At what amperage is a breaker designed to trip at?

[kwired]

180528-2026 EDT

kwired:

At
http://forums.mikeholt.com/showthread.php?t=174880&highlight=amperage+inrush
is a thread I started on motor inrush current being non-existent, but rather there is a starting current that is larger than normal running current. The plots I showed had no observable inrush current.

Inrush current dominantly lasts only about 1/2 to 1 cycle, and is usually very large compared to steady state current. Motor starting current typically lasts for a large number of cycles, and is not as huge relative running current.

If you remember you responded in said thread.

I know that there are many that want to call motor starting current inrush current, but motor starting current is so different in its character from other initial currents that it confuses peoples understanding of what is happening in a circuit when the term inrush current is substituted for motor starting current.

If you don't understand how different components and circuits work, then you are not in a good position to design circuits or troubleshoot problems.

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my reply in that thread is consistent with what I have said in this thread:

All he is saying is that the transient current that happens when starting a motor is not quite the same as the transient current that happens when energizing a transformer.

To most field electricians, there is still a transient current that occurs when energizing either one and they seldom need to know much more then that.

I will concede that "motor starting current" is likely the more correct term to use. I will also say that in casual conversation with most field electricians, it really doesn't matter to be that precise, either way many realize there is an initial surge of current that is quite a bit higher then normal operating current and that overcurrent protection must be able to ride through this surge of current without opening the circuit.

There are many that think this starting current and locked rotor current are the same level of current - which is not true.

 

[gar]

180529-2123 EDT

kwired:

There are many that think this starting current and locked rotor current are the same level of current - which is not true.

What is zero RPM other than locked rotor?

My plots show no difference in current from t = 0 until there is substantial motor RPM. On the three phase motor 8 to 15 60 Hz cycles before current started to drop. Then it took another 10 to 16 cycles to reach full speed. Both friction and inertia influenced these curves. But otherwise there was no loading of the motor.

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[kwired]

180529-2123 EDT

kwired:


What is zero RPM other than locked rotor?

My plots show no difference in current from t = 0 until there is substantial motor RPM. On the three phase motor 8 to 15 60 Hz cycles before current started to drop. Then it took another 10 to 16 cycles to reach full speed. Both friction and inertia influenced these curves. But otherwise there was no loading of the motor.

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Real world situations there is a driven load, otherwise seldom any need for the motor. Sometimes the driven load is mechanically increased once the motor is running.

I can understand that an inductor can't have an instant change in current. It can have a change over time.

A motor goes from nocurrent to high current to a lower current once up to speed every time it is started with across the line starting methods.

Some of that may seem instantaneous but something that takes a cycle or two to happen still has time factor involved, just faster then we can ordinarily observe without help from special instruments.

 

[ggunn]

180528-2143 EDT

Russs57:

You do not need to assume an unloaded motor.

A motor started at a time of 0 RPM, whether loaded or unloaded, has the same initial starting current. What will differ is the time to reach full speed. See my plots at post #27 at http://forums.mikeholt.com/showthread.php?t=174880&highlight=amperage+inrush

.

I am no motor expert but this does not seem reasonable if I am understanding what you have written. I was at an off grid site a couple of weeks ago, a private boathouse with a couple of boat stalls with electric lifts. The lifts are wood and steel frames with cables up to a rotating pipe that the cable wraps around. With the lift frame in the water and a boat over it, the lift works fine and raises the boat to the required level, but if the lift is stopped midway in the process with the full weight of the boat loading the motor, when we try to finish the lifting the motor doesn't spin at all and draws so much current that it shuts down the inverter.

 

[gar]

180529-1132 EDT

kwired:

At 0 RPM or locked rotor the impedance of the motor is the same and therefore the current is the same independent of how much load is on the motor. The amount of load on the motor will determine whether the motor will even start, remain at 0 RPM, or how long for the motor to get up to full speed.

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[gar]

180529-1140 EDT

ggunn:

For your boat example you need to look at the starting torque capability of the motor.

When the boat was in the water there was buoyancy and less starting torque required than after the boat was out of the water. Speed torque curves for an induction motor can have quite different shapes depending upon design (rotor resistance). Look at a book on induction motor theory, and see how speed torque curves for the same motor vary as the rotor resistance is varied.

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[Sahib]

I am no motor expert but this does not seem reasonable if I am understanding what you have written. I was at an off grid site a couple of weeks ago, a private boathouse with a couple of boat stalls with electric lifts. The lifts are wood and steel frames with cables up to a rotating pipe that the cable wraps around. With the lift frame in the water and a boat over it, the lift works fine and raises the boat to the required level, but if the lift is stopped midway in the process with the full weight of the boat loading the motor, when we try to finish the lifting the motor doesn't spin at all and draws so much current that it shuts down the inverter.

With lift frame submerged in water and boat also partly submerged, there is apparent loss of weight due to Archimedes principle. So the motor is able to accelerate quickly to reach a stable speed. But it could not do so when boat load stopped midway as its starting torque lower than load torque and so kept on drawing locked rotor current till inverter shutdown.

