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I'm restoring a 1970s HP Frequency Counter and went to replace the 110V AC tube axial cooling fan on the unit to discover that it has 3 wires. The original fan is a ROTRON 1 PH CAP 0.25MFD fan.

I'm a little baffled by the wiring in the unit which I've shown below.

Why is there both a red lead and green lead from the capacitor to the fan?

And, I can detect continuity between yellow and red - is that likely because of the winding?

My replacement axial fan has only two leads, so I'm unsure where to connect it.

Cooling Fan Circuit
Figure 1. Cooling Fan Circuit

Existing Wiring of Cooling Fan
Figure 2. Existing Wiring of Cooling Fan

Existing Tube Axial Fan
Figure 3. Existing Tube Axial Fan

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    \$\begingroup\$ Starter capacitor? Regardless, your replacement fan doesn't need the capacitor. \$\endgroup\$ – user1850479 Mar 2 '20 at 1:20
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Yellow and Red are live and neutral connections to the run winding on the motor.

The green wire carries the AC voltage phase shifted by the capacitor, to the start winding. This is necessary to define the direction of rotation, to blow instead of suck.

Unlike a big single phase AC motor, the start winding is permanently connected, which is why the cap is an oil filled run capacitor, rather than a short term rated start capacitor (which would be disconnected by a centrifugal switch when the motor got up to speed)

And both windings will be a few ohms; measure on the motor side of the capacitor to see the other winding.

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  • \$\begingroup\$ Ah! So with only two wires on the replacement motor, I'm guessing I can do away with the oil-filled run capacitor, and connect the new fan using only the red (line) and yellow (neutral) leads? \$\endgroup\$ – tjg Mar 2 '20 at 2:01
  • \$\begingroup\$ doesn't that make it a "run" capacitor instead of "start" \$\endgroup\$ – Jasen Mar 2 '20 at 5:17
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    \$\begingroup\$ @Jasen that's why I called itn a run capacitor. \$\endgroup\$ – Brian Drummond Mar 2 '20 at 11:24
  • \$\begingroup\$ why is it called a start winding? \$\endgroup\$ – Jasen Mar 2 '20 at 19:14
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    \$\begingroup\$ good question ... probably because run winding was already taken (by the winding that develops most of the power). \$\endgroup\$ – Brian Drummond Mar 2 '20 at 19:16

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