Replies

1. 
   davcefai | Dec 21, 2009 04:40pm | #1

   Yes, this is possible and often done.

   A VFD works by altering the AC frequency. At 30Hz the motor (assuming you are in th USA) will turn at half speed. If you exceed 60 Hz the motor will go faster than the rated speed.

   However you are then on your own as regards wear and overheating. With good quality motors such as Siemens I have been told that a 10% overrun is safe. Some people will go to 20%.

   Note that the motor may run slower at higher frequencies because of slippage. The armature simply cannot spin fast enough to keep up with the frequency.

   Incidentally running a motor too slowly will overheat it as the integral fan will not be able to move enough air to keep it cool.

1. 
   WillGeorge | Dec 22, 2009 10:39am | #3

   I agree for what it is worth..

   I loved your ..

   Note that the motor may run slower at higher frequencies because of slippage. The armature simply cannot spin fast enough to keep up with the frequency.

   Long ago I had a large printing press giving occasional 'stripes' in the dot pattern. I think I worked on the problem for several days. I thought some mechanical problem. I had a variable strobe shipped in and I could see the motor 'skipping'. Somebody had replaced the 3 phase controller with the wrong type for the motor in use. A call to the controller Mfg. confirmed the problem.

2. 
   JohnWW | Dec 21, 2009 05:08pm | #2

   If your VFD's output
   If your VFD's output frequency goes above the motor's design frequency the motor will run faster than its rated rpms. As noted in the first response this is acceptable up to a point, 10 to 20% overspeed is conservatively safe.

1. 
   Quickstep | Dec 22, 2009 11:08am | #4

   So, last night I used an optical tachometer to determine the speed of the motor. According to the tach, the motor is spinning at 1794 rpm; about 4% higher than it's rated speed. I verified that the VFD is set up so that it's output matches the faceplate on the motor. In the overall scheme of things, I guess the extra 69 rpm doean't matter much in the overall scheme of things, but I'm still curious. Any thoughts?

1. 
   WillGeorge | Dec 22, 2009 12:36pm | #5

   I guess the extra 69 rpm doean't matter much in the overall scheme of things, but I'm still curious. Any thoughts?

   Do you get experience chatter in you work? If not.. Go for it!

2. 
   northhouguy | Dec 22, 2009 12:50pm | #6

   The formula for synchronous speed is

   (120 x frequency)/number of poles. An induction motor must always run slower than the sync speed, (or no induction takes place which means no torque.) That's why your motor has a nameplate speed of 1745 instead of 1800. If the frequency is above 60 Hz, the motor will run faster than the nameplate rating.

   The poles can't change in your motor, the 120 constant can't change. So I suspect that your VFD has some "slop" in the speed setting, (i.e. the frequency of it's output wave.) You could hook up an ocilloscope or a frequency meter to the output and see the actual frequency. Or, if it's not causing you problems, "don't worry, be happy". Four percent regulation is probably within the drive tolerance.

   Best regards,

   Fred

3. 
   JohnWW | Dec 22, 2009 06:18pm | #7

   The error is as likely to be with your tachometer as with the actual motor speed, and the difference is so minor that it is pointless to worry about in any case. The motor could run at considerably higher speeds without harm. In general lower than rated speeds are more likely to cause problems mostly with heat build up.

1. 
   Quickstep | Dec 22, 2009 10:53pm | #9

   John, Interesting observation about the accuracy of the tach. I measured several other motors and found the tach reading on all of them to be about 3% over their rated rpm. I we assume the tach is off by 3%, that still leaves the lathe motor a little bit "hotter" than rated speed, but within acceptable limits. Just for the record, I'm really not interested in running the motor faster than the rated speed; just the opposite. I want to make sure I'm not over driving the lathe so I don't end up with a turning sticking out of my forehead because what I thought was 1,000 rpm spindle speed was really 3,000 rpm.

   One more question - Are the contact tachometers more accurate than the laser models?

2. 
   mlbfreestyle | Dec 22, 2009 06:19pm | #8

   I am an electrical engineer and design control systems for industrial machines. (been in the business for 25 years). I have often run 3-phase motors with VFD's at 150% of 'base speed'. Base speed as stated previously is the speed at 60 Hz. I cannot speak for your particular motor but most industrial rated motors can go much higher than 10% over base. Most of the VFD's that I work with have maximum frequency parameter defaulted at 90 Hz. The problem is not one of over-heating or wear per se, but rather falling apart due to centrifugal force. I also agree with the post about cooling at low speeds. Fan efficiency drops off at low motor speed, so if you run it very slowly for any significant time, pay attention to motor temperature.