PSC Motor vs Constant Torque Motor vs Variable Speed Motor – What’s the Difference Between?

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PSC Motor vs Constant Torque Motor vs Variable Speed Motor – What’s the Difference Between?

PSC Motor vs Constant Torque Motor vs Variable Speed Motor – What’s the Difference Between?

The HVAC industry has come a long way since its first system was put out on the market.  A big part of that evolution has been the technology used to push the air into our homes.  Today we’re talking about three types of motors: PSC Motor vs Constant Torque Motor vs Variable Speed Motor. We’re going to talk about their operation and their uses in today’s residential HVAC systems.

INTRO – PSC Motor vs Constant Torque Motor vs Variable Speed Motor

We have standard PSC motors that are found in the majority of homes today. I’d say 80 percent of the systems we work on today have PSC motors. They’re the ones that have capacitors strapped to them to help start the motor and help it run efficiently. I’ll explain more about them down below.

Next is the constant torque motor.  One we use often is the Genteq X13 motor.  That motor is a high-efficiency motor that helped manufacturers meet the 13 SEER mandate implemented by the federal government in 2006, which is where the name X13 came from.

Manufacturers of these motors usually refer to them as a standard ECM motor or a constant torque motor.  To be clear, X13 is the Genteq brand name.  Other brands offer similar motors.  However, for the purpose of this video, the term constant torque motor will be used to describe all such motors.

A constant torque motor is not a variable speed motor. But both of them are ECM or electronically commutated motors.

Variable speed motors in HVAC systems are the premium option in today’s systems.  They allow systems to run from a very low speed up to 100% of their capacity.  Being able to dial in the desired speed precisely makes these motors the best choice for all types of advanced systems.

And just as a side note for folks in Sacramento County who might be looking for rebates available through our local utility company; SMUD has verbiage in their new rebate structure (and this is in 2021, so double check these rebates) that the new rebates revolve around furnaces and air handlers that have a variable speed motor.  In this case, ECM variable and ECM constant torque motors are the same to SMUD.  But in the real world, when we are talking to each other about these two different motors, it’s constant torque and variable speed motors.

So let’s expand a little now on the differences between the three motors: PSC Motor vs Constant Torque Motor vs Variable Speed Motor.

PSC Motors –

Ever heard of Nikola Tesla? I’m sure you have. He’s the inventor of the first practical design of these split-phase induction motors. He made them easy to use, low-cost, and compact in design, so well the industrial market started using them more and more. They got to be so useful, home appliances started using them in a variety of ways – like heating and cooling.

While simple to build and low in cost, they are being drowned out by today’s modern, higher-efficiency motors.

Three or four wires high voltage wires start and control the speed of the motor. They’re pretty simple to switch out. There’s usually a HIGH, MED-HIGH, MED-LOW, and LOW tap attached to these blower motors. When that winding receives 120 volts from the furnace’s control board, it starts up at that speed. But 120 volts isn’t the only thing that is needed to start that PSC motor. It needs a capacitor to provide the energy to get the motor up and running.

The capacitor winding stays in the circuit the entire time the motor runs, which is where we get the P in PSC for “permanent.”

The S in PSC stands for “split” because it’s a motor that uses single-phase AC which has been “split” by the utility into two equal proportions, just in opposite polarities.

And then C is for “capacitor,” which, as we already know, is the storage bucket of energy the motor consumes to start and run efficiently.

PSC motors are getting phased out because they build up heat, which decreases the motor’s efficiency. This earned them a rating of 60% efficiency.  But hey, that’s been the standard in residential heating and cooling for a long time

If there are restrictions in the airflow, like undersized ductwork, dirty filters, or dirty evaporator coils, PSC motors start to lose performance. They deliver less air because they tend to bog down, drawing higher amps. The system’s static pressure (or blood pressure) builds up and fights back against the PSC motor wearing it and the capacitor down through overheating.

PSC motors aren’t the quietest motors either. In fact, between the three motors we’re discussing today, the PSC motor is the noisiest. Having said that, PSC motors are the least expensive to buy and repair. The next two types of motors are much pricier to repair or replace.

ECM Constant Torque Motors (X-13 Motors)

Probably the most familiar name in ECM constant torque motors is the X-13 motor by Genteq. Genteq acquired General Electric’s motors and capacitors division in 2004, and in 2009 renamed themselves Genteq. I bring this up because the first high-efficiency residential HVAC ECM motors were made by GE back in the 1980s.

Even though they’re like upgraded PSC motors, they are a little more complicated in design because they use an integrated control board to send the signal for operation and at what speed (or torque setting) to run at. Because of this, they’re more expensive. Not quite as expensive as variable-speed motors, but significantly higher than PSC motors.

X-13 ECM or constant torque motors were designed in 2006 to achieve higher SEER ratings for the air conditioners being put into homes. The government sets the standard, so the industry has to keep up.  The SEER rating they were trying to achieve at that time was 13, thus the X-13 motor.

So, what does “constant torque motor” mean?

As a system gets dirty or ductwork is too small, maybe the filter hasn’t been changed in a year, the system’s static pressure (or blood pressure) will increase because it’s becoming restricted. So just like the PSC motor, it will start to decrease the amount of air it can push out, but not as drastic as a PSC motor. As much as 20% less impact on a semi-restricted system.

