How to test these AC motors with three wires - Page 1
Forum section: electronics / repair

author: erikbrenn
author_rank: Regular Contributor
timestamp: April 07, 2019, 08:37:47 pm
content: Hi,I have a 12 year old air handling unit that doesn't work. I'm trying to find out what's wrong to order spare parts. The unit is completely dead and a technician at the company suggested I replace the controller board. I might do that after I've tried to repair it, which is another story.The unit hasn't been running for a few years and I want to make sure the motors in the unit are working before I spend money on a new controller board. There are three motors as shown in the wiring diagram below.To test M4 I will connect L to pin 6 and N to 7 on the X1 connector. This is the way it is connected on the controlling board (L is connected via an NTC current limiter to pin 6, and 7 is switched to N with a relay)But how do I test M1 ? Obviously I will connect pin 12 to N. But pin 11 and 13 are connected via som relays on the controller board and the various AC voltates on the T1 transformer but I can't figure out how. I figured C1/C2 are start-up caps and that these are brushless motors...? The caps are fairly beefy white ones mounted inside the chassis next to the controller board. According to the table in the figure they are 4uF (FLEXIT S4 R)What to apply to pin 11 and 13 to test the M1 motor? (similarly for M2) I guess pin 11 should just have L, but what about pin 13?I might also mention that:- The T1 trafo in the figure is used to drive the motors only. The various voltages are used to vary the speed of motors and hence vary air intake/exhaust )- There is a separate trafo and rectifier circuit on the controller board to create a low side for the logic- I have a variac so I can vary the AC voltage that I feed the motor (can't use T1 because I can't get it out of the unit). I just need to understand what to apply to pin 11 and 13

author: amyk
author_rank: Super Contributor
timestamp: April 09, 2019, 12:30:45 am
content: If you can post pictures of the actual motors themselves, that would be more helpful.M4 is a permanent split capacitor (PSC) type.M1 and M2 may be capacitor start induction run (CSIR) type.

author: duak
author_rank: Super Contributor
timestamp: April 09, 2019, 01:59:29 am
content: I wonder if in the OP's circuit, pin 13 is monitoring the capacitor voltage and is used to determine if the capacitor has failed. If so, do not apply any line voltage to it.I propose this because I have a heat pump at home and two years ago it started throwing a fault code when trying to start the compressor. I found the code meant that the compressor run capacitor had failed and sure enough, it was bad. I was surprised that with such a simple controller in the outside unit, it could give a detailed failure syndrome. If I were to design a unit like this, I would probably have used a current sensor or two or maybe a rotation sensor and I didn't see any of those. BTW, most capacitors for HVAC motors (and probably others) have thermal fuses in them that open when the capacitor overheats and expands.

author: erikbrenn
author_rank: Regular Contributor
timestamp: April 09, 2019, 08:14:07 pm
content: Thanks guys.I have been able to dig up some more information about the motors.The unit I'm trying to fix look like this:M1 and M2 are blower motors like this one:The vendor selling them as spare-parts strongly suggest replacing the cap together with the motor. The CAP looks like this (they provide lowres image only unfortunately).On the product page of the fan it says"We strongly recommend you replace the capacitor that belongs to the fan when you change the fant. The fan motor will not be able to start up with a defect capacitor, but it will still receive power and therefore heat up and become damaged"So I guess C1 and C2 are start-up caps. If so I believe some kind of voltage should be applied to pin 13 to make things start-up. I suspect some of the lower AC voltages from T1 is used for that. I might have to desolder some of the releays on the controller board to see what pin 13 connects to.By the way, is there anything special with these caps being "start-up" caps? I mean other than being beefy and high voltage...Further, the M4 motor is "rotor-motor". I believe it's the motor that drives the rotational mechanism that handles the heat exchange between exhaust air and intake air. Anyway, I have a low res photo of it only.And the M4 cap which is connected to that motor, which I believe is not a start-cap but a plain high-voltage run-cap.I will look some more around to see if I can find som example circuits for start-up caps, and that can be applied to my situation.And by the way, the controller board looks like this. I have desoldered the onboard trafo and the heatzink for the Triacs to see the traces in the powersupply section. I desoldered and tested the VR and it was gone so I have ordered replacement for that (3.3V). But I would really like to test the motors without the controller so I can at make sure they work first.cheers

author: erikbrenn
author_rank: Regular Contributor
timestamp: April 09, 2019, 08:58:48 pm
content: I was able to follow trace-shadows through the board, and as far as I can see pin 13 is not driven with anything from the controller board/relays, there is no trace going to that terminal. It's simply a connection point for the C1 cap sitting in the chassis and terminal 2 on the motor. Thus, the motor completes the start-up circuit internally I guess, makes sense.Using the same method I also see a trace from pin11 to one of the relays which are switching in 230V from T1. So tomorrow I will test M1 (and M2) by simply applying 230V N to pin 12 and L to pin 11 and motor should start.Here is the connector btw. The two black lines from pin 13 is to the cap and the motor respectively. The two blue lines is to the cap and to the N terminal on mains respectively. The brown is to L mains via relay as exaplained above.Will keep you posted.

author: erikbrenn
author_rank: Regular Contributor
timestamp: April 09, 2019, 09:05:32 pm
content: And by the way, any idea what the upper left black transistor type is? It says "344" and "331C" but I can't find a datashet. It's an On semiconductor device. I suspect it's a mosfet and would like to replace it becuase I roughed it up a bit with my heat blower during desoldering of the VR next to it (the darn thing was glued to the board).

author: erikbrenn
author_rank: Regular Contributor
timestamp: April 09, 2019, 09:39:41 pm
content: I keep talking to myself hereJudging from information in this video about start caps vs run caps I would think my caps are actually just run capacitors, after all they're just 4uF. According to this video start-up caps are normally in the hundreds of uFhttps://youtu.be/OMd9QkinXz4On the other hand there are lots of startup-caps of lower capacitance on ebay.... Hmm, confused.From the picture of the caps it seems they have a threaded bolt at the bottom, so I might as well just go ahead and unscrew one of them so I can determine the type. . Must remember to discharge it though...

author: amyk
author_rank: Super Contributor
timestamp: April 10, 2019, 01:38:14 am
content: Quote from: erikbrenn on April 09, 2019, 08:58:48 pmThat means they're PSC motors too, not surprising for a fan motor.The size of the starting cap depends on the size of the motor.

author: erikbrenn
author_rank: Regular Contributor
timestamp: April 10, 2019, 10:49:15 am
content: Quote from: amyk on April 10, 2019, 01:38:14 amPSC... had to look that upI think I understand why you say that, I mean since I said that relay is switching a fixed 230V to the motor, implying a constanct speed, and hence a PSC motor.However, there is a some kind of speed control on the panel in my living room, so this doesn't make sense. I don't remember if that speed control only affected the M4 motor (rotor-motor) to extract hot air from my bathroom or if it also affected the fan motor blowing heated air into my living rooms.Will check some more tonight.