The following are the options to be considered when choosing an appropriate solid state relay according to actual needs:
1) Load voltage-AC or DC
2) Load current-maximum current and minimum current
3) Load type-resistance, inductance or capacitance
4) Input control signal-AC or DC
5) Installation method-PCB, panel or DIN rail installation
6) Ambient temperature-used to calculate the derating factor and the size of the radiator
7) International certification-Underwriters Laboratories (UL), Canadian Standards Association (CSA), British Telecommunications Approval Board (BABT), Verband Deutscher Elektrotechniker (VDE), Technischen Uberwachungs Vereine (TUV), Conformite Europeene (CE) or others.
Load voltage
The first thing to consider is whether the load voltage is AC or DC to determine whether to choose AC-SSR or DC-SSR. Secondly, the voltage of the load power supply should be considered. The voltage cannot be greater than the rated output voltage and less than the minimum voltage of the solid state relay. Then consider the size of the load voltage and the transient voltage. The load voltage refers to the steady-state voltage. The transient voltage refers to the maximum voltage that the output terminal of the SSR relay can withstand. When AC inductive load, single-phase motor load or three-phase motor load is switched or energized, the voltage at the output of the SSR switch may be twice the peak voltage of the power supply, and the voltage cannot be greater than the transient voltage SSR ground to prevent excessively high The impulse voltage damages the electronic switch. Therefore, when choosing SSR, it is best to leave a margin for the output voltage, and choose an SSR relay with an RC circuit to protect the solid state relay and optimize dv/dt.
RC circuit
RC circuit, also called RC filter, RC buffer or RC network, is a circuit composed of resistors and capacitors. It is recommended to choose a solid state relay with a varistor absorption circuit and an RC snubber circuit. The RC circuit prevents certain frequencies from passing, and allows other frequency signals to pass through to filter out interference signals. In addition, the RC circuit can also be used to reduce the rate of rise of output voltage (dv/dt), absorb surge voltage, suppress excessive transient voltage/current, and prevent solid state relays from being damaged by overvoltage. .
Load current:
The output current value of the solid state relay is the steady-state current flowing through the SSR output terminal, which is usually equal to the current of the load connected to the SSR output terminal. Since the switching elements of the SSR switch are very sensitive to temperature, and overcurrent will generate a lot of heat, the overload capability of the SSR is very weak. Therefore, the output current of the SSR relay should not exceed its rated output current, and the surge current should not exceed the overload capacity, especially for inductive/capacitive loads that are prone to surge current and the surge current generated by it. The power supply itself.
The output current needs a certain margin to avoid excessive inrush current, which will shorten the life of the solid state relay. For general resistive loads, the rated effective current value can be selected based on 60% of the nominal value. In addition, you can consider using fast fuses and air switches to protect the output circuit, or add an RC leakage circuit and a varistor (MOV) to the output of the relay. The selection specification of varistor is: for 220VAC SSR, choose 500V~600V MOV; for 380VAC SSR, choose 800V~900V MOV.
Inrush current:
When it is switched on, almost all controlled loads will generate a large inrush current. E.g:
1) Electric heating devices, such as incandescent lamps and electric stoves. They are purely resistive loads with positive stability coefficients, but their resistance is very small at low temperatures, so the current at startup will exceed the resistance several times. Steady-state current.
2) Certain types of lamps have low impedance when burned out.
3) When the motor is turned on, the rotor is locked and turned off, a large inrush current and voltage will be generated. The locked rotor is the rotation speed where the motor is still at zero speed, during which the torque is output, the power factor of the motor in the described situation will be very low, and the current can be as high as 7 times the rated current.
4) If the intermediate relay or solenoid valve cannot be reliably closed and bounced, a large surge current will also be generated.
5) When switching the capacitor bank or capacitor power supply, a similar short-circuit condition will be caused and a large current will be generated.
6) When the capacitor-commutated motor reverses, the capacitor voltage and the power supply voltage are superimposed on the output terminal of the SSR, and the SSR will withstand a surge voltage twice the power supply voltage.
