Protection to Micro lathe power supply?

[minh-thanh]

No it doesn't.....not by the relay...which is why I asked, what is the purpose of the relay?

I understand what you are referring to
That's just a simple switch circuit that I remember and think can satisfy a certain part. Or should I remake a similar circuit with some electronic components for better control that I did many years ago!!!!??? Am I necessary !?

 

[wce4]

Motor starters are often wired with the sealing contact parallel to the starter coil, allowing a motor contactor to close and latch closed.

If there is a power interruption, or a brownout, the coil will release, and the motor will stop, and the start button has to be manually pressed to restart the motor.

In some situations, the operator may want the motor to restart automatically after a power outage, in which case the control contacts are wired directly in series with the contactor coil. This does not provide brownout protection, but I have seen it done, especially in situations where there process is not constantly staffed by people who could otherwise reset the starter, or it is critical that the process restart immediately, such as after a tranfer to a standby generator.

Either way, the latch function does not provide overcurrent (short circuit) or overload protection.

.

'Motor starters are often wired with the sealing contact parallel to the starter coil.' Sorry but no.

If you look at this ladder diagram you will see the sealing contact is in parallel to the start button, not to the
starter coil.

'If there is a power interruption, or a brownout, the coil will release, and the motor will stop, and the start button has to be manually pressed to restart the motor.' Yes but if you replace the Stop button with a fuse and the fuse open it will also release the coil. This type of circuit would be use in a motor control panel where the power supply is also being used to power not only the motor but also a annunciator panel. The fuse will open unlatch the relay, power is remove from the motor but the annunciator panel will still have power. (See bottom ladder circuit.)
UL 508, the Standard for Safety For Industrial Control Equipment.


'In some situations, the operator may want the motor to restart automatically after a power outage',
Not if it is a piece of machinery like a lathe, mill ....
Automatic restart of production machinery is not allowed - NFPA79 - electrical standard for production machinery.


'Either way, the latch function does not provide overcurrent (short circuit) or overload protection.'
Please explain to me how does the latch circuit not provide over current (short circuit) or overload protection ? If the fuse blows does it not open the relay in which it isolates the power to motor. But yet the power supply is still connected to the main.

 

[GreenTwin]

First paragraph, referring to the standard FVNR (full voltage non-reversing) motor starter schematic, you are correct, the latch contact is across the start button, not the coil.

2nd paragraph:
I can't follow this paragraph, and am not familiar with the lower schematic, so I have no idea.

3rd paragraph: The electrical code says don't do anything that will make the situation more dangerous, such as in a refinery, where you can blow the entire plant up if you trip something off randomly (you don't want to trip off exothermic processes).
For production equipment, there is no need for an automatic restart, and that would be dangerous.

4th paragraph:
I don't see any purpose or function of the relay in the bottom diagram.
The control relay and pushbutton could be removed, and that would not have any affect on the fuse at all.
The fuse is in series with the motor, and if the fuse opens, then the motor stops.
If the fuse opens, it does not matter what happens to the relay; the motor stops regardless.
The fuse opening shoud not affect power to any other device.

Perhaps you are saying someone may blow the fuse, and have the lathe turned on, and then replace the fuse without turning off the lathe, in which case the lathe starts automatically.

I would not change a fuse with power still being supplied to the machine.
The disconnect should be opened and locked out per standard safety proceedure.
But if the disconnect is closed after the fuse is changed, then the machine could restart, such as if it had a manual and maintained-position on-off switch arrangement.

Typically electrical devices in the US are UL rated.
I would not use anything that is not UL rated.
Normally the UL rating would indicate that there are safety features with the machine that will make it safe in all situations, at least electrically speaking.

If you modify an electrical circuit on a machine, you are voiding the UL label and potentially creating some sort of hazardous situation under some conditions of operation.

An example is someone using a relay that has contacts rated less than the current or voltage present.
Example: Using 10 amp relay contacts to switch a 20 amp circuit.

For someone who knows what they are doing, they could safely modify a circuit, but it would still void the UL label whether it was done correctly or not. For someone who does not really know what they are doing, they could create a dangerous situation by coming up with some non-standard wiring/relay arrangement that is not safe in every scenario.

Sort of a complex topic.
.

 

[wce4]

3rd paragraph: The electrical code says don't do anything that will make the situation more dangerous, such as in a refinery, where you can blow the entire plant up if you trip something off randomly (you don't want to trip off exothermic processes).
For production equipment, there is no need for an automatic restart, and that would be dangerous.

