Electromechanical Contactors For Heavy-duty Ac Motors: Selection And Application

By Author: Meba Electric
Total Articles: 14
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Relays are frequently used by engineers when isolated switching of relatively large voltages and currents with a modest voltage signal is required. The relay is activated by a standard low-voltage switch, which then turns on the high-power supply. Solid-state relays (SSRs) prevent contact wear and arcing whereas electromechanical relays (EMRs) are inexpensive and can tolerate relatively high voltages. Relays are replaced by an outside ac unit contactor.

Both kinds, however, face difficulties when dealing with continuous switching of several hundred volts and tens of amps (and above). The contacts of the EMR are quickly worn out by arcing at these high loads, while SSRs are overheated by leakage currents. For these in-demand applications, designers want a different solution.

Relays can be robustly replaced with the less common electromechanical contactor (EMC). The gadgets use tried-and-true technology, and there are many reliable sources to choose from. Without a thorough understanding of how the EMC operates, the selection process quickly gets complex because of the large number of alternatives.

The distinction ...
... between carrier ac contactor and electromechanical relays

It is impossible to use a switch to turn on and off high-power equipment, such as a large three-phase motor since it is exposed to the whole circuit current when closed. The switch overheats while in use and arcs dangerously when switched. The approach is to activate the high-power circuit using a low-power circuit that is controlled by a standard switch. The EMR was created with this in mind.

In EMRs, the usually closed (NC) or ordinarily open (NO) contacts are opened or closed using a movable core that receives an impulse from a magnetic field generated by a coil that is powered by a low-power circuit. A DC or AC load can be switched using the EMRs up to their maximum rating. EMRs' inexpensive cost and assured isolation at any applied voltage below the device's dielectric rating are its main advantages.

The amount of electricity that an EMR can manage has a limit, though. The EMC is an alternative, a powerful, tough industrial relay intended to reliably switch heavy loads over tens of millions of cycles.

EMCs are frequently built with mechanisms to regulate and suppress the arc created while switching under a strong load, and they may be securely connected to high-current demand devices. The devices are almost entirely equipped with NO contacts, but NC contacts are available and employ the same energized coil/moving core activation as relays. When the EMC is powered down, NO contacts make sure that the contacts flip to open, shutting off the supply to the high current-draw device. The devices have a single pair of contacts, commonly known as a pole, or many pairs of contacts.

Deciding to choose an EMC versus an EMR is rather simple. EMCs are the sole alternative for high-load applications, albeit being more costly. Once it is decided that an EMC is required, choosing the optimal EMC for the task becomes more challenging. Finding the peak load current needed at the application's operating voltage is the best place to start. This will then calculate the necessary current load capability of the
eaircon contactor.

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