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1.8kw Single Phase Dual Speed 50hz


eugend58

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Hi guys,

 

As a pump manufacturer we have a prototype motor brought in, a 1.8kW single phase, 2 speed motor (2800/1400 rpm), 230V, 50Hz. It is running a pump, centrifugal type. We were doing a temp rise test on the motor, to assess the class F claimed by supplier, and on top of it trying to replicate a 'real-life' loading/working scenario: pump running such to give the motor the highest load, and the motor inside an enclosure where the temp can be controlled. The test was started with the motor connected (by manually connecting the wires) for the low speed, and was running well, the test results were inside the expected values. When the motor was re-connected to run at high speed, it was noticed the motor would not start, noise and vibration, very high current draw (4 times the nominal full load). We disconnected the pump, and tried again on the bench, no load this time - same symptoms. When trying to reconnect as for low speed - still no load, it was noticed the motor was not running well this time, current values double than expected, rpm only around 1300. We are quite puzzled what possible could happened ? There is a 'relay-thermal protection?' device inside the terminal/capacitors box we are not quite clear about. The Chinese supplier is giving us some vague details about it so far. We would highly appreciate any hints about what has gone wrong.

 

Thanks in advance

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Dear eugend58,

 

it is possible that there were a short circuit between some turns in the same coil in high-speed windings.

This short-circuit happened probably for an electrodynamic action at first start in your location.

 

This is only one of possibilities.

Regards

Mario

Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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Dear eugend58,

 

it is possible that there were a short circuit between some turns in the same coil in high-speed windings.

This short-circuit happened probably for an electrodynamic action at first start in your location.

 

This is only one of possibilities.

Regards

Mario

 

 

Dear Mario,

 

Thanks for your reply. It is possible the 'short-circuit' to be caused by a scratch when inserting/extracting the thermocouples used for monitoring the stator windings temperature? Do you have an alternative for this method of temp monitorig? Our guys here are doing it all the time, despite it is the resistance measuring (cold/hot) which is actually giving the accurate temp rise values - and the thermocouples are just inserted there for deciding when the temp has been stabilized. My idea is to use thermocouples only for the external surface of the motor and once this is stable to do the resistance measurement and calculate the temp rise - as I am 'suspicious' about insering those thermocouples inside the stator winding. (potential damage of windings insulation)

 

Best regards,

 

Eugen

 

PS My main area of expertiese is in hydraulical/mechanical design ... I am the pump designer in this case, and we are under time pressure to get the project done ... you can imagine I am stuck if the motor is not performing, as there are a few hydraulic tests I have to do!

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Dear Eugen,

It is possible the 'short-circuit' to be caused by a scratch when inserting/extracting the thermocouples used for monitoring the stator windings temperature?

sure!

Your actual measurement system is risky, and not so reliable. If your 2-speed motors has separate windings for 2 speeds, there is the possibility to measure the "cold" winding, obtaining wrong measurements.

Obtaining the right motor temperatures measuring only surface temperatures is not so easy, you risk to get wrong data. I would suggest to measure copper resistance at ambient temperature (motor stopped) and - after a certain running time - measure copper temperature after switching off power.

From changes in ohmic resistance values you could get perfectly the running temperature.

Regards

Mario

 

Mario Maggi - Italy - http://www.evlist.it - https://www.axu.it

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Dear Eugen,

 

sure!

Your actual measurement system is risky, and not so reliable. If your 2-speed motors has separate windings for 2 speeds, there is the possibility to measure the "cold" winding, obtaining wrong measurements.

Obtaining the right motor temperatures measuring only surface temperatures is not so easy, you risk to get wrong data. I would suggest to measure copper resistance at ambient temperature (motor stopped) and - after a certain running time - measure copper temperature after switching off power.

From changes in ohmic resistance values you could get perfectly the running temperature.

Regards

Mario

 

 

Dear Mario,

 

I spent some time today trying to start the 1800W motor in different wiring configurations and came accross a quite strange phenomenon. As this 2 speed motor has 2 capacitors and a power realy as well in the capacitor box, I decided to elliminate the power relay first, same symptomps noticed at the attempted start-up. After that I was quite 'drastic' and power the main winding only! (for the high speed, 2 pole). Just connected the active and neutral of mains to the main winding, and to my astonishment the motor started spinning!! Od course very noisy and vibrating, basically not much difference from where the wiring was as per manufacturer diagram. The current drawn was close to 40A. If this behaviour does make any sense to you, please let me know, it appears something has definitelly happened inside the main winding for 2 pole.

 

Cheers,

 

Eugen

 

PS As I was busy all day today testing the 1500W motor, we did not insert any thermocluples anymore inside the windings, and are using the resistance measurement to determine the tem rise.

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