for similar problem.