if it exceeds the 1.7 amps, after( 30 mils) it will break the circuit.

30 mils? Do you mean 30 minutes?

 

I am aware about the initial current that is present. This is why the limit of the current is processed before the relay and before the motor.

I don't quite understand, any and all currentl will be 'seen' by the current limit at all times.
Max.

 

30 mils? Do you mean 30 minutes?

I meant 0.3 seconds

 

Depending on load and motor inertia it could be 1sec to 3 sec to attain normal running operation.
Max.

 

I don't quite understand, any and all currentl will be 'seen' by the current limit at all times.
Max.

Yes, that is right... Again I.m just trying to measure from the C.T with comparator op amp and further to broke the circuit if it exceeds the value. I.m sorry Max but I don't quite understand what are you asking.

 

Yes, that is right... Again I.m just trying to measure from the C.T with comparator op amp and further to broke the circuit if it exceeds the value. I.m sorry Max but I don't quite understand what are you asking.

P. S. there may be needed a longer time, you are right.

 

What Max is asking for is the inrush current during the startup conditions.

Which depending on the mechanical load and inertia, could last for a few seconds.

This inrush current, which can be 2 to 5 times the nominal current, will trip a simple comparator based circuit unless it is accounted for.

 

Also if you are going to build in an initial high current 'ignore' time, you need to know what this time may be in order to allow for this in the circuit.
This is built into the normal Motor O/L's and NFB (Non fused breakers) etc.
Max.

 

So, take the output from your current transformer and full-wave rectify it (either with four schottky diodes or an op-amp full-wave active rectifier circuit) then filter it with a time constant of a second or so. If it exceeds a threshold, then turn off a triac (as per post #5)

 

You must provide the answer to these questions, or you may make some very expensive mistakes.

You have a factory installed Motor Protection Relay, already installed in the Circuit, to protect the Motor.

DO YOU KNOW HOW A MOTOR PROTECTION RELAY WORKS ???
When sized correctly, they are very close to perfect in overall performance.
The Motor only needs protection from excessive HEAT.
Excessive Heat is ONLY caused by excessive Motor Current Demand.
The Current demanded by the Motor is exactly the same as the amount of Current flowing through
the "Heaters" of the Motor Protection Relay.
Heat takes a certain amount of TIME to "build-up" and cause damage to the Motor.
A Motor Protection Relay is designed to "build-up", and retain Heat, inside its installed "Heaters",
this is designed to closely duplicate the Heat Build-Up inside the Motor.
So, the Motor Protection Relay completely ignores temporary fluctuations in Motor Current Demand,
and AVERAGES the the total current Demanded by the Motor, over a LONG PERIOD OF TIME.
The Time Period will vary in direct proportion to the amount of excess Current,
and may range from ~5 seconds, to ~30 minutes.
This is exactly what is needed for Motor Protection,
and causes no problems with normal Motor operation,
provided that the Motor has been correctly sized for the job it is expected to do.

Do you know what the "Service-Factor" of the Motor is ????
Motors are designed to provide their "Rated Output" for a
certain PERCENTAGE (%) of running time at full rated output.
This is usually expressed as a number below, or above, ( 1 ), such as ( 0.85 SF ), or ( 1.2 SF ),
or,
sometimes Motors are simply rated as ( Intermittent Duty ), or ( Fan Duty ), or ( Continuous Duty ).

What is the "Duty-Rating" or "Service-Factor" of your Motor ????
Is the Motor being used under those conditions ????

WHY do you believe that the Motor Protection Relay is NOT providing adequate Motor Protection ?????

Motor Protection Relays have interchangeable "Heaters" that can provide
adequate protection for any motor within its Rated Size Range.
Your Motor Protection Relay is probably rated for use between ~10Amps and ~20Amps, and
there are probably ~10 different Heater-Sets,
in one Amp increments,
that are available for that particular Relay Model.
They work really, really well, and, they are simple, low cost, and very reliable.

WHY are you changing a Protection Circuit that a trained Electrical Engineer designed ????

Has the Motor been replaced with a lower rated Model or Style ????

Are the "Heaters" in the Motor Protection Relay improperly sized ????
How do you know this ????
You can install smaller "Heaters" if you have determined that it is not providing adequate protection.
They are very inexpensive, ( less than $10.oo USD for a set of 3 ).

