Less horsepower AC motor for less RPM - Page 1
Forum section: electronics / beginners

author: jrmymllr
author_rank: Regular Contributor
timestamp: June 22, 2020, 12:46:47 pm
content: I'm not an electronics beginner, but I know little about AC motors. Here's the problem. A family member has a residential A/C installation where the air handler blower is way too powerful. It's been like this for so long there's no point in calling the business that installed it. They don't exist anymore.The system short cycles and doesn't dehumidify very well, but also blow air out of the vents with hurricane force. It's always been like this. I'd like to investigate reducing the CFMs of the blower by changing the motor out for a lower horsepower unit, which as I understand, would increase slip and therefore, run at a lower speed. It seems nearly all these multi-speed blower motors run at 1050-1100 RPM, and the different speeds work by reducing power and increasing slip. The big question is, in general is there a limit of how much slip will cause motor overheating? I could also try changing the size of the fan itself, but that's plan B.Now I do realize that pushing too little air through this can cause the evaporator to ice up, and this is something I need to determine. But if the evaporator temperature is already high, reducing air flow should be ok.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: June 22, 2020, 01:54:25 pm
content: I would not reduce HP to accomplish this. You need to determine the speed of the motor you have now and find a replacement with the same frame size and voltage at lower RPM. I am assuming that your existing motor is a standard induction motor and not one of the newer 3-phase BLDC motors with integrated controller.If you reduce motor HP enough to slow the fan to a comfortable speed, the motor will be running at or beyond rated HP and will likely burn up. The existing motor is sized to match the blower.

author: flynwill
author_rank: Regular Contributor
timestamp: June 22, 2020, 02:04:38 pm
content: If the blower is belt driven like many are you might be able to change the size of the pulley on the blower and/or the motor to achieve a lower speed. That will be considerably cheaper than a different motor.You can also simulate the effect of decreased speed by partially blocking the inlet at the filter box. I suspect that if you reduce the airflow very much you will indeed start having icing problems. The only real fix for this problem is to replace the unit with one properly sized for the space or one of the modern units with a multi-speed compressor.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 22, 2020, 02:06:26 pm
content: Obviously the mechanical load drops by dropping RPM, as less air is blown through the fan. Hence reducing the power rating of the motor is completely sensible.Important point is that with fans, reducing RPM reduces torque requirement.So while it's possible to go for a smaller motor rating, don't go too far with motor replacement, though; the required power should be measured or modelled first.I wouldn't replace the motor, but try to slow it down instead.If it has a run capacitor, as they often do, I would play around changing the run cap to a smaller value, first. This would drop the current on the second phase (a capacitor run motor is internally a two-phase motor). No need to look for replacements! See reply #1 here for more information:https://electronics.stackexchange.com/questions/218304/varying-run-capacitor-for-speed-control-of-single-phase-motorAnother option is adding a small autotransformer to reduce a voltage bit.Thyristor control ("light dimmer" circuit) is another widely used technique, but causes audible buzz.All three methods mentioned increase slip and likely decrease efficiency after some point, but as the mechanical power also drops, efficiency drop likely isn't an issue.It's best to verify at least measuring the input power and motor temperature before and after modification.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: June 22, 2020, 02:20:16 pm
content: Quote from: flynwill on June 22, 2020, 02:04:38 pmI didn't consider changing belt sheeves. However, most residential blowers are direct drive squirrel cage fans.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 22, 2020, 02:35:20 pm
content: Many years ago, I had a similar problem.I attempted Triac phase control, and although it works, at lower speeds the motor buzzed.The very best, and it also saved energy, was to use an autotransformer.A couple of things, though:-You can't reduce the speed too much because the centrifugal switch can drop and re-connect the start winding again. As the start winding is rated for intermittent duty, the motor can burn out.-For the same reason, you have to start the motor at full line voltage and then after a few seconds running, operate it at reduced voltage.

author: jrmymllr
author_rank: Regular Contributor
timestamp: June 22, 2020, 07:19:08 pm
content: Quote from: flynwill on June 22, 2020, 02:04:38 pmI failed to mention this information, but it's a direct drive blower. There's no belt.And great idea blocking some of the inlet to test. I wonder, what's the effect on the motor of having some partial block permanently to fix this problem? Other than maybe more wind noise? Could that overload the motor, or does less airflow actually reduce motor load?

author: jrmymllr
author_rank: Regular Contributor
timestamp: June 22, 2020, 07:24:07 pm
content: Quote from: Siwastaja on June 22, 2020, 02:06:26 pmInteresting regarding the capacitor. That's doable. Adding a transformer might be tough as this unit is in the attic and is already a pain to access. There's also some question I have about reducing voltage supply and motor overheating. There seems to be some anecdotal evidence that running a motor on less voltage is ok, but general advice is to not do it. This air handler runs at 240VAC, which is unusual for residential HVAC in the US.

author: jrmymllr
author_rank: Regular Contributor
timestamp: June 22, 2020, 07:25:40 pm
content: Quote from: schmitt trigger on June 22, 2020, 02:35:20 pmI'm guessing this is a PSC motor with no centrifugal switch. I know some motors have this, but I've never heard any evidence on this unit, and two spare blower motors I have do not have them.

