Maximum speed a motor can be run at
Maximum speed a motor can be run at
(OP)
I have a 1hp 480V 1800rpm motor being driven by a 1hp vfd. This motor is on a conveyor feeder application that is feeding material. It is not feeding enough material so production wants to increase the speed of the drive. The drive is already running with a max speed setpoint of 120hz.
I notice on the motor itself it has listed a safe maximum speed of 3600rpm. I am therefore hesitant to run this motor above this setting. When asked why we could not run above this 3600rpm rating the only thing I could think of was that if we go above this rating, the voltage to the motor will be limited and therefore the V/Hz will decrease and there may not be enough flux in the motor to produce the required torque. The motor may then not have enought torque to move the load and will lock up and thus burn the motor up. Is my explanation correct or am I way off?
So I am now looking into options for speeding this motor up. Option A will be to find a higher rpm motor, or a motor that can handle a higher drive frequency output.
Option B would be to purchase a larger motor that would have enough torque at a higher speeds in order to move the load.
Am I on track with the higher speed vs avaliable torque explanation being the limiting factor for not running this motor over 3600rpm or is there something else?
I notice on the motor itself it has listed a safe maximum speed of 3600rpm. I am therefore hesitant to run this motor above this setting. When asked why we could not run above this 3600rpm rating the only thing I could think of was that if we go above this rating, the voltage to the motor will be limited and therefore the V/Hz will decrease and there may not be enough flux in the motor to produce the required torque. The motor may then not have enought torque to move the load and will lock up and thus burn the motor up. Is my explanation correct or am I way off?
So I am now looking into options for speeding this motor up. Option A will be to find a higher rpm motor, or a motor that can handle a higher drive frequency output.
Option B would be to purchase a larger motor that would have enough torque at a higher speeds in order to move the load.
Am I on track with the higher speed vs avaliable torque explanation being the limiting factor for not running this motor over 3600rpm or is there something else?





RE: Maximum speed a motor can be run at
RE: Maximum speed a motor can be run at
*If* I did the math right, your 1 Hp motor developes about 3 Ft/Lb at 1800 RPM. At 3600, it will develope about 0.73 Ft/Lb.
You should really consider a 2-pole, 1 Hp replacement motor.
Ed
RE: Maximum speed a motor can be run at
Thanks edison
Does it have anything to do with the torque as I mentioned above or strictly just mechanical limitations.
So it sounds like if it is a mechanical limitation, then then I would need to buy a higher rpm motor, or one that is capable of being operated faster?
This is going through a gearbox, so I imagine there would be limitations on the gearbox as well.
RE: Maximum speed a motor can be run at
A two pole motor would have a synchronous speed of 3600rpm. Would be then be able to run this motor faster at lets say 120hz to get the higher speed required.
I come up with 3 lb*ft at 1800rpm and 1.45 lb*ft at 3600rpm by dividing the speed into the a HP of 1.
RE: Maximum speed a motor can be run at
The torque is another issue as fangas says.
Your "management" needs some engineering education. This doesn't seem to me a good idea. It might be better to increase the conveyor size (with attendant motor + gearbox upgrade) to achieve more throughput.
RE: Maximum speed a motor can be run at
Edison is correct. You start exceeding 3600 RPM with a motor, all manner of bad things are possible, mechanically.
The conversion to torque in the *approximate* constant Hp range is not linear. Being I'm a lazy sort, I'd like to direct you here;
http
There's a fine description of of what happens and why.
Ed
RE: Maximum speed a motor can be run at
RE: Maximum speed a motor can be run at
http:
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: Maximum speed a motor can be run at
Your torque calcs are a little off. The Tq./speed relationship is not linear above base speed. It varies by the square of the effective voltage / frequency ratio.
