3 wire motor control with delay 3

[EricElec]

My understanding is this circuit causes delay of motor off by 1 second. I do not understand why the CR required and why the timer relay alone cannot be usedbe combined here?

1

 

[Heaviside1925]

OP,
I don't think the circuit as shown provides a delay of motor off. It provides a delay of CR deenergize. Once the stop button breaks, the M contact will fall out and there is no path for power back to the stop switch. No delay, just stop. The CR will remain energized for 1 second after until the TDR drops out.

 

[waross]

That is one confused circuit.
The control relay is used because there are no instantaneous contacts on the timer relay.
Try this basic circuit.
Please forgive my silly mistakes.
Thank you for pointing out my mistake,Mr Che.
CORRECTED CIRCUIT

The control relay is used because there are no instantaneous contacts on the timer relay.
However, if you use a Timer Relay with instantaneous contacts as well as timed contacts you may use this circuit without a control relay.
CORRECTED CIRCUIT

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[EricElec]

Colleague said this circuit is for a "3 WIRE CONTROL WITH UNDERVOLTAGE RESTART"

 

[EricElec]

The CR is not a lathcing relay

 

[che12345]

@ Mr waross (Electrical)
1. Both proposed Circuit 1 and Circuit 2 are incorrect.
2. a) Circuit 1 , CR N.O. contact should be wired to TD coil terminal a. Its function is to "seal-in", maintaining CR and TR are energized when the Start N.O. contact pressure is removed. To Stop, press Stop N.C. push-button.
b) Circuit 2, TD inst. N.O. contact should be wired to the TD coil terminal a.
Its function is to "seal-in" as above a).
c) In both circuit 1 and 2, the O/L N.C. contact placed between M coil terminal b and the grounded line is a violation of NEC/CSA. Reason: In case the M coil terminal b is grounded, the system remains running until the motor is burned out; even the O/L N.C. opens.
d)In both circuit 1 and 2, the symbol for TD timed and TD delay are incorrect.
e) The original circuit shows one line is fused, the other line MUST be grounded per NEC.
Che Kuan Yau (Singapore)

 

[che12345]

@ Mr EricElec (Electrical)(OP)27 Jul 24 05:04
"...Colleague said this circuit is for a "3 WIRE CONTROL WITH UNDERVOLTAGE RESTART" ...The CR is not a lathcing relay".
I have the following opinion for your consideration.
1. "Three wire" method whether with Start N.O. contact pb ahead of Stop N.C contact or vice versa , are widely used. With this wiring, the motor does NOT restart when UV (say < 0.45 rated ac coil voltage). The coil M does NOT re-energized when the voltage resumes. This is the most usual case. See 2. below.
2. In usual applications , IEC/IEEE/NEC "Prohibits" motor to restart automatically upon voltage resumption , after a failure. DANGER !!!. There are exceptions under special applications. Observe the precautions the danger of automatic restart.
2. See also my post dated 27th inst. under separate posting.
Che Kuan Yau (Singapore)

 

[Heaviside1925]

OP,
My original statement still stands. I will add as the others posted; this is an odd / atypical motor control circuit. Is this currently wired field this way in the field or is this just someone's "idea"? Take note of che12345 advice especially concerning IEC/IEEE/NEC recommendations, there are safety concerns which must be considered when designing motor controls.
If a colleague is mentioning undervoltage restart, they either did not actually look at the circuit or they do not understand motor control. There are relays for this purpose and those types of relays are not shown.

this circuit causes delay of motor off by 1 second

No, it does not.

this circuit is for a "3 WIRE CONTROL WITH UNDERVOLTAGE RESTART"

No, it is not.

If I were in your situation, I would print off copies of this circuit, get out a pack of highlighters and go through each logic step and state, it should become clear to you then and you will have the "backup" if anyone proposes something different.
State 1: Start pb is depressed: M energizes - then CR energized - then TDR energizes
State 2: Start pb is released: power continue to flows from terminal: 1 - 5 - 1A - to keep M energized, 1 - 5 - 6 - to keep TDR energized, 1 - 5 - 7 - 8 to keep CR energized
State 3: Stop pb is depressed t<1 : M deenergizes - power flows 1 - 5 - 7 - 8 CR remains energized - TDR deenergizes but remains unchanged - - TDR times out - CR deenergizes.
State 4: Stop pb is released t<1 : M remains deenergized - power flows 1 - 5 - 7 - 8 CR remains energized - TDR remains deenergized but remains unchanged - TDR times out - CR deenergizes
State 5: TDR changes state after 1 sec : CR deenergizes.
State 6: Stop pb is depressed for t>1 : M deenergizes - power flows 1 - 5 - 7 - 8 CR remains energized - TDR deenergizes but remains unchanged - TDR times out - CR deenergizes

 

[EricElec]

This is already implemented at the site for various 480/600vac motors, it is not a concept

 

[waross]

c) In both circuit 1 and 2, the O/L N.C. contact placed between M coil terminal b and the grounded line is a violation of NEC/CSA. Reason: In case the M coil terminal b is grounded, the system remains running until the motor is burned out; even the O/L N.C. opens.

