Solid State Relays

How to calculate a relay for a three phase motor load ?

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 =               15000                   = 26,7 A
       400 x 1,732 x 0,9 x 0,9

The current rating of the relay must be 4 x I of the motor in order to withstand starting currents:
>>> 106,8 A >>> it's necessary to take a 125 A relay.

Advantages of Solid State Relays in comparison to a electro-mechanical relays ?
  • Because of their semiconductor technology-based design, SSRs provide a higher degree of reliability, longer life and reduced electromagnetic interference (EMI), as well as faster response times and vibration and shock resistance, when compared to the EMR counterparts. This is due to the fact that SSRs have no mechanical contacts to wear out or arc, which is the primary cause for EMR failure.

Protection of the Solid State Relays from short-circuit of the load

How to choose the right heatsink ?

Mounting without heatsink possible ?

A curve without heatsink is given for the SO/SC or SG type of solid state relays. When mounted on the back plate of a cabinet, the relay/plate contact must be correct. An aluminium back plate of 150mm x 150mm x 3 mm corresponds to approximately 4°/W An aluminium back plate of 300mm x 300mm x 3 mm corresponds to approximately 2°/W In all cases, it is recommended to perform a test and measure the heat dissipation. A steel plate will have greater thermal resistance..

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.

What does instantaneous or random turn-on mean?

When the control voltage is on the relay will switch on at the same moment, whatever the mains voltage value.

In what application would I use a zero voltage turn-on v. 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.).

I am not driving my relay and the load is powered - what are the reasons?
  • Relay leakage current too high compared to the load holding current
  • Short circuit relay
  • Leakage current on the control device of the solid-state relay
  • Relay undersized in voltage

I am driving my relay and the load is not powered - what is the reason?

Synchronous relay and continuous control: rectified unfiltered control Solution: put capacitor on order

You cannot start your SMCV / SMCW and SVTA / SWTA

Check that there is a connection between terminals 5 and 6 of the control terminal block.

Is it possible to put AC SSRs in parallel?

This is not possible because of the technology used. In fact current will always go through one SSR only

Is it possible to use DC relays in parallel?

This is possible but you should respect some conditions: This must be done for thermal reasons only (in fact each SSR should be specified to switch alone the load current) This is better to use DC SSRs with low turn-on time: SOM range is more suitable

For a three-phase AC-51 application, for a current&gt; 50A, can I use an SGT - SVT, or these relays are limited to 50A by the connections?/p&gt;

For "heating" applications, it is preferable to use 2 or 3 single-phase relays. Our SGT / SVT in 75, 95, and 125A are suitable for applications with non-permanent peak currents (lamps, motors, ...) or for better protection.

What precautions should be taken when using our solid state relays in the mountains or at altitude ( 2000m above sea level)

Altitude affects insulation and heat dissipation, both of which decrease as altitude increases. More informations.

How to ensure the connection of the solid state relay to the earth?

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..

How to choose the SVTA power controller for a transformer?

SVTA range is designed for tranformer 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

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