Contrasting AC and DC relay specifications

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Relay voltage specifications are often misunderstood as there are several closely related parameters that are easy to misinterpret. The root of the confusion may be attributed to a misunderstanding of the relay itself. Recall that there are two electrical components inside a relay including the contacts and the coil. As the coil is energized, the mechanical armature physically and forces the contact(s) to change state.

A classic error is to conflate the contact specifications with the coil specifications.

Let’s start with the simple coil specification. The coil of a relay is designed for either AC or DC operation. This is a binary specification relating to the physical construction of the coil. A relay coil designed for DC operation will not work with an AC power. You can try, but all that will happen is the relay will buzz with no motion on the armature. The DC relay’s armature will not move as the coil magnetic circuit is missing a critical component known as the shade pole winding – a topic for another day. The relay in this picture is an AC relay. The copper shade pole winding looks like the letter D.

Once again, the coil specification is generally considered a binary specification. In most cases you should select a relay with the coil appropriate for voltage used in your design. This will ensure the relay operates as expected. For example, the relay will hold when subject to mechanical vibration in mobile equipment or when subject to vibrating machinery such as an industrial air compressor.

The contact specifications aren’t nearly as straightforward as the coil specifications. You will often find relays with multiple voltages. As an example, a typical relay such as the one shown above has three voltage rating including:

  • 110 VAC, 10 A resistive / 7.5 A inductive
  • 220 VAC, 7.5 A resistive / 5 A inductive
  • 30 VDC, 10 A resistive / 5 A inductive

Additional constraints are included describing the power factor of the inductive load and the L/R time constant.

This complexity captures the physics associated with opening a switch under load. Recall that an electrical arc forms then the contacts are opened. This arc is a function of the voltage, type of load, and the type of current.

The nature of the arc extinguishing is summarized in this graphic:

Observe that higher voltage and more inductive loads cause arcs that are difficult to extinguish. Finally, a DC arc is difficult to extinguish. Consequently, that “10 Amp” relay is deratted for 5A with an inductive load in a 220 VAC system. It is further deratted to 30 V and 5A in a DC system.

It very important that the relay be able to open and quickly extinguish the arc. Prolonged arcing can quickly destroy the relay’s contacts. The contacts can overheat. In extreme cases the load will not be disconnected when the relay’s contacts open. This is a risk to the equipment as well as the operator.

For reference, here is a picture of contactor rated for 10 A in 250 VDC system. You can see that is considerably larger than the previous relay.

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