[Guy]

Hello. It's been a while since I've participated in a discussion. I do various things, electronics related, and enjoy learning the theory of operation of all kinds of stuff. One thing that's come up here and there is variable speed AC motors, which has only become practical and low cost enough for things like variable speed woodworking tools, appliance (washing machine) motors and such. Even larger motors like those on subway cars are now using variable frequency drives, allowing for brushless induction motors to easily vary their speed.

In watching videos on the subject, I came across ECM motors, which I remember being discussed here a while back. From the videos I've seen, these are a type of variable frequency drive, but where the control electronics are within the motor itself. The videos aren't the most technical, but from what I could see, the motors have several low voltage (24v) control input terminals, each one corresponding to a speed that the motor is meant to maintain. A furnace will be designed around a certain blower speed (or rate of airflow). If the load increases such as due to a dirty filter for example, the motor will sense the increased load due to the RPM getting slower, and increase the power to the windings to attempt to maintain the speed.

Then there are actual variable speed ECM motors (this gets a bit less clear) where the furnace controls themselves will determine what speed is needed based on various factors and, a varying control voltage is sent to the motor (like a speed setpoint) to request the optimal airflow due to the current conditions.

Another thing that isn't quite clear is that ECM motors are said to be quieter than regular induction motors (PSC). It seems to me that any type of brushless induction motor would have similar noise levels for a given RPM.

Is this basically right? Correct me if I've misunderstood things.

 

[JayMan7]

I can only speculate because it doesn’t seem like manufacturers give out a lot of information on these motors.

It may be that they are mini VFDs of sorts. And the ones that are continuously variable and in higher end furnaces because of the additional electronics needed to calculate and then signal the proper speed.

Apples to apples I don’t see how they’re quieter. That claim probably applies to their ability to ramp down and be quieter when needed, such as in circulation mode.

 

[RCinFLA]

ECM motors are three phase motors. Drive is from AC to HV DC supply feeding a three-phase inverter located in end of motor. Speed is varied by frequency and voltage of three phase inverter drive to motor.

The control on speed has several protocols, some proprietary to particular manufacturers. Simplest is three to five discrete steps via just wire high-low control voltage setting from 24vac control output. Some have serial two wire digital control where the serial data tells the motor what speed to run which can run at just about any variable rpm speed. They can adjust their rpm rate to run a more constant CFM or constant motor torque based on their measurement of motor load based on its current draw at given rpm speed.

Not so great aspect of most ECM motor is their power factor is poor, in the 0.5 to 0.65 range due to the simple AC to HV DC rectifier-capacitor HV DC power supply to run the three-phase inverter.

ECM motors get more stressed when pushing against high static pressure resistance. They will attempt to keep up air flow against the air flow blockage causing them to overheat.

Their price is a pure rip off. It doesn't cost that much more to make than a standard PSC motor. Many have their circuit board potted in epoxy to curtail any simple component level repair on printed circuit board. A common failure mode is a large disc thermistor to limit startup surge current in the AC to HV DC power supply that can be relatively easily replaced. The thermistor is a couple of bucks.

 

[pecmsg]

ECM motors are three phase motors. Drive is from AC to HV DC supply feeding a three-phase inverter located in end of motor. Speed is varied by frequency and voltage of three phase inverter drive to motor.

The control on speed has several protocols, some proprietary to particular manufacturers. Simplest is three to five discrete steps via just wire high-low control voltage setting from 24vac control output. Some have serial two wire digital control where the serial data tells the motor what speed to run which can run at just about any variable rpm speed. They can adjust their rpm rate to run a more constant CFM or constant motor torque based on their measurement of motor load based on its current draw at given rpm speed.

Not so great aspect of most ECM motor is their power factor is poor, in the 0.5 to 0.65 range due to the simple AC to HV DC rectifier-capacitor HV DC power supply to run the three-phase inverter.

ECM motors get more stressed when pushing against high static pressure resistance. They will attempt to keep up air flow against the air flow blockage causing them to overheat.

Their price is a pure rip off. It doesn't cost that much more to make than a standard PSC motor. Many have their circuit board potted in epoxy to curtail any simple component level repair on printed circuit board. A common failure mode is a large disc thermistor to limit startup surge current in the AC to HV DC power supply that can be relatively easily replaced. The thermistor is a couple of bucks.

Just so wrong on almost all accounts!

 

[R600a]

Just so wrong on almost all accounts!

In what way I don't see anything wrong in his post.

 

[kdean1]

ECM motors are three phase motors.

ECM = Electrically Commutated Motor. "ECM motor" is redundant.

 

[pecmsg]

In what way I don't see anything wrong in his post.

