An important consideration in specification of an automatic transfer switch (ATS) is the method used to safely transfer motor loads between two energized sources, such as when returning from the generator to the utility after an outage or a test.

Residual voltage

When an ac induction motor is operating, the applied voltage creates a rotating magnetic field in the stator, or stationary winding, which in turn induces a magnetic field in the rotor. The interaction between these fields produces the torque required by the load. When the supply voltage is removed from a rotating motor, the energy stored in the magnetic fields is dissipated through current flow in the rotor and its strength collapses to zero. While the field is collapsing, however, the rotor is still turning due to its own inertia and that of the load. Rotation of the collapsing rotor field within the open-circuited stator, winding briefly, turns the motor into a generator, creating voltage at the motor terminals. This residual voltage decays at a rate dependent on the size and design of the motor; the larger the motor, the slower the decay.

If a motor is transferred between two sources that are not in phase and the second source is applied before the residual voltage has decayed, transient current and torque can be as high as 20 times rated values. These transients can damage the motor, coupling, or driven equipment. When the sources to an ATS are a utility and a standby generator, there is no controlled phase relationship between them and it must be assumed that they will not be in phase. Consequently, some means of preventing out-of-phase transfers between energized sources must be provided whenever an ATS is to be used to transfer motor loads.

Time delay in neutral position

The simplest means of protection is to hold the switch in a neutral state between the normal and emergency positions for a preset time period whenever a transfer occurs between two energized sources. This feature is normally provided with a user-adjustable time delay setting; for the size motors found in a typical facility, the residual voltage will decay in less than a few seconds, and a 3- to 5-sec delay is adequate.

In-phase monitor

A drawback to the above method is the length of time that power is interrupted to other loads that may share the ATS while the switch is in the neutral position. This can be overcome by specifying an in-phase monitor circuit in the ATS. This circuit monitors the phase angle between the sources and permits a transfer only when they are within an acceptable window of each other. When an in-phase monitor is used, the switch transfers between positions at normal speed, typically resulting in an interruption of a few tenths of a second. In many cases, this is fast enough that motor starters do not drop out, and for all practical purposes the transfer occurs without interruption to motor-operated equipment.

The in-phase monitor has no control of the generator, and is not able to force it into phase with the utility source. Rather, it takes advantage of the fact that the engine governor cannot maintain frequency as precisely as the utility can, and the generator voltage will slowly drift in and out of phase with the utility voltage. It can take up to several minutes for this to occur when the generator is provided with a high-quality governor that maintains a frequency very close to 60 Hz.

Elevator transfer

Elevator motors and drive systems raise several application concerns in addition to the transient motor behavior discussed above. The design of ac motors for hydraulic elevator application often results in significantly higher inrush currents than standard motors to start with. Traction elevator drive systems are often designed to regenerate power back to the source during a portion of the operating cycle, which can create serious problems during a transfer switch operation.

For these reasons, a control circuit consisting of status and presignal contacts is required between the ATS and the elevator controller. The status contact indicates when the switch is in the emergency position. Usually, this signal will also adjust the operating cycle of the elevators to reduce the load on the emergency generator, for example by allowing only one car in a bank of cars to operate. The presignal is activated prior to transfer, with a time delay to allow the elevator controller to shut the cars down in an orderly fashion before the transfer occurs. When the status contact indicates that the transfer is complete, the controller resumes normal operation.