When a motor is stuck, why does current (amps) go up?

[Jacobson]

When a motor is stuck, it can cause a spike in current.
This can melt a wire, or blow a fuse.

But why? What exactly is happening here?
Which values are going up, and which are going down?

Volts = Amps * Resistance
Watts = Amps * Volts

 

[Fretters]

In simplistic terms, the same reason you have to rev the knacks somewhat on an engine if you bog down. If something ain't moving, it takes more power to shift than it does to maintain movement.

 

[larry_g]

Motor=Generator. As the motor spins it generates a voltage that counters the input voltage. Look up counter electromotive force or CEMF.

lg
no neat sig line

 

[Jacobson]

This is going to sound really really dumb.....

Wait, since it doesn't have a mind, how does the motor know to draw MORE power when it is stuck? "Gee, I can't move, I will request more power!" It is just programmed to draw a fixed amount of power, right?

Which equation applies?
Watts = Amps * Volts ?

Say you have a simple car window motor. Like a 10w motor?
Volts is fixed at 12v for a car.
So, using the equation, how does amps go up? (fuse blows or wire melts)
Either volts has to go down, or watts has to go up.

Does the 10w motor increase his watts when he's stuck?
How does he know to do that?

 

[theoldwizard1]

When a motor is stuck, it can cause a spike in current.
This can melt a wire, or blow a fuse.

But why? What exactly is happening here?

Because the windings of the motor are now a continuous short circuit.

When a DC motor is spinning the commutator interrupts the current flow. For a short period of time when the brushes are in contact with the contacts of a field on the shaft it is a short circuit. But then that circuit opens and as it passes the gap between the contacts on the commutator and on to the next set of windings.

On an AC motor, the slip rings are connected to the field which is rotating. Magnetic reaction with the stator does effect the current flowing through the field.

Which values are going up, and which are going down?

Average resistance (over time) is changes (goes down). Voltage stay the same (assuming an infinite supply like Niagra Falls ) so current goes up.


FYI: The reason why small motors in cars (window, door lock, power seats, etc) don't over heat is there is a small series, "current limiting", resistor built in to them. These resistors are incredibly cheap and are know to open or change resistance over time so that the motor does not receive enough current to operate the mechanical device (or the mechanical load has increased with age and the motor now needs more current to do the same job; slow window motor).

 

[gtermini]

This sums it up very well: From http://forums.mikeholt.com/showthread.php?t=52458

The current drawn by an induction motor is a function of the rotor speed. The applied voltage creates a rotating magnetic field. When the rotor is rotating at the same speed as this magnetic field, the input current is minimum.

When the rotor is stopped, the input current is a maximum. This is because the turning rotor produces a back EMF that reduces the input current. If the rotor is not turning, there is no back EMF, and the input current is the highest. This is refered to as the "locked rotor current" because it is the same current that would be drawn if the rotor were locked in place so it can't turn.

When a motor is first started, the rotor is stopped, so it draws the locked rotor current. The motor quickly starts turning, and the rotor current decreases. The current keeps droping until the rotor gets up to full speed. At that point the current is at minimum. (Full speed is slightly less than the speed of the rotating magnetic field, and depends on how much torque the motor must produce to turn the load).

In most cases, I think sizing the breaker at 250% of rated current is enough to prevent tripping on startup. It might not be enough if the motor load has a large amount of inertia, or if the motor has a particularly large locked rotor current.

See Also: locked rotor condition

Greyson

 

[zkling]

Did your ******** motor stop working to the point it heated up so much that your orifice got burnt?

 

[sberry]

Its part of design on general use circuits. The voltage drop for 20A becomes a fixation point. Many dedicated items with rather small run loads like laundry machine are on 20A but most generals are one user at a time, come with a 16 cord with a 15 end allowed for 20A ocpd,,, it wouldn't overheat a 14 wire to have a 500 watt halogen on the same circuit you zinged off a board with a circ saw but may blow a 15A breaker on start.
Typical problem with small air comps as they age a little, will trip 20A, had a couple chop saws would too if they were wired close to the panel.

 

[MBfreak]

Hi guys.
The question is a bit unspecified and needs to be adressed in depth, since electric motors have very different performance depending on type.

