MOTOR SINGLE PHASING BASIC DEFINITION AND TUTORIALS

A 3-phase motor can be subjected to a situation referred to as “single phasing.” Single phasing means one phase is open—somewhere.

The current values shown in the figures are for a “pure” single phasing condition with one motor only in the circuit.


Above is an example of secondary single phasing. The windings in the motor are wye-connected. When

a secondary single phasing occurs, the current in one phase drops to zero amperes and the current in the
other two phases increases to approximately 1.73 x 10 = 17.3 amperes.

Two sets of phase windings are burned out because the motor did not have proper overload protection. Proper motor overload protection would have overload devices in all three phases sized at 125% of the motor’s full-load current rating.

When an “open” occurs somewhere in the branch circuit supplying the motor while the motor is running, the overload devices will sense the 17.3-ampere current flow and cause the relay to open the circuit, protecting the motor from burnout. (Courtesy of Electrical Apparatus Service Association, Inc.)


Below is an example of secondary single phasing.


The windings in the motor are delta connected. When a secondary single phasing occurs, the current in one phase conductor drops to zero amperes and the current in the other two phase conductors increases to approximately 1.73 x 10 = 17.3 amperes.

Note that the current in one phase winding in the motor increases to 11.6 amperes and the current in the other two phase windings now in series have a current flow of 5.8 amperes. The phase winding with the 11.6 amperes burns out.

Proper motor overload protection would have overload devices in all three phases sized at 125% of the motor’s full-load current rating. When an “open” occurs somewhere in the branch circuit supplying the motor while the motor is running, properly sized overload devices will sense the 17.3-ampere current flow and cause the relay to open the circuit, taking the motor off line and protecting the motor from burnout. (Courtesy of Electrical Apparatus Service Association, Inc.)



Nothing can prevent single phasing. Single phasing can be caused by such things as a downed power line, the utility company’s primary transformer fuse or cutout being open, an open connection, a bad termination, an open splice, a broken conductor, burned open contacts in a motor controller, bad contacts in a switch or circuit breaker, an open winding in the supply transformer, or misalignment.
Figure 7-12 shows a primary single-phase
.
Prior to 1971, the NEC Table 430-37 permitted 3-phase motors to have overload protection in only two phases. At that time, when a primary and/ or secondary single phasing occurred, there were numerous motor burnouts.

Why so many motor burnouts? Because the motor winding for the phase subjected to the damaging increase in current just might be the phase that did not have overload protection—a one-outof- three chance of motor burnout! The 1971 NEC began to require overload protection in all three phases, NEC Table 430-37.

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