Identifying Blade Pass Frequencies with Motor Current

Motor Talk
The Snell Group

Every 21st Century maintenance program should include monitoring of energized critical motors. Monitoring motors can identify many mechanical anomalies within the motor or driven load.

An electric motor in operation simply converts electrical energy into mechanical energy. That energy is in the form of torque that is developed at the motor shaft.  The shaft is coupled by various means to the load.  Any variations in the load that affects the motor torque at the shaft will affect the magnetic coupling between the rotating magnetic field in the stator and rotor.

Motor blades.

Equipment’s such as pumps and fans will develop a fault signature based upon the number of blades and rpm at which they turn. For example; an unbalanced fan or a pump impeller with a broken blade will develop a rhythmic vibration based upon the speed and blade number. This will in turn effect motor torque. This is known as “Blade Pass Frequency.” These effects may be easily calculated.

Blade pass frequency.

Blade Pass Frequency:

RPM x Blade Number

RPM x Blade Number / 60 – value in Hertz

Note: For belt driven fans multiply the RPM of the driven sheave, times the number of fan blades

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