 

[gar]

180529-1311 EDT

In Figure 6.16 at
http://machineryequipmentonline.com...induction-motor-torque-speed-characteristics/
you can see speed torque curves for various rotor resistances.

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[don_resqcapt19]

So does this mean that in practice at 25C ambient any breaker installed in its own enclosure will function as a 100% rated breaker, even if not listed as such? Or can a basic "enclosed circuit breaker" enclosure for plug-on breakers (e.g. QO2100BNF for Square D QO breakers up to 100A) cause more than an additional 15C in temperature rise?

Are 100% rated breaker required to hold indefinitely at their rated current within their enclosures at 40C ambient? I assume the only reason for upsizing the bus is to reduce one source of heat generation.

Thanks, Wayne

Yes, if you keep the temperature of the breaker guts, bus and connected wires to 40°C of less, the breaker should carry its rated current forever. (assuming a 40°C rated breaker.

The combined heat from the I²R losses of all of the conductors, breakers and the bus in a panel can easily raise the internal temperature 15°C.

 

[Jraef]

Gar and I have had this discussion several times, with my contention that "inrush" is inclusive of the magnetizing (flux producing) current, which is limited only by the impedance of the system and the motor windings during that first cycle, because at that time there is no mutual induction yet from the rotor. I was taught that this can be as high as 20x the motor FLA, especially on newer Energy Efficient motor designs. This is the concept behind the 2002 NEC exception to 430.52 that allows mag trip settings to be as high as 1700% of FLA for energy efficient motors, if the lower setting of 1300% is proven to be causing nuisance tripping.

Cooper / Bussman has a paper corroborating my position on this.

Motor Starting Currents
When an AC motor is energized, a high inrush current occurs. Typically, during
the initial half cycle, the inrush current is often higher than 20 times the normal
full load current. After the first half-cycle the motor begins to rotate and the
starting current subsides to 4 to 8 times the normal current for several
seconds. As a motor reaches running speed, the current subsides to its normal
running level. Typical motor starting characteristics are shown in Curve 1.

For the most part, that peak inrush is too fast for most breakers to act, but it absolutely does rear its ugly head once in a while and I have had to use that NEC exception. Most starter mfrs now also acknowledge this and whereas they used to say that you can't change the MCP size in a listed starter, they will now offer a way to get around that by changing to a Thermal-Mag breaker that fits within the limits of 430.52, but offers a higher mag trip range to accommodate the higher inrush, if necessary.

 

[gar]

180529-1556 EDT

Jraef:

I would agree that it is possible to encounter a high inrush current on a motor, but that is not starting current. Most people are using the term high inrush to describe starting current.

For there to be an inrush current to a motor, then there must be a high residual flux in the core after the motor stops, then upon starting the flux in the core must be driven higher causing core saturation.

With older motors the core material was less square loop, there was a large air gap, and residual flux was further from saturation.

In high efficiency motors the core material is more square loop, possibly has a higher retentive flux level, and more quickly goes into severe saturation. I don't have any idea whether the air gap has been reduced. Thus the probability of a high inrush current at times is increased. It won't occur on every random start. Whereas starting current will be essentially the same on every start.

I have no high efficiency motor to play with. I would like to see plots of motor inrush current for such motors.

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[kwired]

180529-1132 EDT

kwired:

At 0 RPM or locked rotor the impedance of the motor is the same and therefore the current is the same independent of how much load is on the motor. The amount of load on the motor will determine whether the motor will even start, remain at 0 RPM, or how long for the motor to get up to full speed.

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When the winding is not energized there is no inductive reactance only resistance- but very low resistance. As soon as you apply full voltage to that low resistance it is almost like a short circuit very briefly and current rapidly drops as reactance increases.

An already energized motor that happens to stall already has magnetic fields established, and current is limited by the impedance of the windings. Yes it will be high current when it stalls compared to full load rating but not as high as short circuit would be.

 

[gar]

180529-2053 EDT

kwired:

When the winding is not energized there is no inductive reactance only resistance- but very low resistance. As soon as you apply full voltage to that low resistance it is almost like a short circuit very briefly and current rapidly drops as reactance increases.

Your statement is invalid. Study series LR circuits with source battery and switch, and the transient analysis.

.

 

[gar]

180529-2247 EDT

A straight piece of wire, a conductor, has inductance. Wind that wire into a coil and the inductance is increased. Add a magnetic core and the inductance is increased further.

A basic characteristic of an inductor is that the current can not instantaneous change.

Create a series circuit of a battery, switch, and a resistor in series with an inductor. In the real world most inductors have internal resistance, and therefore experiments have to be performed such that by calculation you determine what is happening to all of the series resistance in the loop, and in turn what the inductor is doing.

If at time t=0 just before the switch is closed the current in the inductor is 0, then just after the switch closes the current must still be 0 and all of the battery voltage is applied across the inductor and 0 across the resistance.

You can search the Internet for the equations that define current after t=0.

Because a coil of wire is in a motor and there is no initial current thru the coil does not mean the coil has 0 inductance. It has an inductance defined by the coil and the physical properties around the coil.

