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JCole21961
10-07-2014, 06:26 PM
I have a problem with a Bosch refrigerator made in Brazil. The compressor starts and runs normally from cold, but when the thermostat calls for re-start from hot the compressor just buzzes and eventually kicks out on the Klixon overload. After 1-2 hours it starts and runs again normally. I have tried replacing the capacitor, Klixon and PTC with no improvement. When hot, the motor windings check out at 10 ohms on the run coil and 19 ohms on the start coil. Neither winding has leakage to ground. Note that in this system the PTC (disc type) is wired across the capacitor, putting the capacitor in series with the start coil after start-up. The local repair guys say I need a new motor/compressor but I am puzzled that if the windings are not grounded or open-circuit what is wrong with the motor?

JCole21961
10-07-2014, 08:58 PM
Some more info. Running current is 0.6A which I believe is well within limits. In fact the compressor does not even need to get hot. If I power off 2 minutes after starting and try to restart any time 3-60 minutes after this the current goes up to 6A and the compressor just buzzes.

mikeref
11-07-2014, 07:01 AM
Did the original capacitor test faulty?
How old is your fridge + when the compressor starts, is it initially noisy but it soon settles down.

frank
11-07-2014, 09:55 AM
Did you test the winding resistance to earth with a megger @ 500v DC?

JCole21961
11-07-2014, 03:50 PM
The original capacitor is rated 5mfd, it tested 3.5. The new one is rated 6mfd and tested 5.
The frig is 9 years old but only used 4 months/year.
There is a very short buzz on start but within a second or so the compressor is quiet.
I did not test to ground with a mega, only multimeter 9V at 20K.
Is it likely that there is an obstruction in the refrigerant circuit causing very slow equalization after stopping? The frig cools normally.

mikeref
12-07-2014, 07:21 AM
Check if your voltage drops when it attempts to start. Test your fridge power outlet with some other kitchen appliance like a 2000 watt kettle. Compressor should be able to restart after say 5 minutes without any dramas.

JCole21961
12-07-2014, 08:01 PM
When it tries to start the current goes up to 6A and the voltage drops from 214 to 208. When it does start, amps go up to 6 for less than a second and then drop to 0.6 running. Is there a difference between a run capacitor and a start capacitor other than mfd value? The original run cap was small, about 1"x 3" with polypropylene casing with two wires. The replacement is much bigger1.5"x4" with aluminum casing and connector tabs.

nike123
12-07-2014, 09:54 PM
What is compressor model#?

mikeref
12-07-2014, 11:40 PM
214 to 208? What am i missing here. Your location says USA.
Run capacitors have a fixed value. Start caps have an upper and lower variation. eg: 64-72 uf.

nike123
13-07-2014, 10:09 AM
5 mfd capacitor is not start capacitor. It is run capacitor!
Start capacitor for 220V would be in range from 40mfd and above.
Your compresor then would be of RSCR type with PTC.

This is its connection diagram:
11628

Check your wiring against above diagram. Then check insulation of windings with megger, especially of start winding. It should be above 100 Mohm at room temperature and 500V test voltage after 20 sek. Check PTC with ohmmeter at 25C that resistance is between 8 and 16 ohm.

If system was previously charged other than factory charge, it may be overcharged. Standstill pressure for R134a after 5 min of resting should be below 5 Barg to both high and low side to ensure start for low starting torque compressor like that in your fridge (RSCR is low starting torque motor).
5 min of pause between stop and start is minimum for PTC device to cool down. If thermostat calls for start earlier than that, it is normal that it will fail to start.

You then should check for correct thermostat differential and insulation and air ingress in fridge.

Run capacitor is designed for continuous duty (100%). Start capacitor is designed for 3% duty cycle.
You need to fit proper capacitor for continuous duty.

JCole21961
13-07-2014, 11:45 PM
Thanks for the replies. This refrigerator is in Brazil and the compressor is manufactured here. Bosch is the only frig which used the run capacitor and has since discontinued their frig line here. The diagram is not exactly how my compressor is wired. There are three pins on the compressor. The top pin connects to a Klixon overload and line voltage. A PTC module (ceramic disc type) connects between the lower two pins. On one side is the neutral and one capacitor lead, on the other is the other capacitor lead.

The problem must be either low start torque or high mechanical resistance due to line pressure or friction. Regardless of whether the compressor has run for 3 minutes or 90 minutes it will not re-start sooner than an hour after stopping. I personally do not understand the run capacitor system. At start the capacitor is bypassed for a few seconds, so what provides the phase shift to get started? In run mode both capacitor and start coil are operating continuously. Could I help the situation by changing it to the conventional capacitor start whereby the PTC is in series with a larger capacitor to provide a phase shift to the coil for a few seconds and then remain in line at a very low voltage?

nike123
14-07-2014, 05:44 AM
The diagram is not exactly how my compressor is wired. There are three pins on the compressor. The top pin connects to a Klixon overload and line voltage. A PTC module (ceramic disc type) connects between the lower two pins. On one side is the neutral and one capacitor lead, on the other is the other capacitor lead.



