Choosing the right motor means your axial fan, centrifugal fan can run efficiently and cost-effectively. Fan motors are an important option if you are looking for the right axial fan for your business.
How does an axial fan work?
Industrial fans are selected according to the direction of airflow. Axial fans consist of a rotor shaft, blades, impellers and a housing mainly made of steel. The rotor is driven by an electric motor and has an air intake through which air enters the interior once the rotor shaft spins. Air flows in a linear fashion parallel to the rotor axis. This increases the flow rate and can cause a large influx and ejection of air. This can be achieved while maintaining low stress and low power consumption. Air enters the impeller axially. To increase the flow rate, the vanes push the impeller and further deflect the airflow into the guide vanes. The airflow from the guide vanes becomes axial again and enters the diffuser tube. The kinetic energy of the moving blades is converted into pressure, which facilitates the required amount of ventilation.
What specification fan motor is most suitable?
This is a more complex common problem than one might think. There are many variables based on the application. Different technologies also affect fan selection. Torque, horsepower, and fan laws are discussed below.
No one wants to pay more for a motor than the application requires. However, the real question is really about motor sizing and getting proper performance out of the unit.
Working principle of fan motor
The most important thing to understand is that a fan is a variable torque application. This means that when the fan is on, it spins very slowly and moves very little air (CFM). The force (torque) required to turn the impeller is also very low because there is very little resistance (static pressure) to overcome. As the fan speed increases, more and more torque is required to spin the impeller. The reason torque requirements rise with RPM is because the fans spin faster and move more air (CFM). The increased airflow caused by the fan spinning faster also increases the resistance the fan has to overcome.
Torque and horsepower are related
The torque produces a twisting action on the impeller shaft. Horsepower (HP) is the amount of work done in a specified amount of time. In a fan, HP is the application of torque for a given time. There are equations that describe the relationship between torque and HP.
HP = (Torque x RPM)/5252.
That is, the HP required for the fan to operate is positively related to torque and RPM.
Therefore, the more air you want to move, the more HP you need.
Fan law
Several of the terms mentioned earlier have an effect on airflow. The terms RPM, CFM, static pressure, and HP are actually related. This relationship is often referred to as the Law of Sectors or the Law of Affinity. Three basic fan methods address these terms.
The first fan law states that a change in RPM is directly proportional to a change in RPM. (Figure 1) For example, if RPM increases by 10%, CFM increases by 10%.
The second fan law states that as the RPM increases, the static pressure (Ps) also increases. But the change is disproportionate. Ps actually increases the square of the RPM change. Since the change in RPM is the same as the change in CFM, this indicates a large increase in resistance to flow as more air is moved.
The third fan law connects how HP is affected by changes in airflow. This law states that the amount of HP required to move air in a system is multiplied by the cube of the change in RPM.
Torque is directly proportional to horsepower, i.e. more air flow requires more torque.
Air movement is a variable torque process, and the key terms in air movement are all related. A future article will put this information into practice and show how to size a motor in a fan application.
Selection of fan motor
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Voltage and phase
A common economical combination of voltage and phase for a motor is 460 volts and 3 phase current.
This means the lowest initial motor cost and the lowest power consumption to run. If the customer does not have 3 phase power available, we will recommend a fan that requires a motor under 3 horsepower. Single phase motors above 3 hp are more expensive, have larger frames, and consume significantly more power.
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Motor slipping
All AC motors have a slip factor that reduces their actual speed by 1% to 5% from their rated or synchronous speed. The slip factor depends on the speed and horsepower of the motor. For example, a 5 hp, 1800 rpm rated motor will actually run at 1740 rpm, while a ½ hp, 1800 rpm rated motor will actually run at 1725 rpm. This can lead to some confusion as motor and fan manufacturers are inconsistent with the actual or rated speeds used in their specifications.
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Motor speed
The higher the speed of the motor, the lower the cost, the smaller the frame size and the lighter the weight. the
This means that when fan noise is not an issue, a 3600 rpm motor is better than a 1800 rpm motor, and a 1800 rpm motor is better than a 1200 rpm motor.
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Control motor speed
A cheaper and preferred method of controlling the speed of a motor is to use a variable frequency drive. However, it is important to choose a motor that is VFD ready. the
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Service factor
Motors are designed with a service factor that allows the motor to be operated above its rated horsepower. For example, a 5 hp motor with a service factor of 15% can run at loads up to 5.75 hp.
It may be tempting to think about saving money and choose a motor that will run continuously at the service factor rather than upgrading to the next motor size. We do not recommend it as it will be expensive in the long run for two reasons. First, a motor running at a service factor has exceeded its peak efficiency point and now consumes more electricity, converting it into heat instead of horsepower. Second, overheating can shorten motor life and cause premature motor failure. Replacing a motor in a fan located on a roof or high wall is much more costly than investing in a larger motor.
Conclusion
If you are looking for the right axial fan motor for your business, please contact us or view our products. Langdi is a leading manufacturer of HVAC and air-conditioning fans in China, and a gold medal supplier of many well-known brands of centrifugal fans and axial fans. Let Langdi help your business soar.
