Introduction to Three Phase Motors:
Commonly used in commercial and industrial applications, not typically found in residential settings due to the absence of three-phase power.
Example: Cutaway of an open drive three-phase motor showing motor bearings, rotor, and stator windings.
Cooling: The motor is a totally enclosed fan-cooled motor, with an internal fan that pulls air across the motor for cooling.
Motor Connections:
Winding Connections: Three-phase motors can be connected in a Y (Star) or Delta configuration.
Y (Star) Connection:
Three windings are 120 degrees out of phase, eliminating the need for capacitors or starting relays.
Increased starting torque.
Each phase of power (L1-L2, L2-L3, L1-L3) has 208 volts, with two windings across each phase.
Delta Connection:
Windings are arranged in a triangular pattern.
Each phase (L1-L2, L2-L3, L1-L3) carries 240 volts, but only one winding is present across each phase.
Electrical Resistance in Three Phase Motors:
Equal resistance readings across all pairs of motor terminals (T1-T2, T1-T3, T2-T3) when measured with an ohmmeter, unlike split-phase motors.
Voltage Imbalance:
Critical to measure and calculate voltage imbalance as part of preventive maintenance.
Imbalance can cause current unbalance, leading to premature motor failure, contact degradation, and tripping of fuses or breakers.
Measurement Example:
Measure voltage between each pair of terminals (e.g., 218, 226, 230 volts).
Calculate the average voltage (224.67 volts).
Determine the maximum deviation (6.67 volts in this example).
Formula: (Max deviation / Average voltage) * 100 = Voltage imbalance percentage.
Example result: 2.97% voltage imbalance.
Importance: Most HVAC manufacturers do not accept more than a 2% voltage imbalance. Higher imbalance may require inspection of contact wear, loose connections, or contacting the utility company.
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