I don't see how is that different from drawing I posted.

nike123
14-07-2014, 05:51 AM
The problem must be either low start torque or high mechanical resistance due to line pressure or friction. Regardless of whether the compressor has run for 3 minutes or 90 minutes it will not re-start sooner than an hour after stopping. I personally do not understand the run capacitor system. At start the capacitor is bypassed for a few seconds, so what provides the phase shift to get started? In run mode both capacitor and start coil are operating continuously.
----------------------

When low starting torque is adequate, but
improved operating efficiency is required, the
addition of a run capacitor in series with the start
winding can produce the required result. This
may again be on a domestic refrigerator
application, but one where a small increase in cost
is justified by the higher efficiency that is needed
to satisfy market demands or regulatory
requirements.

Again a PTC or current relay is used, but in
this case it does not isolate the start winding,
which remains active whenever the compressor
runs. The relay allows current to bypass the run
capacitor during run-up, producing the necessary
starting characteristics. When the PTC resistance
increases or the current relay opens, the effect is
to bring the run capacitor into series with the start
winding.



http://www.kame.com.tw/data/03%20KK/%A7%DE%B3N%B8%EA%AE%C6/Compressor-Motor-Types-and-Start-Relays.pdf

http://www.allaboutcircuits.com/vol_2/chpt_13/9.html


Start winding is physically arranged in motor with necessary angle (90) for phase shift to produce torque required to start motor. Once started, run capacitor is in series with start winding providing reduction in angle of phase shift, therefore reducing current consumption and heating of motor, and still maintain enough torque to keep motor spinning.


If an auxiliary winding of much fewer turns of smaller wire is placed at 90o electrical to the main winding, it can start a single phase induction motor. With lower inductance and higher resistance, the current will experience less phase shift than the main winding. About 30o of phase difference may be obtained. This coil produces a moderate starting torque, which is disconnected by a centrifugal switch at 3/4 of synchronous speed. This simple (no capacitor) arrangement serves well for motors up to 1/3 horsepower (250 watts) driving easily started loads.
http://sub.allaboutcircuits.com/images/02490.png
Resistance split-phase motor induction motor.
This motor has more starting torque than a shaded pole motor (next section), but not as much as a two phase motor built from the same parts. The current density in the auxiliary winding is so high during starting that the consequent rapid temperature rise precludes frequent restarting or slow starting loads.

nike123
14-07-2014, 06:13 AM
Could I help the situation by changing it to the conventional capacitor start whereby the PTC is in series with a larger capacitor to provide a phase shift to the coil for a few seconds and then remain in line at a very low voltage?

No, you cannot!
Larger start capacitor is used with motor with both windings (start and run) physically positioned at same angle. Then bigger start capacitor is used to make necessary phase shift at starting, and later, when motor is at full speed, run capacitor is used to lower phase shift and current consumption (and torque).

nike123
14-07-2014, 06:25 AM
Still, we did not get answer on COMPRESSOR MODEL NUMBER!

mikeref
14-07-2014, 07:28 AM
If the fridge has a PCB, the thermostat will be part of the low voltage circuit. If that's the case, the PCB relay supplies power to compressor relay. Where did you take the previous voltage reading? Compressor or GPO.

JCole21961
14-07-2014, 03:22 PM
OK, thanks, I understand the principle now. In your diagram it looks as though nothing is connected to terminal 2, maybe I'm misreading it. When I checked voltage it was at the compressor. There is no obvious brand name on the compressor but the logos VDE and CE. Model number(s) TP153-MS-236 and TPH1390YMS. There are two feed wires to the Klixon, red and blue. One black wire to one side of the PTC. To rule out any control issues can I simply disconnect these and connect a 220V supply directly?

monkey spanners
14-07-2014, 05:43 PM
I'm wondering if the ptc is faulty, one way to check is to measure the current in the capacitor wire, or even temporarily replace it with a switch and do the ptc job yourself to see if it cures the starting problems (only if you are competent to be working on electrics)

JCole21961
14-07-2014, 06:07 PM
OK, thanks, I understand the principle now. In your diagram it looks as though nothing is connected to terminal 2, maybe I'm misreading it. When I checked voltage it was at the compressor. There is no obvious brand name on the compressor but the logos VDE and CE. Model number(s) TP153-MS-236 and TPH1390YMS. There are two feed wires to the Klixon, red and blue. One black wire to one side of the PTC. To rule out any control issues can I simply disconnect these and connect a 220V supply directly?

JCole21961
14-07-2014, 06:10 PM
As they are cheap, my first step was to replace the PTC, Klixon and capacitor. No improvement. I am now supplying power to the frig via timer, 75 minutes on, 165 minutes off and it does fine like this but this is not a permanent solution.

mikeref
15-07-2014, 07:58 AM
Ah...Fridge/ freezers need an average of 12 hours run time per 24 hours.
In warmer climates, that 12 hours goes up to around 16 hours.

