Chuck Yung
EASA Technical Support Specialist
One frequent request of EASA’s technical support staff is for help in identifying unmarked motor leads. This article introduces a set of procedures for identifying unmarked leads of 6-lead motors with 1 or 2 windings. For most connections, the only tools required for these procedures are an ohmmeter and surge tester.
An additional benefit is that these procedures can be used to identify the type of connection (Table 1); for example, when a motor is received without a nameplate. With 6 leads, the motor connection could be part-winding start, wye-delta, or a 2-speed design.
Information about the application can save several steps in the identification process. For instance, a vertical motor from a municipal water plant is likely to be connected for part-winding starting. If the application is a centrifuge or other high-inertia load, it is likely to use a wye-delta connection. A fan application or mixer is more likely to have a 2-speed winding.
Determining continuity
When a motor has no nameplate, the first step in identifying the type of connection is to use an ohmmeter to determine which leads have continuity. The meter must be capable of accurately measuring the resistance. If the resistance is less than 5 ohms, use a milli-ohm meter or bridge device.
2 circuits, 3 leads each
If the motor has 2 independent circuits of 3 leads each, it is either a part-winding start or a 2-winding motor. Surge test each set of 3 leads separately.
If the surge test pattern for each 3 lead circuit has two “good” patterns and 1 “bad” pattern, the motor has an adjacent-pole part-winding start. For each set of 3 leads, the good pattern occurs when comparing leads 1 to 3, or 7 to 9. The “bad” surge pattern occurs when comparing lead 2 to either 1 or 3, and when comparing lead 8 to lead 7 or 9. So the leads common to the “bad” surge patterns are #2 and #8. Connect 2 & 8 together, then pair the other 4 leads (1 from each circuit together), and surge test in the run configuration. If the surge pattern is good, the pairsare correct:1&7,2&8,3 & 9. If not, swap the pairs and repeat.
If both sets of 3 leads have good surge test patterns, the motor is either a 2-speed 2-winding motor, or a skip-pole part-winding start.
Test run the assembled motor with each set of 3 leads, and use a tachometer to determine the rpm. If both operate at the same speed, the connection must be a skip-pole part-winding start.
Label one set as 1-2-3 and the other as 7-8-9. Test run each set to establish the correct phase sequence; the direction of rotation should be the same for 1-2-3 as for 7-8-9.
If the motor runs at two different speeds, label the low speed leads 1-2-3, and the leads for the high speed winding as 11-12-13. Be sure to label the leads for the same direction of rotation / phase sequence. To facilitate this, the phase sequence of your test panel leads should be clearly labeled.
Do not attempt to run an adjacent-pole part-winding start motor in the start mode. Surge compare the winding in the run configuration (pairing leads 1 & 7, 2 & 8, and 3 & 9); then test run the motor in the same run configuration. Caution: You must surge test the winding in the run configuration to be sure that leads 1, 3, 7 and 9 are correctly identified. The consequence of swapping 1-2-3 with 7-8-9: None.
3 circuits, 2 leads each
If the continuity check reveals that the 6 leads are divided into 3 sets of 2 leads, the motor has a wye-delta connection. It could be a wye-start delta-run, or a dual-voltage connection with the voltage ratio of 1 to 1.732 (e.g., a 230/400v IEC motor). The pairs are 1-4, 2-5 and 3-6. To correctly identify the leads, arbitrarily label the 3 pairs as 1-4, 2-5, 3-6. Connect leads 4, 5 and 6 together (wye), and surge test leads 1-2-3. If the surge pattern is bad in two comparisons, select the lead that is common to both bad patterns. Reverse the numbers on that lead with the lead in circuit with it (i.e., if lead #3 is the common lead in the two bad surge patterns, exchange leads #3 and #6) and surge test it again.
When the surge comparison results in a “good” patterns with the wye configuration, reconnect the leads delta (1 & 6, 2 & 4, 3 & 5) and surge test them as final verification that all the leads are correctly identified. The consequence of reversing all 3 pairs (1-4, 2-5, and 3-6): None.
All 6 leads have continuity
If all 6 leads are in circuit, the connection is either an extended delta / double delta PWS, or a 2-speed 1-winding. Of the various possibilities of “6 unmarked leads”, the 2-speed 1-winding is the most challenging to identify.
2-speed 1-winding
The 2-speed winding (with a 2:1 speed ratio) is called a “Dahlander” connection. This special connection allows the coil groups to be energized salient-pole for one speed, or consequent-pole to form twice the poles and thereby operate at half the rpm of the high speed. While the most common Dahlander connection is the constant-torque 2-wye/1-delta, other connections are also used.
