Power Relays – The Electromagnetic Switches

The major plight of doubts for Engineer officers onboard a merchant ship is to tackle with electrical problems which, for most, go beyond the understanding.  It’s not always practical to look forward to the presence of Electrical Engineer to solve the minutest of these. It’s imperative for an engineer to understand the basic and simple understanding of the most common parts of an electrical circuit to head-start the fault finding in correct direction.

Power Relays – The Electromagnetic Switches

In this discussion, the most common part of an electrical circuit : Power relays. All across the electrical panels onboard a ship, we find a plethora of power relays and analog timers plugged in and silently controlling the operations of a floating giant. An understanding of these parts can help us a lot in simplifying our fault finding skills.

Power Relays

The word Relay as defined: An electrical actuator that allows a relatively small voltage or current to control a larger voltage or current. 
In even simple language relays are nothing but an electronic switch. When a small voltage, say 24V DC, is applied to a relay it makes or breaks a 220V AC circuit. So a small 24V DC voltage controls a large 220V AC voltage. This 24V DC may be even smaller and 220V AC can be even larger.

Construction of a Relay

The picture shows a relay inside a plastic casing. The outer casing is just for the protection of delicate parts of this power relay. The outer plastic casing once removed and the internal parts of the relay is much easy to understand.

The following pictures give a view of different parts of a relay. It consists of an electromagnetic coil and an armature. The armature is hinged at a point across which it can move towards or away from the coil. A spring is used on the opposite of armature across the hinge and it keeps the armature pulled away from the coil when the coil is not energized.

When the coil is not energized or in de-energized state, the terminals of armature keep in contact with the NC (Normally Closed) contacts and away from the NO (Normally Open) contacts. When a small excitation voltage is applied to the coil terminals the electromagnetism in the coil pulls the armature towards itself against the spring force. When the armature is pulled down, the terminals of armature move downwards too thus breaking the NC contacts and touching the NO contacts. The coil can be energized with a voltage as low as 6V. The NC and NO terminal can be used to make or break a voltage of as high as 220V.

Terminals of a Power Relay

The following is the schematic  diagram of power relays in both the energized and DE-energized state.

Power Relay De-energized

Power Relay energized

Now, if a load is connected across the common and NC contact of the relay, the following happens :

When the coil is de-energized the common terminal and NC terminal are closed and thus the load receives the power supply.

As soon as the coil is energised the armature is pulled down thus breaking the NC contact and the load circuit is broken. The load doesnot receive power and hence switches off.

On the contrary, if a load is connected across the common and NO contact of the relay, the following happens :

When the coil is de-energized, the common terminal and NO terminal are open and thus the load circuit is broken. The load doesn’t receive power and hence remains switched off.

As soon as the coil is energized, the armature is pulled down thus touching the NO contact and the load circuit is energized. The load receives power and hence switches ON.

As already explained, to energize the coil we need a relatively small voltage and current. In lieu of that the relay’s NC and NO contact can handle a relatively larger current and voltage.  A number of different types of relay comes with different coil and terminal parameters.

Usually the coil is energized by either a small DC supply or a 220V AC control voltage supply. The NC, NO and Common terminals are made to handle the usual 440V AC power supply for the load which is required to be switched ON or off.

Here a small doubt may arise among readers, how a coil would be energized with an AC supply. The characteristic of AC voltage to continuously change its direction would cause a coil to continuously change it’s polarity and thus there won’t be any change in position of armature.

An important point to understand here is the resilience of the spring. Though the polarity of an AC coil changes at a high frequency of 60 or 50 times per second (60 Hz or 50 Hz supply), the spring has it’s own delay time. So it can’t pull back the armature to NC contact at same rate. Hence the armature remains pulled towards the coil.

Yet, the AC relays suffer from a characteristic noise of chattering, because of continuously loading and unloading of spring. In addition to it the continuously reversing force on the spring may cause it to fail at much lesser time. To tackle these problems of an AC relay has two coils to make a transformer. The wire of the traditional cil is the transformer primary. The secondary of the transformer looks like a D shaped copper washer or ring. Some magnetism from the primary coil produces current inside the copper ring. The current in the ring or the secondary is actually delayed compared to the current in the primary coil to keep the core always partially magnetized. The lever, hence, won’ chatter against the core because, while the AC is flowing, the continuously magnetized core will never release it.

While choosing a relay it is to be well understood that the coil and terminal parameters should be well matched with the intended use of the relay. 

A DC relay should only be energized by the specified DC voltage and the voltage against NC, NO and Common terminal should never go beyond the rated terminal voltage. If the terminal voltage goes beyond the rated terminal voltage the NC and NO contacts may get burnt or welded at high temperature thus failing the switching action of the armature when the coil is energized or DE-energized.

The following is a snapshot of different ratings of both AC and DC relays with their coil and contact parameters

Coil and Contact parameters of a power relay

From the above table it can be well inferred that a coil voltage as small as 6V AC can switch 250V AC or 30V DC at the terminal. As discussed earlier many different coil voltage can be chosen upon as given in the table depending upon the usage. The Power relay discussed above is a four pole type relay. Meaning, this relay can be used to switch 4 power circuits at one time from single control voltage. Similarly we have 2 pole and single pole relay also.

Single Pole and 4 Pole power Relay

Usually onboard a ship, relays with 220V AC coil voltage and 440 V AC terminal voltage is used. Following is few example of use of relay:

• Control signal of 220V AC signaling a relay to switch on the Boiler FD fan.
• Control Signal of 220V AC signaling a relay to start the bilge pump on receiving high level signal from float switch in the ER bilges.

The vast use of relays in almost all machineries onboard can only be explained better by a good understanding of its working. Power relays are small yet effective link in the control systems of machineries.