A DC motor with a traditional commutator. When the rotor starts spinning a back emf (EMF) is created which is slightly lower than the applied DC when the motor has reached constant speed . Rotor resistance =R. Rotor ( armature) current=I. Applied voltage U
U= EMF+R*I. When the rotor is subjected to a mechanical load, EMF will reduce. R is largely constant, as is U. So to balance , I will increase, generating a higher torque and speed may increase. And on it goes.

An asynchronuos motor with a cage(squirrel cage) rotor winding is totally different, and runs on AC only. In short, it has a poor starting torque and the frequency of the rotor current equals the difference in rotor and applied power frequency to the stator.
The motor draws a largely reactive stator current and accelerates slowly with low torque. As speed increases torque goes up, reactive power goes down and a torque peak of around 85% of syncro speed is passed, motor continues to spin up to about 95-97 % of syncro speed, and stabilize there.
If you apply a mech load, speed will drop, current and reactive power goes up. If you brake it so speed drps below the torque max , the motor will stall if the braking torque is not reduced. Reactive current at start can be 5x the full load (acive) current , or more. ( Syncro speed in RPM = f(Hz)*2*60/n( number of poles)


Then there are asynchronuos motors with wound rotor and external rotor resistors, also on AC. A less common design for "our " motors is a synchronus motor which has an rpm which fixed in relation to the applied frequency.
And the list goes on, but I probably have written too much and boringly already. Good nite.

Regards

Ola

 

[Rossco]

Ohms Law.

When resistance goes up, you need more amps to push the current.

 

[Jacobson]

Tons of replies, well meaning, but I have no idea what anyone is saying. I am no closer to understanding why the current goes up when the motor is locked. Maybe this is just too complex for me?

 

[Jacobson]

Ohms Law.
When resistance goes up, you need more amps to push the current.

Ok, this I can follow.
Why does resistance go up when a motor is stuck?
I thought resistance goes up when the length of cable is longer.
In the case of a stuck motor, the cable is a fixed length.

 

[Rossco]

Ok, this I can follow.
Why does resistance go up when a motor is stuck?
I thought resistance goes up when the length of cable is longer.
In the case of a stuck motor, the cable is a fixed length.

Am no electrician but a motor is a resistance device.

That's why a motor draws allot more AMPS on start up. You need to increase the Amps to overcome the resistance so current will flow. Once the motor is up to speed less amps are needed to maintain current flow.

I see it all the time at work. A starter motor will jam and glow white with heat. The energy has to go somewhere so its converted into heat.

Edit: Iam talking about DC here. Mainly 12-24v

 

[Danglerb]

Until its moving it doesn't create the back emf, faster it moves the more it creates.

 

[Jacobson]

Ok, The motor itself is part of the circuit, and when it's stuck, it is creating resistance.

But wait.
Volts = Amps * Resistance

If the resistance is going UP,
that means Amps must be going DOWN.
In order to keep volts the same (like a 12V auto system)

Motor stops -> Resistance UP -> Amps DOWN.
This is the exact opposite of what supposedly happens!

 

[Jacobson]

Also, why is there resistance when the motor is stopped. When the motor is running, where is the electricity going? Is it flowing through the motor and getting "used up"?

Or is it being passed to ground? And it just goes into the car's body? Where does it go from there? Or is a circuit closed, and the electricity just goes back to the source battery?

Why, when the motor is stopped, is there a traffic jam of electricity at the motor, and the electricity can't find another way to go, like a river, so it just melts the wire and explodes into the ambient air? Why can't the current just go past the motor, even though it's not moving. Circuit is still closed, right?

But, when a fuse blows, there isn't a traffic jam at one side of the broken bridge, b/c the current only flows with a close circuit, right?

 

[todd_fuller]

V0 = (I x R) + Ve

Where:
Vo = Power supply (Volts)
I = Current (A)
R = Terminal Resistance (Ohms)
Ve = Back EMF (Volts)

Also:
Ve = ω x Ke

Where:
ω= angular velocity of the motor
ke = back EMF constant of the motor

Therefore, by substitution:
Vo = (I x R) + (ω x Ke)

Therefore, by subtraction and division:
I = (Vo - ω x Ke) / R

If ω = 0, then:
I = Vo / R


(shamelessly taken from http://www.micromo.com/motor-calculations)

 

[sberry]

To the user or diy designer it often means there is no reason to upsize wire or pipe. Putting a 10 wire on a 20A breaker only helps your comp or saw to trip. Put it on a smaller wire or farther from the panel where there is a little voltage drop and it draws a little less current over a little longer time and it works.
Somewhat the same for air movement, big piping and connectors let it rip during starts and waste energy before the operator can apply it.
Sometimes you are not doing a favor "upgrading" factory cords etc.

