Fan Motor Selection Tips
Selection of the right motor can mean many years of efficient, maintenance free operation for an industrial grade fan. Here are a few tips that we have learned over our 73 years of helping customers economically solve tough ventilation issues.
Electricity Voltage and Phase
A common economical combination of electricity voltage and phase for a motor is 460 volts and 3 phase current. That means that the initial motor cost is the lowest and the operating power consumption is also the lowest. If a customer does not have 3 phase electricity available, then we will recommend fans that require motors under 3 horsepower. Single phase motors over 3 horsepower are more expense, have a larger frame, and will consume significantly more power.
Motor Slip
All AC motors have a slip factor that lowers their actual speed in the range of 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 actual operates at 1740 rpm while a ½ hp, 1800 rpm rated motor actual operates at 1725 rpm. This can lead to some confusion because motor and fan manufactures are not consistent with using either the actual or rated speed in their specifications.
Motor Speed
The higher the motor speed, the lower the cost, the smaller the frame size and the lower the weight. That means that when fan noise is not an issue, a 3600 rpm motor is preferred over an 1800 rpm motor and an 1800 rpm motor is preferred over a 1200 rpm motor.
Motor Speed Part Two
The old way of controlling motor speed was to build the motor with a two speed capability. Now, the cheaper and preferred way of controlling motor speed is to use a variable frequency drive. However, it is important to select a motor is VFD ready.
Service Factor
Motors are designed with a service factor that will allow the motor to operate above its rated horsepower. For example, a 5 hp motor with a 15% service factor can operate up to a load of 5.75 hp. It may be tempting to think about saving money and selecting a motor that will continuously operate in the service factor versus going to the next motor size up. We don’t recommend it because it will be costly in the long run for two reasons. First, a motor that is operating in the service factor has gone beyond its peak efficiency point and is now consuming more electricity that it converts to heat instead of to horsepower. Second, that excessive heat will shorten the motor life and cause the motor to prematurely fail. Replacing the motor in a fan located on the roof or high on a wall is exponentially more costly than investing a larger motor.
Conclusion
We have applied these fan motor selection tips to industrial ventilation application where the fans are still operating after 20 years. When you have an expectation that your next investment in ventilation equipment be the will last one you will ever need, choose Eldridge Fan.