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VPM to VFD Conversion


  2. Remove and/or plug the air controls and supporting hardware from the front of the fan. You can also remove the positioner as no longer be required.
  3. Remove the dust cone cover (match mark the cover to the hub before dis-assembly).
  4. Match mark the aluminum diaphragm cover to actuator section then remove the diaphragm cover only.
  5. Remove the material diaphragm. No longer needed.
  6. Move the aluminum actuator forward by hand pulling away from the fan hub.

You should notice the blades modulating toward maximum pitch. If you find stiffness use lubricant and keep pulling or use steering wheel motion right/left as you pull same time.

Note the maximum blade pitch angle at the end of the mechanical stop rod/nuts.

7.At this point you should be able to see the three mechanical stop rods that control minimum and maximum blade pitch. The three rods bottom out at minimum pitch at the hub wall and bottom out against the back of the reaction plate at maximum pitch.

Without changing or undoing anything else try to move the fan blades by either pulling the actuator section and/or also twisting the blades by hand to achieve maximum pitch and mark maximum position by scribing a line along the curvature of the blade tips at the housing (6 o'clock). Roll each blade to the scribe line to confirm they're all the same. If actuator section is stiff you may be binding along the hub slide way. This condition is rare but keep going and spray more lubricant and keep turning it like a steering wheel (back and forth) and pull away from the hub at the same time.

8. When you're sure maximum pitch has been found scribe another line along the blade flat root to outer hub to confirm maximum.

All of the above steps is assuming you have already recorded the amps at existing VPM maximum pitch angle so you can check it at 100% VFD speed later.

9.Make up (machine shop) three new mechanical stop rods to complete the first max pitch lock mode. The new longer custom rods should be long enough to replace the existing short rods to match the maximum distance travelled and to stop linear travel of the actuator section towards the inner hub and/or up against the back of the reaction plate. Basically long enough to span the distance between the hub wall and the back of the reaction plate = old maximum and minimum blade angle.

10. Next, remove the gold colored steel reaction plate.

11. Remove any of the three existing stop rods from the actuator section and replace with the new longer stop rods. Note: Exact new stop rod length is important.

Add locking nuts to all three new rods tighten up to the actuator wall when maximum blade angle is confirmed.

12. Replace the steel reaction plate.

At this point you should be locked at maximum blade pitch angle. Check and make certain the blades are fixed and no movement on the actuator section.

13. You no longer need the material diaphragm so save for spare part if you have other VPM fans.

14. Before mounting the aluminum diaphragm cover drill the rotary union hole to typically 5/8"NC or different size for smaller fan models.

15. Mount the aluminum diaphragm cover.

16. Make a 5/8"NC redi rod long enough approximately (4"to 5").

Run the 5/8" NC redi rod (double nut at end) down the diaphragm cover (mounted) center hole until it bottoms out against the reaction plate shaft then tighten.

Use only slight torque on the redi rod as the blades are already locked at maximum pitch per the three new longer stop rods.

Then run two nuts or a lock nut down the redi rod to lock the rod at the max pitch position. If the rod sticks out a bit you can cut it off with a zip disc or sawsall.

17. Congratulations you now have a double locked fixed blade pitch fan.

18. NOTE: Throughout the above steps check the internals of the fan make sure all blade rollers (bolts) are snug and no other defects exist.

19. Start the fan (motor) at lowest VFD speed.

20. Increase VFD speed by 2 Hz at a time up to 60 Hz (Canada - USA).

21. Use a vibration instrument to measure velocity peaks (ips or mm/s) on the fan housing at all speeds but most certainly document any unusual increased peaks at any speed of concern. You may have a resonance or critical speed issue.

CAUTION! If you do confirm a resonant frequency or critical speed program that speed/frequency out of the VFD software.

22. If you find high vibration at maximum blade angle it may be unbalance or isolation issue.

Unbalance will require on site balance expert to resolve.

Isolation issues may be due to grounding at one or more springs.

High vibration at maximum or even lower speeds may be due to "Stall".

"Stall" is caused from higher resistance due to damper modulation problems, plugged coils or filters.

"Stall" is typically when the fan suddenly becomes very noisy and performance drops.

Finally, assuming the maximum blade angle is correct and has not been exceeded the motor amperage should be below FLA.

23. Prior to converting your VPM fans to VFD please read my "Learn About EIBD/Shaft Currents" article.

When buying a VFD from any vendors insist on load reactors and/or sine wave filters to avoid electrically induced bearing damage issues in future. Get vendors advice on this subject.


Garrett Sandwell, MET, CVA (ASNT 3) CEO


"Accurate Analysis, Diagnostic Evaluations, Corrections on All Types of Machinery"
Vibrations, Noise, Dynamic Balance, Performance, Laser Shaft & Drive Alignment,
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