30-60Hz range listed on motor nameplate

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philly

Senior Member
I was looking at a 350hp 460V motor on a VFD that were were about to startup today and noticed that the nameplate had the frequency listed for 30-60Hz. Most motors I see list a 60Hz frequency and I am curious why this motor lists a range.

Is the 30Hz a value which we should not operate the motor below? If so why? Is it something to do with minimum torque output?

This motor is on a process fan and I believe is a custom motor.
 

mcclary's electrical

Senior Member
Location
VA
I was looking at a 350hp 460V motor on a VFD that were were about to startup today and noticed that the nameplate had the frequency listed for 30-60Hz. Most motors I see list a 60Hz frequency and I am curious why this motor lists a range.

Is the 30Hz a value which we should not operate the motor below? If so why? Is it something to do with minimum torque output?

This motor is on a process fan and I believe is a custom motor.



It is an inverter duty motor. And the minimum torque would vary with alot of different things. You can operate it lowere than 30 htz depending on the application. Most of the time, that's about the point (or lower) where the motor does not have enough power to overcome the load.
 

philly

Senior Member
It is an inverter duty motor. And the minimum torque would vary with alot of different things. You can operate it lowere than 30 htz depending on the application. Most of the time, that's about the point (or lower) where the motor does not have enough power to overcome the load.

But with a VFD I thought we should be able to produce full torque throughout the whole speed range 0-60Hz? Why then would this 30hz be the cutoff for being able to produce required torque?

I would think this application being a fan we could operate below 30Hz due to the fact the fan is a variable torque load and will require lower torque at lower speeds. There may be a minimum point with the fan however that below which would not move any air.
 

Besoeker

Senior Member
Location
UK
I was looking at a 350hp 460V motor on a VFD that were were about to startup today and noticed that the nameplate had the frequency listed for 30-60Hz. Most motors I see list a 60Hz frequency and I am curious why this motor lists a range.

Is the 30Hz a value which we should not operate the motor below? If so why? Is it something to do with minimum torque output?
You can get full torque at any speed.
What you don't get is full cooling at any speed if the motor has a shaft mounted cooling fan.
I'd hazard a guess that's why the speed variation has a restricted bottom limit.
 

Jraef

Moderator, OTD
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Electrical Engineer
You can get full torque at any speed.
What you don't get is full cooling at any speed if the motor has a shaft mounted cooling fan.
I'd hazard a guess that's why the speed variation has a restricted bottom limit.
I would agree. What they are saying is that this motor, when used with a VFD, has only a 2:1 turn down ratio. Basically that means it isn't really a great inverter duty motor, the better ones would have 20:1, 100:1 or even 1000:1 (although I have to say, I'm not sure if anyone has a 1000:1 ratio at 350HP). But if you are not going to run it below 1/2 speed, and most likely on a centrifugal fan you are not, then the issue is irrelevant other than what it says for the construction of the motor.
 

philly

Senior Member
Usually when starting a large fan we have dampers and ensure the dampers are closed when starting the fan for a sucessfull start.

Because now there are not dampers and we will control airflow with speed of motor will we have an issue with starting this motor from 30Hz instead of from 0Hz and ramping?

I know typically with dampers you have two speed vs torque curves with the lower curve representing dampers closed, the higher one representing dampers open and somwhere inbetween these two represents the incremental damper positions. When using a drive to speed up motor which curve would be more followed with varying the speed of the drive? Since technically with the drive running at 100% would represent full airflow, would this be the same as operating with the dampers 100% open? If so I would think that a varying speed motor on a fan would follow the damper 100% open speed vs torque curve?

If this does corrospond to the 100% curve, then how are we able to start the motor sucessfully by ramping the speed? Is it due to a slower ramp although we will still be at the same required torque values?
 

Besoeker

Senior Member
Location
UK
Usually when starting a large fan we have dampers and ensure the dampers are closed when starting the fan for a sucessfull start.

Because now there are not dampers and we will control airflow with speed of motor will we have an issue with starting this motor from 30Hz instead of from 0Hz and ramping?
With a variable speed drive you will be starting from 0Hz.
You just can't have continuous operation below 30Hz
I think you might be in danger of seeing problems that don't exist.
 

philly

Senior Member
With a variable speed drive you will be starting from 0Hz.
You just can't have continuous operation below 30Hz
I think you might be in danger of seeing problems that don't exist.

