Solid state relays (SSRs) are increasingly being used in various industries, including plastics, packaging, food and beverage, HVAC, semiconductors, renewable and traditional energy, oil and gas, transportation, printing, laboratories, kilns and ovens, lighting, medical, and motion control. SSRs are often used to replace electromagnetic relays (EMRs) because they have no moving parts and a long service life. In addition, they will not be subject to contact erosion and electrical interference due to the presence of arcing on the contact surface.
Due to the multiple configurations of solid-state relays that can support various types of loads, designers must understand how to choose solid-state relays to meet their intended use. This is particularly evident in industrial applications, such as controlling inductive loads such as motors, water pumps, and fans, which require different types of relays than heating and lighting applications, as they are resistive loads.
This article will briefly discuss why SSR is an excellent choice for industrial and factory automation. Then, taking Carlo Gavazzi's devices as an example, the purpose, characteristics, and how to select these devices for applications were introduced.
Why use SSR?
Industrial and factory automation systems require switchgear with the following characteristics to meet requirements: low cost, high reliability, fast actuation time and no contact vibration or arc, minimal electromagnetic interference (EMI), resistance to harsh environments, and strong resistance to mechanical shock and vibration. SSR uses semiconductor devices to replace the armature and contacts of mechanical relays in switch operations, thus meeting these requirements. Due to its fully enclosed nature, SSR also has the characteristics of shock resistance, vibration resistance, moisture resistance, chemical corrosion resistance, and dust resistance. Therefore, the device has a long lifespan and high reliability.
Therefore, when selecting an SSR for an application, it is necessary to understand the controlled load and the basic characteristics of the SSR in order to match the application requirements with the relay specifications.
SSR control and load specifications
SSR can be controlled using AC or DC control voltage. DC controls use low voltage, typically 4 V to 32 V. They can also use 4 mA to 20 mA current loops or 1 VDC to 10 VDC analog inputs. The voltage range used for communication control is 24 VAC to 275 VAC.
SSR loads can be AC or DC. The maximum AC load voltage of SSR is up to 690 VAC, with a rated AC current of 125 A. The DC rating is 500 VDC and 100 A.
Electrical load type
Electrical loads are classified according to their main electrical characteristics. Motors, fans, and pumps are all inductive loads. The load current and voltage are not synchronized, and the current lags behind the voltage. Inductive loads will counteract changes in their load current by generating a back voltage potential called back electromotive force (EMF). Solid state relays used with inductive loads must be able to withstand these voltages.
Equipment such as heaters, ovens, electric stoves, dryers, and lighting fixtures belong to resistive loads. The voltage and current of resistive loads are in phase.
Capacitive loads can withstand changes in load voltage. The current and voltage in capacitive loads are not synchronized, with the current leading the voltage. Most switch mode power supplies and some medical devices (such as defibrillators) have capacitive loads. When voltage is first applied to a capacitive load, its impedance is very low, resulting in a large surge current.
The characteristics of each load determine the type of SSR required to control the load.