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> What type of SSR is used to switch capacitive load
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Ronnie Haiduk
post Oct 19 2005, 01:29 PM
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All capacitive loads should be switched with zero voltage turn on SSRs....the reason for this is that the capacitor appears as a very low impedance when it is first energized (very nearly a short circuit), thus causing significant inrush current if the line voltage is very high. Turning it on with a zero turn on SSR applies voltage to the cap when the SSR is within the zero voltage "window" which is approximately in the range of 5 to 25 volts depending on the version of SSR, rather than a higher line voltage value like the peak of the AC line voltage. By turning on the cap at a relatively low voltage compared to the AC line voltage, the inrush current through the cap and SSR is reduced significantly compared to turning it on at higher line voltages.

The rate of rise of current or di/dt is also a factor when switching a capacitive load. The larger the capacitor value and the higher the applied voltage when energized, the greater the di/dt. If the di/dt value of the SSR is exceeded, it means that the load demanded more current quicker than the SSR output semiconductor could supply it, which results in a hot spot in the die's silicon and the die melts at that spot and is destroyed (shorted, and possibly opened if the I squared t value is also exceeded). Basically, this is a function of the turn on time of the output SCRs verses the rate at which the load demands or accepts current.....the turn on time of the SCRs effects this, with conduction beginning near the gate area and spreading across the die.

After the initial turn on and resulting inrush current driven by the "window" voltage value, the charging current into the cap rises at the same rate as the line voltage rather than being a step function based upon the line voltage and cap impedance (capacitance).which limits the inrush current. The reduction of inrush current means that the cap is not stressed to the same extent, and the SSR can be sized for the smaller inrush current and lower di/dt (rate of change of current), and lower value transients are created.

Here is some additional calculations to demonstrate the importance of the zero turn on function of the SSR:

The impedance of a capacitor can be calculated as follows: X = 1/ (2) (pi) (f) , where X is in ohms, pi=3.1415, f is in Hz, and C is in farads. For the application noted, the capacitor's impedance would be calculated as 1/ (2) (3.1415) (50) (.00004) = 79.5 ohms. With a maximum of 600 volts AC peak applied, the peak continuous current through the cap would be I = V/R or 600/79.5 = 7.5 amps. The rms current would be 600/1.414 x 79.5 = 5.33 amps rms.

However, when the capacitor is first energized, it appears as a very low impedance and substantial inrush current can flow. Inrush current can be calculated as follows: Imax = 1.414 x V x square root of C / L where V is the line voltage at the instant of turn on, C is the capacitance in the system in microfarads and L is the inductance of the system in microhenries. In this application the capacitance is given as 40 microfarads, but the inductance of the wiring is not known or given. Assuming a very low value of inductance (1 micro henry), yields the following inrush current: Imax = 1.414 x 600 x (40/1) = 5365 amps. As should be obvious, any additional inductance in the system serves to limit the current as would any series resistance. Note: a non-zero voltage turn on relay (random or asynchronous) can turn on at any point in the AC sine wave, thus it is possible that at some time during the cycling of the load, a random relay may turn on at the peak of the line voltage, resulting in the very high inrush current calculated above......

Using a zero voltage turn on SSR with a maximum zero turn on inhibit voltage (upper window value) of 25 volts would give the following inrush current:
Imax = 1.414 x 25 x (40/1) = 223 amps.....substantially lower due to the lower line voltage at the moment of turn on of the SSR. Again, it should be apparent that any increase in system inductance or series resistance would serve to reduce the inrush current.

Selecting the SSR then becomes a matter of determining both the continuous current flow and ambient temperatures, as well as the inrush current that the SSR must withstand. Generally, the inrush current will be the limiting factor, therefore it is important to understand the actual inrush current at the turn on value (zero inhibit voltage in this case).

Crouzet SSRs have very good surge withstand capabilities. Single cycle surge withstand capabilities (in peak amps) are ten times the continuous rating, and then derate exponentially to twice the continuous rating at 1 second. In other words, a 50 amp SSR can withstand 500 amps peak (not rms) for 1 AC cycle. A 25 amp SSR can withstand 250 amps. So in the above example, a 25 amp SSR should work OK. However, since few customers actually know what their inrush currents are, we recommend that an SSR with a surge rating of twice the expected surge be applied to provide adequate head room in the event of an underestimation of the true surge value.


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