But to make it simpler to visualize, think of what happens when you apply lower voltage to a motor at base frequency. We already know that torque varies by the square of the applied voltage, so if for example you apply only 230V with a soft starter, the torque is reduced to .502 or 25% of rated torque. To now apply this to a motor running ANOVE base speed, we use the V/Hz ratio. A motor operating at 460V 60Hz will have a V/Hz ratio of 7.666, so at only 230V the V/Hz ratio is then only 3.833. If we calculate the V/Hz ratio when we operate that motor ABOVE base speed, the same effect applies to the torque. So at 120Hz, the V/Hz ratio becomes 460/120 = 3.833 again, so the effective voltage becomes the new ratio times the BASE frequency (3.833 x 60 = 230V), and the motor output torque is again reduced to 25% of rated. In your case, this appears to be enough because we can assume the motor is working (and not overloading), so obviously someone thought about this when sizing it in the first place.
Most motors are built to handle 25% more speed than what the 2 pole speed rating is. So that means you can run a 4 pole 1800RPM motor to about 4500RPM max. In that case though, the frequency applied to that 4 pole motor would need to be 150Hz. So your V/Hz ratio drops to 3.07, which is like having only 184V applied to the motor (3.07 x 60Hz), which is roughly 40%, and the output torque will drop to 16% of rated. So in your case, you will have increased the speed 25%, but cut the operating torque to 64% of what you have now!
If you change to a 2 pole motor, IT can still only be increased to 125% speed, but since it STARTS at 3600, that means the frequency only needs to increase by 25% to 75Hz. So apply the rules and the V/Hz ratio stays higher at 6.133, which corresponds to a torque equivalent of applying 368V, which is 80% of normal and thus the torque will end up at 64% of rated. So you will actually end up with MORE torque at the shaft than you have now. That of course is going to throw your gearbox ratios out the window however, so I doubt that is an option for you.
To ensure then that you get at least the torque you have now, without having to change the gearbox, you would need to increase the motor HP size by 56%. If by chance you still had a little overhead left in the amount of torque you are using, you might get away with a 1-1/2HP motor, but if it were me, I'd go to 2HP.
But before you ASS-u-me the motor can run at 125& of 2 pole speed, you must ask the mfr. I would also do the same with the gearbox mfr as well.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Maximum speed a motor can be run at
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Maximum speed a motor can be run at
The breakdown shortterm torque comes down as the inverse of the overspeed squared.
Thus, the torque at 3/2 base speed is 2/3, at 2/1 base speed is 1/2, etc.
The breakdown torque at 3/2 base speed is 4/9, at 2/1 base speed is 1/4.
If you plot these curves, they come together at double speed so effectively you have no overload torque available anymore.
RE: Maximum speed a motor can be run at
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Maximum speed a motor can be run at
RE: Maximum speed a motor can be run at
Using the existing 1hp, 4-pole, 1800rpm motor I current have I came up with the following toruqe values for the different overspeeds:
@60 HZ and 1800rpm Torque = 2.91 lb*ft
@120 HZ and 3600 rpm Torque = .727 lb*ft
@150 HZ and 4500 rpm Torque = .4656 lb*ft
If I upgrade to a 2hp, 2-pole, 3600rpm motor as others suggested I came up with the following values:
@60 HZ and 3600 rpm Torque = 2.91 lb*ft
@75 HZ and 4500 rpm Torque = 1.86 lb*ft
I notice that the 4500 rpm torque for this motor is much larger for that of the 1hp motor at this speed.
I was trying to run some calculations for the suggestion that Warros made but I'm not sure that I approached it right. Heres what I did.
If the motor is re-wired for 230V then new V/Hz ratio is 3.83 at 60hz. So if we set the drive up for a V/Hz ratio of 3.83 then at 120Hz the V/Hz ratio would be 4. I used the following calculations.
Torque at 60hz = (2.91)(3.83/3.83)^2 = 2.91 lb*ft
Torque at 120hz = (2.91)(4/3.83)^2 = 3.17 lb *ft
The torque value for 120hz seems much larger than that of the previous two cases, but I'm not sure if I'm doing this right by using the torque of 2.91 which was calculated for a 460V 4-pole motor. If I'm off base can you please explain the theory behind wiring the motor for 230V and supplying VFD with 480V?
