# Carrier Frequency VS. Motor Winding Heat



## RePhase277 (Feb 5, 2008)

There's always a trade off. The lower the frequency, the lower the losses and therefore temperature. But with low frequency comes audible noise. 20 feet isn't a long distance but a reactor might help with reflectance at higher frequencies if you need to go up on the frequency.


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## Mike_kilroy (Sep 2, 2016)

Yes, There's always a trade-off. We have found through extensive testing on high performance Motors that the least heat happens at the highest pwm frequency. 8 - 12 kilohertz producing approximately 10% less Heat than 4 kilohertz.

So as you can see, it depends.

Why not just test it yourself since you have steady long time running load?

Then report back results to us curious souls.

Sent from my SM-G900V using Tapatalk


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## JRaef (Mar 23, 2009)

Actually, the motor heat goes DOWN when you increase the carrier frequency, it's the drive losses that increase with carrier frequency (more switching, so more switching losses in the IGBTs). So if you increase it to 4kHz, you should see the motor heat reduce a little. 4kHz is the maximum before you have to de-rate the VFD (only because your room is so cool). If you had more overhead in the VFD, you could increase it to 8kHz, but the drive can only handle about 72% of it's rated load in a 20C room, and assuming this is a 367A ND rated PF753, you can go to 72% of 367A, so only 264A, your problem gets worse.

BUT, even at 4kHz, from a motor standpoint it also gets a lot dicier, there is no free lunch. Increasing the CF will increase the risk of "standing wave" generation that can cause motor insulation breakdown and increase the capacitive coupling effects between the stator and rotor that can create voltage flow across the bearings and EDM (Electric Discharge Machining) damage. So experiment with the increased CF and if it works, add a DV/DT output filter to the output of the VFD, and add shaft grounding rings to the motor (if it doesn't already have them).

Side note: Motors that have a 1.15SF become 1.0SF if run on a VFD. The pump supplier that picked that motor would have known that, but in this case the motor was originally picked to be run at full voltage (the soft starter makes no difference here), so they must have chosen to use the SF. You cannot run into the SF of a motor when running it from a VFD, so by changing to VFDs, you inadvertently changed the maximum flow of these pumps to be lower. Now if this one trick I just gave you doesn't work but they want "more water", they may have to get bigger pumps and motors and VFDs (or another pump).

PS:
Ah, just noticed you said it was only 20' from drive to motor. Don't worry a lot about the filter on the output, at 4kHz on that drive, you can go 25' from drive to motor even if it's an old motor that's not inverter duty. (1000V insulation). A load reactor wouldn't be a bad idea, but it does reduce the motor voltage a little, which means a little less output, and you might need all you can get. Just make sure to let the owners know of the increased risk from turning up the CF. Definitely put in the shaft grounding rings though.


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## Mike_kilroy (Sep 2, 2016)

JRaef said:


> Actually, the motor heat goes DOWN when you increase the carrier frequency, it's the drive losses that increase with carrier frequency (more switching, so more switching losses in the IGBTs). So if you increase it to 4kHz, you should see the motor heat reduce a little. 4kHz is the maximum before you have to de-rate the VFD (only because your room is so cool). If you had more overhead in the VFD, you could increase it to 8kHz, but the drive can only handle about 72% of it's rated load in a 20C room, and assuming this is a 367A ND rated PF753, you can go to 72% of 367A, so only 264A, your problem gets worse.
> 
> BUT, even at 4kHz, from a motor standpoint it also gets a lot dicier, there is no free lunch. Increasing the CF will increase the risk of "standing wave" generation that can cause motor insulation breakdown and increase the capacitive coupling effects between the stator and rotor that can create voltage flow across the bearings and EDM (Electric Discharge Machining) damage. So experiment with the increased CF and if it works, add a DV/DT output filter to the output of the VFD, and add shaft grounding rings to the motor (if it doesn't already have them).
> 
> ...


Since YOUR load requires >335amps, obviously you cannot try 8khz. Most of the drives we have used have a gradual derate curve since that is how it actually works in the extra drive IGBT heat. So as long as AB shared that curve you can go a tad higher than 4khz for testing and still output your 335 or slightly higher amperage...

Great reminder on SF going back to 1.0 with vfd! This would seem to imply why your motor has burned up twice running over its nameplate rating by a few amps.

