3 Phase Solid State Relay: Working Principle, Advantages And Applications

In this article we will provide all the knowledge about three-phase solid state relay from working principle, advantages to applications. If it is a good article, please don’t forget to share it with your friends and now let’s start..

Table of Contents

What is a 3 phase solid state relay?

The working principle of a three-phase solid-state relay is the same as that of a single-phase solid-state relay, both utilizing semiconductor components (such as two anti paralleled thyristors or triacs, two anti-parallel thyristor is used in high current  SSR above 35 amps, triac is used in low current SSR below 35 amps) to control the on/off state of the load. The difference lies in the number of thyristors or triacs used: a three-phase solid-state relay employs three thyristors or triacs connected in parallel, whereas a single-phase SSR has only one. When a control signal is applied to the control terminal of a three-phase solid-state relay, it simultaneously opens or closes the three parallel thyristors or triacs, thereby allowing the controlled three-phase power to be switched on or off simultaneously. This also means that a three-phase SSR is essentially a combination of three single-phase solid-state relays. Therefore, the price of a three-phase solid-state relay is typically around three times that of a single-phase solid-state relay.

3 phase solid state relay circuit diagram

Advantages of 3 phase solid state relay

When talking about the advantages of three-phase SSR, we always compare it with three-phase AC contactor or mechanical relay, so below, you will find its 7 major advantages:

  1. The power consumption of the control part is low: The power consumption of mechanical relays is usually about 1-2W, while the power consumption of solid-state relays is only about 0.1-1W. The voltage drop at the SSR control terminal is about 1.5V, and the current can reach at least 20mA, so according to calculation ( voltage*current), assuming it only takes a few milliwatts to control a heavy load. But if you look at it from the controlled end, SSR will consume more energy because the voltage drop is about 1.5V, which means if the current passing through is large, the power consumption will also be high, which is why SSR should always be used together with the heatsink. The voltage drop across the AC contactor can be considered to be zero or very little and therefore does not generate excessive heat.
  2. Long lifespan: The 3 phase SSR is same as the single phase SSR, they are both not having moving parts, this feature makes it have a long lifespan, generally the life of solid state relays is usually between 100,000 and 5 million operations. So you can use it for 5-10 years.
  3. Fast switching speed: SSRs feature fast switching speeds of up to 120 times per second, enabling fast response times and precise control of load switching, which is critical for applications requiring fast regulation such as precise temperature control.
  4. Noiseless operation: Unlike electromechanical relays that produce audible noise during operation, three-phase solid state relays operate quietly and are suitable for noise-sensitive environments.
  5. Wide load voltage: The three-phase SSR load side voltage range is very wide, from 24VAC to 480VAC. This wide load voltage makes it suitable for many applications without being limited to a specific voltage.
  6. Strong vibration resistance: As a semiconductor switch, it has no moving parts, allowing it to withstand high vibration environments.
  7. Safety: 3 phase SSR has enhanced safety features such as isolation (its control part and controlled part are isolated by optocoupler, which can block voltages below 4000V), reducing the risk of electrical hazards and ensuring the safety of personnel and equipment.

Types of 3 phase SSR

From the switching mode aspect, the 3 phase SSR can be divided into zero crossing type and random turn on type.

Zero crossing SSR switches the load on and off only when the AC voltage crosses zero, this can reduce the risk of electrical noise and extend the lifespan of the connected equipment.

Random turn on SSR switches the load immediately once the control voltage is applied, allowing for more precise control of power delivery.

3 phase solid state relay zero-crossing and random turn-on output waveforms

From the perspective of control methods, 3 phase SSR can be DC AC type and AC AC type. DC AC SSR uses DC3-32V to switch on or off the 3 phase AC voltage, while AC AC SSR uses AC70-280V to control the load part.

3 phase SSR wiring diagram

For three-phase SSR wiring diagram knowledge, read this article on our website: “How to Wire a Solid State Relay?“.

3 phase solid state relay wiring diagram-1
3 phase solid state relay wiring diagram-2

3 phase SSR applications

Due to the significantly higher power and efficiency of three-phase electricity compared to single-phase electricity (the power of three-phase electricity is approximately 1.732 times that of single-phase electricity), using three-phase electricity is a better choice when operating machinery or equipment with a high rated power. Thus, we can use the 3 phases SSR in many applications or equipment, its typical applications are as following:

Heating controls: They are commonly used in three-phase heating circuits such as professional cooking equipment, plastic machinery, packaging machinery and laboratory ovens.

Solid state relay with temperature controller and temperature sensor
  • Motor Control: Three-phase SSR is suitable for motor control applications, especially inductive loads.
  • Industrial Applications: They can be found in thermoforming machines, climate chambers, plastic extruders, welding ovens, air handling units and shrink tunnels.
  • Other uses: Other applications include blow molding machines, coffee machines, conveyor trucks, industrial furnaces and plastic injection machines.


In summary, the three-phase solid-state relay (SSR) is an advanced electrical switching device that has significant advantages over traditional relays, including high efficiency, low power consumption, fast operation, and long life. Its adaptability and safety features make it suitable for a wide range of applications from heating control to motor control and various industrial machinery. Three-phase SSRs are capable of handling high power loads efficiently and reliably, making them the first choice for many industrial and commercial environments.

Are you in search of top-tier, cost-effective three-phase solid state relays (SSRs) from reliable Chinese manufacturers? Look no further than Lorentzzi®—Your Premier Choice for Quality and Value! Contact us now for your 3 phase SSR requirements!

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