So with a VFD do you always start from zero when accelerating even if you have a minimum setpoint of 30Hz? So another words when it ramps up and down it will always start and stop at zero however for continuous operation it will never ramp to anything below 30Hz and operate there. Is this correct?

I was under the assumption that because we had a 30Hz minimum the drive would automatically start its ramp at 30Hz and may cause problems during starting due to the fact that you wouldn't be able to accelerate the load slowly and benefit from the small slip during ramp.

So just to clarify, with my question about the speed torque of a fan, would the "no-damper" curve be the curve that would need to be used when looking at different operating torques with the fan on a vfd since nothing would be restricting the airflow?

I guess with a VFD we can start a fan much more easily because although the torque is the same at all points as using an open damper on across the line, we are able to accelerate the motor slowly and benefit from less current due to the fact that there is less slip during a ramp with a vfd. This less current will not eat up thermal capacity of the motor when trying to start. Do I have the basics of this correct?
 

Besoeker

Senior Member
Location
UK
So with a VFD do you always start from zero when accelerating even if you have a minimum setpoint of 30Hz? So another words when it ramps up and down it will always start and stop at zero however for continuous operation it will never ramp to anything below 30Hz and operate there. Is this correct?
Yes, I agree with that.


So just to clarify, with my question about the speed torque of a fan, would the "no-damper" curve be the curve that would need to be used when looking at different operating torques with the fan on a vfd since nothing would be restricting the airflow?
Yes if the motor is to be run up with the dampers, (if there are any) open

I guess with a VFD we can start a fan much more easily because although the torque is the same at all points as using an open damper on across the line, we are able to accelerate the motor slowly and benefit from less current due to the fact that there is less slip during a ramp with a vfd. This less current will not eat up thermal capacity of the motor when trying to start. Do I have the basics of this correct?
No argument with that either.
 

philly

Senior Member
I would agree. What they are saying is that this motor, when used with a VFD, has only a 2:1 turn down ratio. Basically that means it isn't really a great inverter duty motor, the better ones would have 20:1, 100:1 or even 1000:1 (although I have to say, I'm not sure if anyone has a 1000:1 ratio at 350HP). But if you are not going to run it below 1/2 speed, and most likely on a centrifugal fan you are not, then the issue is irrelevant other than what it says for the construction of the motor.

What do you mean by turndown ratio? It actually turns out that the process engineer wants to run this fan below 50% speed. Should we run this motor below the 50% speed stated on the nameplate? Is it just a matter of cooling as mentioned above, or are there torque issues to worry about?
 

mcclary's electrical

Senior Member
Location
VA
What do you mean by turndown ratio? It actually turns out that the process engineer wants to run this fan below 50% speed. Should we run this motor below the 50% speed stated on the nameplate? Is it just a matter of cooling as mentioned above, or are there torque issues to worry about?

If you have an auxillary cooling fan turning full speed at all times, you should be able to operate at 50 % .
 

philly

Senior Member
One thought that occured to me is that this fan is a variable torque load, and therefore torque and current will decrease with speed. So since operating under 50% speed we will be operating at reduced current do we still need to be concerned about cooling?

I would think that maybe this cooling issue would be a case where a constant torque load could possibly draw full load current at low speeds and thus increase heat and the need for cooling?

Am I way off on this thought?

The motor is indeed a Inverter Duty Motor according to its nameplate.
 

mcclary's electrical

Senior Member
Location
VA
One thought that occured to me is that this fan is a variable torque load, and therefore torque and current will decrease with speed. So since operating under 50% speed we will be operating at reduced current do we still need to be concerned about cooling?

I would think that maybe this cooling issue would be a case where a constant torque load could possibly draw full load current at low speeds and thus increase heat and the need for cooling?

Am I way off on this thought?

The motor is indeed a Inverter Duty Motor according to its nameplate.

That's an interesting question, since you're not turning a gearbox, or load that is not dependant on the speed of the motor, but rather your load is directly proportional to the speed of the motor, you may be ok without a cooling fan, althought of course, it's better with a fan. But I'm curious as to what Jraef thinks of this question.
 

One-eyed Jack

Senior Member
What do you mean by turndown ratio? It actually turns out that the process engineer wants to run this fan below 50% speed. Should we run this motor below the 50% speed stated on the nameplate? Is it just a matter of cooling as mentioned above, or are there torque issues to worry about?