I would just caution against painting with too broad a brush by saying "_at 4kHz *on that drive*, you can go 25' from drive to motor even if it's an old motor that's not inverter duty_." Even at <25' motor cables, you will see 1200v spikes on the output; obviously above many old motor insulation ratings. Unless that model drive has some special output filter built in that reduces the normal IGBT 2x output voltage spikes when switching (even without standing waves): if so, then that "on this drive" makes it still a good rule.


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## RePhase277 (Feb 5, 2008)

Yes drive losses. My mistake.


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## John Valdes (May 17, 2007)

Rob. Have you considered an external blower?


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## Mike_kilroy (Sep 2, 2016)

John Valdes said:


> Rob. Have you considered an external blower?


Wow did none of us say this before now?? 

He has a "300HP hollowshaft pump motor" but it still has windings on OD of motor... 

If we were to put a WATER JACKET around that stator, we would effectively DOUBLE the available rating (torque & amps). Obviously that is not in the cards...

When we put a good airflow axially to the stator of the motor by putting the motor in an OD tube with fan at back, we "rule of thumb" increase the capacity of the motor by 50%. Be VERY conservative, and you can get 25% more capacity. Ditto if you blow good air ACROSS the motor radially. So OP guy can likely go from 335a to 500 amps - and get the same life. Or said better, go back to 60hz for the flow, and stop overheating the motor every 2 years...

LEAVE the internal fan alone! Just consider putting a 300cfm? muffin fan(s) or other type fans blowing radially ACROSS the stator and get 15% more capacity easy. Result can be 60hz operation again, cooler motor, so no motor failure for 10 yrs now...


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## Mike_kilroy (Sep 2, 2016)

RePhase277 said:


> Yes drive losses. My mistake.


In your defense, MANY (most?) people say since the higher freq DOES increase eddy current losses IN THE ROTOR ITSELF... Often they fail to look at the WHOLE picture.


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## JRaef (Mar 23, 2009)

Mike_kilroy said:


> ...
> I would just caution against painting with too broad a brush by saying "_at 4kHz *on that drive*, you can go 25' from drive to motor even if it's an old motor that's not inverter duty_." Even at <25' motor cables, you will see 1200v spikes on the output; obviously above many old motor insulation ratings. Unless that model drive has some special output filter built in that reduces the normal IGBT 2x output voltage spikes when switching (even without standing waves): if so, then that "on this drive" makes it still a good rule.


Yes, "on that drive" is specific based on tested values, available in this document (for all A-B drives), Appendix A toward the back. In that _specific_ class of drive (PowerFlex 750 series), there is some "secret sauce" in attaining some limitation of spike generation. If you look at the older models or cheaper models, the distances are usually lower.


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## ElectricMetalTuba (Mar 20, 2017)

Why are you telling him to increase the frequency? I say lower it to say 1.5Khz or band aid it with a load reactor if you really need the torque. Protect that motor.


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## JRaef (Mar 23, 2009)

ElectricMetalTuba said:


> Why are you telling him to increase the frequency? I say lower it to say 1.5Khz or band aid it with a load reactor if you really need the torque. Protect that motor.


Increasing the carrier frequency lowers the heating effect in the motor, which is his first problem. Lowering my the CF to 1.5 kHz would actually increase his motor heat a little more. But at the same time, increasing CF also INCREASES the heating of the _drive_, however in this specific case he can increase it to 4kHz without affecting the rating of the drive, which may have some small amount of effect in reducing the motor heat. I'm not totally convinced it will be enough, because the customer is wanting even more flow than they have now, and the motor is ALREADY overloaded, so unless he has liquid oxygen or something pouring over that motor, it's going to have a shorter than normal life. But it's something he can do for free at this point, so it's worth a shot.

As I said, adding a load reactor lowers the motor terminal voltage, which lowers the shaft torque, which in a centrifugal pump translates to less flow, so adding a load reactor may help with the heat issue in the motor, but takes him in the OPPOSITE direction than what his customer asked for.


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## ElectricMetalTuba (Mar 20, 2017)

I don't have much experience with motor heating. I do know that the length of cable and the duration of the pulse have a dramatic effect on CW harmonics. I am worried about winding damage from excessive coronal discharge. Does that relate to motor heating? Did you check the bearings for leaks?


