herman nelson steam fan coil honeywell controls . . .

archibald tuttle

Fan coils in a late 1950s (estimated) small (10 classroom) school building. Never seen this blower motor control strategy before. Can anyone tell me how it works? The motor is three wire but not your typical two speed motor that has a neutral and then a hot lead to two different points on the winding to get the two speeds. rather this has a neutral and a hot lead (conveniently white and black) and then the third red lead gets its input from a transformer (no labels left, no idea the VA and I didn't have an ammeter with me to see what the load actually is on that third leg but judging by the size of the transformer and 18 gauge wires, I don't think it is the running load). the hi lo switch alters the voltage to that third prong between 55 (low) and 73 (high).

I am the steam guru around there and they had an electrician replace a motor a few weeks back. It stopped working and they thought maybe, because there are various cutouts and electric solenoids (e.g. anti-freeze control in diagram below) comingled with pneumatic controls that perhaps something at the heating end was preventing motor from running. It turned out that the funkadelic 3 prong plug was just not making contact but the electrician came back to see if i had any wisdom because he just hooked up the motor according to the lead colors but had never seen a control system like this either. on that subject it was the blind leading the blind for me to show up cause it was news to me to. So the unit is running fine now that the plug is cleaned and seated but for future reference I'm trying to gain insight into what this control strategy actually does . . . and whether when the transformers go (still strong after 60 years so maybe i shouldn't worry) do we just look for equivalent 2 speed motors and discontinue the transformers or is this a useful and efficient control strategy that does better by the motor life when running/starting at low speed and such transformers are still available. . . .

thanks to any tech heads out there with experience on units of this sort. have attached the wiring diagram. important subject area is upper two thirds of the right half which shows the leads to the motor and the transformer setup (although it doesn't specific voltages anywhere, those were what I measured).