 

[sberry]

Ok, The motor itself is part of the circuit, and when it's stuck, it is creating resistance.

But wait.
Volts = Amps * Resistance

If the resistance is going UP,
that means Amps must be going DOWN.
In order to keep volts the same (like a 12V auto system)

Motor stops -> Resistance UP -> Amps DOWN.
This is the exact opposite of what supposedly happens!

A battery is unregulated unlike a car system, its not constant voltage like a wire welder but V drops as current goes up like a transformer.

 

[pokey459]

Ok, The motor itself is part of the circuit, and when it's stuck, it is creating resistance.

But wait.
Volts = Amps * Resistance

If the resistance is going UP,
that means Amps must be going DOWN.
In order to keep volts the same (like a 12V auto system)

Motor stops -> Resistance UP -> Amps DOWN.
This is the exact opposite of what supposedly happens!

you are looking at it backward, as resistance decreases the number
value [ohms] goes down , lower number of ohms=less resistance.
less resistance = harder short. resistance/impedance slows/restricts
the current flow [amps/watts]. when an electric motor is stopped/stuck
it has the least resistance of any point in it's run cycle. similar to twisting
two wires [short circuit] together and applying power , result ? heat and
blown fuse.

and electricity does not go away , it is energy that is converted to motion
and/or heat.

 

[404]

Many of these replies are well meaning but incorrect.

To keep things simple lets consider a 12 volt dc motor with permanent magnets out of a cordless drill.

If we were to spin the shaft of this motor, it would generate electricity. The faster the spin rpm the more voltage is put out.
For fun we will call this voltage Back EMF.

Now, keeping in mind the above, we apply 12 volts to the motor with no load on the shaft, and the motor spins up to full speed but no faster. If we up the voltage to 13 volts for example the motor spins a bit faster.

The key point is that even when the motor is fed 12 volts and allowed to spin it still acts as a generator. The motor will speed up untill the voltage it generates (back emf) is nearly equal but opposite polarity of the voltage put in. Due to the very small voltage difference between input voltage and generated back emf voltage only a small current will flow, just enough to overcome bearing and brushes friction, cooling fan friction, etc...as the motor is spinning.

Now we apply a load to the motor and the RPM is reduced. Slower rpm means less opposite voltage (back emf) generated by the motor. The voltage difference between the feeding in and the generated voltage is bigger, so more current flows.

When motor is at zero rpm, no generation occurs and current is limited by resistance of the motor wire windings. Usually a big number of amps and the motor will soon overheat or blow a breaker.

Regards,
404

 

[Wamsutta]

Ohms Law.

When resistance goes up, you need more amps to push the current.

I think you mean volts; don't you?

 

[PBCampbell]

I'm not an Electrical Engineer but I want to take a stab at this. Be forewarned that my "explanation" may not be technically correct but is simply an explanation that may help some "get their head" around the subject. Electricity moves from it's origin (point of generation, say Hoover Dam) to Earth (this big thing we're living on). Everything in between is resisting this movement. Basically when you flip a switch you're allowing electricity to flow from a source to earth, like opening a gate.
Making an analogy with water (this used to be popular), water pushing against a "water wheel" (like at an old fashioned grist mill) that isn't turning is encountering resistance. This resistance results in a pressure differential between the two sides of this motor. High pressure on the supply side and low pressure on the opposite side (the load (earth) side). Of course Physicists tell us that electricity doesn't have pressure and doesn't push. It's actually more analogous to the load being a vacuum sucking the electrons along their path.
At any rate the higher current reading on a non moving motor is simply due to the higher resistance due to the motor not moving and electrons that want to move on getting piled up and on one side opposed to the other.

 

[Jacobson]

you are looking at it backward, as resistance decreases the number value [ohms] goes down , lower number of ohms=less resistance.
less resistance = harder short. resistance/impedance slows/restricts
the current flow [amps/watts]. when an electric motor is stopped/stuck
it has the least resistance of any point in it's run cycle. similar to twisting
two wires [short circuit] together and applying power , result ? heat and
blown fuse.
and electricity does not go away , it is energy that is converted to motion
and/or heat.