author: rstofer
author_rank: Super Contributor
timestamp: June 23, 2020, 01:16:42 am
content: Some control systems have the ability to vary the fan speed with jumpers on the PCB. I'm not sure how they do this because, unless the motor is of a 'shaded pole' design, you can't control RPM by controlling voltage. AC motors don't work that way.You could add a variable speed drive, they aren't terribly expensive.There is no point in reducing the HP rating because the motor uses as much power as it takes to move the air and that is determined by RPM and fan laws. If you back off on air volume by reducing RPM, you will need less HP and the motor will just not work as hard.If you are determined to make this thing work, look in to variable speed drives. Controlling slip works with shaded pole motors, not so well with induction motors. Shaded pole motors are used for room fans and things like that.Automation Direct has variable speed drives. I did a search for 120V 1 HP because I didn't know what else to do and here's the resulting page:https://www.automationdirect.com/adc/shopping/catalog/motor_controls/ac_variable_frequency_drives_(vfd)#Horsepower_Rating_ms=%221hp%22&start=0&Nominal_Input_Voltage_ms=%22120%20VAC%22

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: June 23, 2020, 01:36:52 am
content: Are you sure the blower motor is a standard induction motor and not BLDC with electronic control module. Just about anything installed in the last 15 years or so is BLDC with integrated electronic control. If it has a capacitor it’s induction.Also, ensure all of the supply vents are completely open and not blocked to prevent pressure build up.Been through all this myself.

author: wizard69
author_rank: Super Contributor
timestamp: June 23, 2020, 01:41:17 am
content: That is a huge bit of information.If the motor is single phase you don't want to slow it down by increasing slip, that should just drive power usage up. More importunity I would burn up the motor.From my perspective you have three options that might be viable.By a motor rated for a lower speed.Install a DC motorInstall a 3 Phase motor and drive it from a VFD. You still have to be careful about mating the rotors BASE speedQuote from: jrmymllr on June 22, 2020, 07:19:08 pm

author: bdunham7
author_rank: Super Contributor
timestamp: June 23, 2020, 01:54:06 am
content: Can you have a look at the motor and see how it is wired? Many older (10-20 years or so) air handlers and blowers have direct drive squirrel cages and the motors have a plethora of winding leads, typically only two are connected, one for heat and one for AC. You might have a choice of 5 or more speeds right there. The answer to your question depends on what sort of system you have. Go there, take it apart enough to see the motor, the circuit board, the model number and the wiring diagram if there is one (there often is, right on the inside of the first panel you take off). It would be truly exceptional if there were not a way to vary the speed--when these are installed they are supposed to be setup with a magnehelic pressure gauge to set the plenum pressure--and that requires speed changes.

author: rstofer
author_rank: Super Contributor
timestamp: June 23, 2020, 03:33:30 am
content: Quote from: bdunham7 on June 23, 2020, 01:54:06 amThis is the way mine works. They use a different speed for cooling than they do for heating.Pictures of everything would be a great help.

author: james_s
author_rank: Super Contributor
timestamp: June 23, 2020, 04:35:04 am
content: Quote from: schmitt trigger on June 22, 2020, 02:35:20 pmI've never seen a centrifugal switch in one of the motors used in the blower of furnaces/air handlers in the US, at least not the direct drive type that have been common for at least 40 years. They use a PSC motor which typically works well on a phase control motor speed controller. The furnace in my mom's house has just such a speed controller installed on it, works fine.

author: WattsThat
author_rank: Frequent Contributor
timestamp: June 23, 2020, 05:42:10 am
content: Several unrelated points/observations. Comments pertain to ac induction motors.Motor slip is a function of design (it’s rotor resistance) and while slip typically rises as motor power goes down, higher slip for a smaller motor, you don’t control slip or buy a higher slip motor. You buy speed and power ratings which indirectly specifies the torque available. Slip is a design constant and is proportional to shaft load. Unless you’ve got a special duty motor designed for high slip, like a Nema class D, slip range must be observed.With a centrifugal blower, the HP required will go down at the cube root of the speed.Attempting to reduce the speed of a single phase induction motor by reducing the voltage will typically result in burned out motors. Why? Because you’re actually overloading the motor by dragging the rotor speed below the rated slip which increases stator current dramatically. In order to control an induction motor, you’ve got to apply both voltage and frequency in a fixed ratio. If you lower the voltage, you’ve got to lower the frequency as well. The only problem is that it’s not easy to do with single phase motors, not easy at all.Without reducing both voltage and frequency together, as a vfd does, the rotor wants to follow the rotating field in the stator. Lowering just the voltage only affects the speed when the motor no longer can produce enough torque to maintain the speed. While it appears to have slowed down, the stator is overloaded due to the excessive slip.It’s a very fine line. A variac is about the only realistic way to even attempt this. It is far better to to throttle the inlet flow with a flow damper, this will reduce the motor load. Placing a flow damper on the output will reduce airflow but it will not change the motor load.99.999% of the variable frequency drives you see in the market are 3 phase output devices. Single phase output drives are rare and problematic as single phase induction motors are all over place, design wise. Capacitor start, capacitor run, both, some with electronic starting, some with mechanical, everyone is different making is extremely difficult to apply a one-size-fits-all approach to controlling one with a VFD.The vast majority of the residential a/c spilt units are now all polyphase ac inverter controlled motors. The energy efficiency gained out of being able to have a fan that can run slow or fast with low airflow or high airflow makes the difference between a just okay system and a great system that is inexpensive to operate.Here is some reference material on damper and flow controls.https://www.nrel.gov/docs/fy03osti/29166.pdf

