VFD Torque

[ptonsparky]

Customer changed the max hz of a VFD from 60 to 120. We had set it up as constant torque. It is on an auger with gear reduction so my initial thought was no big deal, if nothing else it will trip on overload at higher rpm. Sounds like they are having some problems at low end...should I rethink this? (May be some other problem entirely, like bad bearings, or wet product)

 

[Besoeker]

Customer changed the max hz of a VFD from 60 to 120. We had set it up as constant torque. It is on an auger with gear reduction so my initial thought was no big deal, if nothing else it will trip on overload at higher rpm. Sounds like they are having some problems at low end...should I rethink this? (May be some other problem entirely, like bad bearings, or wet product)

If it is now constant V/f ratio all the way from zero to 120Hz with maximum voltage at 120Hz, the available torque will be reduced at all speeds.

 

[Jraef]

If you didn't catch the subtlety of Besoeker's response:

There are multiple ways to change the maximum frequency limits of the VFD output. If they changed the maximum frequency by using the V/Hz selection process, then they are telling the VFD to reach maximum voltage at maximum frequency of 120Hz. So let's say it's a 460V motor so the motor's design V/Hz ratio is 460/60 or 7.67:1. By using the V/Hz pattern programming to change the maximum frequency, they are saying that the output of the VFD reaches 460V RMS at 120Hz instead of 60Hz. So that means that the motor V/Hz ratio becomes 460/120 or 3.83:1. Since torque is directly related to the V/Hz ratio applied to the motor, you are only able to produce 1/2 the torque the motor is capable of. And since HP is a factor of torque and speed, you are essentially at 1/2 the HP at 60Hz compared to what the motor is capable of, but you have full HP at 120Hz only by virtue of having twice the speed.

If on the other hand they left the V/Hz ratio alone for max. V at 60Hz and had simply programmed the maximum allowable frequency to be 120Hz, then what you happen is that the VFD output would remain at 7.67V/Hz until you got to 60Hz. But after that you would be dropping it and by the time you got to 120Hz, you are back at the 3.83:1. So you are reducing the HP at 60Hz and under while maintaining full torque, but above 60Hz you are remaining at a constant HP while losing torque. So you can have one or the other, not both.* But when operating at 60Hz and under, everything would be normal.

*There is a trick though.
If you have a dual voltage motor, you can strap the motor for the lower voltage but feed the VFD with the higher voltage, then program the VFD to reach the lower voltage at 60Hz and the higher voltage at 120Hz. So you end up with a 230V motor, who's V/Hz ratio is 3.83:1. This allows you to have the same feature of having full torque up to 60Hz and then allows you to continue increasing the output voltage beyond that so that you end up with full torque at twice the speed (and thereby twice the HP). The VFD has to be twice the size, there can be no issues with cooling (the motor will run hotter) and the motor bearings have to be rated for the speed. Not for the faint of heart or the novice.

 

[Besoeker]

If you didn't catch the subtlety of Besoeker's response:

Berated by you for too much detail in another thread and, by implication, insufficient in this.
Can't win, eh?

My response was intentionally simple, addressed the issue, and went no further.
"Simplicity is the ultimate sophistication" - Leonardo da Vinci.

 

[ptonsparky]

Both answers help, although I struggle with the concept of hp and torque changing.

I have read about the * solution on another forum but they never mentioned the problem of higher operating temperatures. I would love to try it but not at my customers risk. Being a novice does not scare me...well not much. Gotta be deflowered some time is my thought.

Thanks

 

[Besoeker]

Both answers help, although I struggle with the concept of hp and torque changing.

Motors have what's called a base speed. This is the speed they would run at if rated volts and frequency are applied - the situation that would apply if the motor was directly connected to the supply. Say 460V and 60Hz. At that, you can get rated motor output of say 100HP.
Note that power is speed times torque. It's a simple but important concept.

The 460V at 60 Hz sets the level of magnetic flux in the stator and that determines available torque. If you maintain a constant voltage to frequency, you get normal flux and can get rated torque at any speed. So, for example using a VFD at 230V and 30Hz, you will get rated torque. But only half the speed of course because the applied frequency has been halved.
Remembering that power is speed times torque, you would then get only half rated power - 50HP for the example I gave above.

Above 60Hz, the situation changes. You can't get a higher voltage out of a VFD than you put in so you are limited to 460V out. Thus, if you increase the frequency above 60Hz, the voltage to frequency ratio drops and consequently available torque drops. So you have speed going up and torque coming down and the speed times torque - the power - remains roughly constant. This is often referred to as the "constant power" region of operation.

The above is based on a standard three-phase cage induction motor, a standard variable frequency drive and for common standard applications.

 

[John120/240]

A little off topic but: Thank You Besoker & Jraef I now understand why for

cieling fans we must use a fan rated speed control & not a dimmer switch.

We must vary the frequency not the current.

 

[Besoeker]

A little off topic but: Thank You Besoker & Jraef I now understand why for

cieling fans we must use a fan rated speed control & not a dimmer switch.

We must vary the frequency not the current.

Frequency and voltage.
And it's Besoeker btw.