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1) How do you define a relay for a three-phase motor load? For example:
U: 400V
P: 15kW
Efficiency: 0.8 for a motor < 5kW
Efficiency: 0.9 for a motor > 5kW
Power factor: 0.8 for a motor < 5kW
Power factor: 0.9 for a motor > 5kW
V3 = 1.732
Nominal current absorbed by the motor:
I = 1500/ = 26.7A
(400 x 1.732 x 0.9 x 0.9)
The relay’s current rating must be 4 x I of the motor in order to withstand starting currents:
>>> 106.8A >>> a 125A relay must be used.
2) Advantages of Solid State Relays vs. Electromechanical Relays
With their semiconductors and a technology-based design, SSRs are more reliable, have a longer service life and less electromagnetic interference (EMI) than EMRs. They also have faster response times and are vibration and shock resistant, when compared to the EMR equivalent. This is because SSRs have no mechanical contacts to wear out or an arc, which is the primary cause for EMR failure.
3) Protection of the Solid State Relays from short-circuit of the load
4) How to choose the right heatsink
5) Installation without a heatsink or installation on the back plate of a cabinet
A curve without a heatsink is associated with SO/SC or SG types of solid state relays.
When installed on the back plate of a cabinet, it is essential that the relay/plate contact is correct.
A 150 mm x 150 mm x 3 mm aluminium back plate corresponds to approximately 4°/W.
A 300 mm x 300 mm x 3 mm aluminium back plate corresponds to approximately 2°/W.
In any case, we recommend testing and measuring the heat dissipation.
A steel plate has greater thermal resistance.
6) What does “zero voltage turn on” mean? (or synchronous or zero cross)
The relay will only turn on when the mains (output) voltage is near zero.
7) What does instantaneous or random turn on mean?
When the control voltage is on, the relay will switch on simultaneously, irrespective of the mains voltage value.
8) In what application can I use a zero voltage turn on vs. a random turn on relay?
Zero cross relays are used with resistive loads while random turn on relays are used with inductive loads (motors, transformers, coils, etc.).
9) Control is off but the load is still energised
-
Relay leakage current is too high in relation to the load holding current.
- Relay short circuit.
- The relay input is on because of the leakage current from the device controlling the relay (PLC, temperature controller, etc.)
- The relay’s output voltage has been underspecified.
10) Control is ON but load is off
This mainly happens with zero cross SSRs with DC control: the input signal is rectified but not filtered; at zero cross, the input signal is too low to make a valid control signal.
Solution: install a capacitor across the input.
11) You cannot start your SMCV/SMCW or SVTA/SWTA
Check whether there is a connection between terminals 5 & 6
12) Is it possible to place AC SSRs in parallel?
This is not possible because of the existing technology being used. In fact, current will always flow through one SSR only.
13) Is it possible to use DC relays in parallel?
This is possible, but it must comply with the following conditions:
- This can only be performed for thermal reasons (in fact, each individual SSR should be specified to switch the load current).
- We advise using DC SSRs with low turn on time: SOM range is more appropriate.
14) For an AC-51 3-phase load > 50A, can I use an SGT product, or are these relays limited to 50A because of the connections?
2 or 3 single-phase relays can be used. Our 75, 95, and 125A SGTs are used for loads with non-permanent peak currents (in lamps, motors, etc.), or to provide better protection.
15) What precautions should be taken when using our solid state relays at high altitudes (2,000 metres above sea level)?
Altitude has an impact on insulation and heat dissipation, both decrease as altitude increases.
Heat dissipation is also less, so it might be necessary to increase the size of the cooling element (heatsink) used with the SSR. Tests must be carried out using a nominal current, in an ambient temperature with a T°C measurement on the heatsink near the relay The temperature should not exceed 100°C.
For more detailed information.
16) How to ensure the connection of the solid state relay to the earth?
There are several ways to ensure the grounding of Solid State Relays: - solid state relays have an earth connection; or - their construction ensures a good connection to their mounting support (Din rail, plate, panel, etc.) which itself has an ensured earthing.
17) How to check the correct functionality of a solid state relay?
The easiest way is certainly to unplug the control and place an ammeter clamp on the power wire that goes to the load. On the control, you apply the required voltage in DC with a battery or an independent cell, or in AC with a power supply independent of the installation, the voltage sources having been checked beforehand. If you have a current without the control present, the relay is short-circuited, and if you have no output current while the control is applied, the relay does not work.
Watch the video tutorial
18) How to choose an SVTA power controller for a transformer?
SVTA range is designed for transformer control. The starting ramp must be adapted to the load (of the transformer) to avoid current pulse. In the worst case with a full conduction switch ON, the current can increase up to 30*In, in your case, the SVTA4694 will support this current pulse according to Non-repetitive peak overload current, but the margin of safety with this thyristor rating is not very important. Regards
19) Which parameters do you need to know in order to select the correct AC SSR for your application?
The following is a list of common parameters that are helpful to know ahead of time for proper SSR selection :
■Line voltage
■Load type
■Load rating
■Switching mode
■Control voltage
■Control current
■Mechanical packaging
■Ambient temperature
....
Technical tricks - Here is an example of questionnaire to use : https://www.celduc-relais.com/wp-content/PDF/Questions_AC_UK.pdf
20) How to install a Solid State Relays (SSR) to get optimal performances ?
In order to get the optimal performances of your solid state relay (SSR) you need to follow our mounting instructions. Here are our top 6 tips for an optimal installation of SSRs.
1-Heatsink mount to allow heat dissipation
2-Using a thermal interface
3-Do not mount SSR on a plastic or painted surface
4-Screw terminals tightening torque between 1,2 and 1,8Nm maximum
5- The heat sink should be positioned with the fins in a vertical position
6- Care must be taken when mounting multiple SSRs in a confined area