Motors! Motors! Motors! (Topic 3 of 3)

For my third and final question of the day, I want to reach out to the motor gurus. I’ve asked these questions of several sources and they all gave hesitant answers. Again, thanks for the help, Tom. This topic could become very political for me, so please let me know if I can contact you on your feedback.

1) When wiring an overload & contactor combination it is customary to connect the overload on the load side of the contactor. Every motor SCHEMATIC I’ve ever seen has this arrangement. I stressed schematic because although my shop draws the circuit this way, the shop wires them with the overload FEEDING the contactor. Even if you neglect the fact that the schematics become confusing, this still doesn’t feel right to me. Is this arrangement completely Kosher, or does it violate some regulation somewhere? The NEC doesn’t seem to specify the preferred sequence.

2) When using two contactors to create a reversing start circuit, is a hard-wired connection between the two required by code? Currently, only the PLC prevents the two from pulling in at the same time. The only hardwired protection in the circuit is an E-STOP, which disables a master control relay and cuts all control power to motor contactors and control circuits.

3) When multiple motors (of HP ratings typically 5 thru 15) are sharing a connection to a secondary fuse (assuming this also meets code), can that fusing be replaced by a circuit breaker? I have read a few pamphlets on supplimental breaker protection and asked a few “experts.” As far as I can tell, it is OK to use a breaker for group motor protection as long as the panel has a primary fuse. The NEC does specifically mention branch circuits with motors under 1HP, but is less forthcoming on the arrangement I’m speaking of. So, at this point I do know that my hairdryer, sump-pump, and power drill can share a breaker but I’m not so sure I can group three pumps on the same circuit. All motors also have individual overload protection, if this makes a difference.

akreel I am going to give basic answers at this time, I will followup later with more detail info like the NEC or NFPA code that applies.

1. The reason an overload is AFTER the contactor is to protect the contacts, they are rated for a specific load carrying capacity, if that is exceeded too many times the contacts may burn or weld together.
2. I will have to check the NFPA code but if I am not mistaken this is considered along the same lines as an estop ckt, the "mechanical interlocks" / auxillary contacts are supposed to be hardwired BUT there is no reason you can't do both...ie do it with the mechanical interlock but also use it for a plc input.
3. Look at NEC code 430.37 and up to decide on applicable parts. NOTE: a controller may be a switch, circuit breaker etc.

If the NEC code book is confusing at times (which is all the time for me) then get the NEC HANDBOOK, its offers detailed explanations in layman terms to sections of the code. This section is full of examples and explanations.

It doesn't matter if the thermal overloed is before or after the contactors since the current is the same throughout the circuit, but we do it after because there is less chance of someone tee-ing off some power for a later circuit addition, and its drawn that way, and most th' overloads are designed to snuggle in under the contactor.
With 2 direction contactors, probably no code goes into this detail, but it make sense to have hard wired prevent interlocks on the auxillary contacts. I had a contactor burn it's contacts closed yesterday. It was only for one direction, but if another direction existed -- "poof".
Not sure about question 3. I always use individual cct brkr or M.fuses for each 3 phase motor. Extra motors on this cct would need bigger fuses, so less protection. Fuse size should JUST fail to blow at start up current, so with an extra motor on there, it should blow. Dont forget fuses must always protect the cable, so you can't upsize your fuses to 100 amps when you have 5 cables rated at 20 amps going out to 5 motors. A 50 amp short circuit wont blow the fuses and the cable insulation will melt away.

Dont forget fuses must always protect the cable, so you can't upsize your fuses to 100 amps when you have 5 cables rated at 20 amps going out to 5 motors. A 50 amp short circuit wont blow the fuses and the cable insulation will melt away

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I partially agree with this, what I do not agree with is the 5 cables from one branch fuse. If you are going to use multple devices/motors on one ckt then you should run ONE (1) wire to the contactors/relay input side and jumper across. The cable/wire should be rated to handle the total current capacity of the devices/motors by being spec'ed at 125%. The NEC code and NFPA code address this issue in detail.

