Should a VFD Be Derated for "Low Speed" Motors?

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Rho_HV

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It is a 2100HP induction motor, 12 poll. Inverter IGBT's fire at about 12Hz (electrical frequency) so at about 2.5% slip motor runs at 117rpm (full load HP).

Here is my question: Should the inverters be derated because they are firing at such low frequency (12 Hz)? My boss thinks so and I am not sure because I don't see why and I have not found any reference online indicating so... :?
 
all we can give on this forum is guesses to that question because only your drive mfgr can answer it for sure. contact them.

that said, none of the drive mfgrs I work with need any derating of the DRIVE down to 0hz output at rated current. Good reason for it too: the slower you go the less power you put thru the output IGBTs so they should run cooler anyway. Likewise most motors easily handle down to 1/10th base speed w/o heating issues at their nameplate rated torque; most to 1/6th base speed - on motor going your speed you should be fine but if this were first time I was running it that low loaded I would keep my IR temp gun handy for the next 8 hours to monitor temperature before blessing it.....
 
LMAO said:
It is a 2100HP induction motor, 12 poll. Inverter IGBT's fire at about 12Hz (electrical frequency) so at about 2.5% slip motor runs at 117rpm (full load HP).

Here is my question: Should the inverters be derated because they are firing at such low frequency (12 Hz)? My boss thinks so and I am not sure because I don't see why and I have not found any reference online indicating so... :?
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Unusual application.
Presumably the motor is designed and rated to run at 12Hz speed for full load? But, if it's mains frequency motor being run at 12Hz speed you might need to consider the motor cooling.
Can you get the correct V/f ratio?


From my experience with induction motors, those with more poles often have a lower power factor. That would make the current higher for a given power rating.
That could be an issue.
Can you compare the motor nameplate current with that of the inverter rated current?

To answer your question, I don't see the 12Hz within it's rated current being a limitation of the inverter.
 
LAMAO... If by inverter derating, you mean GBTs having lower current-rating, the answer is NO! Current carried by the IGBTs are limited by the motor winding inductance and resistance. The former is quite low at 14 Hz. And even though the voltage is reduce proportionately, remember the V/Hz limit, inductance is still a concern!

Regards, Phil
 
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LMAO said:
It is a 2100HP induction motor, 12 poll. Inverter IGBT's fire at about 12Hz (electrical frequency)
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perhaps you should relook at motor specs also: 12 pole motor has base speed of N=120*f/p = 120*60/12= 600rpm.

a 6 pole motor has base speed of 1200rpm........ if this is indeed a 12 pole motor with base speed of 600rpm, running to 2x base for 1200rpm, then your 100 some "low speed" is not very low speed anyway.....

and you will not get full 2100hp at 117rpm either. something is wrong with all these specs so it is unclear exactly WHAT slow speed you will be running what base speed rated motor at.....
 
mike_kilroy said:
perhaps you should relook at motor specs also: 12 pole motor has base speed of N=120*f/p = 120*60/12= 600rpm.
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Yes, for 60Hz. But remember that this is an inverter application and the OP states that it is to be run at 12Hz which equates to a synchronous speed of 120 rpm.

mike_kilroy said:
and you will not get full 2100hp at 117rpm either. something is wrong with all these specs so it is unclear exactly WHAT slow speed you will be running what base speed rated motor at.....
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Unless the motor is designed to produce 2100 hp at 117 rpm. See post #3
 
Motor HP is torque x speed. Current is related to torque. Remember that for a given HP, the slower the speed at 60 Hz, the higher the current will be. The inverter must be rated for the maximum current it will see. You must have all the motor operating parameters and load conditions to contact the VFD manufacturer for this one.
 
High pole count motors have higher FLA ratings than typical 4 pole motors, yet most VFDs are catalogged by HP based on 4 pole ratings. If you just make sure that you select the VFD based on the motor nameplate FLA rather than the motor nameplate HP, you should be OK. That might be where he got that idea.

