How to Avoid Overloading the Motor

When servicing hvacr equipment, technicians are sometimes required to increase the airflow of a system. This may be necessary in order to correct some type of system problem or to achieve the designed airflow upon start-up.

An easy method for increasing airflow on a centrifugal blower application is to increase the rpm of the drive motor. There is a direct relationship between the rpm of the motor and the cfm of airflow delivered by the fan system. If you increase the fan’s rpm by 10%, you increase the cfm delivered by 10%.

However, there is a potential problem with this “easy” solution. There is also a relationship between the cfm of airflow produced and the required brake horsepower (bhp) of the drive motor.

As the cfm is increased, the required bhp will increase by the cube of the cfm increase. For example, if the cfm is increased 50%, the required bhp will have increased by approximately 700%. This increase can easily overload a motor and cause it to fail.

Calculating BHP

Any time a technician increases the rpm of a motor in order to increase cfm capacity of the fan system, that tech must make sure the motor can handle the increase. A technician can calculate the projected required brake horsepower by using this formula:

Bhp new = (Cfm new/Cfm old) 3 x Bhp old

For example, assume the original airflow equals 1,000 cfm and it needs to be increased to 1,400 cfm. If the original calculated horsepower requirement of the motor is 0.5 hp (the horsepower requirements may need to be calculated separately since the nameplate hp does not always equal the required horsepower), the new horsepower requirement is:

Bhp new = (1,400/1,000) 3 x 0.5

Bhp new = 1.372 hp

If the original motor is not rated at 1.4 hp or higher, the motor will be overloaded.

An easy way for a technician to ensure that a motor is not overloaded is to measure the amp draw of the motor. As long the motor does not go 10% beyond its rated amp draw, it will work satisfactorily and will not become overloaded.

A motor should also be checked to see that it is not underloaded. The current draw should not fall below 25% of its rated amp draw. If the existing motor cannot be adjusted to provide the necessary cfm requirements within the operating limits of the motor, the motor should be replaced with one that is suitable for the required application and cfm requirements.

Any time a technician replaces a fan motor, he should also check the amp draw of the motor before leaving the job. This will ensure that the motor has been properly sized for the application and will not be underloaded or overloaded.

The motor should be tested under a full-load condition. This may require the technician to close all the access doors to the unit and measure the amp draw. Sometimes this is hard to do, but it will give a true measure of the amp draw of the motor.

If possible, the motor should be rechecked after approximately 2 hrs of operation, as most motor problems develop within this time frame.

Marchese is owner of Arctic-Air Refrigeration, Pittsburgh, PA.

Publication date: 07/02/2001

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