Funny you should mention a refinery. I used to be the lead electrical engineer on the second shift at a sugar refinery located in NY (First civilian job after I served my first 4 years in the navy back in 81) own by a London-based company Taten Lyle and its subsidiary Toronto-based Red Path. Any how my job there was to automate the plant. ( Program and install PLC's, back then we use to refer to them Industrial PC's ) A similar latching relay circuit I posted was used on the sugar evaporators agitators motors.

4th paragraph:
I don't see any purpose or function of the relay in the bottom diagram.
The control relay and pushbutton could be removed, and that would not have any affect on the fuse at all.
The fuse is in series with the motor, and if the fuse opens, then the motor stops.
If the fuse opens, it does not matter what happens to the relay; the motor stops regardless.
The fuse opening shoud not affect power to any other device.

The function of the relay is to isolates the power to a faulty motor but maintain DC power to the rest of the circuit. In the evaporator you have more then one agitators motor supplied from the same DC power.
The refinery had it own power house it supply both DC and AC power to the plant. That reminds me of the time when the 4160 power feed blew, but that is a story for another time.
The purpose of this circuit is so you can replace the faulty evaporator motor and fuse with out shutting down the dc power to the other part of the circuit / motors. By the way the relays were not control relays they where 30 amp open frame Power relays. The Power relays and push button switch were located in a control room not in the operators control cabinet / console. The only one allowed to push the button in that room was the duty electrician when deemed safe by him .

Sort of a complex topic.

Agree with you I do. Been active in the trade for over 40 years fell way behind in the last 10. Sorry for being long winded on here, but I am bored out of my mind lying here with a collapsed lung and heart failure. If you or anyone find my post unwanted please feel free to remove it. No harm no foul I am just board.
By the way GreenTwinn congratulations on becoming a Moderator. I find this site to be very helpful in my endeavors .

 

[minh-thanh]

Simple: my circuit diagram is incorrect,

 

[wce4]

Simple: my circuit diagram is incorrect,

Agree with you I do. I thought you may have drawn the circuit wrong, but your intention was correct. That is why I stated

It looks like he is trying to use the relay as a circuit to latch the (Low Voltage DC) power circuit to the motor.

But I guess some over look that part. Oh well no harm done. We all learn from each other, that is why I enjoy this site. Thank you minh-thanh for understanding my intentions.

 

[Lloyd-ss]

I still think the re-setable breaker presented in post #12 is an effective and inexpensive solution to the OP's question.
However I agree with Minh That that avoiding stalling the motor is the best solution.

 

[SmithDoor]

I still think the re-setable breaker presented in post #12 is an effective and inexpensive solution to the OP's question.
However I agree with Minh That that avoiding stalling the motor is the best solution.

The motor on the micro lathe is 24 Volts DC , 150Watts and 6¼ amps.
They make a time delay fuse 6¼ amps. A time delay fuse will easily go over 5 time the rated amps for start and stalking. There is a graph on the overload amps.

The other is a cooling fan for low speed . This is because the motor fan is turning to slow to cool the motor.

You find electronics that does same job at higher cost.

The time delay fuse great for both motor and welders.
The welders need a high current for starting the weld and the delay fuse give extra ampage needed like for motor starting too.

Dave

 

[Nikhil Bhale]

I recently converted my lathe to 3 phase power by using VFD. To protect the VFD and motor I used a fuse inline with the power input to VFD. I used a 10 A fuse, my motor is 1.5 kW and mains power is 220V.

As far as I know start push button on motor control circuit are non latching type. So to keep the relay energised after the start button is released latching contact are used. The latching contact can be a NO contact pair on the relay itself or it can be a piggyback contact.

 

[GreenTwin]

Funny you should mention a refinery. I used to be the lead electrical engineer on the second shift at a sugar refinery located in NY (First civilian job after I served my first 4 years in the navy back in 81) own by a London-based company Taten Lyle and its subsidiary Toronto-based Red Path. Any how my job there was to automate the plant. ( Program and install PLC's, back then we use to refer to them Industrial PC's ) A similar latching relay circuit I posted was used on the sugar evaporators agitators motors.