But if you are planning on using vector control on this motor, there is another issue with regards to low frequency operation of a high pole count with regards to higher saturation capability that you may want to know about. If you are using this on a centrifugal pump or fan and vector control is not that important to you, I won't bore everyine with the details. If however you are planning on using vector control, then let me know (and I will bore everyone).
 
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mike_kilroy said:
perhaps you should relook at motor specs also: 12 pole motor has base speed of N=120*f/p = 120*60/12= 600rpm.

a 6 pole motor has base speed of 1200rpm........ if this is indeed a 12 pole motor with base speed of 600rpm, running to 2x base for 1200rpm, then your 100 some "low speed" is not very low speed anyway.....

and you will not get full 2100hp at 117rpm either. something is wrong with all these specs so it is unclear exactly WHAT slow speed you will be running what base speed rated motor at.....
Click to expand...

12 pole = 600RPM, 2.5% slip = 585 RPM, applying 12hz = 20% base speed, x 585 RPM = 117 RPM.

But I agree on the other point. If it is designed to deliver full 2100HP at 12 RPM, that is one big mother... i.e 10,500HP at full speed!

You are talking about a VFD capable of 1500A at 4160V!
:jawdrop:


I'd dearly love to sell and install that beast... :angel:
 
rbalex said:
On occasion the text "wrap-around" algorithm may cause this. Try refreshing after resizing the window.
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Thanks. Didn't help though, I even deleted the word "sure" in edit mode and retyped it after refreshing.

No big deal, just odd that's all, I'm sure nobody will think me stupid, at least not for that reason...
 
Jraef said:
12 pole = 600RPM, 2.5% slip = 585 RPM, applying 12hz = 20% base speed, x 585 RPM = 117 RPM.

But I agree on the other point. If it is designed to deliver full 2100HP at 12 RPM, that is one big mother... i.e 10,500HP at full speed!

You are talking about a VFD capable of 1500A at 4160V!
:jawdrop:


I'd dearly love to sell and install that beast... :angel:
Click to expand...

120 RPM is the full speed; VFD will not exceed that. It is a 550V motor with max output power of about 3800HP at about 114 rpm. I modeled it in Excel.

Made in USA here on our facility.
 
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I am very confused I guess... :dunce:

It is a 2100HP induction motor, 12 poll. Inverter IGBT's fire at about 12Hz (electrical frequency) so at about 2.5% slip motor runs at 117rpm (full load HP).
Click to expand...

It is a 550V motor with max output power of about 3800HP at about 114 rpm.
Click to expand...

Motor base synchronous speed (N) = 120*f/p where f = frequency and p = number of poles. So unless your motor is designed for something other than 60Hz, the base synchronous motor speed for a 12 pole motor in a 60Hz environment, from which nameplate HP is usually calculated, is 600 RPM. As you reduce frequency in a constant V/Hz ratio, HP reduces proportionately. If you are saying that your motor develops 3800HP at 114RPM, which is 19.48% of base (60Hz) motor slip speed, then your base motor HP would be 3800/0.1948 = 19,507HP at full slip speed.
 
Jraef said:
I am very confused I guess... :dunce:





Motor base synchronous speed (N) = 120*f/p where f = frequency and p = number of poles. So unless your motor is designed for something other than 60Hz, the base synchronous motor speed for a 12 pole motor in a 60Hz environment, from which nameplate HP is usually calculated, is 600 RPM. As you reduce frequency in a constant V/Hz ratio, HP reduces proportionately. If you are saying that your motor develops 3800HP at 114RPM, which is 19.48% of base (60Hz) motor slip speed, then your base motor HP would be 3800/0.1948 = 19,507HP at full slip speed.
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Motor nameplate HP is about 2300HP. I modeled the motor in Excel using regular induction motor models (not considering VFD) so what I said about 3800HP is not right because I totally forgot about V/Hz deal (I thought voltage remains the same at lower frequency). Motor base voltage/frequency is 550V/12Hz, so VFD/motor will never exceed 12Hz (120 rpm at no slip).
Sorry about confusion, not a motor expert.
 
