Maximum speed a motor can be run at 4

[rockman7892]

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?

 

[rockman7892]

edison123

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.

 

[LionelHutz]

First off, in North America the motor will really be a 460V rated motor, operated on a 480V line power.

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.

 

[rockman7892]

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?

 

[LionelHutz]

I would just try it. Then you will know if it works with no further estimating required.

As for the motor speed, just balance the rotor as they suggest.