Author Topic: Max winding temp  (Read 205 times)

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Offline Pete

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Re: Max winding temp
« Reply #15 on: April 24, 2022, 03:14:19 am »
Hi LH , I came across this explanation that may help.
As the frequency of the drive is reduced the voltage has to be reduced also.
So in your case your motor is rated at 460 volts at 60 hz.
That is a ratio of 7.666.
if the voltage is reduced to 400 as in your case then the frequency needs to be reduced too. that would mean the drive should be putting out 52 HZ.
The motor would then be running at 120 x 52/2  which equals 3120 RPM.
If possible back the VFD frequency back of to 52 hz and keep the voltage at 400 and see if it runs cooler and has the power and torque needed to run the compressor.
Pete

AC motor characteristics require the applied voltage to be proportionally adjusted by the VFD whenever the frequency is changed. E.g., if a motor is designed to operate at 460 Volts at 60 Hz, the applied voltage must be reduced to 230 Volts when the frequency is reduced to 30 Hz. Thus the ratio of volts per hertz must be regulated to a constant value (460/60 = 7.67 in this case). The most common method used for adjusting the motor voltage is called pulse width modulation (PWM). With PWM voltage control, the inverter switches are used to divide the simulated sine-wave output waveform into a series of narrow voltage pulses and modulate the width of the pulses.

With a standard AC across-the-line motor starter, line voltage and frequency are applied to the motor and the speed is solely dependent on the number of motor stator poles. In comparison, a VFD delivers a varying voltage and frequency to the motor, which determines its speed. The higher the frequency sup plied to the motor, the faster it will run. Power applied to the motor through the VFD can make the motor working speed lower than the nameplate base speed, or increase the speed to synchronous speed and higher. Motor manufacturers list the maximum speed at which their motors can safely be worked.

Offline lighthunter

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Re: Max winding temp
« Reply #16 on: April 24, 2022, 07:01:37 am »
Hi Pete! This is great information!  Reminds me of a time years ago when i had a pump failure on a laser and no replacement, i found an R.O. pump and plumbed it in but it didnt have enough flow. I put an AB vfd on it and turned the freq up and it worked, i want to say 76hz it ran very warm but got us through till oem part came. In this case we'd do the opposite. I have already tried reducing max output to 70% which helped but still overheated. I'm guessing by what your saying, the scaling needs to be adjusted. So that we hit 400v at 52hz instead of 60hz. Some drives have an autotune for this.

Trying to wrap my head around all this... if switching connection to wye works that would mean the 400v is too high for 100% duty cycle in delta, (magnetics are saturated causing excess current wasting energy in heat?) If reducing freq and maintaining 400v in delta works, then magnetics were too weak and allow too much slip for load applied causing excess heat?

So there are at least two maybe 3 possibilities to improve this situation without a motor change. Thats good news!
 So far the reps from CP have not been people who know anything about motors or vfds, its been more like... "let me talk to someone and get back to you"  I look forward to trying this information and getting rpm vs freq information for before and after.
Thanks and have a great day!
Health Warning: May contain traces of nut!
LH

Offline Pete

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Re: Max winding temp
« Reply #17 on: April 24, 2022, 05:12:37 pm »
Hi LH, hopefully lowering the frequency will sort it out. Of course it will run slower but hopefully not so hot.
My understanding is that the frequency needs to be lowered when the voltage is lowered otherwise the motor will draw more current as the frequency falls.
Inductive Reactance  XL =2 x pi x F x L
So pi doesn't change 3.14 is a constant
F is the frequency so if it is lowered then
L inductance stays the same but
XL inductive reactance will fall, which means that the motor will draw more current, hence dropping the voltage compensates for the lower inductive reactance.

I have not had a real lot to do with VFDs over the years, I did my apprentiship as a motor rewinder. VFDs were not very common then.
Good luck
Pete

Offline Pete

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Re: Max winding temp
« Reply #18 on: May 02, 2022, 05:34:17 pm »
HI LH just wondering if you have had any luck with the compressor.
Did you manage to drop the frequency to test the current draw and heat at 52hz,
anyway just wondering if you had any luck
Pete

Offline lighthunter

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Re: Max winding temp
« Reply #19 on: May 06, 2022, 02:27:46 am »
Hi guys!  :)

We've had a myriad of other breakdowns to attend too lately. The guy from tech support finally responded to the question this week (attached) and we got some time yesterday to try a couple things.

So the first thing was i brought in a phototach i had at home, the control max speed was set to 75% so max rpm for test was 45.75hz, the measured phototach rpm was 2733 rpm and 175A 400V as read on vfd display. Vfd display also read 2725 rpm.

Nameplate rpm calculation is 60hz=3585rpm so 45.75 hz = 76.25% * 3585=2733.56.

When i saw that my head went blank because i was so sure we were underpowering the magnetics of this motor causing slippage thus the thought to raise voltage to cure problem, in fact, if no other solutions were obtainable, i had in mind to install an orfice on intake and run it at a vacuum to reduce load. When i saw this, all those ideas vaporized.

So, it appears the reason motor temp climbs too much is because we are magnetically saturated and pushing current that isnt necessary to spin the mechanical load... (screw). Correct me if im wrong but if anything, we need to reduce voltage correct?? 

So Pete must be right about the wye choice, we looked on net and seemed like a 30% change by switching connection to a wye. We did so and fired it up. As it spun up, the current meter on vfd seemed to be wildly hunting to 170s then down and up. Not being able to hear with tons of other noise we hit the kill switch. We envisioned a situation where the vfd freq climb did not match the acceleration of the shaft under load which could cause a bad event. As yet its still connected as wye but locked out.

I plan to switch it back to delta next and verify nothing is hurt which i dont think it is.