LMAO said:
Motor nameplate HP is about 2300HP. I modeled the motor in Excel using regular induction motor models (not considering VFD) so what I said about 3800HP is not right because I totally forgot about V/Hz deal (I thought voltage remains the same at lower frequency). Motor base voltage/frequency is 550V/12Hz, so VFD/motor will never exceed 12Hz (120 rpm at no slip).
Sorry about confusion, not a motor expert.
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Thank you. That completely answers the question I asked in post #3.

Presumably the motor is designed and rated to run at 12Hz speed for full load?
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I think what possibly caused some confusion was the usual expectation that it would be a 60Hz machine rather than your original post being confusing.
As I also mentioned in my post, and a point reiterated by Jraef in post #9, motors with many poles often or mostly have a lower power factor than those with fewer poles.
Compare the motor nameplate current with that of the inverter rated current.

And, will the selected inverter allow the parameters for nominal voltage and frequency to be set at 550V for 12Hz respectively.

I curious about the application. The last one I had similar to that was a direct drive on a steel rolling mill.

For the units we usually use, the nominal voltage can be set for 180-690V and nominal frequency for 8-320Hz so you would get what you want. I don't know if all have a similar range of settings.
 
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Jraef said:
But if you are planning on using vector control on this motor, there is another issue with regards to low frequency operation of a high pole count with regards to higher saturation capability that you may want to know about. If you are using this on a centrifugal pump or fan and vector control is not that important to you, I won't bore everyine with the details. If however you are planning on using vector control, then let me know (and I will bore everyone).
Click to expand...

It sounds like this doesn't pertain to the OP, but I would like to be bored with these details.
 
LMAO said:
It is a 2100HP induction motor, 12 poll. Inverter IGBT's fire at about 12Hz (electrical frequency) so at about 2.5% slip motor runs at 117rpm (full load HP).

Here is my question: Should the inverters be derated because they are firing at such low frequency (12 Hz)? My boss thinks so and I am not sure because I don't see why and I have not found any reference online indicating so... :?
Click to expand...

LMAO, there have been a lot of off question replies here, including lots of questions about whether your motor that is apparently running fine without overheating running 12hz, 117rpm, is capable of running fine at 12hz, 117rpm...,,,

Your OP was whether the DRIVE needs derating. You have seen lots of questions about what the motor nameplate rating is. Can you provide that? If so, a lot of the off topic replies can be answered.

Bottom line I see is you asking if a particular drive can run at 12hz w/o derating. Again, we cannot answer this but your drive mfgr can. I suggest you not get sidetracked with the motor heating issues if that is not a problem but rather find if your particular drive fgr can handle 12hz full nameplate current at 12hz. Have you asked them? what have they replied?

so since your motor IS rated 117rpm@12hz BASE speed, back to your OP question: can your present drive handle this without derating: Again, you must ask your drive mfgr if it can - we will asll give you guesses based on our experience. The only way we can say NO IT WON't is IF YOU GIVE US MOTOR NAMEPLATE DATA AND DRIVE NAMEPLATE DATA. Then we can compare the two and show you if the drive AMPERAGE rating is LESS than the motor amperage rating. That is all we can tell you. As for a motor rated same or less amperage as your drive which is normally run at 60hz you must ask your drive mfgr. What have they told you? If they are unwilling to tell you, then it would be nice to know their name so some of us can avoid them in the future. But I suspect you have not asked them yet. Have you?
 
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mike_kilroy said:
As for a motor rated same or less amperage as your drive which is normally run at 60hz you must ask your drive mfgr.
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There isn't really a "normal" operating frequency for an inverter. If it was normally operating at 60Hz, that would obviate the need for a variable frequency inverter.
Their capability of running at different frequencies is the raison d'?tre.
We have some in operation that can, and do, run for extended periods in "crawl" mode at 0.5Hz or lower and they suffer no deleterious effects.

Thus, if the drive is rated for the at least the motor current (why do you guys call it amperage?), then operation at 12Hzshould be fine.
 
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