Fitting a test lead from mains supply directly to the compressor relay is a proceedure only trained and certified engineers can legally perform.:(

JCole21961
18-07-2014, 03:26 PM
After several days I conclude that heat and possible back-pressure have little to do with the problem. Sometimes the compressor will start and sometimes it will stall regardless of previous run time. It seems the start torque is right on the edge. Could low voltage be the culprit? Mains voltage at my house is 214-215 and when stalled, it drops to 208 at the compressor. Also, when stalled, the noise is not the traditional hum but a much higher pitched buzz. Could it be dirty contacts on the start relay on the control board causing higher than normal resistance under load?

nike123
18-07-2014, 07:40 PM
After several days I conclude that heat and possible back-pressure have little to do with the problem. Sometimes the compressor will start and sometimes it will stall regardless of previous run time. It seems the start torque is right on the edge.

I don't see any measurement that can support these subjective conclusions.
Start taking pressure and current and voltage and resistance and temperature measurments. Than everything will be clear. ;)

JCole21961
18-07-2014, 09:43 PM
I have taken all the measurements you mention except refrigerant pressures which you can see in my previous posts. The frig has never been worked on before and it cools well. They are all normal except the line voltage of 215 is a little low but within tolerance. I have also connected power directly to the compressor, bypassing the control board. (I am a licensed mechanical engineer, no constraints in Brazil) No improvement.

nike123
19-07-2014, 09:12 AM
We don't see any insulation measurement. Also, there is no record on how pressure is equalizing (time/pressure relation) after compressor is stopped and what is standstill pressure at what ambient (refrigerator compartment) temperature. Also, we don't know if refrigerant charge is correct (by weight) or not.
Therefore, lot of guessing, and this trade is all about measuring!

mikeref
19-07-2014, 09:55 AM
If the fridge/ freezer combination reaches designed temperatures, then there really is no need to question the gas charge as OP's problem is compressor start issues after the thermostat cycled on temperature.

So what is the issue? Connecting compressor directly to line voltage eliminates possible circuit problems.
That leaves just 3 possibilities. Low voltage, compressor relay...(A new relay doesn't mean it is guaranteed to work)...or the compressor piston locks due to tolerance constraint after shutting down.

JCole21961
19-07-2014, 12:49 PM
Thanks. I shall try to borrow a mega to check insulation. The way I directly connected the compressor by-passed any relay, the only other device which remained in the start circuit was the PTC ceramic disc (a new one). Cold this checked out at 26 ohms. If this increased too rapidly when power was applied I guess this could be the culprit but I think this is unlikely. The original one checked out about the same cold. When running, the compressor is quiet. That leaves low voltage (drops to 208 on start). The local Bosch rep insists I need a new compressor and perhaps he is correct, I'm just curious as to why all the criteria seem normal.

mikeref
20-07-2014, 09:41 AM
What is your regular (Nominal) supply voltage? You didn't answer my question from post #9.

JCole21961
20-07-2014, 12:46 PM
Sorry. I live most of the year in USA but spend a few months in Brazil. Brazil is unique in that some areas are 110V and others 220V. The official voltage in my area is 220/60 and this is the nameplate voltage on the compressor. This morning the frig would not start after 4 successive tries and on the 5th it started normally. All these within 5 minutes. Start winding somewhat damaged? I assume once running, although the start winding provides an increase in efficiency, the motor can run without it. Running amps only 0.6 so it's unlikely insulation breakdown between start windings.

The MG Pony
20-07-2014, 03:39 PM
Sounds like mechanicle clearence issues on the piston assembly its self, wonder how well they purges while brazing! Or bad piston ring and has degraded or expands too much, there is a large list of mechanicle cuases, but in the end non are repairable with the can in place.

Time for new pot or new fridge! imo

nike123
20-07-2014, 06:50 PM
If stator windings are partially short-circuited, than power supply voltage, although in normal range, can be inadequate to overcome breakdown torque and result is motor stalling. If stator winding insulation is in poor condition, than multiple partial small short-circuits in stator winding can be of more influence on torque when motor is hot, than when motor is cold. Check insulation of motor with correct instrument to eliminate that possibility.

Sometimes, pretransformer (heightening power supply voltage - previously used here with fridges and freezers when power grid was weak) can overcome that problem, but if insulation of motor is real problem, than it is only matter of time when motor will fail again.

Correct insulation resistance, for modern compressors made after 1977, is 100Mohms at 500V test voltage. If any lower, you have faulty motor windings.
When I test them, correct insulation is usually outside of megger range at that test voltage (10Gohms).

I use this one which is availiable here under 100$ and perfectly good for job:

https://www.youtube.com/watch?v=HalIsuguUEk

JCole21961
21-07-2014, 01:19 PM
Thanks guys. As an ME my electrical knowledge is limited so bear with me on the questions. As I understand it, there are two possibilities electrically:
1. The start winding is partially short-circuited due to insulation breakdown. If this is the case I would think it would show up as abnormally low resistance and abnormally high operating current. It may also have the effect of raising the resistance of the ceramic disc too quickly. So what would be a normal resistance and operating current?
2. If the start winding is short-circuited to ground The resistance may be normal across the winding but operating current would also be high. I'll check the ground issue with the mega as described.