So are we right in thinking the delta performance is real close just maybe a bit oversaturated? After all it ran for 120hrs. with subzero F° cooling. Maybe im just wierd for thinking winding temp should stay below 100C?  Shouldnt there be more slippage than that?

I feel like if tech support werent so absent all this needs is a vfd tune which is a standard procedure for all large HP vfd setup...

I get their argument about motor should be identical to original and you wont have issue, still vfd tune is always done with new motor install regardless if oem or not.

I think the wye connection would be fine also with new vfd settings. Part of me wants to call Danfoss and get original software reload for vfd and do the normal setup an this would likely be running. Then im faced with wiring my own pressure control loop which im not crazy about modifying an oem machine...liabilities.

Sorry if i sound confused, Ha! I am.


(I meant to include what i wrote to tech support but it wasnt in photo i will try to add, it said something like.  We need this machine running and if you wont give us lock codes, can you send a technician on site to make corrections? That was his response.)


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LH

Offline Pete

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Re: Max winding temp
« Reply #20 on: May 06, 2022, 04:07:32 am »
LH slippage is not your problem there.
Synchronous speed = 120 x Frequency divided by number of poles.
So 120 x 45.75 divided by 2 = 2700 rpm.
As the frequency is reduced the voltage must also be reduced, otherwise the motor will draw too much current.
As the frequency is reduced so it the inductive reactance. Reducing the voltage then keeps the maximum current within specs.
If the motor is designed to run on 480 volts at 60 hz
then at
45.75 hz it needs to have a supply voltage of 366 volts.
Looks like in Star the motor was not getting enough voltage to run at full power under load.
In Star 400 volts line voltage would mean each phase only had 230 volts. Not enough at that frequency.
Put it  back in Delta and drop the supply voltage to 366 volts at the VFD.
Pete

Offline lighthunter

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Re: Max winding temp
« Reply #21 on: May 06, 2022, 08:44:19 am »
Hi Pete! I agree, I wish i could. The VFD is locked as far as changing anything. I can select one of about 6 programs but they are for different specific machines including I/O differences.

The voltage reading above was not a new reading and i need to check it since it was from the 60hz value. The other numbers were read at the time of test.

Am i right in thinking if no slip is present, the volts are high enough? Perhaps higher than needed?


Thanks for your comments Pete!
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LH

Offline Pete

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Re: Max winding temp
« Reply #22 on: May 06, 2022, 05:12:45 pm »
HI LH, if there is no slip in a squirrel cage motor then it would have hardly any torque.
There needs to be some slippage so that the lagging magnetic field creates torque.
Unless your gear is Laboratory standard there will be some errors in the readings you get.
Like I said the voltage has to fall as the frequency goes down or current will rise and the motor will run hot.
It is hard to figure that whoever built the machine did not set it up correctly in the first place with a decent program in the VFD that varied the voltage to suit the speed of the motor.
It may have been a rush job
good luck
Pete

Offline lighthunter

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Re: Max winding temp
« Reply #23 on: May 07, 2022, 07:09:03 am »
Hey Pete,  yes this inverter is programmed to ramp voltage with freq/rpm. From memory, ive seen it down to 120v/phase at the minimum freq. around 700rpm.

We had chaos on thurs and no work on friday due to a computer network outage.  I will get the voltage reading at 45.75hz on monday.

 Since the slippage is (normal) and current is high, This much points to volts being too high at that particular frequency? Am I understanding right here?  Assuming all other aspects of the motor and load are correct of course.

My reasoning for doing the rpm/slippage test was to determine if for some unknown reason the shaft was loaded too much or volts were too low, i thought there would be more slippage. Maybe my equipment was not accurate enough to draw that conclusion.

I cant thank you enough for your input on this. Ive just never really had to mess with induction motor behavior before. The only other time it got close to this was a new  fan/filter unit kept tripping 20A 480v breaker like once a week. (I dont install this stuff, just try to fix it) so this went on, company was crap to zero support
from italy i believe. Then burned up disconnect switch and breaker. I got those replaced under warranty. Motor then burned weeks later. I had it rewound because they couldnt supply new one quick enough. I called. the motor rewind guy and gave him measurements and he said its gonna fail again if you leave it that way, so i ordered a new motor next size up thinking this would cure problem now the electrical supply was too small to handle it so put a vfd on it and dialed it to 50 hz and its been about 8 years since weve had trouble with it. Im pretty sure the company fitted the wrong centrifugal fan blade in it causing the grief.

You dont usually question the design of this stuff but this was a clear example where they got it wrong. Because the motor on this compressor failed under warranty and twice since leaving all things OEM i think that proves thats the case here as well. They never bothered to put a temp sensor on motor. I could just remove it and push the button and walk away, let it cook. Who knows it might last another two years. Ha! 
Health Warning: May contain traces of nut!
LH

Offline Pete

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Re: Max winding temp
« Reply #24 on: May 07, 2022, 09:53:06 pm »
Hi LH, one other thing to check if the motor is ever apart again is the Rotor.
Squirrel cage rotors do fail.
The bars in the slots usually break at the rings that connect them together at the ends.
This causes the motor to hunt and make strange noises, they sound like they are trying to get up to speed but can't quite make it so they emit a strange humming noise as they run.
I once walked into an autoelectric shop I worked at a few years earlier. They had a 100hp motor that they could not find anything wrong with, They were just about to start chopping the stator windings out when I looked at the rotor.
The rotor had copper bars that were silver soldered to a brass ring. Sure enough some of the bars were broken.
I managed to stop the fellow with the air chisel just in time. He was just about to start chopping the stator winding up.
You could always slip some thermistors into the windings if you wanted.
Unfortunately as the controller is setup, the motor will probably just keep tripping the thermistor relays anyway.
Good luck
Pete