Perhaps I was unclear rsdoran. My answer to akreel was intended to encourage him to use maximum protection for each circuit, and NOT to share fuses. The example of 5 cables ended in tears, see.
I didn't mean to contradict you on the themal overload thingy only I was typing my answer whilst you posted yours. Sorry, mate. I come here for a look every day unannounced, and know you well enough to say you are a huge help to a lot of people. Jon.

jon no need, I have a tendency to misread/misinterpret things and jump to conclusions. I think overall we were saying similar things in our own way.

I think what akreel was referring to was the control circuit, not the motor circuit.
In ancient times, when you purchased a motor starter it typically came with one side of its auxiliary contact connected to the coil (phase or line side) and the neutral side of the coil was connected to the NC overload contact. If you purchased a reversing starter, then, in addition to the mechanical interlocking, the coils were pre-wired through NC auxiliaries of their opposite numbers and the coil neutrals were again connected to a common NC overload contact.
And typically only two motor conductors ran through the overload relay.
In more modern times, it has been considered bad practice to switch the neutral in a control circuit since it is a safety hazard.

my 2 penny's worth

Akreel

I stressed schematic because although my shop draws the circuit this way, the shop wires them with the overload FEEDING the contactor.

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In my opinion, all circuits should be wired as per the schematic, and I always wire the overload after the contactor, not before.

As for using 2 contactors to make a reversing circuit, again I will always hard wire as additional protection against both contactors pulling in at the same time. PLC outputs fail and so do contactors, so I will always put in as many safe guards as I can to protect a motor.

I cannot comment further on your questions, as I am not totally convinced that us lot this side of the pond work to the same standards as you lot over there, I suppose that there is an international standard that we should all work to, but I just make sure that I comply with the IEE regulations (Institute of Electrical Engineers), which is also a European standard,

Paul

Another point about not putting the overload before the contactor is, if it is directly above the contactor then heat from the coil will flow upwards into the overload.
Overloads work on heat bending a bi-metal strip and so may cause nuisance tripping.
And what usually happens when something keeps tripping for no reason? Somebody turns the overload up.

Akreel,

1) Seems there is some confusion over your first question. I take it you are referring to the control circuit and not the supply power to the motor. When you say, “load side of the contactor”, I’m assuming you mean neutral or grounded conductor of the motor starter coil. Same as you I learned to wire the N.C. of the overload relay block in what actually is the ground conductor. And it looks a little odd to see it on the left side of the motor starter coil. Most motor starters even come from the manufacturer with a wire from the coil to the N.C. contact of the overload relay block that way, and you land your white wire to the other terminal of the N.C contact. (i.e. AB NEMA Rated Starters) But some people believe in never switching the neutral/grounded conductor. And that is a good rule, but this is one of those exceptions that are allowed. I can’t seem to locate the NEC code section right now, but will continue to look for it.

2) About interlocks on your reversing contactor ,I don’t think you’ll find this one in the NEC. But the NFPA 79 Electrical Standard for Industrial Machinery, 1997 Edition addresses this in section 9.11 and related material 9.12.
9.11 Motor Contactors and Starters
Motor contactors and starters that initiate opposing motion shall be both mechanically and electrically interlocked to prevent simultaneous operation.
9.12 Relays and Solenoids
Relays and solenoids that are mechanically interlocked shall be electrically interlocked.

3) Fuses or Circuit breakers can be used, I think it’s more of a personal preference by the designer and/or customer specifications. NEC Art. 430.62 specifically address your question on multi-motor, feeder short-circuit and ground-fault protective device ( i.e. fuses and breakers). NFPA 79 Section 8 Protection also has information you seek.
Fuses and Circuit Breakers protect the supply conductors to motors and must allow for starting the motor, or inrush current. Generally this is usually six to eight times the Full Load Amperage (FLA) of the motor. The overloads or heaters are used to protect the motor on an overload condition a motor might encounter.

Very often motors are over dimensioned for the mechanical construction or for process needs. OL/Moment-settings must do for construction/process, not for motor nominal values, in other case mechanical construction could break and any case OL-relay cant protect mechanic depends of motor rotor 'massenergy'.

Motor enviroment temperature, dust etc. must keep in mind when setting OL-relay etc. as for conductors and is it enough for protection, will we need motor temperature measure or PTC-protection
etc. if there are dust explosion hazard...

