Why Motor Size Isn't Everything

Does size matter? In the case of pump motors the answer is yes, but theres reason behind it! We'll break down the importance of correct motor selection.

Why is my pump fitted with a larger motor than is actually required?

Although your pump may be fitted with a certain sized motor the power it uses is actually likely to be much less during operation. The power used during the operation of a pump at duty point is known as the absorbed power.

A pump is fitted with a larger motor than is actually required due to a number of reasons. Motors are sized according to operating conditions so depending on what these depend on what power in kW your pump may absorb (use).

Factors of motor size choice include:

• Humidity (especially within humid environments or tropical countries)

• Environmental temperature
  

• Power absorbed at end of curve
  

• Allowance for wear and tear over time
  

• Operation via VFD

• Rounding of motor sizes due to availability of models from motor manufacturers

• High Starting Torque
  

Design tolerances have to be accounted for and as a rule, it is always better to have an oversized motor which ensures extra capacity rather than not enough meaning the entire unit may require replacing and the pump would suffer from nuisance tripping.

But why shouldn’t too much emphasis be placed on motor size?

Just because a motor is oversized, doesn’t mean a pump utilises the rated power of the motor.

Pumps operate at a point known as a duty point. At this point flow and pressure are produced, causing the pump to use a particular unit of power.

The power used at this duty point is known as Absorbed Power. If the absorbed power is close to the motor power and something unexpected changes, then it is possible for the motor trip, bringing your process to a screeching halt!

Unexpected Changes are usually quite rare but can happen

Changes such as:

1. Shortages of key ingredients meaning a higher viscosity product is used, as current ingredients are short in supply or prices have risen above budgetary constraints

2. Changes in consumer tastes meaning different recipes are higher in demand

3. Pumping temperatures are reduced to lower heating costs, leading to fluid viscosity increasing

The technical reasoning:

Just because your pump is fitted with a 30Kw, 50kw or 150kw motor doesn’t mean it uses the full motor power available.

The Real Saving

Typically, more energy savings are made by utilising the correct pump technology and ensuring a pump is operating in the correct area of its curve or Best Efficiency Point (BEP) than by replacing the motor for a different size, or type.