AC Electric Motor Starter Failure

What I understood from your question is:

the motor is fed via one of two contactors (each for each direction), while the two contactors are mechanically interlocked. What will happen to the coil of (contactor/motor ?????) if the two contactors are energized simultanuously.

If so, off course nothing will happen to the coils of the motor, but the coil of the contactor which doesn't feed the motor will burn up as its coil is energized while mechanically the plunger has no motion. So there are always an electrical interlock to prevent such a case.

Regards... SAMAK

You are right, electrical interlock is essential.

hello reber, if q0.1 is forward command and q0.2 is reverse then interlock the outputs internally in the PLC. But if 0.1 comes in before 0.2 is not OFF then contactor coil will burn. Good luck

It is not good engineering practice to energize both forward and reverse contactors at the same time.Normally the auxiliary nc contact of forward contactor will be wired to the coil of reverse contactor and auxiliary nc contact of the reverse contactor will be wired to coil of forward contactor.Inthis case both the contactors will not be on at the same time.

IF plc is controlling the same logic will be applicable

It may or may not! It all depends on the time element!

It take a nano to switch a contactor but if they were both engaged at the same time you would blow out a safety device (like a breaker) before you burned the motor up! However if your safety went off or disconnected power and continued to do so then I would say to check your contactors 1st before saying the motor was bad! Unless the obvious was that you seen smoke out of the motor! then yes the motor is bad or just burnt and some motors may still run after smoking but this would not be a proper safety hazard to not just a human being but to a production line! (time out lose of product delivered thru the line!

further more: the motor is bad!

Coils, like light bulbs, do occasionally burn out. For longevity, I ALLWAYS oversize any reversing contactors by at least double the required motor amperage. Consideration should be given to the increased coil current as to not exceed the control voltage supply. You don't give the coil voltage. If you're using the 24vdc from a plc, this may be part of your problem. Check the actual current & voltage at the coil when energized. If the voltage is low, the current will be high and cause overheating/burnout.

Naturally, always electrical&mechanical interlock reversing 3 phase starters unless you want to check your circuit protection devices.

You could always find someone with some dirt & mercury under their fingernails and let him fix it!

Luck

General electric makes the most rugged reversing motor contactor I have ever experienced.I saw one in constant cycling about once per second, 24/7/The motor was used for a clip control on a textile web-drying oven.These controllers come pre-wired to prevent both coils from energizing,as well as mechanically interlocked.After about a year of operation (millions of cycles), the starters would become noisey, due to the armature being hammered down into the shading coil, causing it to open.About one minute or less is required to swap out the contact set, and no tools are required,only one clip to remove.

If both coils are allowed to energize simultaneously, there will be a short circuit on the supply, and a fuse will blow, as well as the heaters(thermal overloads).

Hope this helps

As I see your issue, the problem really is that you had a mechanical interlock that prevented the 2nd contactor from closing while the first was already on, and you are wondering why the coil burned up. Is that correct?

First off, you should always use electrical interlocks in addition to the mechanical ones to prevent just this sort of thing. PLCs are great for complex control, but not so good at making sure things like this don't happen. They are only as good as the programmer.

Second, the reason why the coil burned up was that when you cannot make the armature faces come together to complete the magnetic circuit and decrease the reluctance of the electromagnet, the impedance will be low in the coil circuit and therefore the coil will draw massive amounts of current. If you look at a typical contactor's coil specification, the ratio of "inrush power" to "sealed power" is usually anywhere from 8:1 through as much as 10:1. So that means that when the magnet poles are at their farthest apart, the coil current is 8-10 times as high as when they are together. Your mechanical interlock prevented the pole faces of the magnet from coming together, so your coil was continuously drawing 8-10 times the power that it should have. That lets out the magic smoke.

Now if your mechanical interlock had not been there and both contactors were allowed to close, the result would have been either catastrophic (a bolted fault) or hopefully your short circuit protective device would have acted to prevent a fire. Still not good no mater how it would have turned out.

Most 3 phase motor starters I am familiar with have a normally closed auxiliary contact for each coil. Activation currents for the oposite direction coils are cross fed through these contacts. Properly wired, only one coil at a time can be activated.

Right, but he obviously didn't use them, that's why he smoked his coil.

As i said in my comment,many people do not follow certain basic and fundamental engineering practices ,due to this failures,coil burn outs etc happen.

The simplest solution is to program the two outputs so that they cannot both be on at the same time, no matter what the logic upstream of these output is telling them. If one output is called A and the other called B, put a 'notB' in front of the A output command and a 'notA' in front of the B output command. This logic is sometimes called 'XOR' - 'exclusive OR' - 'either A or B but not both A and B at the same time'.

At the worst, do it outside the PLC using two hard-wired relays, so that the A relay opens a contact to the B relay coil, and vice-versa.

Alternatively use a normally-closed contact on each contactor to disconnect the coil circuit on the other contactor.

No more risk of burnt contactor coils.

Do not disconnect and remove the mechanical interlock.