Where is OL-relay or some contact? I think it is insiders thing.

some differrence answers

Dear all folks,
May I share some diff. answer...
When I use pLC for control motors .
If the motor using Start-Star Run delta .I has one more timer
(a half sec delay)in program for change from Start Star to Run Delta...
because of plc run faster than relay...otherwise short circuit if do not care of this case..
I use Mechanical interlock between Contactor Star and Contactor Delta, include interlock contact in program and hard wire between boths.
Also the same in Forward - reverse circuits.(without timer)
I protect motor by using overload nc. contact 95-96 series with contactor and place overload no. contact 97-98 as input of PLC...
Why I have to done this...
In some case, some machines when I use plc control with small protection forward contactor work at the same time with reverse contactor (I still surprise)..

platootod

Thought I would pass on these changes in the code.


Art. 430 — Motors IX. Disconnecting Means
430.102 Disconnect Means Location
(B) Motor Disconnect. A disconnecting means must be located in sight of the motor location and the driven machinery location. The controller disconnecting means, in accordance with 430.102(A), can serve as the disconnecting means for the motor, if the controller disconnect is located in sight of the motor location and the driven machinery location.

Exception: The motor disconnecting means is not required to be in sight of the motor and the driven machinery location under either condition (a) or (b) below, provided the controller disconnecting means required in accordance with 430.102(A) is capable of being individually locked in the open position. The provision for locking or adding a lock to the disconnecting means must be permanently installed on or at the switch or circuit breaker used as the controller disconnecting means.

(a) Where such a location of the disconnecting means is impracticable or introduces additional or increased hazards to persons or property.

(b) In industrial installations, with written safety procedures, where conditions of maintenance and supervision ensure only qualified persons will service the equipment.

FPN No. 1: Some examples of increased or additional hazards include, but are not limited to, motors rated in excess of 100 hp, multi-motor equipment, submersible motors, motors associated with variable frequency drives, and motors located in hazardous (classified) locations.

FPN No. 2: For information on lockout/tagout procedures, see Standard for Electrical Safety Requirements for Employee Workplaces, NFPA 70E-2000.

Intent: The revised subsection and exception are intended to improve worker safety by requiring the motor disconnecting means to be within sight of a motor and its driven machinery location as long as it does not add or increase hazards. There is much less chance that the worker will attempt to work when the equipment is “hot.” However, (b) of the exception recognizes that industrial establishments have a greater degree of control over the workplace, and those facilities that have an effective lockout/tagout program could use the exception. The exception also clarifies the provision for locking the disconnecting means for the controller must be permanently installed on or at the switch or circuit breaker used as the controller disconnecting means so an installer/maintainer can apply it and work safely.

Goody as you said

And what usually happens when something keeps tripping for no reason? Somebody turns the overload up.

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Yep this happens all the time, particually on someones last night shift. This is why I like the overloads where you cant change the setting the turning the pot and increasing/decreasing the distance of the bimetel strip, you physically change out the strip for one with a different rating. You dont see these to often but I think the are a good idea.

This is why I like the overloads where you cant change the setting the turning the pot and increasing/decreasing the distance of the bimetel strip, you physically change out the strip for one with a different rating. You dont see these to often but I think the are a good idea.

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Then that same nite shift or whoever has a problem with it kicking out and when they should use a W35 they use a W53 or whatever is handy that is rated higher than what was there. I cant count the motors I have changed out because of the overloads being too high, at least with the contactors that have the adjustments its isolated to a small range but in many cases that range may be too high too.

I will have to look closer at the codes involved but I have never seen the overloads put in front of the contactor, if you look at NEC 430.1 it shows a motor system sequence with the overloads shown after the contactor, all subsequent drawings show the overloads after the contactor. I have never seen anyone, US or European, wire the overloads before the contactor. I always thought it was to offer some protection to the contactor too. I can see where it may make more sense if the power lead for the contactor went thru the auxillary contacts for the overload instead of the neutral/common.

I haven't seen it doesnt mean it isnt done, I hope someone can offer some reason or a guideline that would explain why, especially why anyone would put them before the contactor when the schematic/drawing specifies after.

Reversing starters: I prefer mechanical interlocks along with using auxillary contacts, if safety is an issue then the use of redundant contactors is necessary with safety monitoring relays or a safety plc.
This was taken from an application guide for STI safety relays, note that in this picture it also shows the overload/thermal protection between the motor and contactor.