Three-phase solid-state relays (SSRs) were introduced after the initial development of single-phase SSRs in 1970. Early versions of these relays comprised three individual SSRs placed together in a single housing with a common input connection. This fundamental design concept remains almost unchanged today. However, while single-phase and three-phase relays function relatively similarly, their application usage can vary. This is mostly due to the distinct characteristics of three-phase power circuits and the features/demands of three-phase loads, with specific regard given to inductive loads.
Therefore, when choosing and deploying three-phase SSRs in motor control applications, a deep understanding of three-phase SSRs, the three-phase power system, and the typical loads that will be applied in given scenarios is almost a prerequisite.
This article reviews three-phase power systems, the attributes of common three-phase motor loads, and an in-depth examination of three-phase SSRs and their application to such loads.
Solid-state relays are typically used in an extensive range of electrical load-switching applications. These include beverage dispensing equipment, HVAC&R systems, laboratory ovens, lighting systems, medical equipment, packaging machinery, plastics machinery, professional cooking equipment, and other industrial or commercial applications.
However, to fully comprehend how these applications relate to using SSRs for control guidelines, most loads should be categorized into two main groups: heating and motion control. This is not an all-inclusive grouping because other applications exist outside the framework of these two categories, including lighting and power distribution systems. However, many design engineers who specify the use of SSRs group them under one of these two general types of three-phase loads, as illustrated in Figure 1 below.
While every application has unique areas of focus that should be evaluated, concentrating on these two main categories allows for general observations related to the applications' characteristics and the requirements that SSRs must meet to perform the different switching functions for each category.
Furthermore, narrowing the focus to high-power systems that use three-phase networks to energize the load(s) encompasses many of the most difficult commercial and industrial applications that use SSRs.
This information has been sourced, reviewed and adapted from materials provided by Sensata Technologies, Inc.
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