Selecting the right relay is important for long life and application efficiency.
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Here is guide to selecting and installing (SSRs) Solid State Relays.
A solid state relay or SSR is an electronic device that switches loads on or off when an external voltage or signal is applied to its control terminals. The lack of moving parts in an SSR allows it to switch much faster that electromechanical relays while avoiding the creation for electrical arcs. Solid Sate Relays are also silent, so users won't be subjected to the annoying noise associated with electromechanical relays. Furthermore, because SSRs have no moving parts or contacts that can wear out, SSRs last longer and require less maintenance than electromechanical relays.
SSRs are used in a variety of applications where quick, silent switching is advantageous. Some of these industries include: plastics/rubber, Food and beverage, processing, packaging, HVAC, semiconductor, oil/gas, transportation, conveyors, power or energy systems, printing, laboratory,kiln/oven, lighting, medical/pharmaceutical, and motion control.
How will your solid state relays be mounted/installed?
SSRs are available in a few mooting configurations. Printed circuit board (PCB) mont SSRs are limited in load size due to electric creepage and clearance distance and heat dissipation constraints.
Chassis munt SSRs are often installed to a back panel of an electrical panel but may also mount directly to a metal surface within a machine or appliance. Typically, the SSR can be mounted without a heat sink if the load is un 5 amps or under 8 amps if the mounting surface is an unpainted metal plate. To achieve the desired maximum SSR output rating, a sink sink must be properly sized and installed along with the SSR.
Engineers wo tont want to worry about properly sizing and mooting a heat sink can opt for an SSR with an integrated heat sink. These typically snap directly onto the DIN rail and are ready to wire up and use. Many of the more advanced Ssts, such as those enabling proportional control, are available in this design. Is it also easier to obtain agency approval with SSRs integrated heat sinks, which are typically cUL listed and carry horse power ratings.
How Many Poles Need to be Switched?
When selecting a Solid State Relay, you will need to know how many polkas (or lines of voyage) are to be switched to the load. If your application uses a DC load, you'll need a one pole DC switching SSR. For a single-phase AC load you'll need a one pole AC switching SSR. For a three phase AC load, you'll need to determine if you want to switch all three poles to the load or if you can get by with only switching 2 poles. This can be achieved with a two-pole or three-pole three phase switching SSR.
What is Your Load Voltage and Current?
You will need to determine the maximum AC or DC voltage and current for your load. These variables are typically found on the specification of your motor, heater or other electrical device. For AC loads, SSRs usually switch on phase at 120 or 240 VAC or for three phase applications: 208, 240, 480 or 600 VAC. Also factor in any potential power sags or surges in the application location when determining the minimum and maximum load voltage. DC loads are gong to be powered by DC voltage.
For motors, you will need to know the maximum horse power or kilowatt rainy at the respective voltage of your application. Check for these specs on the motor name plate.
What is the control voltage or input signal to turn the SSR on?
Next you'll need to determine another kind of voltage, the pick-up voltage that is required to energize your load as its turn off voltage (at which your load will de-energize). These are the critical control voltages for your SSR, and are usually found on your loads spec sheet. Unlike electromechanical relays or contractors which typically are controlled by a fixed voltage, SSrs have input ranges either VDC or dual VAC/VDC.
If you wish to proportionally control your load, you'll need some additional specs to choose the proper SSR. Proportional control is typically done with a 0-5, 1-5 or 0-10 VDC input or a 4-20mA control signal. Be sure to consult with a SSR manufacturer to determine the optimum out-put switching type for your load application, as it can get a bit sophisticated. Proportional output types include phase angle, distributed full cycle, burst full cycle, soft start and burst full cycle with soft start.
What is the Ambient temperature where the SSR will be installed or located?
The maximum SSR current rating depends upon the ambient temperature where is will be used and installed. Heightened temperatures can reduce the SSRs current rating. Any SSRs are mounted on a chassis and require a heat sink t ptomaine their performance. you will need to know the ambient temperature, as well as the mounting orientation upfront, to properly ensure the right type of heat sink is specified. Highly competent SSR manufacturers provide online tools to help match single or multiple SSRs with the appropriate heat sink. With Din rail mountable SSRs the heat sink is already integrated and the ratings are typically listed on the side of the device or data sheet for various ambient temperatures. Keep in mind that higher ambient temperatures will require you to de-rate the current rating of the Solid State Relay.
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