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## ElectricMetalTuba (Mar 20, 2017)

I would also like to add that the loss of power going from 2khz to 4khz is around 7%. you will go down to 279 HP. If the pump needs 300hp you will see significant slip. I don't think that is going in the right direction. Your cooling may suffer.


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## Mike_kilroy (Sep 2, 2016)

ElectricMetalTuba said:


> I would also like to add that the loss of power going from 2khz to 4khz is around 7%. you will go down to 279 HP. If the pump needs 300hp you will see significant slip. I don't think that is going in the right direction. Your cooling may suffer.


Please do not make a blanket statement like that. I know of NO vfd that will loose ANY power going from 2 to 4khz. 

If you do, please share the brand, model, and spec sheet!

Please also share why you think slip is affected.

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## ElectricMetalTuba (Mar 20, 2017)

Please understand I have seen mostly drive failures and few motor failures. I like cool VFDs.

What I understand is that the on a 200 HP Motor you get from 3 to 7% less power going from 4khz to 2khz. (Ignoring the air gap design ect.) 
This is due to losing waveform reconstruction accuracy which then causes ripple. Rotors current is effected and leads to slip causing a loss in motor output. This can heat the motor to some degree. It may also overload the motor. But you could try to filter that out.

While a 10 degrees over motor design can cut the life expectancy by 50%. You can kill a motor much faster by letting the insulation see too much skin effect. This is becomes a problem at high carrier wave frequencies. In an ideal situation you would buy a motor with a high CIV rating. Then crank it up to achieve desired noise reduction. You could ultrasound the bearings.

I'm pretty certain this motor is too small for the load and not inverter duty. I am suggesting at the very least leave to CF alone, keep it at 60hz and add inductors. Time will tell.

I would like to hear if you guys have experienced different.


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## Mike_kilroy (Sep 2, 2016)

ElectricMetalTuba said:


> Please understand I have seen mostly drive failures and few motor failures. I like cool VFDs.
> 
> 1) What I understand is that the on a 200 HP Motor you get from 3 to 7% less power going from 4khz to 2khz. [Typo - u meant 2 to 4 from previous posts]
> 
> ...


1) You are flat out wrong.
2) So? we all know the 10degree life rule of thumb. Does this rule prove your point?
2a) skin effect? You suggest there is significant difference in something called skin effect going from 2 to 4 khz? PLease provide documentation or proof of this.
3) We ALL have told that to the OP many times already
4) Yes; your comments are totally wrong and indefensible (except #2 &#3).


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## ElectricMetalTuba (Mar 20, 2017)

Telling me I'm wrong is helpful. How so? 

Go put a watt meter on the input of a VFD that runs a 200hp motor. You lose 10 HP turning it up.


As for rotor skin effect heating. I will quote: "For motors, an unbalance of 5% will result in capacity reduction by 25%. [4] That means, the motor current will increase to match the equipment’s torque needs which will result in proportional copper losses in motor. The voltage unbalance of 3% increase the heating by 20% for an induction motor." 

This is from unbalanced current caused by a intermittent floating earth due to eventual wear in one phase winding. Such is the issue with older continuous motors.


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## Mike_kilroy (Sep 2, 2016)

ElectricMetalTuba said:


> 1) Telling me I'm wrong is helpful. How so?
> 2) Go put a watt meter on the input of a VFD that runs a 200hp motor. You lose 10 HP turning it up.
> 3) As for rotor skin effect heating. I will quote: "For motors, an unbalance of 5% will result in capacity reduction by 25%. [4] That means, the motor current will increase to match the equipment’s torque needs which will result in proportional copper losses in motor. The voltage unbalance of 3% increase the heating by 20% for an induction motor." This is from unbalanced current caused by a intermittent floating earth due to eventual wear in one phase winding. Such is the issue with older continuous motors.