Shown here is typical factory pre-wiring of a typical magnetic motor starter in North America.
This is an approved assembly, and the NEC rules for field wiring do not apply.
Regarding your concern;
"Reason: In case the M coil terminal b is grounded, the system remains running until the motor is burned out; even the O/L N.C. opens."
The wire connecting the M coil to the O/L N.C. is a short jumper between the coil and the overload relay, both of which are mounted on the same base plate.
There are exceptions, but they are special orders for very rare special applications.
[Anecdote Alert] Yes, I have encountered special applications where the factory wiring of the O/L circuit was altered.
I specially remember it as the alterations included mistakes in the circuit that required the field rewiring of over 20 MCC buckets in the field.
The unintended consequence of the change was the self destruction of contactors under unanticipated operating conditions.[/AA]
I repeat, that the wiring of the overload relay is part of an approved assembly and therefore is not an NEC violation.
Well over 99.99% of magnetic motor starters supplied and installed in North America are pre-wired this way.
TYPICAL STANDARD WIRING OF A MAGNETIC MOTOR STARTER

Automatic Restarting:> This is application specific;

(a) when automatic restarting is liable to create a hazard, the motor control device shall provide low-voltage
protection; or
(b) when it is necessary or desirable that a motor stop on failure or reduction of voltage and automatically
restart on return of voltage, the motor control device shall provide low-voltage release.

Low Voltage Protection: No Automatic Restart.
Low Voltage release: Automatic Restart.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

This is already implemented at the site for various 480/600vac motors, it is not a concept

Is it working as intended?
As I read the circuit, A momentary tap on the start button will drop out the M contactor momentarily and then re energize it.
This may lead to instantaneous protection trips, and/or mechanical damage to the motor up to and including broken shafts.
Safety concerns for both persons and machinery suggest two possible modifications:
1. The circuit should be re-designed to avoid the possibility of momentary drop out.
2. Depending on the application, personal safety may require, in addition to the service stop button, an over-riding emergency stop button that will immediately stop the motor. This is requirement is application specific. You don't want to have to defend your circuit at an inquiry.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Heaviside1925]

A momentary tap of the of the stop will drop out M and its contacts between 7 and 8 and 5 and 6 will open. The CR will remain energized since the TDC contact 3-5 is closed and CR contact 1-3 is closed. Since CR is still energized Both M 5-6 and CR 1-4 will remain open, and the motor will stop.

Not how I'd design it, but I can see the momentary low voltage restart working if both M and CR drop out from the low voltage. CR contacts 1-4 will close allowing M to reenergize.

 

[Heaviside1925]

OP,
Now to your question. Yes, you could get rid of the CR in this specific design but..... This is likely a standardized bucket design across your site. This design, as shown, all operates on AC control voltage tapped off of the line, but with the removal of a few jumpers and swapping the AC CR out for a 24VDC CR, allows for low voltage control (remote or DCS) as well. It's more about standardized design allowing for functionality than it is, what's the minimum number of components needed for this specific application.

 

[EricElec]

Its a standard design, the tap off main line to CPT sec is 120vac

 

[EricElec]

@Heaviside1925

Not how I'd design it, but I can see the momentary low voltage restart working if both M and CR drop out from the low voltage. CR contacts 1-4 will close allowing M to reenergize.

But that's if the undervoltage is <1 sec. , cct has undervoltage ridethrough AND restart capability, but if the drop is >1sec, the start seal will be broken...Am I correct in what you stated?

 

[waross]

