HVAC Fan Motor Speeds: The Truth May Not What You Think
We’ve all seen it: a multi-speed PSC (Permanent Split Capacitor) blower motor with four wires promising “high,” “medium-high,” “medium-low,” and “low” speeds. You wire it up, and you assume each speed is delivering a precise amount of airflow, right?
Not exactly.
Here’s the thing many of us in the trade don't stop to consider: those different speeds are a lot more like a "best guess" than an exact measurement. And the “guess” is based on a whole bunch of assumptions about the system that are rarely true in the real world.
Let's break down the truth behind the PSC motor.
The "Secret" to Multi-Speed Motors
First, let's get one thing straight. You don't have four separate fan windings inside that motor. That would be expensive and inefficient.
Instead, the motor has a single main winding with multiple taps—extra wires that are "tapped" into the winding at different points. When you wire your power lead to a different tap, you're changing the electrical resistance of the circuit. This changes the motor's speed, but not in a precise way.
Think of it like an old-school dimmer switch on a light bulb. You're not changing the bulb; you're just restricting the flow of electricity to make it glow differently.
The "Best Guess" is Based on Assumptions
So, if those taps aren't a guarantee of speed, what are they for? They're designed to achieve a specific RPM (revolutions per minute) on a test bench, under perfect conditions. This perfect scenario, however, is full of assumptions that your real-world install will almost never match.
The speeds are "calibrated" for things like:
A perfectly clean filter: How often do you find a brand-new, perfectly clean filter in a service call? Never. A dirty filter chokes off airflow and changes the dynamics of the system.
Ideal ductwork: The motor assumes the ductwork has a specific static pressure—the resistance air encounters as it moves through the system. But every installation is different. A new home with straight, clean duct runs will have a different static pressure than an old house with long, kinked, or undersized ducts.
Perfectly clear coils: A dirty evaporator or condenser coil adds resistance, forcing the fan to work harder.
When you have a dirty filter or a restricted duct, the static pressure increases. And this is where the PSC motor's "best guess" falls apart.
The Fan Chart and the Truth About Airflow (CFM)
Here’s the biggest takeaway: a PSC motor does not maintain a constant airflow (CFM) when the static pressure changes. A fan performance curve, which is provided by manufacturers, plots the relationship between airflow and static pressure for a specific fan at a fixed RPM. The chart shows that as static pressure—or resistance to airflow—increases, the actual airflow (CFM) from the fan decreases.
This is where the fan laws come into play. A fundamental fan law states that airflow is directly proportional to fan speed. A 10% increase in RPM results in a 10% increase in CFM, but static pressure changes with the square of the fan speed. This means a small change in speed or airflow results in a significant change in pressure. The fan chart visually represents this inverse relationship.
When a PSC motor is faced with an increase in static pressure, like from a dirty air filter, its amp draw and airflow will both decrease. While it may seem counterintuitive that the motor is "working less" (lower amps), it's actually a sign that the system is failing to move the proper amount of air.
In short, those speeds are a "dumb" setting. They don't have any feedback from the system. They don't know the filter is clogged, or the duct is pinched. They just spin at the pre-set rate, assuming everything else is perfect.
The Bottom Line for Technicians
When you're working on a system with a PSC motor, remember this: the speed setting is not a guarantee of airflow. The actual CFM is a direct result of how much resistance is in the duct system.
This means that to properly diagnose and service a system, you can’t just trust the fan speed. You need to verify the actual airflow. Use your static pressure gauges and airflow hoods to see what's really happening. It’s the only way to know if your fan is delivering what the system and the customer truly need.
Works cited
Understanding PSC Motor: The Essential Guide to AC and Induction Motors - Dadao, https://dadaoenergy.com/blog/psc-motor/ 3. Speed Control For Fractional Horsepower Motors | Clarage - FAN ENGINEERING, https://www.clarage.com/wp-content/uploads/sites/6/2021/12/Speed-Control-for-Fractional-Horsepower-Motors-FE-1000.pdf 4. HVAC Blower Motor Maintenance: Boost Efficiency and Safety Tips, https://larrycookhvac.com/boost-hvac-blower-motor-with-maintenance/ 5. What is static pressure and how does static pressure affect the ..., https://www.aaon.com/resources/what-is-static-pressure-and-how-does-static-pressure-affect-the-performance-of-an-hvac-system 6. Fan Basics: Air Flow, Static Pressure, and Impedance - Engineering Notes - Oriental Motor, https://blog.orientalmotor.com/fan-basics-air-flow-static-pressure-impedance 7. An Engineer's Guide to Understanding Fan Curves - Q-PAC, https://www.q-pac.com/resources/engineers-guide-to-understanding-fan-curves 8. The 3 Fan Laws and Fan Curve Charts - HVAC School, http://www.hvacrschool.com/the-3-fan-laws-and-fan-curve-charts/ 9. The 3 Fan Laws and Fan Curve Charts - HVAC Know It All, https://hvacknowitall.com/blog/the-3-fan-laws-and-fan-curve-charts