You're saying when a motor is stuck, there is LOW resistance?
Everyone else in this thread said resistance is HIGH when motor is stuck.
Which is it?

If resistance is HIGH when motor is stuck, then the following paradox occurs:
Volts = Amps * Resistance

Motor stops = high resistance = high Ohm number = Amps MUST decrease (if volts stay the same)

If Volts don't stay the same
Motor stops = high resistance = high Ohm number = Volts decrease (and amps stays the same)

This makes no sense, so I guess RESISTANCE goes to 0 when the motor stops,
which makes current spike UP ?

This also makes no sense, b/c everyone here has said when the motor stops, the electricity has nowhere to go (high resistance, high Ohm number). If there was no resistance, and the motor was stopped, the current would just pass on by the motor and go on it's marry way, where ever that happens to be.

Now we apply a load to the motor and the RPM is reduced. Slower rpm means less opposite voltage (back emf) generated by the motor. The voltage difference between the feeding in and the generated voltage is bigger, so more current flows.

Ok, but who or what higher being is making the current increase? I am picturing little elves inside the wire saying "Whoa! Getting harder to push! We need some back-up here! MORE CURRENT please!" What is actually happening to increase current when resistance increases (meaning lower ohm number)

At any rate the higher current reading on a non moving motor is simply due to the higher resistance due to the motor not moving and electrons that want to move on getting piled up and on one side opposed to the other.

Again, I get the metaphor, who who is telling the electricity to increase current? Is God watching and seeing we need more power to break that water wheel free? So he turns up the current knob ?

 

[Snaps1992]

Many of these replies are well meaning but incorrect.

To keep things simple lets consider a 12 volt dc motor with permanent magnets out of a cordless drill.

If we were to spin the shaft of this motor, it would generate electricity. The faster the spin rpm the more voltage is put out.
For fun we will call this voltage Back EMF.

Now, keeping in mind the above, we apply 12 volts to the motor with no load on the shaft, and the motor spins up to full speed but no faster. If we up the voltage to 13 volts for example the motor spins a bit faster.

The key point is that even when the motor is fed 12 volts and allowed to spin it still acts as a generator. The motor will speed up untill the voltage it generates (back emf) is nearly equal but opposite polarity of the voltage put in. Due to the very small voltage difference between input voltage and generated back emf voltage only a small current will flow, just enough to overcome bearing and brushes friction, cooling fan friction, etc...as the motor is spinning.

Now we apply a load to the motor and the RPM is reduced. Slower rpm means less opposite voltage (back emf) generated by the motor. The voltage difference between the feeding in and the generated voltage is bigger, so more current flows.

When motor is at zero rpm, no generation occurs and current is limited by resistance of the motor wire windings. Usually a big number of amps and the motor will soon overheat or blow a breaker.

Regards,
404

This is the right response, but obviously lacks a few explanations on things you don't understand (yet).

A motor is not a simple resistance - Ohms law (V=I*R only applies to a simple resistance). The coils within the motor act like an inductance in series with a resistance.

The way an inductor works is that any current flowing through it will create a magnetic field through the coils. Due to the laws of physics, this produces a force on any magnetised object (as the object's magnetic field tries to align with the coil's magnetic field).

Now, the opposite effect also happens (and is EXACTLY what a generator is) - when a magnetic object is moved/rotated within a coil, an opposite current is INDUCED (hence the reason a coil is called an inductor) within the wire, and due to that opposite current, a voltage with the opposite polarity will be created on the ends of the coil.

So, what happens is that when you run a current through the motor windings (the inductor), it turns the motor. The motor (the magnet) then speeds up, and causes SOME opposing current to the original current that was used to drive the motor (note that the created/induced current is always less than the original input current). Essentially you have a large amount of current in one direction, and a slightly lesser amount of current in the other direction. The overall current from the addition of these two currents will be a small positive current.

Now, when the motor is not turning, or is loaded too much to turn, the magnetic object (the part that turns, or the 'rotor'), is not turning, and so does not induce any opposite current. The result of this, is that the overall current is MUCH larger.

Hope that answers your question... If anyone's wondering, I'm a final year mechatronics engineering student. I have completed an entire paper on this subject, so that's where the information comes from!