author: Circlotron
author_rank: Super Contributor
timestamp: June 23, 2020, 06:07:55 am
content: It's probably a capacitor run motor. No starting switch. These are used when the application needs very little starting torque. With a fan load the torque varies with the square of the rpm and the power needed varies with thecubeof the rpm. So if youhalvethe rpm youhalve halve halvethe power needed. So with a reduced voltage supply, even though the motor is running in a high slip condition with very poor power factor (the now much higher frequency induced into the rotor means it's reactance is much higher than it's resistance) the power demanded from it is greatly reduced. As for overheating, just measure the input current. The stator winding losses are an I^2R thing. Rotor losses probably lower than normal but not quite sure. So, yeah, I'd just try reduced voltage, preferable a nice sine wave from a transformer, and see how you go. Just be sure to check the input current to the motor leads before and after.

author: jrmymllr
author_rank: Regular Contributor
timestamp: June 23, 2020, 12:51:57 pm
content: Quote from: Ground_Loop on June 23, 2020, 01:36:52 amDefinitely not BLDC. I've had the cover off and looked up the part number of the motor. I don't have it now, but it's a 3 speed AC motor controlled by switching wires. It's also over 20 years old.

author: jrmymllr
author_rank: Regular Contributor
timestamp: June 23, 2020, 12:56:13 pm
content: Quote from: bdunham7 on June 23, 2020, 01:54:06 amI've looked it at by taking the cover off and looking at the diagram in the manual. It's a 3 speed AC motor direct driving a squirrel cage and it's already set on the lowest speed. This is also not part of a furnace, it's a plain air handler in the attic for A/C only. I believe the air handler is oversized, and possibly the entire system is which is an even bigger issue.

author: jrmymllr
author_rank: Regular Contributor
timestamp: June 23, 2020, 12:57:51 pm
content: Quote from: rstofer on June 23, 2020, 03:33:30 amPictures are tough right now. It's in the attic of a house some distance from me. I know it's a direct drive 240VAC 3 speed motor, almost certainly PSC but I have to confirm. Already set to the low speed.

author: jrmymllr
author_rank: Regular Contributor
timestamp: June 23, 2020, 01:03:11 pm
content: Quote from: Circlotron on June 23, 2020, 06:07:55 amI've thought about running the thing from 120VAC instead of 240, which would be easy. The only change would be to change the power transformer in there so a 120:24 unit instead of the 240:24 that's likely in there. This unit has a delayed off feature where the fan runs for a few minutes after the command to turn it off so there's some electronics in there. It also supplies the 24VAC for the A/C compressor contactor.I'm just leery about halving the motor voltage as this seems to be a almost universally accepted bad idea.

author: james_s
author_rank: Super Contributor
timestamp: June 23, 2020, 05:56:38 pm
content: Quote from: jrmymllr on June 23, 2020, 12:56:13 pmI'm 99% confident that a triac motor speed controller will work just fine. As I mentioned earlier there's one on my mom's furnace, and we had one on the furnace in the house I grew up in too. You can talk to a HVAC guy and see what they recommend.

author: bdunham7
author_rank: Super Contributor
timestamp: June 23, 2020, 07:07:04 pm
content: Quote from: jrmymllr on June 23, 2020, 01:03:11 pmIt sounds as if the AC unit may be grossly oversized. Some HVAC guys like to do this out of sheer ignorance. If so, unless it has a TXV (thermally regulated expansion valve) instead of usual fixed orifice, it will be difficult to lower the speed below the lowest selectable speed of the unit because the evaporator will freeze. You could add a temperature sensor and cycle the compressor, but you need to limit the number of hourly cycles, typically to 5 or less.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: June 23, 2020, 07:22:45 pm
content: Hi,Assuming the motor is a PSC motor this is what should happen:Reproduced from here:https://electronics.stackexchange.com/questions/441251/controlling-speed-of-psc-induction-motor-questions-about-operating-at-high-slipThe blue lines show the torque versus speed curves for 4 different voltages.The red line shows the torque required to spin a fan versus speed. This is approximately a square law. This will change with the back pressure on the fan.The circles indicate the operating points. This is where the torque provided by the motor equals the torque required by the fan.The efficiency will go down as the slip frequency increases. Although the efficiency is lower, the power losses may also be lower.The motor may have a fan to cool itself. If you slow it too much it will overheat.I suggest that you try a suitably rated transformer, or a variac to reduce the voltage.Regards,Jay_Diddy_B