When the airflow decreases on an air conditioning mode, the system can freeze up with ice and completely restrict airflow. While in heating mode, low airflow means the system can overheat and shut down, wait for itself to cool off, start back up, overheat, shut back down, and over and over until it completely locks itself out. So, with a constant torque motor, the chance of the system shutting down because of airflow restriction is less because the drop in airflow produced isn’t as drastic as a PSC motor.

If you decrease the amount of air that a PSC motor moves because of ductwork, a dirty system, or a dirty filter, it will use less power and not deliver the same amount of air as if it were clean. It’s not a “smart” motor.

In the same situation, a constant torque motor will increase its power output to the shaft, which spins the wheel harder, to try and maintain more airflow through the duct system, preventing those issues we just discussed. And, there are safeties programmed into the motor to prevent it from destroying itself if airflow gets really bad.

The power block to the constant torque motor receives a high voltage and a low voltage signal. The upper portion of the block has connections for the high, and the lower terminals are for the low voltage taps. So instead of having high voltage determine the speed like the PSC motor, low voltage taps are plugged into either the 1,2,3,4, or 5 terminals which have been pre-programmed to a specific torque setting by the manufacturer.

And only some of those taps may have been pre-programmed. So maybe only the 1,2, and 3 terminals work on the low voltage side. The maker of the air handler decides it.

A constant torque motor is a brushless motor, which reduces heat within the rotor and stator area. They’re 80% efficient compared to PSC motors which are rated at 60% efficient. They also don’t need a capacitor to start and run.

They have integrated electronics built into the motor, which means if either the motor windings or the circuitry goes bad, the whole motor has to be replaced.  When I talk about variable speed motors, I’ll explain why I mentioned that.

Variable Speed Motors –

GE, which I said before was acquired by Genteq (on the motors side), introduced the first variable-speed motors for use in HVAC systems in 1987. So, it’s not a new technology.

So, you know how when you get in your car on a 100-degree day and set the AC with that AUTO button to reach 70 degrees? When the car first starts, the AC fan turns on about as high as it can to match the cooling effect the system is trying to produce. Then, as the system starts reaching that 70-degree set point, you may notice the fan actually starts to ramp down little by little because the demand isn’t as high.  Once you reach 70 degrees in the car, the airflow doesn’t really stop, does it?  No. The air stays on lightly while keeping you at 70 degrees.

Do you notice how that motor ramps up and down instead of the constant torque and PSC motors that have set speeds for specific demands from the thermostat? Variable speed motors can fluctuate between very low speeds and high speeds with their brushless design, which helps maintain an impressive 80% to 90% efficiency rating.

When airflow in variable speed systems becomes restricted, airflow volume stays the same (constant airflow) because they’ve been programmed to know exactly how much torque and airflow the system needs to function properly.

So, variable-speed motors are constant airflow motors. Constant torque motors can apply a little more power to the blower wheel in an attempt to maintain the airflow needed by the system.

The motor is made of two pieces – the mechanical part (the motor) and the control module. The control module has microprocessors and electronic controls, which increase or decrease the speed of the motor. One part or the other can be replaced if needed, so you don’t always have to replace the entire motor and module together.

That saves quite a bit of money when it comes time for repair. But – remember that these need to be programmed by the manufacturer, so you can’t just buy one on e-bay and think it will work for your specific application. Variable-speed systems are not cheap to fix. So, it’s like, yes, they save money by being more efficient, but they cost an arm and a leg to repair, so who’s really coming out ahead?

If the shaft is stuck and won’t spin, or the windings are out of sort, you can just replace the mechanical portion of it. If the shaft spins freely and ohms out correctly, and the motor is getting the proper voltage and communication, then you could have a bad module. It’s kind of hard to troubleshoot a module without special tools that can give the proper signal to the motor, in which case, if it didn’t respond properly, you’d know the module was bad.

The variable-speed motor is powered by high voltage plugged into a power block, just like the ECM constant torque motor. It also receives another signal to the power block very similar to the constant torque motor. Instead of individual taps with 24 volts applied to speed terminals, variable speed motors have a multi-pin communication connector. Previous versions of variable speed motors have used AC signals, DC signals, and in today’s motors, serial communication – another reason why technicians need those special devices to troubleshoot them.

Conclusion –

PSC motors are simple, fairly inexpensive, moderately efficient motors with a reputation for being loud and not very dynamic. They also come hitched with the always unpredictable capacitor, which can go out due to overheating.

ECM constant torque and ECM variable-speed motors are pretty complex in design, high-efficiency motors, known for being much quieter on start-up and shutdown. Their ability to control airflow better than PSC motors makes them smart motors because they can ramp up power as needed (the variable speed more so than the constant torque motor.)  All of this comes with a higher price tag, though, when it comes to repairs.

The practical side of me asks, “So the ECM motors are more efficient, saving money on the operation (of a low amperage blower motor) by 20%. But when it comes to repairs, I’m going to pay thousands for a technician to install a new motor?”

And this is the world we live in.

Thanks for reading the blog this week, and we’ll see you on the next one!

 

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