.

 

[kwired]

180529-2247 EDT

A straight piece of wire, a conductor, has inductance. Wind that wire into a coil and the inductance is increased. Add a magnetic core and the inductance is increased further.

A basic characteristic of an inductor is that the current can not instantaneous change.

Create a series circuit of a battery, switch, and a resistor in series with an inductor. In the real world most inductors have internal resistance, and therefore experiments have to be performed such that by calculation you determine what is happening to all of the series resistance in the loop, and in turn what the inductor is doing.

If at time t=0 just before the switch is closed the current in the inductor is 0, then just after the switch closes the current must still be 0 and all of the battery voltage is applied across the inductor and 0 across the resistance.

You can search the Internet for the equations that define current after t=0.

Because a coil of wire is in a motor and there is no initial current thru the coil does not mean the coil has 0 inductance. It has an inductance defined by the coil and the physical properties around the coil.

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Current has to rise sometime, whether it be during first cycle, the tenth, the 300th or several million cycles, over multiple cycles...

Rise might not be instantaneous, but is a rapid rise with typical AC motors started across the line. Fast enough we perceive it as instantaneous. And it typically rises a large amount then drops off rapidly as motor accelerates. High speed and/or high inertia loads do factor into how fast motor may accelerate and how rapidly current will drop, as well as what the source can deliver.

 

[Sahib]

Current has to rise sometime, whether it be during first cycle, the tenth, the 300th or several million cycles, over multiple cycles...

Rise might not be instantaneous, but is a rapid rise with typical AC motors started across the line. Fast enough we perceive it as instantaneous. And it typically rises a large amount then drops off rapidly as motor accelerates. High speed and/or high inertia loads do factor into how fast motor may accelerate and how rapidly current will drop, as well as what the source can deliver.

You may think of inductance as open circuit at t=0 when a voltage is first applied across it.

 

[kwired]

You may think of inductance as open circuit at t=0 when a voltage is first applied across it.

I conceded to that, but at some point in time the current has to rise or we would never have any current to operate the motor. And when it rises it rises at a rapid rate. This time is apparently so short that without using specialized measuring equipment it seems to be almost instantaneous.

I was told there is no "inrush" current when a motor starts. Exactly how you look at this is all that is being mentioned as well as some terminology being thrown in the mix. Bottom line is even it it isn't at the same instant the circuit gets closed, there is a rapid rise in current at some point and sure looks like it is some point less then a tenth of a second, which is a long time with 60 Hz being the measuring stick.

Bottom line - there is a surge of current that flows at some point during the process of starting that motor that is higher then it's normal running current.

And I am fairly certain that it often is higher (though only for very short time) then what the motor would draw steady state at locked rotor condition.

 

[gar]

180530-0833 EDT

kwired:

You need to get some instruments, various components, and then run experiments, and correlate this with theory.

I have provided actual plots of two different motors, one a single phase with centrifugal switch, and the other a three phase with no switch. These show no inrush current, but do show starting current that lasts a long time.

I have also shown inrush current for an incandescent bulb, and a transformer driven into saturation.

No one else has provided any actual plots.

We should get back to the question of the original post. But the diversion to a discussion of loads is important because it relates to the original question.


Jraef:

Can you provide any plots from any of the sources that show an inrush current on a high efficiency motor resulting from core saturation, as distinguished from just a higher starting current resulting from new design criteria? It is possible that higher starting currents are simply a result of different values of L and R in the high efficiency motors of a given HP rating?

Starting current is many cycles long. Inrush current from core saturation is about 1 cycle in general.

.

 

[kwired]

180530-0833 EDT

kwired:

You need to get some instruments, various components, and then run experiments, and correlate this with theory.

I have provided actual plots of two different motors, one a single phase with centrifugal switch, and the other a three phase with no switch. These show no inrush current, but do show starting current that lasts a long time.

I have also shown inrush current for an incandescent bulb, and a transformer driven into saturation.

No one else has provided any actual plots.

We should get back to the question of the original post. But the diversion to a discussion of loads is important because it relates to the original question.


Jraef:

Can you provide any plots from any of the sources that show an inrush current on a high efficiency motor resulting from core saturation, as distinguished from just a higher starting current resulting from new design criteria? It is possible that higher starting currents are simply a result of different values of L and R in the high efficiency motors of a given HP rating?

Starting current is many cycles long. Inrush current from core saturation is about 1 cycle in general.

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You earlier said this inrush current doesn't exist.

I don't doubt most the theory you have posted, it all comes down to what is your base reference for "a long time" and how you define what "inrush" means.

You earlier mentioned a straight piece of wire has inductive characteristics - so with that said and with your other mentioned rules - even an incandescent lamp can't have an inrush current - because of the inductance in the supply conductor.

My point is somewhere along the line there is still a sudden surge of current with these applications we have been mentioning, it just may not be at the very same instant the circuit gets closed.

 

[Sahib]

kwired: An induction motor cannot have inrush current because of presence of air gap.