The best of luck to you.
No one who has traditional experience with motor controls would design nor would they accept that circuit.
But it looks as if your are stuck with it.
Be aware that using that circuit to control larger motors may result in motor damage, including bent or broken shafts.
It may also result in contactor flash-over with some, but not all designs of contactor.
[Anecdote Alert]I remember a failure of a 400 HP wound rotor motor and fan caused by a faulty design.
During the installation, the electrical foreman queried the design. The engineer responsible was cruel.
He belittled the electrician.
He claimed that after testing they had determined that the design was satisfactory. (They apparently missed one important test.)
He touted his PHD in engineering.
He finished his tirade with words to the effect;
"You, sir, should know your place and stay in your place!"
Several weks later, the circuit failed catastrophically.
The contactor was of the design that was prone to flash-over.
Some event caused the contactor to flash-over when the motor was running at speed.
This caused a
Crow-bar effect", that is a short circuit across the motor terminals.
The motor immediately became an induction generator.
A generator driven by inertia and with shorted terminals tends to decelerate rapidly.
The deceleration was so rapid that the coupling exploded.
Pieces of the coupling were found in all parts of the machine room.
The 1"x 1" key rolled out of the keyway, leaving one side of the keyway deformed into an almost perfect 1/2" radius.
The motor shaft was bent.
The fan shaft was bent.
Added to the cost of the motor and fan shft and impeller was the cost of a crew electricians changing the motor and a crew of millwrights changing the fan shaft.
The crews worked through a long weekend at 200% premium pay.[/AA]
But hey, what do I know.
I will strive to know my place and to stay in my place.
With luck you may never experience a catastrophic failure.
Playing Russian Roulette when you don't know how many empty chambers the weapon has is still Russian Roulette.
Good luck with that.
Bin there, Dun that.
PS Send pictures when it goes.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Heaviside1925]

Essentially yes. Not ride through really since M will open briefly, but it will restart if less than 1 sec if CR changes state.
Please STRONGLY consider waross's advice in the previous post. As I stated is the section you quoted, Not how I'd design it. Why?
- There are plenty of reasonably priced, reliable power monitoring relays on the market. They have all the logic
required and are meant / built for this purpose.
- The design "as is" will Only work if both M and CR drop out due to the undervoltage. Do relays have a minimum
holding voltage, yes, but it should never be a property that is used to design a control circuit because this is
not how the relay is designed to operate. I think the relay manufacture will tell you this.
- The design "as is" will only function as intended if the undervoltage is on the leg that the control voltage is
tapped from. This is not a reliable way to detect /protect undervoltage on a 3P system.
- The design "as is" requires M and CR to drop out together. Consider if they don't. They could begin to chatter
back and forth, damaging the CR and M's contacts.
- Consider the TDR is field adjustable. What is preventing maintenance personnel from adjusting this delay causing
unintended consequences?
- I am assuming these are MCC buckets. Reach out to the manufacture, tell them what you are trying to do and let
them provide you with guidance.
Control circuits should be reliable and dependable and should stop and start the motor as needed for safe operation. IF the power is of poor enough quality that it cannot be depended on for your control circuit to function properly, then another source of control power needs to be used. I prefer to keep controls as segregated as possible from the actual motor but in terms of a motor bucket, it makes sense to use what is there but if what is there in unreliable, then either the unreliability needs to be addressed or the control circuit needs to be designed in a more reliable way. The way it's been designed is a "Band-Aid" which should have never become a site standard.

 

[EricElec]

Yes it is in a bucket.


So a Power Monitor that would trip on on UnderPower with a delay would function as Ride through and the restart circuit can go away? Or a powernitor can do both ride through and restart?

 

[waross]

You initially mentioned a stop delay.
Do you want a stop delay or a power outage ride through?
A delay on shutdown is unusual. It may be employed to empty a conveyor during a programmed shutdown to oid starting a loaded belt.
In that case, the delay would start when a conveyor feeder stops to give the conveyor time to empty.
These installations often have an over-riding emergency stop to stop the conveyor immediately in an emergency.
A one second delay is strange.
When I design an Automatic Transfer switch, I select a power monitor relay that triggers on a voltage below the set point on any phase.
A power outage on one or more phases is a voltage below the set point.
If I am protecting refrigeration, I may use the same relay or I may select a relay that incorporates a time delay BEFORE RESTART. These are commonly available for smaller refrigeration and there are plug in models for residential refrigerators and freezers.
But. I haven't designed either for a few years now and new things are always coming on the market.
Low voltage ride is a bad idea for large motors.
A delay on restart is more useful.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Heaviside1925]

OP,
As a rule of thumb, my preference for motor control is to have a little between a stop circuit and the motor stopping. The simplest being, having a stop switch in series with the motor contactor, I press it, the motor stops. If you had a reliable control power, this wouldn't even be an issue but since you don't, any device to do what you are looking for, which is automatic restart, is going to add complexity and reliability issues at the cost of machine safety. There are options to perform this function with a latching relay and the stop and start buttons changing its state, but I would not be comfortable designing it this way unless there was more positive control of the stop function.

IF providing a more robust and reliable control circuit is not feasible, I would consider the safest option would be the use of a small UPS in each bucket for the control power. The power monitor could protect the motor and drop things out as needed, i.e. loss of phase, low voltage, etc. but the machine safety inherent to the stop function would remain intact. This may also be more cost effective because it could be done as an "as needed" basis with minimal effect on the standardized wiring already in place.