I would be grateful for assistance in identifying an old motor from Matsushita Electric Industrial Co. Ltd. I have been unable to find a datasheet for the motor. An image of the partially disassembled motor is included below. I am able to take more specific pictures if requested and directed. The text reads:


4P 30W

220V 50Hz 3.5uF

240V 50Hz 2.8uF

The cylindrical case has an outer diameter of 90mm and height of 110mm.

I assume this is a single-phase four-pole motor. I would be grateful for guidance in determining whether the capacitances listed are for start capacitors or run capacitors, and which of the three wires - white, brown, black - is which.

I have measured resistances and inductances. The resistances were measured directly with a multimeter. The inductances were measured by driving a square wave through the windings in series with a known resistance of 270R, and measuring the RL time constant from the captured waveform. The results were as follows:

white to brown: 210R 127mH

white to black: 94R 87mH

brown to black: 120R 94mH

In calculating inductance via the resistance and time constant, I summed the 270R and winding resistance and multiplied by the time difference between the peak of the waveform and 0.37x that peak with respect to the waveform's minimum value.

I believe that the winding with the largest inductance is the main winding. I also believe that the winding with the highest resistance tends to be the auxiliary winding, but this makes the results contradictary. However, I could understand a lower resistance for the auxiliary if it is substantially shorter.

I hope my inexpert research can help an expert to answer my questions on capacitors and winding identification. I would also welcome any corrections, especially if I am way off base!

Thanks for considering my question.

enter image description here

  • \$\begingroup\$ Thanks for your comment. I understand the relationship between poles, frequency and RPM. My question says "single-phase four-pole motor", is that incorrect somehow? \$\endgroup\$
    – user133493
    Dec 17, 2016 at 7:22
  • \$\begingroup\$ I deleted my earlier comment because I misread what you wrote. Unfortunately you'd already replied. Sorry for the confusion. \$\endgroup\$ Dec 17, 2016 at 7:40

1 Answer 1


Based on the capacitor values, I would say that the motor is the permanent split capacitor type (PSC). The capacitor is a run capacitor that remains connected at all times. The highest resistance measurement bust be the sum of the main and auxiliary windings. Therefore the black lead must be the common connection between the two windings.

It is difficult to determine which is the main winding and which is the auxiliary winding, but is seems more likely that black and white would be the colors used for power. I found one reference that shows the connection this way:

enter image description here

Additional Thoughts:

It would be useful to carefully examine or measure the winding wire diameter. If there is a difference in diameter between the two windings, the winding with the smaller diameter would be the auxiliary winding.

Motors like this are sometimes constructed with two identical windings so that the direction of rotation can be reversed. One method of reversing a single-phase motor is to reverse the connection of one winding end-to-end. That requires a 4-wire motor. The other method of reversing is to connect the capacitor to the opposite winding.

Wire colors are not very consistently used to signify functions. Where they are used, their significance varies from one country to another and from one manufacturer to another. Manufacturers ship motors all over the world for use by other manufacturers. They may youse their own color codes or those requested by their customers.

For products that they encounter locally, repair professionals may become familiar with colors used by various end-product and motor manufacturers. Repair people seldom if ever work on items that have been dismantled to the point that internal wire connections need to be determined. They may decline to work on such items if they encounter them.

It is probably not possible to use motor theory to confidently determine the connections of a motor like this. Careful testing with good instruments and a load would reveal which connection results in the most efficient operation.

  • \$\begingroup\$ I should have noticed that one resistance was the sum of the other two. Two follow-ups: 1. Does the 16.5 in the code IB30-16.5 mean anything to you? 2. You say that determining which winding is which is difficult. Could you outline any experiments a professional would undertake to find the answer, or the theory behind them? \$\endgroup\$
    – user133493
    Dec 18, 2016 at 3:06
  • \$\begingroup\$ I have added to my answer. I know nothing about the code marked on the motor. It likely means nothing descriptive of the motor design. It could be just an arbitrary model number or it could mean something about the product it is intended for. \$\endgroup\$
    – user80875
    Dec 18, 2016 at 15:29

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