A complete, brand-new, Motor Protection Relay, should not cost more than ~$45.oo (USD).
When you call the Electrical Supply House and give them the normal running Current of the Motor,
they will select for you a set of Heaters from a chart that will perfectly match your Motor and application.

Sometimes the Motor Manufacturer will specify a model number, and Brand Name of,
a Motor Protection Relay, and the matched set of "Heaters" that have been tested with
that particular Model of Motor, and found to provide adequate protection.

What information, and testing, causes you believe that
the already existing Motor Protection Relay needs to be replaced by
a complex, and potentially expensive, and problematic, DIY electronic circuit ????

If you do not provide accurate answers to these questions, you will very likely cause an expensive problem.

Installing a larger Motor, (if necessary), would be less expensive,
when you add together all the Time and Materials required to design, and adjust,
a DIY electronic version of a Motor Protection Relay.
Add to that the cost of "down-time" , while the Motor is not doing its job.
.
.

 

You're basically talking about engineering an entire motor protection relay from the ground up. Major firms like Allen Bradley, Square D, Cutler-Hammer, Eaton, etc. spend millions of dollars designing, testing and listing these sorts of devices. There are probably entire departments within these companies dedicated to engineering motor protection and control devices.

What I'm trying to say is; an op-amp and a CT aren't going to perform anywhere near as well as an electronic motor protector that has been engineered with non-volatile thermal memory, I^2R functions, configurable trip classes, etc. for protecting against thermal overloads.

Bottom line is in order to provide reliable overload protection that does not nuisance trip, you need to simulate the thermal conditions that are occurring in your motor. That either requires a properly sized motor overload heater or a microprocessor that is constantly sampling the motor current and predicting the temperature of the motor windings.

A suitable dual-element, time delay fuse could technically do the job too - but thermal-mag circuit breakers tend to nuisance trip on motor circuits due to the high starting currents - which is why they are usually oversized up to 250% above table FLA such that they only serve to provide short circuit and ground fault protection. (That's why you often see "mag-only" circuit breakers inside of MCC buckets.)

 

Unfortunate I don't have any access to the motor information. Been told that it operates in these margins and that is all. ....

I would tell anybody within earshot that they can't expect me to do my job if I don't have access to ALL of the available information, and I would interpret the second statement with a jaundiced eye. Just because somebody tells you something doesn't make it true. In this case I think there is ample evidence to conclude that you are maybe being misled. If you choose to be misled, then that is on you. If you have some way of ascertaining the veracity of what you have been told -- so much the better. You are absolutely doing your job. Believing what you have been told could be interpreted as negligence in some professional settings. Guess who takes the fall between an intern and a supervisor? Engineering is about knowing what you are doing so well that you can defend every action in a court of law.

Do you know what happened to a famous group of people whose primary defense was: "I was just following orders".

 

Well said, Papabravo.
It depends a lot on what the task is. If the thread-starter is responsible for one part of a design (just the over current protection) that is one thing. If he is responsible for the entire motor protection circuit then it as quite a different matter.
We're all trying to help, but don't even know if we're trying to protect the motor from some sort of overload, or trying to protect the supply from over-current. I think we're all guessing that it's the former.
Is auto-reset an important feature here? So that it doesn't require being manually reset after an overload situation? That presents its own dangers - such as a stopped motor starting unexpectedly.

 

Is auto-reset an important feature here? So that it doesn't require being manually reset after an overload situation? That presents its own dangers - such as a stopped motor starting unexpectedly.

That's a very good point. We don't even know for sure if this motor is being manually controlled by e.g. a three wire start-stop arrangement or if it is part of an automated control scheme. Generally speaking, motor overload protection (and undervoltage drop-out) always requires some sort of human interaction to permit re-starting of the motor after an overload has occurred. The only real exception to this is with small motors that are completely enclosed or guarded in such a way that an unexpected start up cannot possibly cause injury (e.g. self contained direct-drive air compressor pumps, etc.).

Usually the latter rely on a bi-metal thermal switch embedded in the windings, whereas everything else relies on either pushbutton-type thermal-only circuit breakers or entirely separate motor overload protection.

What kind of motor are we dealing with? A custom number designed to fit into an appliance - or a NEMA/IEC/etc. standard frame size that can be ordered with an internal overload protector built-in?

