Solid state relays (SSRs) are increasingly used in a wide variety of industries, including plastics, packaging, food and beverage, HVAC, semiconductors, renewables and conventional energy, oil and natural gas, transportation, printing, laboratories, kilns and ovens, lighting, medical and motion control. SSRs are often used in lieu of electromagnetic relays (EMRs) because they have no moving parts and have long service life. In addition, they are not subject to contact erosion and electrical interference due to the presence of arcing on the contact surfaces.
Because solid-state relays are available in a variety of configurations to support a variety of load types, designers must understand how to select solid-state relays to meet their intended use. This is particularly true in industrial applications where inductive loads such as control motors, pumps and fans require different types of relays than heating and lighting applications, which are resistive loads.
This paper will briefly discuss why SSR is the best choice for industrial and plant automation. Then, taking the devices of Carloas an example, the application, characteristics and how to select these devices for application are introduced.
Why use SSR?
Industrial and plant automation systems require switchgear to have the following characteristics to meet the requirements: low cost, high reliability, fast actuation time and non-contact chatter or arc, minimum electromagnetic interference (EMI), immunity to harsh environments, and strong resistance to mechanical shock and vibration. SSR fulfills these requirements by replacing the armature and contacts of mechanical relays with semiconductor devices in switch operation. Since SSR is totally enclosed, it is also resistant to impact, vibration, moisture, chemicals and dust. Therefore, the device has long service life and high reliability.
Therefore, when selecting SSR for an application, it is necessary to understand the load being controlled and the basic characteristics of the SSR so that the application requirements match the relay specifications.
SSR Control and Load Specifications
SSR can be controlled with AC or DC control voltage. DC control uses 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 AC control is 24 VAC to 275 VAC.
SSR load can be AC or DC. The SSR has a maximum AC load voltage up to 690 VAC and a rated AC current of 125 A. The DC ratings are 500 VDC and 100 A.
Electrical load type
Electrical loads are classified according to their main electrical characteristics. Motors, fans and pumps are inductive loads. The load current and voltage are not synchronized, and the current lags behind the voltage. An inductive load counteracts changes in its load current by producing a counter-voltage potential called the counter electromotive force (EMF). Solid state relays used with inductive loads must be capable of withstanding these voltages.
Equipment such as heaters, ovens, stoves, dryers, and lamps are resistive loads. The voltage and current of resistive load are in phase.
Capacitive loads withstand variations in load voltage. The current and voltage in the capacitive load are not synchronized, and the current leads the voltage. Most switching power supplies and some medical devices such as defibrillators have capacitive loads. When the voltage is first applied to a capacitive load, its impedance is very low and therefore a large inrush current is generated.
The characteristics of each load determine the type of SSR required to control the load.
Type of SSR
There are five types of commonly used SSRs (Figure 1): zero or zero crossing, instantaneous or random, DC, peak, and phase angle or analog.