GE LIMIT AMP MCC LOUD HUMMING NOISE 2

[genoc]

I have an old GE Limit Amp, 4160 MCC with 500HP motors tied to it. The gear was installed in the mid 70's as I understand. We have a loud humming sound comming from the gear.
What is suspect is the M1 coil.
It has also been said that the control circuitry was DC at one time. With that being said, will a DC coil care if the supply voltage is AC? And if so what is the differance between the coils?

 

[waross]

In an old MCC dirt or corrosion on the pole faces of the magnetic contactor will cause humming.
Yes a DC coil will care if it is fed AC.
Typically a DC solenoid has two coils, a pull in coil and a holding coil. The pull in coil takes quite a heavy current and is switched out of the circuit as soon as the contactor closes. The holding coil stays in the circuit to keep the contactor closed.
The fall off in strength of a magnetic field with the length of the air gap means that a much greater current is needed to pull the coil in than is needed to hold the contactor closed once the armature contacts the pole faces.

In an AC coil, an increased air gap reduces the inductive reactance and the impedance of the coil. This allows the heavy current needed to pull the contactor in with the air gap. As the air gap lessens the inductive reactance increases and the current drops. When the armature contacts the pole faces the inductive reactance increases sharply, the impedance increases and the current drops.
Most AC contactors have part of the pole faces encircled by a single short circuited turn. This is called a shading coil. The relatively high inductance and low resistance cause a phase shift in the current through the shading coil. The result of this is that the magnetic flux through the shading coil lags behind the main flux. The main flux and the shaded flux then go through their zero crossings at slighly different times and there is always some flux to avoid the chattering that may result if the pole face was not "shaded".
A DC contactor does not have shading coils, another reason not to apply AC to a DC contactoe coil.
In the coils of small relay the air gap is relatively short.
In the DC coil the current is limited by the resistance.
In an AC coil the current is limited by the impedance and the inductive reactance often predominates.
Feed AC to a DC coil and the inductive reactance added to the resistance will often limit the current to a value insufficient to pull the relay in.
Feed DC to an AC coil and there will be no inductive reactance to assist in limiting the current and the coil will often burn up.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[oldfieldguy]

One more problem with changing from DC to AC: The armature of an AC contactor is laminated to decrease eddy currents. DC coils are sometimes a solid piece. Simply swapping coils without changing the armature will cause unusual heating of the armature and eventually may cause coil failure from overheating.

old field guy

 

[SceneryDriver]

I've attached an interesting article regarding running DC relays from AC and vice versa. Its from Tyco, so it references their products, but the principles they cite are correct. Interesting read, and something to keep in your back pocket when you're in a pinch at 3am. If you have a choice though, it's always wisest to replace like for like.


SceneryDriver

 

[waross]

I agree with the article that mixing AC on DC relays and DC on AC relays is a bad idea, but a couple of points jar against my understanding.
Presented for discussion:
Due to the action of the diodes, the current through both coils is always in the same direction.
The magnetic flux will induce the same EMF in both coils. The EMF in the unpowered coil will cause a current to circulate through the diode.
As the sine wave voltage decreases, both diodes will act as freewheeling diodes and the current caused by the back EMF will decay at a rate dependent on the resistance of the coils. With any amount of induction the time constant will be more than sufficient to carry over until the sine wave rises again across the opposite coil and will prevent chatter.

The other issue is the copper pin;
A copper or brass pin may be used in small size relays but not in contactors. At one time sticking was a problem for AC powered contactors as well as DC contactors. Is has to do with the ratio of permeability of steel to air of about 10,000:1 The magnetic force rises alarmingly when the air gap closes and there is only steel in the circuit. The steel will probably be driven into saturation. Brass pins were used for both DC and AC contactors to prevent sticking but were problematic. Under high usage, the repeated impacts tended to damage the pins. They may deform or be knocked loose and fall out. Then about 50 or more years ago, an unsung hero realized that an air gap anywhere in the magnetic circuit would reduce the flux density enough to prevent sticking. Now many AC contactors have an air gap to prevent sticking. In a contactor with a "U" shaped magnet stator there will be a small air gap in the back side of the stator. The gap will be at right angles to the direction of movement of the armature so as to prevent impacts on the air gap.
I seem to remember a three legged stator in an old contactor where the center pole was slightly shorter than the outer poles so as to introduce an air gap.
I do have (had?) some old texts that showed contactors with the brass pin. Very old texts. Probably dating from the 20's or 30's, possibly the 40"s.
Skoggs, you are probably familiar with the brass pins. Have you actually seen any in the last 30 or 40 years?? grin.
Good catch on the laminations, Oldfieldguy
Discaimer; I am speaking about contactors. The design of relays, the contact areas of the moving parts and the forces involved are so much different than contactors that these comments should not be applied to small relays. There may also be hidden air gaps.
Comments on small relays jraef? Thanks in advance.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[SceneryDriver]

Sorry waross, I should've been more explicit in stating that the application note I attached was for RELAYS and not for contactors. I had that app note squirreled away, and I remembered it when I read this thread. Apologies if I muddied the waters at all. I too hope Skoggs and jraef chime in.


SceneryDriver

 

[waross]

Slightly off topic:
When I hear Tyco I always think of Dennis:

Leonard Dennis Kozlowski[1] (born November 16, 1946) is a former CEO of Tyco International, convicted in 2005 of crimes related to his receipt of $81 million in purportedly unauthorized bonuses, the purchase of art for $14.725 million and the payment by Tyco of a $20 million investment banking fee to Frank Walsh, a former Tyco director. He is currently serving 8.33 to 25 years at the Mid-State Correctional Facility in Marcy, New York. His earliest release date is January 17, 2014, when he becomes eligible for parole.

http://en.wikipedia.org/wiki/Dennis_Kozlowski

Bill
--------------------
"Why not the best?"
Jimmy Carter