Electromagnetic Relay Working Principle & Testing

How does an electromagnetic relay work?
As the below figure showed, electromagnetic relay consists of electromagnet, armature, spring, movable contact and stationary contact.
Usually an electromagnetic relay has two circuits, low-voltage control circuit and high-voltage working circuit.The low-voltage control circuit includes an electromagnetic relay coil, a low-voltage power supply and a switch. The high-voltage working circuit includes a high-voltage power supply, a motor and the contacts of the electromagnetic relay.
The working principle of electromagnetic relays is not complicated, and it operates mainly according to the principle of electromagnetic induction. Switching on the power in the low-voltage control circuit, the current goes through the coil of the electromagnet to generate a magnetic field. Then the armature generates a suction force to making the movable contact and stationary contact touching. Thus the working circuit is powered on and the motor begins to work. When switching off the power in the low-voltage control circuit, the current in the coil will disappear and the armature under the action of the spring will separate the movable contact and stationary contact. The working circuit is disconnected and the motor stops working.

Generally speaking, an common electromagnetic relay uses electromagnet to control “on” or “off” status of the operating circuit. When placing voltage to both ends of a coil, the coil will be flowed with current and generate electromagnetic effect. The electromagnet will attract armature to the iron core against tension of spring, so as to pull the movable contact of the armature to the stationary contact (normally open contact, or NO). When cutting off power, attraction of the electromagnet will disappear and the armature will restore its position under tension of the spring to release the movable contract from the stationary contact (normally closed contract or NC). The pulling and releasing are used to control opening and closing of the circuit. Normally open and closed contacts respectively refer to the stationary contract is in the state of “on” when the coil is cut off from power and the stationery contract is in the state of “off” when the coil is connected to power.

How to test an electromagnetic relay?
After knowing about the electromagnetic relay' s working characteristics, it is really helpful to learn how to test an EMR, thus you can figure out whether the electromagnetic relay good, or check whether there is any problem with the EMR?

1. Testing coil resistance
   Use multimeter to measure resistance of relay coil and judge whether the coil is in a state of open circuit. Resistance of the relay coil is closely related to its operating voltage and operating current. Operating voltage and operating current of the coil can be calculated according to its resistance.
2. Testing contact resistance
   Turn the multimeter to its resistance mode and use it to measure resistance of normally closed contact and movable contact. Their resistance is supposed to be zero. If the resistance is unstable or bigger than the value, it means the contact is under the condition of poor contact. If the resistance of the normally open contact and movable contact appears infinite, the condition shall be judged as contact adhesion. By this way, users can distinguish which one is normally closed contact, which one is normally open contact and whether the relay is in good condition (especially for used relay).
3. Testing pull-in voltage and pull-in current
   Connect the adjustable regulation power supply to the relay and input a set of voltages to the relay. Also, connect the ampere meter into the power supply circuit for monitoring. Turn up voltage slowly, and when hearing pull-in sound of the relay, write down the pull-in voltage and current. For accuracy, try more times and calculate its average value.
4. Testing release voltage and release current
   Carry out the test in foresaid ways. When the relay is pulled in, gradually reduce the voltage of power supply. When hearing the release sound of the relay again, write down the voltage and current. Generally, release voltage of the relay is 10-50% of the pull-in voltage. If the release voltage is too low (lower than 1/10 of the pull-in voltage) it will not be able to work normally. It will have negative influence on stability of the circuit and reliability of the operation.