Investigated blower motor startup - start relay is defective

DETACHED AND OPENED THE START RELAY

I removed the relay and opened it up to see the mechanism. The contacts are clean and make good connection when manually pushed down. The coil appears intact and I saw no obvious issues.

Relay with cover removed

A split phase AC motor is manufactured with two windings, one for the main operation of the motor and the second just for starting. A single phase motor won't rotate on its own because the oscillating magnetic field is symmetric thus imparting no rotation to the magnet in the rotor. Three phase motors don't have this issue as the magnetic field rotates due to the phase differences on the three power legs. 

The start winding should have a magnetic field that is out of phase with the main winding, which would cause rotation of the combined field and begin the rotation of the motor. The motor used by IBM uses a technique called resistance start, where the start winding has a higher resistance than the main winding. 

The effect of this resistance and the different inductance of the windings means that the current in the start winding is out of phase with the main winding. Due to the higher resistance, however, the winding will heat up thus the start winding must be disconnected once the motor gets nearly up to speed. 

The start relay is in series with the main winding. As the motor is first energized, the current is relatively high in the main winding, causing the coil to close the contacts. The contacts connect the start winding so that the rotation can be forced to begin. As the motor speeds up the current in the main winding declines until the coil in the start relay drops out, disconnecting the start windings. 

Start relay is K4

POSSIBLE DEFECTS CAUSING FAILURE TO START

It is possible that the relay is defective, failing to close the contacts to start the motor. Since I changed the motor to one that superficially appeared compatible, it may be due to differences in the motor itself. I will verify the relative resistance of the two windings as I just assumed that the color code from the same manufacturer (Robbins & Myer) is the same between motors. The current in the motor may not be as high leading to inability of the coil to close. The design may require a start capacitor to add additional phase delay in the start windings, if the difference in resistance isn't enough. 

WATCHED OPERATION OF THE RELAY

The relay was erratic - sometimes failing to connect the starting winding, sometimes clicking quickly and achieving a good start, and sometimes locking the starting winding on leaving the motor to spin slowly. 

I adjusted what I could inside, also burnished the contacts to be certain they had good low resistance. The relay continued its undependable operation. I imagine that the reason the original blower motor had an open connection was due to thermal damage as this relay cooked the start winding. 

BUYING SIMILAR START RELAY FROM EBAY

The part on the disk drive was 91252-1613 which I didn't find anywhere, however there was a 91252-1616 on eBay for under $20 delivered so I took the gamble. The difference in model number means something - likely the current specifications for activation and dropout - but I couldn't locate any material online that would explain the differences. 

At worst case, I will need to pick up some kind of universal or adjustable motor current start relay but that wouldn't fit in the same space and look the same. Hopefully my purchase will make things work properly.

BLOWER BACK IN DRIVE 

The blower motor itself is working well. I installed it into the disk drive so that once the start relay issue is revolved the drive will be ready for further testing.