We really need to see the motor nameplate here. When sizing anything for motors, we generally need to know horsepower (kW), voltage, full load amperage, phase, frequency, rated speed, service factor, duty rating, temperature rise, locked rotor kVA, NEMA design (if applicable) and any other "special" information such as whether it is rated for use in classified locations (explosion-proof), requires an auxiliary blower, is designed for VFD service or is fitted with a dehumidifier/heater, etc.

 

That's a very good point. We don't even know for sure if this motor is being manually controlled by e.g. a three wire start-stop arrangement or if it is part of an automated control scheme. Generally speaking, motor overload protection (and undervoltage drop-out) always requires some sort of human interaction to permit re-starting of the motor after an overload has occurred. The only real exception to this is with small motors that are completely enclosed or guarded in such a way that an unexpected start up cannot possibly cause injury (e.g. self contained direct-drive air compressor pumps, etc.).

Usually the latter rely on a bi-metal thermal switch embedded in the windings, whereas everything else relies on either pushbutton-type thermal-only circuit breakers or entirely separate motor overload protection.

What kind of motor are we dealing with? A custom number designed to fit into an appliance - or a NEMA/IEC/etc. standard frame size that can be ordered with an internal overload protector built-in?

We really need to see the motor nameplate here. When sizing anything for motors, we generally need to know horsepower (kW), voltage, full load amperage, phase, frequency, rated speed, service factor, duty rating, temperature rise, locked rotor kVA, NEMA design (if applicable) and any other "special" information such as whether it is rated for use in classified locations (explosion-proof), requires an auxiliary blower, is rated for VFD service or is fitted with a dehumidifier/heater, etc.

I hope they don't expect the "intern" to quickly pull something out of a dark place, because I think we have just given him at least a years worth of important questions to run down. If they are willing to pay him to do that for a whole year, it would be real "helluva deal" for him. Oh wait....is this a paid internship or is he working for free..,ah, I mean ancillary benefits?

 

Also, is this going to be utilized in a field installation or as an integral part of an appliance? If the former, you'll need to adhere to your local electrical codes, à la NFPA-70, article 430.

I suppose a carefully tuned current-driven R-C circuit might be able to loosely approximate the thermal characteristics of a motor. Emphasis on carefully tuned. You'd need to factor very particular details like capacitor leakage, life expectancy, degradation characteristics, etc. as well as short-time overload withstand ratings for your resistors in the event the circuit sees short circuit current.

 

The bottom line, IMO, is that an "Intern" should not be responsible for creating "a small system that will take place of the fuse highlighted".
Any attempt at this project should be overseen and OK'd by qualified superiors to ensure compliance with safety and current regulations.
From the DWG style and the voltages shown, the OP may not be under N.A. jurisdiction of NFPA79/70.
But I am am sure that they most likely have to conform to some equivalent body.
Max.

 

The bottom line, IMO, is that an "Intern" should not be responsible for creating "a small system that will take place of the fuse highlighted".
Any attempt at this project should be overseen and OK'd by qualified superiors to ensure compliance with safety and current regulations.
From the DWG style and the voltages shown, the OP may not be under N.A. jurisdiction of NFPA79/70.
But I am am sure that they most likely have to conform to some equivalent body.
Max.

"Sketchy" is the only word that comes to mind to describe this situation. If I was the intern, I'd be running, not walking away as fast as possible.

 

Looking at the diagram in post #1 where it has a designation of F1 it looks to be switching the Neutral. Do you really want neutral switched?

If all you want to do is monitor current to a motor and disconnect when current exceeds a preset threshold you can use a current transformer or hall effect design, just a matter of how you want to sense the current. I assume if current exceeds a limit you want the motor shutdown requiring a manual reset. Since the motor normally operated in the 1.3 to 1.7 amp region the motor start up or locked rotor could be 13 to 17 amps. K1 in your drawing is your motor starter contactor which has overload heaters. What are the ratings? The dotted line off the heaters goes somewhere?

Ron

 

Looking at the diagram in post #1 where it has a designation of F1 it looks to be switching the Neutral. Do you really want neutral switched?

As I mentioned in post #9, if it is fusing, it is not allowed, if a DP breaker it can be used, but not usually recommended.
Max.