There are times when a defective motor will not operate and the nameplate is missing or not legible. How do you determine a replacement? The following steps should help determine the “mystery” motor's identity:
• Determine the diameter by measuring the diameter of the motor.
• Determine motor type:
• If no capacitor is used, it has to be a shaded pole, split-phase or polyphase.
• If no capacitor is used, but it has a starting switch, then it is a split-phase.
• *If no capacitor is used, it has no starting switch, but it has two leads and a stator (excluding ground), more than likely it is a shaded-pole.
• *If there is no start switch and it uses a run capacitor, it is a PSC.
• *If there is no switch, relay or capacitors, but it has three line leads and layered winding, then it's a three-phase motor.
• *If it has a start capacitor and switch, it is a capacitor start motor.
• If it is the same as “F,” with the exception of a run capacitor, it's a cap run-cap start motor
*Start capacitors generally have higher MFD valves (100 MFD+) than run capacitors.
• Determine speed by counting number of poles (wound coils) in the outside of winding the stator.
• Determine the number of speeds by the number of leads. If it's a shaded pole, the number of speeds normally will be one less than the total number of leads (excluding ground lead end). If there are three leads, it's a two-speed motor (one common – one for high and one for low).
If the motor is PSC, the number of speeds is two less than the total number of leads (excluding ground lead) if only one capacitor lead is used and three less if two capacitor leads are used.
• Determine rotation on a shaded-pole motor by viewing the stator. The shading coil is in a copper insert on each pole that determines which way the rotor will turn. It will rotate from the side of the pole without a shading coil to the side that has a shading coil.
A PSC is more difficult and one must observe such things as what side of the motor fan blades, fan or blower wheel dust has collected. Belted drives are generally reversible and polyphase are easily reversed by interchanging any two of the three line leads.
• Determine voltage, which is difficult by sight. But indicators can include the type of receptacles or application. To be certain, check the power source with a crimple voltmeter.
• Determine horsepower by measuring the stack length.
• Determine mechanical mounting requirements by making notes or taking the “dead motor” to your nearest dealer or distributor.
Source: A.O. Smith
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