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Re: Contactor
Re: Contactor -- ongjie	Post Reply	Top of thread	Forum

Posted by: Bruiser ® 05/10/2007, 01:08:41 Author Profile eMail author Edit

Randykimball has covered the subject well! If you're talking about a small, dc operated control relay, there may be one other thing to check. This type of relay is often fitted with a 'back emf diode' fitted in parallel with the coil. The diode does not conduct when the coil is energised, but when it's de-energised the diode 'snubs' the back emf generated by the coil. This protects the controller from harmful voltage spikes. If the dc supply to the coil is reverse polarity, the diode will conduct, almost a short circuit. Depending on what stage it's at, a voltage of 0.7 or 1.4V would be observed at the terminals. If it's steamed away for a while, there'll be burnt components.

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Re: Contactor
Re: Contactor -- ongjie	Post Reply	Top of thread	Forum

Posted by: randykimball ® 05/09/2007, 22:36:16 Author Profile eMail author Edit

8 times out of 10 when contactors get hot it is because of a loose wire at a terminal on the contactor block. Once the contactor has gotten hot the contacts themselves are often oxodized and need cleaned or replaced. If not, they can become a source of the heat from that point on. Be sure to trim off the end of a wire that has been hot and get a fresh length of cooper to fit into the terminal block. If the terminal block is damaged by heat, replace it. You want a good connection. Aluminum wire is asking for trouble at a terminal block in a contactor. While aluminum is an excellent conductor it has no tolerance for a loose connection and will oxodize too quickly. Heat is caused by high resistance at the loose connection or damaged contact. Whatever section of a circuit has the highest resistance will be the place that creates the most heat. You want that to be a motor, heater element, lamp, or transformer, .. not the contactor block. 1 out of 10 times it is caused by using a contactor not large enough for the current load it is expected to carry (wrong contactor for the motor size). .... or a motor with burned windings... (same effect) The more often a contactor operates, the more important it is that it is properly sized for the motor. If a contactor is operated extremely often it is wise to size it some larger than the normally required size for that motor or load current draw. The last chief reason for hot contactors is: If the coil in the contactor is weak or is the wrong kind the contactor may vibrate (chatter the contacts). This will cause the contacts to burn. This is often the case when the contactor coil is rated for 240VAC and the supply voltage is 220VAC or 208VAC. This is the difference in Delta and "Y" supply grid at your user transformer cluster or bank. This problem is common. Frequently in America the power supply company provides a 208VAC 3 phase grid for comerical use but most machine tools require the 240VAC 3 phase. This causes machines to trip off line often and contactors with coils rated for the full 240VAC to chatter and burn the contacts. It is a simple matter to change to the correctly rated coil, if enough damange has not made it necessary to replace the whole contactor. Some newer machine tools will operate on either grid type by doing their own conversions to DC then providing the required AC version they need, or simply refuse to run until the problem is corrected. If you suspect the delta "Y" 240VAC / 220VAC problem, ask an electrical contractor to check your equipment and your power. If you need to change from one to the other most often the power company can make the change at the power bank where your transformers are. Ask about the advantages and disadvantages or each for your situation from your electrical contractor. There are good reasons why both are available. The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them. Modified by randykimball at Wed, May 09, 2007, 23:14:58

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