Motor with Gearbox vs. High Efficiency High speed motor 4

[enverd]

I have a situation where I replaced an older motor that was running a process along with a gear box and the set up was controlled with an ABB VFD, we have replaced the entire set up with a new high efficiency high speed (5500RPM) motor but the new motor is still controlled by the existing ABB VFD.

Once everything was replaced and set up, we fired everything up and as the result we see the current used is by about 10% higher.

Why is that? Why would the current go up when we are using higher efficiency motor?

Thanks in advance for any replies.

Enver

 

[LionelHutz]

these "small" facts that you can take a 230V motor and run it at 460V I can't say that I knew that.

Never ignore the frequency when talking about a VFD and running the motor at some voltage. You need to talk V/Hz ratio or voltage @ frequency. This makes it much clearer.

So by the time you hit 120hz, your v/hz ratio is now only 1.91, so you produce 1/2 the torque but are going 2x the speed - aka, 175hp still.

Good explanation but you made a mistake here. It's much more complex then this. The torque drops by the square of the voltage reduction. So, when supply 1/2 of the motors rated V/Hz ratio you can only get 1/4 of the torque. This is why posts earlier mentioned the motor will have a higher slip when operating above the base frequency which could be hurting the efficiency of the motor and causing a little higher current.

There is an easy way to visually look at this. Get a motor speed vs torque curve which shows the motor characteristics operating at rated voltage and frequency. Now, to produce a motor curve when you are running the motor at double the frequency you will double the frequency or speed scale and quarter the torque scale. Notice how the torque at double the rated motor speed has dropped to 1/4 of what it was before? Well, to maintain a constant HP you need the motor to produce 1/2 the toque, so the motor has to run slip more to do so.

 

[mikekilroy]

Quote (mikekilroy)
Drive did not change: it is the same as it should be.
Quote (lhoash)
The new setup has the gear box removed and a different but same type of AC drive (ABB ACH550)
It sure sounds like the drive changed....

Sorry for the confusion about vfd model, I must have misread lhoash's 3 references:

1) new high efficiency high speed (5500RPM) motor but the new motor is still controlled by the existing ABB VFD
2) old setup is....an ABB ACH550 drive.
3) new setup has....same type of AC drive (ABB ACH550

http://www.KilroyWasHere<dot>com

 

[mikekilroy]

Good explanation but you made a mistake here. It's much more complex then this. The torque drops by the square of the voltage reduction. So, when supply 1/2 of the motors rated V/Hz ratio you can only get 1/4 of the torque.

Good point that the MAXIMUM available torque decreases at square of velocity above base speed (we of course still have same voltage). Since this MAXIMUM did not dip under the AVAILABLE rated torque at this 120hz yet, I chose to not confuse the issue with breakdown torque, simply mentioning it in passing as it had no impact on the 175 vs 350hp discussion at that point.

http://www.KilroyWasHere<dot>com

 

[LionelHutz]

I'm just pointing out that the torque curve of the motor does not drop linearly with speed as you enter field weakening. I'd hate for someone to go away from this thread and think that with 1/2 the rated voltage the motor will produce 1/2 rated torque.

And yes, the motor rated voltage is 460V for a 3.38V/Hz ratio when running at 120Hz so it is running at 1/2 of the rated voltage when supplied with 230VAC, 120Hz.

 

[enverd]

Thank you Lionel and Mike.
Great points and like Lionel said these threads should be dwell-ed on to eliminate the confusion and any wrong information.

Just to clear some confusion that I created.

The VFD drives didn't change at all.
Both drives are ABB ACH550 which used to feed old setups and will eventually feed both new setups.
(At this time we still have one old and one new installation)

This website is honestly the best one so far and all of the younger and upcoming engineers should feel lucky to have experienced and knowledgeable engineers like you guys giving feedback and actually finding time to answer all these random questions.

 

[mikekilroy]

Lhoash, I did not want to get a discussion going on breakdown torque since it was not really relevant to your questions, but since it has been brought up and is being discussed as if it is a limit for your case, it probably should be examined more....

I did a quick google search for a chart showing both breakdown and rated torque above base speed on a vfd but did not find one; so I made one up. A typical breakdown torque rating can be around 250% of rated.

First a description of rated, available, and breakdown (Max) torques to be clear....

- Rated is what the motor can put out all day long and as you know can be dependent on speed (constant below 'base' speed, drop linearly above) - this is a thermal limit.
- breakdown is the absolute max torque you can get out of the motor at its rated voltage - again, changes with speed, but in this case, it drops as the SQUARE of the speed increase above 'base.' - this is a physical design limit
- available is the lower of continuous or breakdown.

So attached is my chart (first one) showing these on a typical 10hp motor, as well as my spreadsheet (second one) that generated it.

You can see that breakdown torque has no effect on the motors rated output torque, since it is well above the rated torque to over 4500rpm: this is the reason I did not bring up breakdown torque or torque dropping at the square of anything rating.

Lionelhutz' discussion about why your motor is less efficient at these higher speeds (due to more slip) is a good one; I simply showed previously that the less efficiency was due to 'stuff' going on at these higher speeds - including more slip, more eddy current losses, more dynamic friction losses, windage, and POSSIBLY breakdown torque effects.

Feel free to modify the spreadsheet attached and put in YOUR new motor data as you know it! Unlike this typical motor, you will find YOUR motor's BD torque is WAY above its rated torque well out past its 5500rpm rating due to their new 'base' speed of 3600rpm! LOTS & LOTS of tricks can be played with motors.....





www.KilroyWasHere<dot>com

 

[enverd]

Great stuff Mike,

Thanks,

 

[LionelHutz]

but since it has been brought up and is being discussed as if it is a limit for your case{/quote]

I never posted anything about it being a limit, but it always is one of the limits when running in field weakening.

But since you bring it up and want to expand on the curves, you shouldn't call the first curve the rated torque. That is nothing more than the theoretical constant HP curve. There are a number of possible reasons the speed and/or torque can be limited besides hitting the breakdown torque.