The Basics of Variable Frequency Drives, Part 3

In Parts 1 & 2, we discussed the basics of how VFDs work, looked at how to size them for centrifugal pumps and the pitfalls that need to be avoided. Today, I would like to discuss the advantages of VFDs.

Ease of Varying Speeds

It goes without saying that VFDs are great for varying the speed of a pump. Simply pressing a button can raise or lower a motor/pump’s RPM. Do keep in mind that selected speeds may be, or need to be, “locked out” to avoid a pump’s critical speeds. This is particularly applicable to cantilever pumps that often have low critical speeds.

Power Savings

VFDs offer huge energy savings for fans and pumps. Adjusting the operating speed of the driven unit to match the required output drastically reduces overall power requirements.

The graph below illustrates an example. The yellow line represents a 20 % reduction in flow from a centrifugal pump.  The bold red line shows the reduction in HP when a throttling valve is employed. The yellow line intersects the bold red line at 93% indicating a 7% reduction in power. However, where the yellow line intersects the curved blue VFD HP  line, a 40% reduction in power is indicated.

33% less overall power!

Valve & Pump Wear

Reducing flow by introducing a partially closed valve may be a smaller capital expense than a VFD, but it does add to wear /maintenance costs, especially when handling slurry. Obviously, the valve becomes a wear item (in some slurries, it’s an item with a very short life expectancy); however, it also affects the pump itself.

If the pump was originally sized correctly, that is to say just to the left of BEP, then reducing the flow by throttling will move the operating point away from BEP.  The new operating point will then be in an area on the pump curve that is marked by lesser ISO efficiency lines. Operating at a less efficient point increases overall wear.

On the other hand, reducing the flow by the use of a VFD  moves the operating point down while still staying close to the original ISO efficiency line. This is illustrated in the graph below.

The best efficiency curve (red line) intersects the n2 speed head/flow curve at point #2, indicating a flow rate of q2.  Reducing the flow by slowing the pump motor with a VFD moves the operating point to the lower flow rate of q1. This is done while staying on the BEP line, as marked by point #1.

The arrow marks the lower flow rate achieved when a throttling valve is used, illustrating how the operating point has moved to an area of lower efficiency.

Tip Speed

Tip speed is often used to compare anticipated wear rates.  The lower the impeller tip speed, the less wear. Reducing the flow by using a VFD reduces RPM and, therefore, tip speed.  Regardless of how you size it up, using a VFD to control flow rates is superior to a fixed speed and a throttling valve!

Closing

We have covered a lot of territory in this three-part review of VFDs, so before I sign off for today, I would like to leave you with a summary of what I would consider some of the important takeaways.

• If the pump should not exceed a certain percentage of the first critical speed, has this percentage been programmed as a locked-out maximum speed?
• Pumps may have been sized at 60Hz to run between critical speeds. Will the VFD pass the pump through those speeds quickly, and are the dangerous speeds locked out from operation?
• Have minimum speeds been locked in when splash lubrication is required by the equipment (e.g., PD pump cranks)?
• Over speed may be at the touch of a dial, but are the mechanics of the equipment up for the task? Especially the motor!
• Can the standard fan on the motor shaft supply enough cooling at less than 60 Hz? A critical question when operating at altitude.
• Are bearings insulated against current circulation?
• Are line reactors installed on both sides of the drive?
• VFDs generate heat and require airflow that may not be available in a dirty environment.

Bye for now!

RJ