Impeller Trimming vs VSD

When a pump is oversized or operating above system requirements, the next question usually is: “How do we reduce energy consumption?”

Two common solutions come up:

• Trim the impeller
• Install a Variable Speed Drive (VSD)

Both can reduce energy usage.
Both have advantages.
But they are not the same — and the best option depends on your system.

Let’s simplify the decision.

First: Why Energy Is Being Wasted

If a pump is too large for the system:

• It operates away from its Best Efficiency Point
• Flow is often controlled by throttling
• Electricity consumption stays high

You are paying for performance you don’t need.

So the goal is simple:

Reduce pump output to match system demand — without damaging efficiency.

Now let’s look at the two main methods.

Option 1: Impeller Trimming For Energy Saving

Impeller trimming means physically reducing the diameter of the impeller.

When the diameter is reduced:

• Flow decreases
• Head decreases
• Power consumption decreases

It’s a mechanical adjustment. Once done, it’s permanent.

When Impeller Trimming Makes Sense For Energy Saving

Impeller trimming works best when:

• System demand is stable
• Flow and pressure requirements don’t change much
• The pump is slightly oversized
• No variable demand exists

For example:

A fixed irrigation system that runs at the same flow daily.

Once trimmed correctly, the pump operates closer to its Best Efficiency Point — and energy usage drops accordingly.

Advantages of Impeller Trimming

• Lower upfront cost than a VSD
• No electronics involved
• Simple, reliable solution
• Reduced power consumption permanently

Limitations of Impeller Trimming

• Not adjustable once done
• Cannot adapt to changing demand
• Requires workshop modification
• Over-trimming can create new performance issues

It’s a good solution — but only if the system is steady and predictable.

Option 2: Variable Speed Drive (VSD)

A VSD changes the motor speed instead of changing the impeller size.

And here’s the important part:

In centrifugal pumps, power consumption drops dramatically when speed is reduced.

Even a small speed reduction can create significant energy savings.

When a VSD Makes Sense

A VSD is ideal when:

• Flow demand varies
• Pressure fluctuates
• The system does not always require full capacity
• Energy efficiency is a priority

For example:

• Booster systems
• Municipal supply systems
• Industrial processes
• Irrigation with varying zones

Instead of running full speed all the time, the pump adjusts to actual demand.

That’s where major savings happen.

Advantages of a VSD

• Adjustable performance
• Significant energy savings in variable systems
• Reduced mechanical stress
• Soft starting (less mechanical shock)

Limitations of a VSD

• Higher upfront cost
• Requires correct setup and programming
• Electrical components need protection

It’s more advanced — but often more powerful in the right application.

So Which Saves More Energy?

Here’s the honest answer:

It depends on the system.

If demand is constant and stable →
Impeller trimming is often sufficient and cost-effective.

If demand varies throughout the day →
A VSD usually saves significantly more energy over time.

In systems with high daily operating hours and fluctuating demand, a VSD can deliver substantial long-term savings.

But if your system runs at one fixed duty point 24/7, trimming may achieve similar results at a lower cost.

A Practical Way to Decide

Ask yourself:

1. Does my flow demand change throughout the day?
2. Is the pump running continuously at full speed?
3. Am I throttling valves to control flow?
4. What is the annual electricity cost of this pump?

If electricity cost is high and demand fluctuates, a VSD often makes financial sense.

If the system is stable and slightly oversized, trimming might be enough.

The Real Goal Is Efficiency — Not Just Modification

Both solutions aim to move the operating point closer to the pump’s Best Efficiency Point.

The wrong decision can create new problems.

The correct decision reduces:

• Electricity bills
• Mechanical wear
• Maintenance costs
• Long-term operating risk

Need Help Deciding?

At Pumps Africa, we help customers across South Africa evaluate pump systems properly before making modifications.

Sometimes trimming is perfect.
Sometimes a VSD is the smarter investment.
Sometimes the issue lies elsewhere in the system.

The key is analysing the duty point and operating conditions first.

Because once you understand how your system really behaves, the right solution becomes clear.