Understanding HVAC Motors – PSC vs. ECM

New technology- ECM – electrically commutated motors, are allowing breakthrough performance improvements for your personal comfort and for saving money on your heating and cooling bills. It is important to understand the mechanical characteristics of the motors used in HVAC to understand these fundamental differences in performance and and all the opportunities to impact your comfort for the better.

We will first talk about mechanical features here and then tie these features  into a conversation about actually saving you money  while improving your levels of comfort, here.

There are two basic motor choices.

1. The traditional PSC – permanent split capacitor motor, a purely analog motor that runs at a particular speed due to the windings in the motor control circuit, and
2. The new fangled (stu humor, they have been around for years) electronically commutated motor which can change speeds by manipulating currents with a controlling mechanism.

Let’s briefly discuss the inner workings here and then on this basis we can figure out how each impacts your comfort and quality of air differently.

PSC schematics - single and multi-speed configurations

This is not really an image of the PCS motor, but rather a schematic of the electrical circuitry that makes it a PCS motor. Also included is a diagram for a multi-speed PSC motor. Notice the taps on the inductor. We talk about multi-speed in our larger conversation on HVAC motors and comfort and saving money

A PSC motor is as good at it gets with basic analog motors for the HVAC industry.

A permanent split capacitor motor has a non polarized capacitor connected in series with the start winding. The capacitor creates a lagging voltage that acts as an extra hand to help start a load, in our case a blower motor in our air handler. As the motor picks up speed the capacitor and induction of the circuitry changes the current pattern and delivers steady current at a rated speed.

permanent split capacitor motor

OK, here is a image of a PSC motor. Notice the capacitor in series on the side. Thank you GE for this nice image.

These components, selected by the manufacturer, give the best efficiency and power factor at rated load that analog systems can provide.

The PSC motor offers smooth acceleration of the load, when compared to other analog electrical motor configurations, help extend relay and switch lives, because your heating and air conditioning systems are regularly called to action with repeated load starts from a standstill. Starting current might be only 200% of rated current. For the price range, these are the ultimate solution for analog electrical motors. Do you notice how I keep repeating analog?

Now we need to talk abou ECM motors.

ECM motors do not have brushes as do analog motors. Brushes track along the armature, the spinning shaft portion of the analog motor, and pick up the progressive windings and driving voltages and these new driving voltages generate the currents that produce the magnetic field force that actually adds the torque to the rotor to make it turn. These currents work just like an electromagnet in principle. Brushes and electric motors were a huge new technology in the last half of the 1800’s, being much more compact and flexible than stream driven belts, but brushes do have issues.

In brushless DC motors, electronics track the position of the rotor and fires off semiconductor switches to drive precise currents that drive the rotation. ECM motors have permanent magnets attached to the rotor and the driving currents push off against the permanent magnets to create torque and speed.

An ECM - electrically commutated motor.

Here is an ECM motor disassembled for visibility. The key difference is a dozen windings through which an electronic motor control can drive individual currents to create torque to act on the permanent magnets on the rotor.  No brushes, just magnetic fields to create torque.

There is no need for actual currents to cross over with brushes. Driving magnetic fields cross over the small gap and eliminate the need for brushes  having to provide current through the winding of the moving armature. The digital control allows precision and just the right amount of current to do what is needed at that instant. The precise currents create precise magnetic force that precisely push off and create the precise torque pattern to turn the rotor at the precise speed dictated by the controller.

Currents are provided in microseconds at the precise level needed. The starting current is precisely metered to provide the exact starting torque required for critical ramping up. Switches receive no jolt of power at starting. The controlling mechanism provides the rotation speed and torque needed to provide the air volume called for, we are again using the example of the air handler motor…

These mechanical differences in the motors in heating and cooling systems are critical to any conversation about opportunities to add to your comfort and saving money on heating and cooling bills. That conversation starts here.

Otherwise, do you have a question, or an added thought. Feel empowered to add them below.