The function of the relay is to isolates the power to a faulty motor but maintain DC power to the rest of the circuit. In the evaporator you have more then one agitators motor supplied from the same DC power.
The refinery had it own power house it supply both DC and AC power to the plant. That reminds me of the time when the 4160 power feed blew, but that is a story for another time.
The purpose of this circuit is so you can replace the faulty evaporator motor and fuse with out shutting down the dc power to the other part of the circuit / motors. By the way the relays were not control relays they where 30 amp open frame Power relays. The Power relays and push button switch were located in a control room not in the operators control cabinet / console. The only one allowed to push the button in that room was the duty electrician when deemed safe by him .
View attachment 155110

Agree with you I do. Been active in the trade for over 40 years fell way behind in the last 10. Sorry for being long winded on here, but I am bored out of my mind lying here with a collapsed lung and heart failure. If you or anyone find my post unwanted please feel free to remove it. No harm no foul I am just board.
By the way GreenTwinn congratulations on becoming a Moderator. I find this site to be very helpful in my endeavors .

Collapsed lung and heart failure ?
Wow, take care of yourself, that sounds rather serious.

I remember using about 20 of those open relays inside of an enclosure many years ago, and we were doing final inspection with the Owner's rep.
I had not been in the business very long, and so was just copying what I had seen used perviously.
The rep opened the cabinet, and said "What is with all the open relays? You guys never heard of safety relays with enclosed touch-safe parts?"
We had to change all the relays out.

The good part about those open relays is they were rated 30 amps, and were real workhorses.

I have worked in medium voltage for many years, up to 35 kV, and do a lot of PLC control work too.
The programs have changed a lot with PLC's, with all the color graphic screens, and the advent of high speed ethernet, and fiber.

And the relays have gotten very sophisticated these days, including the overload relays.

I also do sychronous motors, brushed and brushless, up to 5,000 hp or so, and the controls for those have changed over the years.

I don't get into DC stuff much, and prefer to stay AC, even in control systems, just to avoid power systems and their failure.
You can't fail equipment that you don't have.

I had an old pinball machine, and I assumed that the coils and lights must be DC (before I got into electrical), but it was all AC at various voltages.
That pinball machine is what really got me into electrical.

Edit:
When I first started in electrical, many/most/all? of the medium voltage switchgear relays were the old style mechanical type.
You don't see those anymore. Those were what I call dinosaur relays, but they did function well.
So much has changed over the years. There were no microprocessors when I started. It was mainframes, punch cards, and FORTRAN.
There was a mini-computer in the lab, and we programmed it in octal.
It was a big deal to write a program that closed a relay contact and lit a light.
.

 

[wce4]

Collapsed lung and heart failure ?

All thanks to my stupidity and a little help from Uncle Sam , Smoking, asbestos exposure
Aqueous film forming foam exposure ,lead and all other nasty exposures with little or no protective gear.

When I first started in electrical, many/most/all? of the medium voltage switchgear relays were the old style mechanical type.

The sugar refinery I work at was refining sugar since 1862 talk about some old switch gear,
there were knife switches with fuses on the old power distribution panels.

So much has changed over the years. There were no microprocessors when I started. It was mainframes, punch cards, and FORTRAN.

First microprocessor was Intel's 4004 it was released in 1971,
I mostly did Assembly language ,Basic, Ladder Logic then C. Had a hard time with C still do .
Now I am feeling very old

 

[GreenTwin]

I really avoid DC stuff like the plague, just because of the title of this thread, ie: power supply failure.
I didn't like electronics in school either.
It is a point of failure that I don't need in anything I do, so I basically live in an AC world.
.

 

[wce4]

I really avoid DC stuff like the plague, just because of the title of this thread, ie: power supply failure.
I didn't like electronics in school either.
It is a point of failure that I don't need in anything I do, so I basically live in an AC world.
.

A lot of your smart relays and PLC mix both AC and DC signals . There are a lot of 4 to 20 ma sensor signals which is typically powered by DC voltage ranging from 8-32 VDC .
The reason for using the current signal is that it is not easily disturbed. And the internal resistance of the current source is infinite, and the resistance of the wire in series in the loop does not affect the accuracy, and it can transmit hundreds of meters on the ordinary twisted pair. The upper limit is 20mA because of explosion-proof requirements in a hazardous environment. (The spark energy caused by the on-off of the 20mA current is not enough to detonate the gas.)
So how do you avoid using DC in your panels ?

 

[bluejets]

See here for more info.

Well now see...I've been an electrical contractor for 50 years or more and certainly do not require a link to a site that explains how you amateur mob think a circuit works.

 

[GreenTwin]

I am on the road to see the eclipse at the moment.
I will start a new thread.

 

[wce4]

I am on the road to see the eclipse at the moment.
I will start a new thread.

Be safe, talk at you later.