If you want to run the fan at or below 50% it would appear than less than full capacity will be required from the fan. Size the sheave on the motor so that it runs at 30 hz with the fan at the lowest speed desired.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
That's an interesting question, since you're not turning a gearbox, or load that is not dependant on the speed of the motor, but rather your load is directly proportional to the speed of the motor, you may be ok without a cooling fan, althought of course, it's better with a fan. But I'm curious as to what Jraef thinks of this question.
We don't know why the motor mfr only gave that motor a 2:1 turn down ratio, so anything we say MIGHT be applicable, but it equally might not. A separate cooling fan will only help if it's an ODP / TEAO / or TEFC motor. If it's TENV (as I suspect because of the low turn down ratio), an external cooling fan will not help.

As to it being a VT load, that might make a difference. Usually motor thermal modeling is done based on a CT load; i.e. worst case scenario where the motor power varies linearly with speed. A VT load will vary the HP at the cube of the speed. So IF the 2:1 ratio is CT based, they are assuming at 1/2 speed the motor is at 1/2 load. But on that VT load, at 1/2 speed you are at 1/8 load, so theoretically you are putting a lot less heat into that motor.

Is there a reason why you can't just ask the motor mfr?
 

mcclary's electrical

Senior Member
Location
VA
We don't know why the motor mfr only gave that motor a 2:1 turn down ratio, so anything we say MIGHT be applicable, but it equally might not. A separate cooling fan will only help if it's an ODP / TEAO / or TEFC motor. If it's TENV (as I suspect because of the low turn down ratio), an external cooling fan will not help.

As to it being a VT load, that might make a difference. Usually motor thermal modeling is done based on a CT load; i.e. worst case scenario where the motor power varies linearly with speed. A VT load will vary the HP at the cube of the speed. So IF the 2:1 ratio is CT based, they are assuming at 1/2 speed the motor is at 1/2 load. But on that VT load, at 1/2 speed you are at 1/8 load, so theoretically you are putting a lot less heat into that motor.

Is there a reason why you can't just ask the motor mfr?[/QUOTE]



We just like to hear you talk:grin:

As usual, very helpful, Thanks
 

Besoeker

Senior Member
Location
UK
Somewhat relevant to this topic, we have recently taken an order for an ac drive and motor rated at about 300 hp. It is on a process line and the motor will run at or about 4-pole 60Hz speed.
But for some conditions it could run at about 1% of that speed at rated torque for an undefined period. Just to add to the mix, it will be in an area that can get wet. For this duty, a standard shaft mounted fan isn't an option so there will be a separately powered blower.
 

philly

Senior Member
We don't know why the motor mfr only gave that motor a 2:1 turn down ratio, so anything we say MIGHT be applicable, but it equally might not. A separate cooling fan will only help if it's an ODP / TEAO / or TEFC motor. If it's TENV (as I suspect because of the low turn down ratio), an external cooling fan will not help.

As to it being a VT load, that might make a difference. Usually motor thermal modeling is done based on a CT load; i.e. worst case scenario where the motor power varies linearly with speed. A VT load will vary the HP at the cube of the speed. So IF the 2:1 ratio is CT based, they are assuming at 1/2 speed the motor is at 1/2 load. But on that VT load, at 1/2 speed you are at 1/8 load, so theoretically you are putting a lot less heat into that motor.

Is there a reason why you can't just ask the motor mfr?

I went ahead and got ahold of the motor manufacturer and they confirmed that operating the motor on a variable torque load we are able to run the motor below 50% and that the speed ration for a variable torque load on this motor is actually 10:1. They said that the 2:1 ratio is the rating when running constant torque loads with this motor.

So I'm assuming the 10:1 speed ration is the same at the turndown ratio mentioned? What does this imply? Is this simply the ratio of the maximum speed to the minimum speed of the motor?

So it looks like I am able to run this motor continuously down to about 6Hz or so with no issues.

All our other motors operated from drives of this size have an external additional fan used for cooling. Why is the cooling of these motors run from a drive much more important then a similar size motor running across the line? Does the PWM output of the drive cause that much more heating on the motor?
 

One-eyed Jack

Senior Member
All our other motors operated from drives of this size have an external additional fan used for cooling. Why is the cooling of these motors run from a drive much more important then a similar size motor running across the line? Does the PWM output of the drive cause that much more heating on the motor?[/quote]

It is the speed of the fan on the motor that provides cooling. At low speeds,typ. below 30 hz this is not adequate for motors under constant torque conditions. With a fan the load decreases with the speed of the fan so there is less heat to dissipate.
 
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