Increasing frequency till it Hertz 1

[OperaHouse]

I've been watching for a while and haven't seen much on the general rules for running a VFD at higher than 60Hz. Even the basic ones go to 240 and others go many times that. What is the general rule for drop in power? Should this be avoided with older motors because of bearing issues? For this discussion let's avoid the mechanical issues that you can get into with gear boxes.

 

[electricpete]

General rule for drop in power? Generally power is limited to nameplate power above rated speed.... results in torque derating below torque computed from nameplate conditions.

One consideration is that volts/hertz decreases as speed increases above nameplate. V/f ~ 1/N means Tbd ~ 1/N^2 ie breakdown torque decrease in inverse proportion to speed squared. You can get into a situation where momentary overloads can push the motor past breakdown and cause dramatic increase in current.

Also there are mechanical considerations for motor overspeed.

Sorry if I have told you what you already knew.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[Marke]

Hello OperaHouse

In addition to Pete's comments.
Above the rated speed of the motor, the power output is limited to the rated power output of the motor provided that the input line voltage is equalt to the voltage rating of the motor. i.e. 415 volt motor with 415 v input to the drive. In the case where the line voltage input to the drive is higher than the motor rated voltage, it is possible to operate as a constant torque to a higher frequency. i.e. if we take a 60Hz case and connect a 220 volt motor to a drive operating from 440 volts with the drive set up to provide 220 volts at 60 Hz, then the voltage can continue to increase as the motor is accelerated beyond 60Hz and the motor will become power limited at 120 Hz rather than at 60Hz. This will double the effective power rating of the motor when operating at 120 Hz.
Care must be taken to ensure that the insulation of the motor is capable of withstanding this voltage.

At speed beyond rated speed, one of the major limitations is the bearing life, especially with 2 pole motors.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[GGOSS]

Hello Operahouse,

VFD's control both the voltage and frequency to the motor and are for that reason often referred to as Variable Voltage Variable Frequecny (VVVF) Drives.

As motor torque is proportional to V/F maintaining a constant ratio between these ensures constant torque is available up to 60HZ.

Because the drive cannot output more voltage than that available from the supply, a constant V/F ratio cannot be achieved above 60Hz. Therefore operation above 60Hz will result in reduced motor torque.

Hope the above helps.

Regards,
GGOSS

 

[OperaHouse]

I guess I was really fishing for applications other than spindle motors. Anyone running a standard 60Hz motor at 200 or higher? It seems that having this feature over 100 Hz is very limited.

 

[Warpspeed]

It can be handy sometimes because running belts or gears to increase Rpm, can introduce other problems.

I have used it to reach high spindle speeds in machine tools, where a smaller high Rpm cutter requires less drive torque, and it is a simple and compact way to do it.

The loss of horsepower can be a disadvantage, but not always. Be aware of the speed ratings of the motor bearings.

 

[DickDV]

I've done numerous (over two dozen) hydraulic pump test cells for the aircraft engine industry which require speeds ranging from 6000 to 18000 rpms.

We use special built two pole water-cooled induction motors and operate as high as 300Hz.

The motors vary with some having base speeds requiring overspeed to double base speed while some others have their base speed right at max speed. It all depends on the desired torque profile to operate the load.

Some of these cells require four-quadrant operation since the pumps can also be used as actuators in some applications.

It's been a tough challenge but fun when the results are good.

 

[Skogsgurra]

Too late to add anything of value to this thread. But I think that the heading deserves a star!

 

[jraef]

"I guess I was really fishing for applications other than spindle motors. Anyone running a standard 60Hz motor at 200 or higher? It seems that having this feature over 100 Hz is very limited."

Not standard 60Hz motors, but VFDs are used on centrifuges now on a regular basis that run from 150 to 200Hz, but the motors are designed to run at those frequencies.

"Venditori de oleum-vipera non vigere excordis populi"

 

[djs]

I had an application on a bottle packaging machine where we were having trouble with the bottles jaming at the infeed. These machines have a "shaker" which moves the bottles side to side in order to prevent this.

In trying to fix the jamming one of the things I did was to connect a VFD to the shaker motor. The motor was a four pole 60 Hz gearmotor. I set the VFD to output 120 Hz. This doubled the shake speed. Fized the bottle jam on the machine. The machine was left like this for a period of several months.

On a further note, we modified all of the packaging machines to double the shaker speed. The way we did this was to change the shaker motor stator from a 1 hp 4 pole unit to a 2hp two pole unit. The replacement stator were of course from the same mfgr and same model series. To my knowledge these shaker have been running for 10 ten years without problems.

 

[DickDV]

jraef, the main reason you don't see ordinary design motors running way up there in overspeed is that the torque starts to fall off faster than the speed increases thereby causing a drop in net hp output.

In fact, if you take a small 4 pole induction motor with nothing coupled to it and start increasing the speed, you will find that somewhere around 200-240hz, the motor no longer can develop enough torque to overcome its own friction and windage losses and simply stops turning.

I conclude that, at that point, it ceases to be a useful machine and therefore, would be rather unpopular!

 

[Warpspeed]

Exactly DickDV.

You need to look for applications where load torque falls with increasing Rpm, and there are not many common ones apart from cutting tools.

 

[OperaHouse]

I believed that would be true as I said in my first post at considerably higher frequencies. The same thing happens in stepper motors because the inductance reduces the current at higher frequecies. Talk about seeing funny things happen, I have a drive now that rotates 1/4 turn back and forth at very low (2-3Hz)frequencies. Has a period of about one second.