Home Forums General Discussion Technical Questions Need info on an old Otis dual RPM motor

I am currently modernizing an old Otis car that has a 7.5 HP, 208 volt 1200/300 RPM motor. The motor has 12 leads 1 through 6 and 1S through 6S. Am I correct in assuming that the 1-6 leads are the high speed winding and the 1S-6S leads are the low speed? For both sets of leads 4,5and 6 are joined together and 1,2 and 3 go to the respective high and low contactors. I am installing an MCE 4000 controller and my plan is to abandon the 1S-6S leads and use the 1-6 leads to go to the new drive. Can anyone tell me if this is correct?

Thanks

hi Chris,

yep you logic sounds right to me. It appears the pdo is a 2 speed motor and I would imagine the old controller would've a separate fast and slow door contactor and simply drive the winding needed. It seems they are star connected on each winding. If your connecting to a vf drive i'm pretty sure you always connect to the higher rpm winding.

Thanks for the info, I am going to try it today.

The easiest way to check is to measure the impedance the high speed winding has a lower impedance than the slow speed winding.

Thanks for the tip that's a great suggestion. I don't have a motor schematic, but each group of leads are wired with 1, 2 and 3 as individual leads and 4,5,6 joined together. Would I get a proper measurement by reading ohms from 1 to 4,5,6 ... 2 to 4,5,6 etc.

Chris, what willem stated is an important lesson when modernising or reparing ac motors as its very common to have to test a burnt out motor or to test and install a repaired motor that the winders have forgotten to number the leads. Remember most of the time with a 3 ph motor they will be marked either 123/u1,v1,w1 and 456/u2,v2,w2. If you get a 3 ph motor with no markings remember that the're many ways to wire them but essentially you test between all leads for a cct. I bet it would be 1 to 4, 2 to 5 and 3 to 6 as the 3 windings of the slow speed and the same for the fast speed. You can also identify the fast and slow windings like willem said as the higher resistance will be the slow speed windings and vice versa. Just to note that most modern 2 speed ac motors wont have 12 wires as they would have an internal star/ delta connection and all that would be at the terminals would be the three phase slow and fast speed connections.