Excessive surge current will damage the semiconductor switches inside the SSR. Therefore, when choosing a relay, you should first analyze the surge characteristics of the controlled load, so that the relay can withstand the surge current while ensuring steady-state operation. The rated current of solid state relays should be reduced according to actual requirements. If the selected relay needs to work in a place with frequent operation, long life, and high reliability, the rated current should be divided by 0.6 according to the known derating factor to ensure the reliability of operation. In addition, resistors or inductors can be connected in series to the output loop to further limit the current.
Note: Do not use the SSR surge current value as the basis for selecting the load starting current. Because the surge current value of the SSR relay is based on the surge current of the electronic switch, but the premise is that the power cycle is half (or one), that is, 10ms or 20ms.
Load type:
According to electrical impedance, the load can be divided into three types: resistive load type (or pure resistive load), inductive load type and capacitive load type. In ordinary electrical appliances, there are no pure inductive loads and pure capacitive loads, because these two types of loads do not provide active power. In a series-parallel circuit, if the capacitive reactance is greater than the inductive reactance, the circuit is a capacitive load. vice versa.
Resistive load:
In short, a load operated only by resistance type components is called a resistive load. However, some loads have low resistance at low temperatures, which can lead to larger starting currents. For example, when the electric furnace is first turned on, the current is 1.3-1.4 times the stable current. When the incandescent lamp is turned on, the current is 10 times larger than the steady current.
Q1: What is the characteristic of resistive load (when working)?
A1: In a DC circuit, the relationship between current and voltage conforms to the basic Ohm's law, I = U / R; in an AC circuit, the current phase is the same as the voltage phase (compared to the power supply).
Question 2: What are resistive loads?
A2: Heating devices heated by resistance (such as resistance stoves, ovens, electric water heaters, hot oil, etc.), and lamps that rely on resistance wires to emit light (such as iodine tungsten lamps, incandescent lamps, etc.).
Inductive load:
Generally speaking, inductive loads refer to loads that apply the principle of electromagnetic induction (with inductive parameters), such as high-power electrical products (such as refrigerators, air conditioners, etc.). The inductive load will increase the power factor of the circuit, and the current flowing through the inductive load will not change suddenly. When starting, the starting current required by the inductive load is much larger than the current required to maintain normal operation (about 3-7 times). For example, the starting current of an asynchronous motor is 5 to 7 times the rated value, and the starting current of a DC motor is slightly larger than that of an AC motor; some metal halide lamps have a maximum turn-on time of 10 minutes, and their pulse current is as high as the steady-state current 100 times.
In addition, when the power is turned on or off, the inductive load will generate a back electromotive force (usually 1-2 times the power supply voltage), and the back electromotive force (abbreviated as back electromotive force or abbreviated as CEMF) will be superimposed with the electromotive force. The power supply voltage, the final voltage is up to three times the power supply voltage. Therefore, when the load type is an inductive load, the output terminal of the solid state relay should be connected to a varistor whose withstand voltage is 1.6-1.9 times the load voltage. Back-EMF is an indeterminate value, it changes with changes in L and di/dt, and if the current rate of change (di/dt) is too high, SSR will be damaged. In practical applications, CEMF can be reduced by series inductance L, the size of L inductance depends on the size and cost.
Q3: What are the characteristics of the inductive load (at work)?
A3: The inductive load lags behind (current lags behind voltage). In a DC circuit, an inductive load causes current to flow, energy is stored in the inductor, and the current lags behind the voltage. In an AC circuit, the current phase lags the voltage phase (compared to the power supply), and the phase lags by a quarter of a period (or 90 degrees) at most.
Q4: What are inductive loads?
A4: Lamps that rely on energized gas to emit light (such as fluorescent lamps, high-pressure sodium lamps or HPS lamps, mercury lamps, metal halide lamps, etc.) and high-power electrical equipment (such as motor-based equipment, compressors, relays, etc.).