1) Helpful in that you should stop posting nonsense. I tried explaining why your various comments were totally wrong. If nonsense is posted then others who do not know better may believe it, not knowing better.
2) OK, so what? Actually most good vfds will be 97% or so efficient, so 200*.97= 194 so 6 hp loss, not 10hp. but ok, so what does this have to do with the OP?
3) Who was talking about voltage phase inbalance? Sure, losses,, but this has nothing to do with skin effect. Do you know what skin effect is? Do you know what frequencies skin effect is in effect at? It is NOT anywhere near 2 or 4khz - skin effect is in mhz and ghz freqs. there is no place in this threads discussion about skin effect. And what IS skin effect? It is that the current moves out of the wire onto the skin of the wire. Sure this can have an affect on resistance, but not in ac induction motors. Now build yourself a nice 450mhz ham high power transmitter and you can begin to consider the effects of skin effect.


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## ElectricMetalTuba (Mar 20, 2017)

If you continue to dissect my comments. What are you justifying? I am working toward a problem. Please try to have a discussion and contribute to the thread. This is not a pissing match.


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## ElectricMetalTuba (Mar 20, 2017)

Mike_kilroy said:


> 2) OK, so what?


If you turn it up you have to derate the VFD.



Mike_kilroy said:


> 3) Who was talking about voltage phase inbalance? Sure, losses,, but this has nothing to do with skin effect. Do you know what skin effect is? Do you know what frequencies skin effect is in effect at? It is NOT anywhere near 2 or 4khz - skin effect is in mhz and ghz freqs. there is no place in this threads discussion about skin effect. And what IS skin effect? It is that the current moves out of the wire onto the skin of the wire. Sure this can have an affect on resistance, but not in ac induction motors. Now build yourself a nice 450mhz ham high power transmitter and you can begin to consider the effects of skin effect.


I was pointing out that the ROTOR SKIN EFFECT is producing the heat. This is typical of old motors. Go read about it. Ask a motor tech if you need to learn more about VFD issues. 

I'm done trying to simplify it.


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## JRaef (Mar 23, 2009)

ElectricMetalTuba said:


> If you turn it up you have to derate the VFD.
> 
> 
> 
> ...


Well, you might thing so, but in reality you are mixing things up a bit and if left unchallenged, others may end up getting the wrong impression.

The rotor skin effect in motors has to do with the HARMONIC frequencies (between the 7th and 11th harmonic) that are present in the _*INPUT*_ circuit of a VFD , *not on the output*, and travel on the line getting to _*other *_connected motors that are running across the line in the same system. This is often used as an argument as to why harmonic reduction is important even if you already meet IEEE519 requirements. But there are numerous other factors affecting whether those higher order harmonics get to these other motors in a way that affects them negatively. The "rotor skin effect" issue is real, but the issue under discussion was however NOT about line side harmonics.

So where you are apparently misinformed is in the connection to CARRIER FREQUENCY. If you increase the Carrier Frequency going TO a motor from a VFD, rotor skin effect heating is NOT made worse. Yes there are _some _voltage harmonics on the motor side just because of it being a PWM signal and those harmonics do cause skin effect heating, in fact that's why when you run a motor with a 1.15SF from a VFD, the motor mfr will tell you that it becomes a 1.0SF motor. BUT, those harmonics on the output side actually go DOWN when carrier frequency increases. That is why a motor runs slightly cooler at a higher carrier frequency. You were conflating the higher CARRIER frequency with being a higher HARMONIC, but the relationship is the OPPOSITE of what you are thinking here.


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## ElectricMetalTuba (Mar 20, 2017)

Thank you for clearing that up. I wanted to test the idea that it was worn windings and bad grounding (bearing leak) were possible cause for most of the heat. These have been known to cause phase imbalance seen in the rotor. Perhaps my initial mistake was to hijack the tread. :cowboy:


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## JRaef (Mar 23, 2009)

ElectricMetalTuba said:


> Thank you for clearing that up. I wanted to test the idea that it was worn windings and bad grounding (bearing leak) were possible cause for most of the heat. These have been known to cause phase imbalance seen in the rotor. Perhaps my initial mistake was to hijack the tread. :cowboy:


A turn to turn short due to first turn insulation damage from reflected waves will indeed cause an imbalance. 

The current flowing through the bearings to ground is however not. It's a capacitive effect between the stator and rotor, so its source is essentially equally distributed across all 3 phases and won't show up as a phase imbalance. It's also a very very low voltage, lower than most people realize will cause bearing damage. It's actually in the mV range, that's why it's hard to detect, but although the damage it causes is microscopic at first, it's just time and repetition that kills the bearings.


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