This procedure works regardless of the number of circuits. For visual reference, Figure 1 illustrates schematics of the constant-torque, constant hp and variable torque (CT, CH and VT, respectively) connections.
There is continuity among all 6 leads of a 2-speed winding, so we can apply basic principles of paralleled resistors to determine the lead markings. The ratio of resistances for the variable torque connection is intuitive, so we will first cover the more complicated CT / CH procedure.
Constant torque or hp
From Figure 1 we can see that the constant torque and constant horsepower Dahlander connections are symmetrical, so there is a 50-50 chance of correctly labeling 1-2-3 and 4-5-6 by the resistance alone. It will be necessary to test-run the motor, as the final step to confirm the lead markings.
The path between any 2 leads can be traced in either a clockwise or counter-clockwise direction (Figure 2). Therefore, the resistance between any 2 leads may be treated as 2 sets of paralleled resistors. The ratio of the resistance of the 2 parallel paths (CW and CCW) can be discerned from Figure 2. The resistance of paralleled resistors (R1 and R2 in the formula below) can be calculated by: Resistance of circuit = (R1 x R2)/(R1 + R2)
Since the winding is comprised of identical coil groups, the ratio of the resistances can be used to determine which leads are which. Regardless of the winding resistance or number of circuits, the ratio of the resistances is constant.
Start by using letters to temporarily identify the leads. Use an accurate ohmmeter to measure the resistance from each lead to the other 5 leads. If the resistance of the winding is at least 5 ohms (winding resistance is inversely proportional to the hp/kW rating) a normal volt-ohmmeter is sufficient. For larger hp ratings, the winding resistance is usually so low that a DLRO, milli-ohm meter or bridge is required to obtain useful values. The meter should be capable of measuring to at least 3 significant digits. Use the ratios from the Table 2 to interpret your result.
There will be 3 lead-pairs with resistances higher than the others. It should be evident from Figure 3 that those combinations are 1-5, 2-4, and 3-6. We will not know for certain whether the two sets of numbers are correctly identified (i.e., 1-2-3 might actually be 5-4-6, and vice versa) until the motor is test run, so affix temporary labels. Caution: Whether we have the 3 pairs reversed or not, at this point the winding surge test will yield good results.
The resistance between 1, 2 and 3 will be approximately 89% of the resistance between 1-5, 2-4 and 3-6. (The resistance between 4-5-6 is also 89% of the maximum values.) The lowest resistance pairs will be: 1-6, 2-6, 2-5, 3-5, 4-3, and 1-4. The lead with the lowest resistance to both 5 and 6 is therefore lead #2.
Likewise, lead #1 will have the lowest resistance to leads #4 and #6; and lead #3 to leads #4 and #5.
Next, surge test the windings using both the low and high-speed connections. The surge test pattern should appear normal, even if leads 1-2-3 were exchanged with 5-4-6.
Assemble the motor, and test-run it using both high and low speed connections. If it runs correctly on both speeds, the leads are correctly identified. If leads 1-2-3 and 5-4-6 are swapped, the motor will probably run on the high speed connection, but it will be noisy and the speed will be too low. On the low speed connection, it will not run and will draw high current.
If the motor does not run properly, reverse the markings of all 3 pairs:
Swap the lead markings on 1 with 5, lead 2 with 4, and lead 3 with 6. Repeat the test run with the high and low speed connections as final confirmation.
Variable torque
Because the variable torque connection has an internal wye, and 3 of the leads appear as center-taps of their respective phases, identifying unmarked leads is even simpler. The ratio of resistances should be intuitive from Figure 4, and is shown in Table 3.
The highest resistance will be measured between leads 1-2-3. It does not matter which leads we identify as 1, 2 or 3. The lead with the lowest resistance to the lead we designate as #1 is lead #4. The lowest resistance with lead #2 is lead #6, and the remaining lead #5 must have the lowest resistance to lead #3.
As with the earlier CT and CH examples, test run the assembled motor using both high and low speed connections.
To convert from NEMA to IEC lead markings, after following the above procedure, use Table 4.
These procedures should be helpful the next time you receive a 6-lead motor with missing or questionable lead markings. If the nameplate is missing, you can further refine the identification process by comparing the number of slots, grouping and coil pitch.
One easily overlooked source of information when determining the correct motor connection is the customer. They may not know about connections, but there is a very good chance they can send a schematic of the controls. Labels like “high speed” or “low speed” on the contactors can end uncertainty in a hurry.
AVAILABLE IN SPANISH
Print