The issue I have with the 230V operation is that the motor is only a three lead motor wired for 460V. I guess I would have to send it out to be re-wound at 230V, which in that case it may be worth just buying a 2-pole 2hp motor.
RE: Maximum speed a motor can be run at
The one thing I thought of after posting was determining how much torque is being used in the current operation at 120hz.
If I were to measure the current being drawn by the motor and calculate the % of full load current being used (factoring in reactive current)by the motor and equate this to a % torque could I find aproximately how much torque if being produced by the motor? Could I then compare this torque to the calcualted value I have for 4500rpm and see if at 4500rpm we will still have enough torque to move the load? If I see that we do then could we run the existing motor up to 4500rpm.
Is the 125% or 2-pole speed the rule of thum for all motors? This is the first motor I have ever seen a max rpm listed before. Since all 2-pole motors are 3600rpm then does this mean that the maximum any motor can be run at on a 60hz system is 4500rpm? What if a faster speed was required, would you have to find a higher speed motor at nominal frequency?
RE: Maximum speed a motor can be run at
Ed
RE: Maximum speed a motor can be run at
In regards to my questions about monitoring the torque I went and looked at the VFD manual and see that you can meter torque current, estimated torque, and commanded torque. This should give me an idea of the torque being used to move the load at the current speed of 1200rpm. Maybe I could then compare this toruqe value to the calculated torque value for higher speed as described above?
RE: Maximum speed a motor can be run at
I was able to look at the drive an find an paramater that showed the torque being used. At the particular time I looked at the drive the motor was being run at 65HZ and the drive showed that is was using 11% of the avaliable torque. If I watch this torque paramater and see what it is showing when the drive is run at 120HZ should this be a good estimate of the torque required for the motor?
How does a motor have an rpm of faster than 7200 rpm (1 pole)?
RE: Maximum speed a motor can be run at
RE: Maximum speed a motor can be run at
You answered my question which I guess should have been what is the minimum number of poles that a motor can have. You answer of two makes it clear that at 60hz the fastest base speed of a motor can only be 3600rpm. If a motor ever needs to be faster then 3600rpm then it needs to be installed with a vfd. So when you see higher rpm ratings on such things as spindle motors these motor are always assumed to be on a vfd and their rpm ratings account the the electrical and mechanical limitations of the motor on a drive.
Unfortunately as Warros suggested this motor is only a 3-lead motor and therefore cannot be re-connected at 230V. It would need to be sent out to get re-wound which I'm guessing would be just as expensive as purchaseing a 2hp 2pole motor.
RE: Maximum speed a motor can be run at
To make it simple, the reconnection basically creates a 120Hz motor at 2X the rated HP. If you have a 230V rated motor with a supply capable of operating a 460V rated motor then you can increase the frequency to 120Hz and still maintain the V/Hz ratio.
You will also need a new VFD unless you can run the motor you already have up to the speed you require.
If you replace both, I would consider a 2HP/230V/3600rpm/60Hz rated motor. Connect it to a 480V VFD with a current rating to match the motor - most likely the VFD will be 4hp rated but check the current rating. Set the VFD to operate either a 480V/120Hz motor or a 230V/60Hz motor. Then, you can over speed from 0-7200rpm while maintaining rated motor torque over that speed range.
RE: Maximum speed a motor can be run at
I spoke with Toshiba about this particular motor and they informed me that this motor could safely be run at a speed of 4500rmp (limited by rotor. Bearing limitation was 13000rpm). They mentioned that if we wanted to go faster then 4500rpm then we would need to have the rotor balanced.
Seeing that we can run up to 4500rpm at 150Hz from a mechanical standpoint, this then leaves the torque issue. If I do as I suggested above and verify that the amount of torque being used now is lower than the calculated torqe at a speed of 150Hz then can I try to run this motor at 4500Hz. I will verify the toque by a metering function on the drive that displays required torque.
Toshiba mentioned that they do not make any motors capable of running faster than 4500rpm becuase they are not into the definite purpose motor market. Does anyone know a good manufacturer to look at for higher speed motors?
RE: Maximum speed a motor can be run at
As for the motor speed, just balance the rotor as they suggest.