How would I go about identifying unmarked leads on a 9-lead motor? I also need to establish whether this is wired delta or Y.

  • \$\begingroup\$ Large motor? Any idea on the kW rating and any other nameplate information? \$\endgroup\$
    – Joshyp00
    Commented Nov 14, 2016 at 18:21

2 Answers 2


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You would need to measure the continuity of each winding with a digital multimeter or similar device. Doing a quick google search found this schematic. It should become fairly obvious whether it is in star or delta once you start taking the measurements as there can only be a certain pattern of readings - providing the windings are in sound condition.

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    \$\begingroup\$ Conceivably it could be an 8 way connection (e.g. a 4 coil stepper) with an earth. \$\endgroup\$
    – abligh
    Commented Nov 14, 2016 at 9:12
  • \$\begingroup\$ If there are just 9 unmarked leads, and it is a three phase AC motor, you will also need to determine the phasing of each winding before attempting to connect it. \$\endgroup\$
    – R Drast
    Commented Nov 14, 2016 at 12:12

The schematics above provided by Joshyp00 are helpful in determining the leads, but alone do not provide enough information to mark all 9 leads. To do that, you can use a battery flashing method, outlined below. You will need a 6 or 12 V lantern battery and preferable an analog voltmeter, like the old Simpson 260. You will also a sensitive ohm meter. Before proceeding, it is a good idea to check the motor for grounds. A megger is the best way to see if any of the leads show leakage to the frame of the motor.

Next, use an ohm-meter to determine if the motor is Y or Delta connected. A Y-connected motor will have 3 groups of 2 wires each which show nearly 0-ohms between each pair, and a 4th group which shows near 0-ohms between 3 wires. A Delta connected motor will have 3 groups of 3 wires each which show near 0-ohms (see diagrams above). There should not be any ohm reading, i.e., infinite ohms, between the groups. (Don't hold the meter leads with your fingers as you will get a resistance reading through your body). If there is a reading, the motor may be bad or it is a different type of motor other than an AC induction. Put different colored tape on the wires in each group to keep the groups separate. The remaining steps below are different, depending on whether you have a Y or Delta connected motor. You will need a 6 to 12V lantern battery.

Y-connected Motor

  1. Randomly assign the numbers 7,8,9 to the 3 leads which are part of the 3-lead group.

  2. You will be touching the battery between leads 8 and 9 (polarity not important on this step), and monitoring the voltage deflection on each of the other 3 pairs of wires, each in succession. Find the pair with the minimum or no voltage deflection. This will be the 1-4 pair. Use this pair for the next step.

  3. Differentiate which is 1 or 4 by moving the battery to the 7-8 wires, with + on 7 and - on 8. Connect the volt meter between 1&4, swapping the leads in such a way as to get a positive deflection when the battery is connected (will go negative when removed). The + lead of the volt meter will be lead #1 and negative voltmeter lead will be lead # 4.

  4. Move the battery to the 7-9 leads with positive the #9 lead and negative on the #7 lead. Find lead #3 by looking for a positive deflection on one of the other pairs. The other lead is #6.

  5. Finally move batter to 8-9 pair with + lead on #8 and negative flashed to #9. Positive deflection on the meter will identify #2 lead and the other is #5.

Delta-connected Motor

  1. For the delta connection, a sensitive ohm-meter usually works at least on smaller motors. Measure the resistance between any 2 of the 3 leads in a group. Find the lead that is commonly the lowest reading between the two and that will labeled either 1,2 or 3. Just pick a label for each group. If the ohm reading is too low to determine the center wire, you can pass a low-voltage DC through the pairs of wires and measure voltage drops to determine which on is in the center.

  2. Connect the minus of the battery to both of the two of the leads from group #1 which are not labeled. You will be flashing the positive battery lead to the other lead which should be labeled #1. Connect one of the voltmeter leads to lead #2. You will be monitoring the other two leads from the #2 group to find the one with the largest deflection. That lead will be #7. The other lead in the group will be #5. The deflection occurs when the battery is connected.

  3. Keeping the battery the same as above, repeat the procedure for the other group. Put the voltmeter between lead #3 and one of the other leads in the same #3 group. Look for the largest deflection between lead #3 and the other two wires. The wire with the largest deflection will be lead #6 and the other one lead #8.

  4. Last step is to determine which is #4 and #9. Move the battery negative lead to both #5 and #7, and the other battery terminal will be flashed to #2 during the test. Connect the voltmeter between #1 and one of the other 2 wires in the same #1 group. The wire with the largest deflection will be #4. The other wire is #9.

  • \$\begingroup\$ Otto quick question for you. I totally understand the science behind this operation however when I am using it on known leads the two leads that I'm looking for have the most inflection on my meter. Can you please explain. \$\endgroup\$ Commented Dec 9, 2021 at 3:01

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