Contactor Chatter 5

[BoogWar01]

Hi, I'm a newbie (Mechanical Engineer). I have been trying to install a set of CX312A6 Eagle timers on a wye delta 100 hp mixer motor. I've basically ironed out the control side, but when I apply juice, I get contactor chatter from both circuits on the Cutler - Hammer switchgear (I dunno the model or series number of the contactors, but they're huge as contactors go; the machine they're in is a really old AMF mixer that's at least sixty years old). The timers i'm trying to install are digital timers but they were built in 1987. Plant power is 208V 50Hz 3 Phase, the timers are rated at 120 - 250V at 50/60 Hz. I think that there is a low voltage condition in the panel I set up, but I'm not able to find it - maybe it is causing the oscillation? The control coils are rated at 120V and I am using power upstream of the coil for the control gear. I thought this might be the problem, but after completely disassociating the panel from the machine and testing I could not find any appreciable voltage drops anywhere; could someone tell me what kind of voltage drop would cause chatter? The most impedance I saw while I was checking on the panel was 1.8 ohm. Every thing else was dead short, like it is supposed to be. I know that under load conditions will be different, but I think If I know what kind of voltage drop to look for I can trouble shoot more effectively...

 

[ScottyUK]

Contactors usually drop out somewhere around 70% of nominal voltage. During motor starting and acceleration the motor draws about 6-8x rated running current. That's almost certainly where the volt-drop is occuring.



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If we learn from our mistakes I'm getting a great education!

 

[BoogWar01]

OK, ScottyUK, I hear you, and I've been reading some of your most informative posts. The problem is that when I connect straight to coil without the panel, and just use the pushbutton / toggle switch setup I get no chatter, and it runs quite well. So it is a control issue, I just don't see how my control wires - 1.5mm solid, and the two timers -could cause a drop to 80V. But I will run the clocks on a seperate circuit and just use the hot coil side on the relay terminals and not to power the clocks, as I have been doing. Will post results, and thanks!

 

[BoogWar01]

Also, I have included a jog button on the delta side of the motor to help push out doughout of mixer bowl. Good / bad idea?

 

[jraef]

ScottyUK is giving you the voice of experience here. 9 times out of 10 that is going to be the issue. You try to start the motor, it causes the VD, which chatters the contactor, which causes the VD to be worse and it cascades until something fails.

One of those failures COULD be that your timer contacts have now become seriously compromised and are causing a VD of their own on the control side; one that you might not see until there is a load on it. That could happen almost immediately the first time you chattered the contactor coil.

Another possibility is your frequency. In looking for info on that timer, I saw what I assume were your other posts asking for help elsewhere, and it appears you are in Jamaica where you have 50Hz 208Y120V power. This is highly unusual and if you used 120V 60Hz coils, they are NOT going to operate correctly at 50Hz. They may pull in, but will be significantly weaker. Are you sure you are using 120V 50Hz coils?

If not, the best thing however would be to ensure you have a 50Hz rated coil. If that is impossible, then I would switch to a coil voltage that is as close as you can get to 130V if they are 60Hz rated. Coils, like motors, work best when the V/Hz relationship is within design specs. A 120V 60Hz coil is looking for a V/Hz ratio of 2:1. Applying 50Hz to it makes that 2.4:1 and you are reducing the inductive reactance of the coil, which has the effect of reducing the impedance (similar to resistance) and thereby increasing the amount of current the coil takes to operate. Couple that with even a slight voltage drop as mentioned above, and you can easily get chatter.

By the way, in that other forum posting, there was mention that the starters are Y-Delta. Being that you are an ME, are you comfortable with your understanding of how that works and how that might be having an effect as well?


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

If it works with a push button andnot with the timers, start a daisy chain test. This is not difficult but considering the number of people that I have seen screw it up and missthe fault as a result, please read carefully.
1, Connect your volt meter to one incoming line and to the input of first device in the circuit. This will check the wiring to the first device. Try a start and look for excessive voltage drop.
2, WITHOUT CHANGING THE FIRST VOLTMETER CONNECTION TO THE INCOMING LINE, Connect the other voltmeter lead to the input of the second device in the circuit. This will check the contacts of the first device and the wiring to the second device. Try a start and watch for excessive voltage drop. If you find excessive drop, move your voltmeter connection back to the output terminal of the first device. This will indicate whether the problem is the contacts of the first device or the wiring to the second device.
3, Repeat the process connecting the voltmeter to each device in the control circuit in turn until you find an excessive voltage drop.

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

 

[BoogWar01]

First thanks to all who have replied to this thread!

Seeing that I got no help a couple of weeks ago when I posted in the PLC forum and the MechE forum this is like a breath of fresh air.

OK. The timers are CX312A6's (Eagle) and they are rated at 50 /60 hz. After reading Jraef's post I vaguely remember talking to an Eagle tech rep and him saying that there was a problem on some of these timers on 50HZ. Now I'm wondering if the internal timer relay coil has issues at 50hz... But on the bench it tested fine. Will tear into it and check tomorrow.