Their price is a pure rip off. It doesn't cost that much more to make than a standard PSC motor. Many have their circuit board potted in epoxy to curtail any simple component level repair on printed circuit board. A common failure mode is a large disc thermistor to limit startup surge current in the AC to HV DC power supply that can be relatively easily replaced. The thermistor is a couple of bucks.

 

[R600a]

Their price is a pure rip off. It doesn't cost that much more to make than a standard PSC motor. Many have their circuit board potted in epoxy to curtail any simple component level repair on printed circuit board. A common failure mode is a large disc thermistor to limit startup surge current in the AC to HV DC power supply that can be relatively easily replaced. The thermistor is a couple of bucks.

And all of that is basically true so I don't get your point.

 

[pecmsg]

The circuit boards are sealed to prevent condensation.
They do cost more to manufacture than PSC’s.

 

[R600a]

The circuit boards are sealed to prevent condensation.
They do cost more to manufacture than PSC’s.

Well I've seen 25-year-old ECM motors that were not sealed and ran just fine until the system was replaced for other issues so I can just suspect that the common opinion that they are doing it more for planned obsolescence then increased longevity is a very likely possibility.
The statement that they don't cost that much more to manufacture is really a statement of opinion.
Personally I agree with most of what he said.
At the same time everything's expensive these days so I don't really care I like ECM motors because they have some handy features I like to use. Thankfully aftermarket ECM motors aren't quite as crazy expensive and I'm able to install them in almost every OEM application for a lot less money once they are out of warranty.

 

[JayMan7]

ECM motors are three phase motors. Drive is from AC to HV DC supply feeding a three-phase inverter located in end of motor. Speed is varied by frequency and voltage of three phase inverter drive to motor.

The control on speed has several protocols, some proprietary to particular manufacturers. Simplest is three to five discrete steps via just wire high-low control voltage setting from 24vac control output. Some have serial two wire digital control where the serial data tells the motor what speed to run which can run at just about any variable rpm speed. They can adjust their rpm rate to run a more constant CFM or constant motor torque based on their measurement of motor load based on its current draw at given rpm speed.

Not so great aspect of most ECM motor is their power factor is poor, in the 0.5 to 0.65 range due to the simple AC to HV DC rectifier-capacitor HV DC power supply to run the three-phase inverter.

ECM motors get more stressed when pushing against high static pressure resistance. They will attempt to keep up air flow against the air flow blockage causing them to overheat.

Their price is a pure rip off. It doesn't cost that much more to make than a standard PSC motor. Many have their circuit board potted in epoxy to curtail any simple component level repair on printed circuit board. A common failure mode is a large disc thermistor to limit startup surge current in the AC to HV DC power supply that can be relatively easily replaced. The thermistor is a couple of bucks.

Potted electronics are such a bummer!

We quoted a motor today…ecm condenser, descent size motor (80lbs), trane unit. Our cost was thousands and a 52 week lead time. ****ing joke. Trane commercial seems like they’re going downhill fast…a lot of what I’ve been seeing looks like typical decay from corporate greed.

 

[poppa]

Well I've seen 25-year-old ECM motors that were not sealed and ran just fine until the system was replaced for other issues so I can just suspect that the common opinion that they are doing it more for planned obsolescence then increased longevity is a very likely possibility.
The statement that they don't cost that much more to manufacture is really a statement of opinion.
Personally I agree with most of what he said.
At the same time everything's expensive these days so I don't really care I like ECM motors because they have some handy features I like to use. Thankfully aftermarket ECM motors aren't quite as crazy expensive and I'm able to install them in almost every OEM application for a lot less money once they are out of warranty.

i concur.

 

[JayMan7]

The circuit boards are sealed to prevent condensation.
They do cost more to manufacture than PSC’s.

I thought they were sealed to prevent vibrations cracking solder joints or maybe it’s for all three reasons!

 

[R600a]

I thought they were sealed to prevent vibrations cracking solder joints or maybe it’s for all three reasons!

Probably reduces the warranty issues in the short term and reduces the number that get repaired in the long term. So it's a win-win for them.

 

[TechmanTery]

I thought ECM's use B-EMF somewhere, somehow.
RSES has 4 books and/or 4 SAM's on the subject.

 

[Guy]

Interesting responses. Thanks!

Yes, the points that come up in the various videos I've seen on the subject is that ECM motors are very expensive to replace, fail more easily, and (from an electronics standpoint) nothing seems to justify their high cost. I've even seen people explaining how to replace a bad one with a regular PSC motor, mostly due to the replacement cost. The HVAC industry seems to be one of the few remaining ones clinging to proprietary designs and avoiding standardizing on parts, to seemingly make money on replacement parts, or for marketing reasons to make their system sound better than their competitor. I'm grateful that my own furnace has a PSC motor!