 

[minh-thanh]

A lot of your smart relays and PLC mix both AC and DC signals . There are a lot of 4 to 20 ma sensor signals which is typically powered by DC voltage ranging from 8-32 VDC .
The reason for using the current signal is that it is not easily disturbed. And the internal resistance of the current source is infinite, and the resistance of the wire in series in the loop does not affect the accuracy, and it can transmit hundreds of meters on the ordinary twisted pair. The upper limit is 20mA because of explosion-proof requirements in a hazardous environment. (The spark energy caused by the on-off of the 20mA current is not enough to detonate the gas.)
So how do you avoid using DC in your panels ?

Agree 101%

----
When someone tries to do everything to prove it is wrong, and unnecessary...don't argue.

 

[GreenTwin]

Starting a new topic, here:

https://www.homemodelenginemachinist.com/threads/power-distribution-and-control-system-chat.36036/

.

 

[Steamchick]

I have not read this in full. But it seems to me that a simple calculation to study ratings of fuses and breakers is to use the Isquare R value.
SIMPLY:
Determine the current drawn when the motor stalls = 6.25AMPS.
Continuous CURRENT rating should be nearest (just over) 120% of that = 7.5Amps.
MAX. Current rating should be not exceeding 150% of the continuous current rating = 9.375A. (Perhaps break the rules and use a 10A fuse!) - So an 8A SLOW BLOW fuse should manage this.
But please be aware that fuses are simply resistors, with a thermal "inertia" - so take time to heat to failure point.
So the energy needed to blow a fuse is time dependant. For starting motors - or resisting stalling - or preventing stall currents from cooking the motor, you have to be aware of the time element.
Here's a graph of I versus Time.

I presume the motor will take maybe 5 or 10 seconds of stall without vaporising the internal insulation? (Burn-out).
But if you had a momentary stall on the lathe and quickly hit the stop button - say within 2 seconds - you would not want the motor to burn-out or fuse to blow?
That leaves you (say) between 3 and 10 seconds of "safe" use - but something that will blow before the Motor fries and lets all the smoke escape! - Then that can give you an "IsqR" value for the fuse.
This is what manufacturers of proper equipment do for "the layman" so he can but his drill, lathe, washing machine or TV and it "works OK".
But you are asking about doing all that yourself.
You can see from the graph that at RATED current, the fuse relating to the graph will virtually NEVER blow. But at 150% of rated current it will blow in something like 2 seconds. So following this logic, you want something that is rated a bit closer than 150% of rated current, maybe 130% of rated current? - which is close to the 8A rating for a fast blow fuse - like regular cheap fuses...
A slow-blow fuse is a different curve to a fast blow fuse - this graph is for industrial fast-blow fuses. You could try a 6A slow-blow, as this will probably manage your use as the motor isn't actually drawing rated current when cutting, (stop-start). But will blow in seconds (not minutes I reckon) on a stall. Just time for a quick button hit ?? - and if it blows it is cheaper and quicker to replace than the motor.
A 10A fuse "regular" (= fast-blow) - I reckon - will put the motor at a lot of risk as it will take the starting current for your motor if starting on full load. But if the motor fries in >10 seconds at less than 10A then the fuse won't give you the protection you need.
Hope this doesn't sound too confusing or technical?
Conclusion - try a 5A or 6A SLOW-BLOW. - to be safe.
K2

 

[TSutrina]

This is the curves for the operation of a DC motor. It is for a PM stator and wounds rotor with brushes to control current flow. The curve for a brushless DC motor and a Universal brush motors will be similar unless the electronic limits current. A Brush motor is extremely robust. Do not have to worry about over voltage. The major failure is the destruction of the insulation coating on the windings. The second is a significant over voltage that punches through the wire insulation. If you look up the voltage of the wire it is well over what is possible by your power supply. So temperature is the critical factor.
If you smelled the insulation burning you have already reduced the life of the motor. I would go for a thermal fuse approach. Your power supply to take AC to DC is oversized so it should never have a thermal problem even when the motor is stalled.
The spike voltage that will cause the failure of the power supply is unlikely, however; it is possible. The best example is the electronic controlled coffee K cup maker. When built or some previous home owner to our present house connected the 1.5HP induction sump pump to the same circuit breaker as a kitchen outlet. Likely that outlet was at the mid point between the breaker and the pump. At start up or shut down of the pump if the coffee maker was making coffee a spike exceeded the filter and blew up some part in the coffee maker heating circuit or it's driver. This arrangement violates the building code so is unlikely to happen.
The wiring of shops and the choice that people make for the equipment connect to the circuit could create the above problem for the power supply. There is a filter that will withstand some level of voltage spike but they are not mind readers in sizing the filter.