Capacitive load:
Generally, loads with capacitive parameters are called capacitive loads, and capacitive loads will reduce the power factor of the circuit. During charging or discharging, the capacitive load is equivalent to a short circuit, because the voltage across the capacitor cannot change suddenly.
Q5: What are the characteristics of the inductive load (at work)?
A5: Leading capacitive load (current lead voltage). In a DC circuit, a capacitive load prevents current from flowing, but can store energy. In an AC circuit, the current phase leads the voltage phase (compared to the power supply), and this phase can lead by a quarter cycle (or 90 degrees) at most.
Q6: What are inductive loads?
A6: Equipment with capacitors, such as compensation capacitors. And power control equipment, such as switching power supplies, IT equipment, etc.
How to choose solid state relay according to load type
1) For inductive and capacitive loads, if the dv / dt (voltage exponential rise rate) applied to the relay output terminal is large during the on of the AC solid state relay, it is recommended to use a solid state relay with a higher dv / dt / off.
2) For AC resistive loads and most AC inductive loads, zero-cross relays can be used to extend the life of loads and relays and reduce their own radio frequency interference.
3) As a phase output controller, random solid state relays should be used.
*Power factor:
In electrical engineering, the power factor of an AC power system is defined as the ratio of the actual power flowing to the apparent power circuit of the load, and it is a dimensionless number in the -1 closed interval 1. If it does not specify that the load power of general products is apparent power (including active power and reactive power). However, the general specifications of inductive loads usually give the magnitude of active power. For example, although fluorescent lamps are marked with 15 to 40 watts (its active power), the power consumed by their ballast is about 8 watts, so 15W to 40w should be added to 8W to calculate the total power. The inductive part (that is, the amount of reactive power) of the product can be calculated from the given power factor.
Input control signal:
1) Input control voltage: The input control voltage range is 3~32V.
2) Input control current: The input current of DC SSR and AC single-phase SSR is usually around 10mA, and the input current of AC three-phase SSR is usually around 30mA, or it can be customized to be less than 15mA.
3) Control frequency: The control operating frequency of AC solid state relays generally does not exceed 10HZ, and the control signal period of DC solid state relays should be greater than five times the sum of the relay "on time" and "off time".
Installation method:
In many cases, the load power will limit whether the SSR is mounted on the PCB, panel or DIN rail.
Ambient temperature:
When the relay is in the conducting state, it will withstand the dissipation power of P = V (on-state voltage drop s) × I (load current), and the load capacity of the SSR is greatly affected by the ambient temperature and its influence. Own temperature. If the ambient temperature is too high, the load capacity of the SSR will inevitably decrease. In addition, the SSR switch may be out of control or even permanently damaged. Therefore, it is necessary to set a certain margin according to the actual working environment and select a suitable radiator size to ensure heat dissipation conditions. For load currents greater than 5A, a radiator should be installed. For currents above 100A, the radiator and fan should be equipped with strong cooling devices. If the SSR relay is operated at high temperature (40°C~80°C) for a long time, the load current can be reduced according to the maximum output current and ambient temperature curve provided by the manufacturer to ensure normal operation, and the load current is usually controlled at Within 1/2 of the rated value.
*Derating factors:
The following table shows the recommended derating factor for the rated output current of solid state relays applied to various loads at room temperature (overload capacity and load surge current have been considered).
There are two ways to use the derating factor:
1) The rated current value of the solid state relay can be selected according to the derating factor of different environments and different load types. The rated current of the SSR relay is equal to the continuous current value of the load divided by the derating factor.
2) If a solid state relay is selected and the load type or environment changes, the load current should be adjusted according to the load curve and derating factor in a specific environment. The adjusted current multiplied by the derating factor must be lower than the rated value of the solid state relay.
In addition, when running SSR in applications that require more frequent operation, longer life, and more stable reliability performance, you need to further multiply the derating factor by 0.6 based on the data in the table. However, the load current should not be lower than the minimum output current of the solid state relay, otherwise the relay will not conduct or the output status is abnormal.