As I thought, ECM motors seem to be self contained 3 phase motors with built in controllers. As for the variable speed ones, I imagine that the speed signal is adjusted by the furnace controller, using parameters such as pressure differential (to compensate for dirty filters for example) in demanding the speed that it deems optimal, for efficiency and/or comfort. This is approaching the design of a PID (proportional, integral, derivative) control system, that attempts to achieve an ideal setting through progressive control of various parameters instead of just being on/off like a good ole thermostat. That in itself is a laudable goal, if the cost (of both acquisition and repair) can be kept reasonable. I just wonder how a HVAC tech repairing a furnace with a bad ECM motor can keep a straight face when quoting the price of a replacement motor.

 

[JayMan7]

Funny you mention that because my last video was on converting an ecm fan coil to a standard blower motor. And of course the ecm was stupidly expensive and ridiculous lead time lol!

 

[Guy]

One thing that's been popular in the woodworking hobby world is to convert wood turning lathes to variable speed. Drill presses are also the object of such conversions. One way had been to use VFDs which have become quite affordable (often a couple of hundred dollars), and have them power 3 phase motors, which can often be had second hand. Another even cheaper method has been to get treadmill motors (from treadmills being given away because of a broken part), which are often DC powered and a couple of horsepower, and run them with DC motor controllers that can be had for as little as $35 on ebay. I did my own lathe this way, with very satisfactory results.

In the case of HVAC blower motors, can you find the approximate required speed by installing a variable speed motor (even a multi fixed speed PSC motor) and then using parameters like the pressure differential across the return and supply, and/or calculating the air flow rate? From what I've seen in reading on these things, there are some pretty standard rules of thumb on things like airflow (eg: 400 CFM per ton) that should get you close.

 

[RCinFLA]

Most of the U.S. ECM motors are quite archaic in their design. Manufacturers seem to put more effort into making their ECM motor proprietary in some operational way to keep prices high. Air conditioner manufacturers are partners in this proprietary protection.

A counter intuitive thing about centrifugal squirrel cage blower fans is as the static pressure rises the motor load drops. Along with this is a large drop in cfm air movement. Centrifugal squirrel cage blowers are like a woodchipper. The more tree limbs you throw into the chipper the more load on the motor. For squirrel cage fan, the more air entering it the more power it consumes (and greater cfm produces).

ECM motors brag about being able to take up to 1" wc of static pressure. They do this by over-rpm'ing the motor to keep the cfm moving. An ECM motor efficiency will drop significantly with high static pressure and draw more current stressing the motor. This is counter to PSC motor which just drops in cfm and current draw due to high static pressure. (pushing to problem to a possible frozen evaporator)

When there is high static pressure, an ECM motor can go into oscillations on rpm control. In the industry it is called 'huffing' where the noise of the blower changes pitch louder and softer as the ECM motor oscillates in rpm.

ECM motors usually have a permanent magnet rotor. This makes them a good alternator, producing three phase power if shaft is driven. When there is high static pressure, when motor shuts off, it creates a high back EMF voltage while the motor is spinning down. High static pressure also means less braking load on motor so it spins down longer. This back EMF voltage can rise quite high to point of damaging the driver module electronics when shut off while motor rpm is being run at very high rpm's to compensate for high static pressure. If your AC unit has a step down blower speed control setting option as it turns off, it is good to use it. It may save your expensive ECM motor if you have high static pressure.

When you manually turn the shaft on an ECM motor you will feel some bumps in resistance to turning. This is due to some residual magnetism in the stator core against the magnetic force of the permanent magnet rotor.

When ECM electronic driver shorts out it places a short circuit across the three phase motor so when you try to manually turn the shaft you will have a strong resistance to the back EMF produced by the motor acting as an alternator. Many ECM motors are mis-diagnosed as having a bad motor bearing because of this strong rotation resistance when what they have is a bad ECM electronic module.

 

[Guy]

ECM motors usually have a permanent magnet rotor. This makes them a good alternator, producing three phase power if shaft is driven. When there is high static pressure, when motor shuts off, it creates a high back EMF voltage while the motor is spinning down. High static pressure also means less braking load on motor so it spins down longer. This back EMF voltage can rise quite high to point of damaging the driver module electronics when shut off while motor rpm is being run at very high rpm's to compensate for high static pressure. If your AC unit has a step down blower speed control setting option as it turns off, it is good to use it. It may save your expensive ECM motor if you have high static pressure.

When you manually turn the shaft on an ECM motor you will feel some bumps in resistance to turning. This is due to some residual magnetism in the stator core against the magnetic force of the permanent magnet rotor.

In that regard, an ECM motor is much like a stepper motor, but on a larger scale.