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How to reduce a 24V DC Power Supply

The introduction of CA7-9E…55E electronic coil contactors and CS7E control relays with electronic coils helps to reduce the size of the 24 VDC power supply by reducing the demand of the coil during closing of the contactor. However, this still leaves the customer with a 200 watt inrush on conventional two winding 24 VDC coils for CA7-60…85 and 325 watts needed for a CA7-97 (see E-catalog).

Furtively, we have one more option for DC control circuits and that is an external electronic interface. The CA7 contactor family includes a CRI7E-24 accessory (see diagram below).

This device allows the customer to use a 110 or 240VAC coil which can be taken from the line voltage or a control circuit transformer (CCT) and still use 24 VDC in the control circuit to determine when the contactor should pull-in (start) or drop-out. Simply connect the CRI7E-24 to the CA7 coil terminal A1 and then connect 120 VAC (Hot) to the CRI7E-24 terminal A1 and 120V(N) to A2 as normal for a 120V coil (similar connection for 240VAC coil).

The device will not conduct the 120 VAC to the contactor A1-A2 terminals and nothing will happen until 24 VDC is applied to terminals E1 and E2. The E1-E2 terminals of the CRI7E-24 are connected internally to the gate of an SCR which only conducts when the 24 VDC gate is energized.

This method places the coil burden on the 120 VAC and everyone knows that a CCT is much less expensive than up-sizing a 24 VDC power supply. There is only a 15ma burden placed on the DC power supply or PLC card directly connected to E1 & E2 terminals of the CRI7E-24. Now that represents advancement in DC control circuit design for small HP contactors.

100 HP and larger contactors increase the pull-in (pick-up) demand on DC power supplies and this factor is often overlooked by designers who focus on the hold-in or continuous current requirements which leads to coil burn-out. The Sprecher + Schuh CA9-EI contactor incorporates a 24 VDC electronic interface which can be used to avoid high power demands on the DC power supply and therefore reduce cost while increasing the reliability of the system. When the CA9-EI option is chosen for a 24 VDC application, an electronic interface option (similar to CRI7E) is built-in to every CA9-EI contactor.