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AC Motor - 120v to 220v static conversion problem

  1. Mar 29, 2006 #1
    I'm currently working on a senior project that consists of a frequency controlled AC motor that statically converts 120vac out of the wall to 220vac three phase required for the motor. It is a 1/3 HP, 208vac, 2.4A, 1.35SF, 1720 RPM motor. The frequency driver says it is capable for 2.6A 208vac 3 phase output. The issue i am having, is when i run the motor at approximately 45Hz of it's maximum 60Hz, it will reach it's current of 2.4A under no load. Of all things I find, I should be able to run the motor at full speed at anywhere from 66%-80% of it's rated horsepower.

    The motor will be mechanically coupled to a DC generator to simply power a 12v DC fan. If I keep it on an electrical load to demonstrate the output say desired for a given application, I can alter the electrical load to simulate say a device that will draw 10mA or 2A.

    The frequency driver I have is a reliance, yet it is an older model (don't have the model numbers on me at this time). If anyone can give me some insight into static phase conversions and why I cannot run the motor at max speed without the motor running at maximum overload capacity. (Measured leg current = 3.2A: 2.4A (Rated) * 1.35 (SF) = 3.24A)
     
  2. jcsd
  3. Apr 14, 2006 #2
    Does that Reliance reduce the voltage as it lowers the frequency? It should. At the lower frequencies, the inductive reactance of the motor goes down and, therfore, the current will go up.
     
  4. Apr 19, 2006 #3
    As I have found in further investigation, i was able to determine that it comes down to more of the age of my motor. There was a tag on the motor saying the last inspection time was in 1971. Figuring that the motor is maybe another 10 years older than that, it wasn't mechanically made to handle a VFD. The solutions I was informed of was to add in a snubber or upping the size of the VFD. Another thought I had was possilby installing a flywheel to the shaft of the motor, so once it was up to speed it would be able to makeup for the overload.

    What I didn't know at the time of my post, was that normally VFD's are AC to DC to AC and use pulse width modulation in order to drive the motor. The motor sees it as AC so it makes it alright, but due to the motor being of an older construction, it will burn holes through the insulation. I did notice some burning within the motor that may be from older missuse, but possible that the VFD is responsible for it.

    What I cannot find is documentation talking about using 120V as the input and having a 208V output of the VFD. Most everything is on either a larger scale, or uses 220V 2 phase in, or 220 3 phase in.
     
  5. Apr 19, 2006 #4
    As I have found in further investigation, i was able to determine that it comes down to more of the age of my motor. There was a tag on the motor saying the last inspection time was in 1971. Figuring that the motor is maybe another 10 years older than that, it wasn't mechanically made to handle a VFD. The solutions I was informed of was to add in a snubber or upping the size of the VFD. Another thought I had was possilby installing a flywheel to the shaft of the motor, so once it was up to speed it would be able to makeup for the overload.

    What I didn't know at the time of my post, was that normally VFD's are AC to DC to AC and use pulse width modulation in order to drive the motor. The motor sees it as AC so it makes it alright, but due to the motor being of an older construction, it will burn holes through the insulation. I did notice some burning within the motor that may be from older missuse, but possible that the VFD is responsible for it.

    What I cannot find is documentation talking about using 120V as the input and having a 208V output of the VFD. Most everything is on either a larger scale, or uses 220V 2 phase in, or 220 3 phase in.
     
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