The motor control for a small winch I have has the following diagram:

enter image description here

Note: (4, blue) and (5, gray) are connected together.

I do not understand what the voltage divider connected to the (4) connector is for?

Both resistors have the same value, which means that for an nominal input of 220VAC, the voltage at (4) is 110VAC. Why is it connected to the output of the bridge rectifier?


I have disassembled the motor controller, here is an image of the voltage divider. It consists of 1/4W 120k resistors and it is clamped together to the varistor (DNR14D471K).

enter image description here

  • \$\begingroup\$ it is not connected to the bridge rectifier per se ... it is connected to the AC input \$\endgroup\$ – jsotola Sep 15 '20 at 23:53
  • \$\begingroup\$ When no button is pressed, the pendant switch (CPB-313) connects (3, brown) with (4, blue) together, which shorts the motor armature with the D-resistor. This is used for braking. When the up button is pressed, the output of the rectifier (1, red) is connected to (5/4, gray/blue) and (2, black) to (6, yellow). On the other side, when the down button is pressed the connection is the opposite. The motor gets the output of the bridge rectifier but it is also connected to the voltage divider. \$\endgroup\$ – Guillermo Oliver Sep 16 '20 at 6:05

It's a universal (AC / DC) series motor, wired for DC operation through a bridge rectifier.

The motor reverses when its polarity is reversed, with the field winding polarity kept unchanged through the second bridge rectifier.

Dynamic braking is through the 'D' resistor, which is connected across the motor terminals when it is switched off.

The winch would function even without the two identical resistors, connected in series across the mains supply, at the junction of which one terminal of the motor is connected all the time.

The following is an analysis to determine their function.

  1. The junction of the two resistors is a virtual neutral point to which one terminal of the motor is connected. The positive and negative terminals of the main bridge rectifier alternately get connected to the virtual neutral point when the motor is run in the forward and reverse directions. The purpose is not apparent.

  2. Either resistor gets connected across one pair of diodes, when the motor is run in the forward direction, and across the other pair in the reverse direction. Had a resistor been shunted across every diode, their purpose could have been presumed as reverse voltage equalisation resistors.

The analysis is inconclusive.

  • \$\begingroup\$ The DC voltage from the rectifier is connected to the terminal of the motor in (4, blue) with direct or inverse polarity depending on the switch. (1, red) and (2, black) are connected to (4, blue) and (6, yellow) one way or the other and it is this first rectifier which supplies the power to the motor, while the upper rectifier connects armature and field in series maintaining always the same polarity to the field to be able to switch rotation. I do not see the usefulness of the 2 resistors, the controller should work if they were not present, right? \$\endgroup\$ – Guillermo Oliver Sep 16 '20 at 13:52
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    \$\begingroup\$ Hi Guillermo Oliver, Welcome to Electrical Engineering Stack Exchange! The short answer is a yes! I have edited my answer accordingly. \$\endgroup\$ – vu2nan Sep 17 '20 at 6:16
  • \$\begingroup\$ I have edited the original post with a photo of the voltage divider. \$\endgroup\$ – Guillermo Oliver Sep 17 '20 at 21:51
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    \$\begingroup\$ Hi Guillermo Oliver, That's a real neat and rugged sub-assembly that would also be sold as a spare should the MOV fail. Having the MOV and 120 kΩ resistors together could point to both of them having the job of protecting the bridge rectifier - the MOV to clamp transients from the line or load side and the resistors to equalise the reverse voltage across the diodes. The question still remains - why only two of them and not 4? \$\endgroup\$ – vu2nan Sep 18 '20 at 7:10
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    \$\begingroup\$ Hi Guillermo Oliver, Such equalising resistors are generally used with series-connected diodes in high voltage power supplies (not done with bridge rectifiers with 1000 PIV diodes). \$\endgroup\$ – vu2nan Sep 19 '20 at 9:45

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