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How to Optimize Transfer of Inductive Loads

Several approaches are available to transfer motor loads between two dissimilar sources of power. When selecting transfer switches, it is important to consider the available options for minimizing electrical effects that could disturb operations or damage equipment. The following narrative provides some “rule-of-thumb” guidance.

Open Transition Switching

The most basic form of transfer switch operation is Open Transition switching, where the contacts for the connected source open before closing the contacts for the alternate source. This “break-before-make” sequence results in a momentary sub-second interruption in power. What happens next depends on the type and size of the load. Figure 1 shows the output voltage of an open transition transfer switch resulting from a transfer of load to an alternate power source.

Many facilities can tolerate a momentary power outage because they operate mixed loads that are primarily resistive. Conventional practice shows that, without additional provisions, open transition switching ideally are applied to motors up to ~25 horsepower. Nevertheless, this approach does not mitigate the effects of inrush currents that could occur during source transfer.

The Importance of Inrush Currents

Inrush currents occur at the moment when switches connect circuits that exhibit voltage differences, such as when there are relative differences in phase angle at the instant when two alternating currents are connected (Figure 2). These currents flow from the location of higher potential to the location of lower potential.

Notably, when outages involve inductive loads such as motors and transformers, the amount of energy available from the load device (such as the energy in inductive fields of a transformer or the inertial energy of a rotating motor) is available as residual voltage in the load circuit. If this circuit is connected to an alternate power source before that source has reached a similar voltage, current could rush from the load circuit to the alternate source.

When inrush currents occur in either direction, the rotating generators and motors will respond by instantly changing speed. Where these currents are large, the resulting forces stress electrical and mechanical equipment. Importantly, electric motor components, such a shafts, pulleys, and even bases, can be damaged, either instantly from a single occurrence or cumulatively from repeated events. Figures 3 and 4 show examples. When inrush currents can exceed the capacity of circuit breakers and fuses, their occurrence can result in interruption of operation due to nuisance tripping.

Avoiding Inrush Currents

ASCO Power Technologies identified the following four approaches for managing motor loads in its white paper entitled Transferring Motor Loads Between Power Sources. The first of these involves transferring load when two sources are in or near synchronicity.

Transfer when Phase Angles Match

As noted, open transition switching is often used with motors up to ~25 horsepower. However, transfer switch controllers can be equipped to monitor phase angle differences between power sources so that they close on the alternate source when ac currents are closely synchronized. This is particularly useful during retransfer after the primary power source is restored, or in any operation involving two live sources. In most applications, In-Phase Transfer occurs when frequency differences cause the respective sources to passively drift into synchronicity before executing transfer. Advantages of this approach include gaining the ability to mitigate or avoid inrush currents merely by adding an electronic accessory to a transfer switch, without the need for more complex switching arrangements.

Certain parameters must be controlled to ensure transfer objectives are met. For example, the following criteria are applied to in-phase transfer involving ASCO transfer switches. The desired relative phase angle difference is zero degrees, and transfers are limited to instances when the relative phase angle difference is 60 electrical degrees or less. This value ensures that inrush currents do not exceed the nominal voltage of the circuit.

Introduce a Transfer Delay

There are two ways to introduce delays that assist in the orderly transfer of motor loads. Motor Disconnect Circuits and Delayed Transition Transfer Switches are explained as follows.

Temporarily Disconnect Motors

Where facility processes can tolerate short power interruptions, one way to avoid inrush currents is to enable the residual voltage in load circuits to decay before connecting that circuit to an alternate power source. For example, allowing a spinning motor to stop removes all residual voltage so that the transfer can proceed without any inrush current. This can be done by providing a Motor Disconnect Circuit that signals a motor or motors to shutdown prior to transfer switch operation, then restart after a transfer switch has connected the alternate power source. Motor starting then proceeds according to the features and sequences designed into the electrical distribution system and load equipment. This arrangement is useful where individual motors must be controlled to avoid inrush currents. An example of a motor disconnect circuit is shown in Figure 5.

Allow Motors to Slow or Stop

When a transfer switch serves primarily motor loads, it is unnecessary to install and operate multiple disconnect circuits. Instead, a Delayed Transition Transfer Switch can be used to introduce a user-settable delay that lasts from seconds to minutes for all downstream loads. During this interval, motors can slow or stop, reducing or eliminating residual voltage on load circuit conductors. The transfer then occurs with little or no inrush current and the motors restart and/or return to operating speed. Figure 6 shows the output voltage during delayed transfer switching operations.

Application Considerations

First, introducing a delay is most useful for transferring circuits that primarily supply motor loads at facilities that can tolerate short power interruptions. Nevertheless, large motors can require more than 10 seconds to slow and stop. If the motor is part of a life safety system (for example, a smoke control system in a hospital or high-rise building), it may not be possible to introduce the delay and then restore power to the motor within the 10 seconds maximum time permitted by Article 700 of the National Electric Code®. This application would require a different approach.

Second, the duration of the delay must be developed on a case-by-case basis after considering the nature of the motor(s) that are present downstream of a transfer switch. Facility specific delay recommendations should be provided by the designer of the power distribution system or another qualified electrical professional after consulting residual voltage decay information that may be available from motor and load equipment manufacturers.

Third, transfer delays are not to be confused with transfer switch timing delays. The latter refers to various control settings used in transfer switch controllers to ensure that transfers occur only when needed and proceed in optimized event sequences. For more information, review the ASCO Power Technologies Technical Brief entitled Elements of Time Delays and White Paper entitled Timing Delays for ATS Transition Modes.

Momentarily Parallel Two Power Sources

Closed Transition Transfer Switches close on the contacts of the alternate power sources before releasing the contacts of the original power source. When a Closed Transition Transfer Switch senses that both power sources are present, acceptable, and in synchronism, it parallels the power sources for an instant, typically for less than 100 milliseconds. Transfers of any kind of load, including motor and inductive loads, thus proceed in a “bumpless” manner without any power interruption. This can be the best option for:

• life-safety motor loads that must receive backup power within prescribed timeframes
• sensitive, high-value, digital, or electronic loads downstream of a transfer switch
• facilities that desire to minimize any momentary effects from transfer switching operations

Conditions for closed transition transfer require that both power sources are present, exhibit a voltage differential of five percent or less, and exhibit a frequency differential of less than two Hertz. Figure 7 shows the output voltage during closed transfer switching operations. For more information, review the ASCO Power Technologies publication entitled Transferring Loads with Zero Power Interruption.


While open transition switches can and do provide adequate service for some motor loads, the need to mitigate inrush currents increases with the size of motors and the criticality of the operations they power. Inrush currents can be mitigated by the use of (1) an in-phase monitor on an open transition transfer switch, (2) a motor disconnect circuit, (3) a delayed transition transfer switch, and (4) a closed transition transfer switch. The attributes of each are compared in the following table:

While each solution can provide reliable service for motor and inductive load applications, closed transition switching is the premium solution because it can complete transfers without discernible effects on the operation of load equipment.

The observations presented herein are provided for informational purposes only. Project-specific support for transfer switch selection should be obtained directly from a transfer switch manufacturer or qualified electrical design professional.