Tip & How-To about AO Smith Rheem - Rudd Furnace Motor 51-21853-01 1/6 hp 1075 RPM 208-230V
The indoor blower motor is powered by 115 volts or 230 volts, either way it wires the same.
Every motor be it a single speed or multiple speed will get the 2 legs or power,
Line 1 and Line 2 or L1 and L2 to make it easy.
Common always gets a leg all to itself, you cannot run both legs of power to the speeds as the speeds are on the Run winding, that is a direct short, and you cant run a leg to Start or it will turn backwards! This is how we unlock compressor's that get stuck!
1 leg or power ALWAYS goes to Common.
The remaining leg of power, say it is L2, it goes to the speed desired or if a single speed motor, it can go to the terminal on the Run capacitor that is wired to the Run winding, A Run capacitors terminals are Run and Start.
Note the capacitors leads coming from the motor are both brown, 1 has a white stripe
the capacitors leads are obvious in that respect as well as being the only 2 leads that have female type ends that are for sliding over the run capacitors terminals, it does not matter which lead goes to what terminal as long as both get their own.
The remaining leg of power we are calling L2 will be going to the speeds desired,
Cool uses High while Heat uses Low speed, High speed is controlled by a fan relay or fan board on the high speed terminal or cool.
While the low speed is controlled via temperature delay, time delay or a fan board on the low speed terminal or heat.
There is no need to get an exact replacement motor, as long as the shaft diameter, frame size, rotation, voltage & HP rating equal or better,
(you can go bigger HP, never smaller) you will be ok.
Some universal or trip saver motors have bushings for various shaft sizes, and an ability to reverse rotation by inverting a plugged set of leads. Belly bands are used for most applications so even if your OEM motor has built in motor mounts there is a belly band that can be used to replace that motor perhaps cheaper if not faster, some OEM motors can be days away from being available
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