ECN Forum Forums General Topics of the Trade General Discussion Area 208 volt contactor

The night I installed this it was late. I wanted to go home, the customer wanted to go home so I didn't get ample time to look at everything. The original plug that was on the starter, yes it was pluged in, had a 250 volt plug on it. Perfect, mount the new starter, install the start stop and all should be well.
What I found the next morning was that the motor was wired for 480v. It would run but it ran wierd.
Now that it is re-wired for the 208 volt being supplied it is running fine.
It's been a while since I took something for granted (original wiring being 208 volt based on attached cord and plug). Another reason it is important to check everything and not take it for granted.

The shading coil is a rather large copper band around the laminations of the metal core of the contactor. When a magnetic field is present on the core from the main coil, it induces another current in the shading coil through basic transformer action. This current is about 90 degrees out of phase with the primary current and peaks when the when the current in the primary coil is crossing the zero point, therefore maintaing a magnetic field on the contactor keeping it closed duiring this time.

It the shading coil were to burn out, which happens quite often because of the large amount of current present on the coil, it cannot hold the contactor closed during the zero crossover causing a chatter to become obvious on the contactor.

From Wikipedia: If the coil is designed to be energized with AC, a small copper ring can be crimped to the end of the solenoid. This "shading ring" creates a small out-of-phase current, which increases the minimum pull on the armature during the AC cycle.[1]

One trick that I was taught if the shading coil was broken or fell off was to take some scrap wire and loop in where the shading coil was. Make a good connection at the ends and it should hold together long enough until the new coil that you ordered arrives.

Interesting.
I have seen big contactors with 2 coils. One is a low resistance coil that pulls in the armature and then the side switch flips to a higher resistance coil that holds it. You will see wires running around the outside of the coil to the switch. Get that wired wrong and it won't pick.

A shading coil is a copper loop that is inserted into the steel 'core' of the electromagnet assembly. The copper loop causes the flux in the steel core (that it surrounds) to change slowly. The result of this, is a portion of the steel core remains magnetized during the zero cross over point of the AC voltage.

A broken shading coil can cause a 60 cycle hum (not including a shading coil is one way to make an electric buzzer).

There are contactors available that are energized electrically, then held mechanically. They have a name but I cannot remember what they are called. I will check further for you.
This solved chattering problems in the past.

"latching relays"

Or "Mechanically Held Contactors"

Yes, mechanically held contactors. I got this from micromind today. I hope this helps.

"These devices are known as mechanically held contactors. I've seen them made by square D, GE, and others. They come in multi-pole (2 through 12 poles), and panelboard types. The panelboard type mounts inside of a panel, just like a main breaker. It turns the whole panel on and off.

The multipole type that I installed last was a Square D class 8903 type LX. It was 30 amp, 6 pole, and controlled 6-277 volt parking lot light circuits.

They work like this; if the contacts are open, the close coil is energized. As soon as the contacts close, there is a 'clearing contact' built into the unit that de-energizes the closing coil. The contacts are held closed by mechanical means. Even if all power is lost, the contacts will stay closed. The control voltage can be pulsed or continuous.

When the contacts need to be opened, control voltage is sent to the open coil. There is a similar clearing contact here as well. The contacts are now held open by mechanical means.

The main advantage here is no coil noise. Either coil is energized only momentarily, so no buzz or hum. The disadvantage is that if you're using a photocell or time clock, you'll need a small 'ice cube' relay to control the contactor coils. NO is connected to the close coil, NC to the open".

Courtesy Rob (micromind)

IBM called them a latch/trip relay (used in logic circuits). When relays were replaced by silicon they became flip latches. (AKA flip flops)