Lake of Phase Voltage (One Phase is Missing) for 3 Phase Motor

For the fixed torque load, the motor current of the other phases will increase to approximately 1.732 times of the normal operating current. It can be more than this because of the power factor also goes down in the case of single phasing. See this link for more detail: http://www.progress-energy.com/custservice/carcig/resourcectr/presentations/MotorProtection_VoltageUnbalance.pdf

In your case, it is air blower which has the nature of variable torque load. The current of the other two phases will still increase but not that much. However the blower performance will be decreased, it will blow less air than usual.

Running a 3 phase motor with the phase loss is harmful to the motor. So, 3 phase motors generally have the phase loss protection. Sometimes, small motors may not have this sort of protection but the overload protection works in this case due to increase of current.

- MS

There are two distinct situations to consider here;

1. The motor is NOT running and started when one phase is missing (or attempts to re-start after a phase is lost). In this case the power applied to the motor no longer has relative rotation and the motor will not start, it will simply sit there vibrating back and forth. The starting current will not decrease because the inductance does not increase with rotation, so the current remains at the Locked Rotor Current level. Your Over Load Relay should trip in accordance to the selected trip curve, i.e. if it is a Class 10 OL, it should trip within 10 seconds.
2. The motor is already running and a phase is lost. In this case, the existing rotation will remain and as previously mentioned, the current on the other two phases will increase by the Sq. Rt. of 3, so 1.732X whatever it would have been with all 3 phases present in accordance with the applied load. While true that if the load has decreased to about 58% of normal, the motor may not overload, there is nothing inherent in this situation that will cause the load to decrease, the motor will continue to ATTEMPT to run at full speed if the load demands it. If the load is still heavy, this is actually a better situation because the current increase will eventually cause an over load trip. But if, as previously mentioned, the load demand is low at the time of the phase loss, i.e. the damper on your fan is restricting air flow, the 1.732X increase in current for the load may still be lower than the trip threshold for the OL relay. This is actually worse, because the lost phase causes the creation of Negative Sequence Currents in the motor, which creates counter rotating torque, essentially making the motor fight itself and more of the current used is being converted to heat in the windings. This means that even though the overall current in the 2 remaining legs may be low enough to avoid tripping the OL, the heat effect is much much greater on the motor and can destroy the insulation before tripping the OL relay.

Because of this second situation, most IEC over Load Relays incorporate what is called a Differential Trip Mechanism that biases the trip time lower if there is no significant current on any one phase. It's a crude form of Phase Loss protection. While this is better than nothing, it is by no means the best method of protecting against phase loss damage. I always recommend using either a new Solid State OL relay that has Phase Loss detection (most now do), or use a separate Phase Monitor Relay that will shut down everything in a control panel if a phase is lost, not just the motors.