After machine gunning for about three seconds, I'm afraid to do further damage to the unit, so I've ordered a set of contacts for it. I will also try waross's method, and see if it brings the fault to light. As for being comfortable with the Y / delta setup, my first year at polytechnic was all about this stuff; but after 20 years as a MechE the memory kinda fades... If I remember correctly, Delta is a parallel connection (two coils in series and one in parallel, and y is two coils in series across any two points on the connection. The current therefore across the Delta is more, I think 1.7 times more, which explains why delta mode is faster. Correct me if I'm wrong, but that's what I remember. Any hoo, I still want to now if JOGGING the delta connection withouit engaing the WYE first will have any ill effects...

And THANKS AGAIN!!

 

[waross]

Google wye delta on this site. Your memory is quite rusty.
Wye reduces the current by reducing the actual applied voltage across the individual windings. Yo get about 1/3 torque but with a light load the motor will still approach full speed. Many motors are suitable for Direct On Line starting and some of those are suitable for DOL jogging. The delta connection is equivalent to DOL, but if the machine was supplied with a start:delta starter, I would not go to delta jogging without a lot of investigation.

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

 

[BoogWar01]

Waross,
Thank you for your patience! The motor is wired wye delta so it can achieve two speeds, ie lo and hi. I think it delivers constant torque on both windings, but I'm not sure. I think the start:delta starter you are refering to is a means to get the motor from rest using the wye coil - and then to transfer to delta. This is not the case in our scenario - there are two distinct speeds. So can the machine be delta jogged?

 

[ScottyUK]

Assuming the problem is in the control section, the most likely source of the high current is the main contactors. I don't know just how big these are - my idea of 'huge' is probably a little different to yours - but an old-fashioned 100HP contactor on 110V control supply is likely to draw well over 10A during initial pull-in until the iron yokes touch. If you have an unexpected 1.8[Ω] in circuit somewhere then that resistance combined with the high current could well be the problem. You should be able to measure through the control circuit and find out where the high resistance joint or contact is.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

Wyeelta is inherently single speed. You may have a consequent pole motor. That gives you two speeds and both the vector sketch and the starter are similar to star:delta.
A star delta starts in star and then reconnects in delta.
A consequent pole machine shorts the center taps of the delta windings together to double the number of poles and as a result halves the speed. Not all motors are suitable for this use.

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

 

[jraef]

Bill, that was probably the best and most succinct explanation of 2S1W motor theory I have ever hear. Nicely done, PLS for you.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[BoogWar01]

Thanks to Jraef and waross for the humbling lesson in Motor control. I should have known that y / delta wouldn't give me two distinct speeds. Digging up old notebooks now. Will confirm (or not) waross's hypothesis tomorrow as I am going to have a talk with AMF. I also found out that a switch in the circuit was rated at 12V, 10 amps, not 120 V 10 Amperes. So it seems as if I've found my bottleneck. Thanks again, will report if successful (or not) tomorrow!

Mario

 

[BoogWar01]

Oh yeah, one more thing... IF it is indeed wired as a 'consequent pole' motor, can it be jogged without damage in the high speed?

 

[BoogWar01]

And how many wires would come out of a consequent pole motor...six or nine?

 

[LionelHutz]

If you still have the problem today, check that you are not using a normally closed auxilary contact from the contactor in your circuit that opens and disconnects the power to the coil as soon as the contactor begins to close and this auxilary contact opens.

 

[BoogWar01]

PROBLEM SOLVED! The timer circuit was too much for the circuit the coil was on. isolating them from the contactor circuit did the deed. Thanks to all who participated in this thread, couldn't have done it without you. Will be posting photos of this rather grungy installation in short order...
Mario

 

[dpc]

Glad you got it sorted out. If this timer is only used for this starter, a better solution might be to increase the size of the control power transformer that is powering the contactor coil and put the timer circuit back on this power source.

"The perfect is the enemy of the good." -- Voltaire

 

[BoogWar01]

This whole situation was funny, in hindsight. I guess the clues were there all along, but my very rusty electrical background let me down. Any hoo, I learned alot and am now posting a blog for the timers - what I've learnt, and my problems - and whose solutions I used to find mine. It started with a set of Eagle Timers (CX312A6's) that I got to install on an old AMF Mixer with a consequent pole (thanx, waross) 100 hp motor. Out of the box, the timers misbehaved - keypad was totally locked out. After a week of calling the manufacturer, surfing the net, conversing with the local dealer and getting nowhere fast, I deceided to break into them and see what I could do myself. The solution was simple - the pad that connects the membrane switch pad to the PCB had either shifted during shipping or the twenty year old timers (they were in such good condition I was completely fooled into buying them as new products) had some kind of corrosion in them. I took them apart, used some rubbing alcohol and cleaned them up, and guess what, they WORKED!! I was a happy camper. After stringing them into the new panel, though, heartache set in again, as the main contactors they were meant to control just started chattering away. I think the term folks use around here is 'machine gun'. Between this thread and experimenting I managed to find out that I had mistakenly wired the timers in series with the main contactor coil circuit. The voltage to the coil dropped excessively, and caused the chatter. What a journey it has been! A site search really clued me in on the answer - I was told to Google wye delta and came up with an answer from either Jraef of waross that basically fit my problem. Thanks, I owe you guys a real frosty one if you're ever in this neck of the woods! Mario