# powerfleX 700. erroneous rpm



## jhankovs (Oct 15, 2014)

Hi there. I have a question about an odd anodd rpm reading I was getting on a motor ran off a powerflex 700. 

It is a 1760 rpm 5 hp motor and the vfd was set up to run the motor at 60 hertz. I measured the frequency to be 60 hertz but I was getting something like 1808 rpm on the motor. It's an asynchronous motor and last I checked that speed would be impossible unless I was above 60 hertz. Is there something that would cause this with powerflex vfds? I've only seen this with powerflex. Same results occured on the same configuration of motor and vfd.


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## Black Dog (Oct 16, 2011)

jhankovs said:


> Hi there. I have a question about an odd anodd rpm reading I was getting on a motor ran off a powerflex 700.
> 
> It is a 1760 rpm 5 hp motor and the vfd was set up to run the motor at 60 hertz. I measured the frequency to be 60 hertz but I was getting something like 1808 rpm on the motor. It's an asynchronous motor and last I checked that speed would be impossible unless I was above 60 hertz. Is there something that would cause this with powerflex vfds? I've only seen this with powerflex. Same results occured on the same configuration of motor and vfd.


Welcome to the forum...:thumbsup:


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## JRaef (Mar 23, 2009)

jhankovs said:


> Hi there. I have a question about an odd anodd rpm reading I was getting on a motor ran off a powerflex 700.
> 
> It is a 1760 rpm 5 hp motor and the vfd was set up to run the motor at 60 hertz. I measured the frequency to be 60 hertz but I was getting something like 1808 rpm on the motor. It's an asynchronous motor and last I checked that speed would be impossible unless I was above 60 hertz. Is there something that would cause this with powerflex vfds? I've only seen this with powerflex. Same results occured on the same configuration of motor and vfd.


Do you have the slip compensation enabled? If so, why?

Also if you are using Vector Control, and have set parameter 79 to a 2or 3, it converts speed to Hz or Vice Versa not only for the display (as most other drives do) but also internally in the DRIVE'S speed references. If you don't want that, change it to 0 or 1, which affects only the display, just like the cheaper drives.


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## jhankovs (Oct 15, 2014)

I don't remember all that to be honest. It was a while back and the units we shipped all ready. It's not critical for a certain speed. I was only wondering what could cause a direct drive fan getting 60 hertz (measured) from a VFD could possibly be going faster than synchronous speed. Slip compensation maybe?


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## glen1971 (Oct 10, 2012)

Were you using a tach to measure the speed at the motor? Measured at the motor or on the load? Or are you reading it off the drive? The motor NP Speed (44) may be set at 1800 (default is 1750)... 
When they were commissioned was an autotune done? What type?


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## jhankovs (Oct 15, 2014)

I used a strobe right at the motor. When I got the weird reading I double checked with a laser and got the same result. 

We ran the ama if I remember correctly. 

Having greater than sync speed at 60 hertz is the weirdest thing I've seen with a VFD. Thats why i actuall checked the output frequency with a fluke 87. Sure enough it read 60 hz. I've also seen a vfd blow on us. It made quite the bang. Turns out that the cassing of the vfd was labeled 600 volts but once we took it apart we found that the inside was labeled 208 volt.


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## jhankovs (Oct 15, 2014)

The vfd was set up as 1760 to match the motor name plate.


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## Jhellwig (Jun 18, 2014)

Is it a standard or vector control drive? Was a static and/or a rotational tune done? Is everything programmed to accurately run a sycronus motor instead of a squirrel cage motor?

If accurate speed control is critical you might need to install an encoder or tach.

Allen Bradley is good about leaving some of the information out of their manuals you need to set everything up.


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## KennyW (Aug 31, 2013)

Do you have drive tools? Go on line and look at the speed regulator diagram, it will show you what the speed reference and feedback are (what the drive thinks they are anyways). Then go from there. 

Also keep in mind that 1760rpm is at 100% load. At low load it will be much closer to synchronous. Seriously though if you have drive tools this is easy to solve. If not, and you are working with applications where 1760 vs 1808rpm matters, you should get the software.


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## Jhellwig (Jun 18, 2014)

KennyW said:


> Do you have drive tools? Go on line and look at the speed regulator diagram, it will show you what the speed reference and feedback are (what the drive thinks they are anyways). Then go from there.
> 
> Also keep in mind that 1760rpm is at 100% load. At low load it will be much closer to synchronous. Seriously though if you have drive tools this is easy to solve. If not, and you are working with applications where 1760 vs 1808rpm matters, you should get the software.


I think you can see that through the him also. It might have to have advance parameters enabled though.


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