Nate Electrical Practice Exam

How does changes in CFM of a blower affect a motor’s horsepower?

1. Hp changes directly proportional to changes in CFM

2. Hp changes inversely proportional to changes in CFM

3. Hp changes by the square of the change in CFM

4. Hp changes by the cube of the change in CFM

The correct answer is: Hp changes by the cube of the change in CFM

When analyzing how changes in Cubic Feet per Minute (CFM) of a blower affect a motor's horsepower, it's essential to understand the relationship between airflow, pressure, and power. The correct assertion is that horsepower generally changes by the cube of the change in CFM. This is based on the principles of fluid dynamics and motor performance characteristics. The relationship indicates that as the airflow (CFM) increases, the horsepower required increases exponentially. This is because when the operating conditions of a blower or fan are altered to increase airflow, the motor has to do significantly more work, which translates to an increase in horsepower needed, and this requirement grows disproportionately due to the cube relationship. For example, if the CFM is doubled, the horsepower will not just double; it will increase by a factor of eight (2^3), reflecting the cubic relationship. This indicates that substantial power is required to maintain higher airflow rates in a blower system, hence the significant increase in horsepower necessary to achieve the desired CFM output. Understanding this relationship helps electricians and technicians accurately size motors for specific applications and manage the operational efficiency of HVAC and ventilation systems effectively.