Fan Motor Burning Mystery

In spite of the accustomed general perception suggests that rising resistance of a system overloads the motor, the mystery of burning motors is very specific – it depends on the type of the system.

The peculiar difference between a mechanical and aerodynamical systems are:

- rising resistance of mechanical systems overloads the motor.

- rising resistance of aerodynamical system unloads the motor.

Graph 1 illustrates interaction of fan pressure, capacity, and motor HP.

GRAPH 1

The more resistance in the system, (ducts, filters, coils, dampers, etc.) the less air capacity and lower motor load, and vice versa, the less resistance the more air capacity and higher motor load.

Vent system sizing eventuates in total resistance at the required capacity, which are criteria for the fan selection. There is a tendency to select a fan with some spare total pressure over calculated one.

This case is considered in the Graph 1.

The blue lines on the graph are for the designed system, and the red ones - for inappropriate fan-motor selection with following it consequences.

The design working point 1 with the design system characteristic 0-1, CFM=1a-1 and TP=1b-1, at 3hp motor capacity and Fan Curve A.

Overestimation of TP=1b-2 at designed CFM=1a-1 creates the imaginary working point 2 and system 0-2 on the Fan Curve B with motor capacity 10hp.

The real working point 3 of the improperly selected fan is the intersection of the designed system 0-1-3 with the Fan Curve B. So, the field data is: CFM=3a-3 and TP=2b-3, at 20hp motor capacity.

Deduction.

The selected point 2, estimated over the design point 1, in the field becomes the point 3. The CFM capacity of the point 3 is larger than in points 1 and 2.

In spite of the resistance of the system in the field (TP in the point 3) is lower than the resistance (TP in the point 2) of overestimated system the power consumption in the field (point 3) is higher (20hp) than the power consumption (10hp) of the overestimated system (point 2).

Thus, the increase in energy consumption in the field overheats the selected 10hp motor and causes it to burn.

Note:

A number of manufacturers do not show the Mechanical Efficiency (Mef) of their fans. The possible reason for it - they do not want to unveil a low efficiency. Their tables show BHP and you shall run through catalog pages in order to find the lowest BHP for your system. But it does not mean that you selected fan with sufficient and acceptable MEf. Just the lowest BHP alone is not an assuring criterion for fan selection. The MEf can be still low. Therefore in order to clarify the picture you have to proceed with calculation of MEf.

MEf= power output (air horsepower) / power input (brake horsepower)

Air horsepower (AHP) is a power which moves the desirable air volume at the desirable pressure. Brake horsepower (BHP) is a power input the fan requires.

MEf= AHP/BHP = {(CFM x TP) / 6356} / BHP

Now, when you know the value of MEf, you can be frequently disappointed with the result. In spite of the lowest BHP, selected from table, the MEf is still disappointedly low.

Conclusion

- The fan oversizing must be avoided.

- The attention-grabbing fact is that it must be recognized that no matter what methods have been applied to calculate the vent systems (Equal Friction, Velocity Reduction, Static Regain, or Constant Velocity, etc.,) the results eventuate in a value of the Total Pressure, not in a value of the Static Pressure.

- Thus, Fan Total Pressure, evidently and logically, is the only proper basis for the fan selection.

- Fan Static Pressure (Fs) is an unacceptable basis for fan selection. "Most importantly, remember that Ps is a defined term." (ASHRAE Handbook, Fundamentals). And "It cannot be read directly from the calculation sheet." (Industrial Ventilation, SMACNA).

- As the essential measures for motor safety and energy saving, the fan manufacturers must provide graphs illustrating System / Fan trade-offs: air capacity and total pressure, fan and motor curves, fan and motor mechanical efficiencies, energy consumption, and noise levels.