 

[91bronc300]

I'll take a stab at it with the little I know. Once the switch is flipped there will be a rotating magnetic field in the stator (more just flip flops in a single phase motor, doesn't truly rotate unless it's a multiphase motor like a 3 phase), this magnetic field rotating in the stator (the stator is the windings on the outside i.e. that encircles the rotating bit in the middle) will rotate according to the frequency of the voltage, 60hz (or times per second) in this country. That comes out to 3600RPM for a 2 pole motor (1800RPM motors are 4 pole).

When you hit the switch there is instantly a magnetic field in the stator that is rotating at 3600RPM. There is nothing mechanical rotating in the motor at this point. Only a magnetic field that you can't see. And the stator NEVER rotates, it's fixed to the motor housing, all the stator ever does is creates this rotating magnetic field. Inside the stator and also inside the stator's rotating magnetic field is the rotor. The part of the motor that spins. It also has windings but these windings are not connected to input voltage like the stator windings are. What these windings are for is to accept energy from from the stators magnetic field, they do that through induction.

Induction- if you were to have a magnet in your hand and you were to move it over a wire it would INDUCE a voltage in that wire. You can't just hold the magnet near the wire and get current, you must move it. But if you do (you can also move the wire and leave the magnetic standing still, same effect) then that movement makes a voltage (or a current in a closed circuit). The faster you the relative movement between the magnet and wire and the greater the current.

Back to the motor. You have a magnetic field in the stator that is now rotating at 3600RPM (60 cycles per second times 60 seconds = 3600RPM) and the rotor, which is not moving, sitting in the middle of this field. The relative movement between this rotating magnetic field (always 3600 RPM) and the rotor (0 RPM) induces a large current, much larger than the current listed on the motor spec plate, in the windings of the rotor. With a large current now in the windings of the rotor the rotor will also have it's own magnetic field (current flowing in a wire makes a magnetic field). And the magnetic field of the rotor will be a strong one because there is a large current in it's windings. The magnetic field of the rotor rotates at the same speed as whatever the rotor is spinning at (which is still zero right now, right after the switch was flipped) This magnetic field in the rotor wants to synchronize with the stators magnetic field and that causes the rotor to begin turning, to mechanically accelerate. The rotor wants to be at 3600 RPM. The stator's magnetic field is always 3600 RPM even though the stator itself is stationary, the rotor's magnetic field is fixed to whatever the rotor itself is spinning so in order for the two to try to synchronize the rotor must accelerate.

When the rotor starts to accelerate the speed difference between the magnetic fields in the stator and rotor will get smaller. For example when the rotor has accelerated to 2400RPM there is now only a 1200RPM difference between the rotor and stator's rotating magnetic field. Because how much current in induced in the rotor is dependent on this speed difference (more speed equals more current) this smaller speed difference causes a smaller current to be induced in the rotor and thus a weaker magnetic field in the rotor. This weaker magnetic field means less actual work going into turning the rotor. As the rotor keeps accelerating eventually you reach a point of equilibrium where this magnetic field is so weak it's only creating enough power to turn the rotor against whatever little resistance exists in the motor itself (bearings).

This is why motors will say their no load RPM is something like 3500 instead of 3600 RPM. There needs to be a 100 RPM speed difference between the rotor speed and thus the rotor's magnetic field speed and the stators permanent 3600 RPM magnetic field speed in order to make enough power to just overcome the resistance in the bearings. As you load the motor the rotor speed (the output shaft is connected to the rotor) will decrease thus causing a greater speed differential, thus cause more current induced in the rotor, which cause a stronger magnetic field in the rotor, which causes the two magnetic fields to work harder again to synchronize (or speed back up).

So if the stator's magnetic field is rotating at 3600 RPM and the rotor (and thus it's magnetic field) is rotating at 3600 RPM there is NO induction of current in the rotor, NO magnetic field in the rotor, and NO power accelerating the rotor anymore. This is impossible because there will always be some power needed to turn the rotor, even against no load so the maximum rotor speed is a little less than this by some bit. On the other hand when the stator is receiving power from the wall and thus it's magnetic field is rotating at 3600 RPM but the rotor is a zero speed there is MAXIMUM current induced in the rotor, MAXIMUM strength of magnetic field, and the rotor goes zipping up to speed. If the rotor happens to be locked up on something and can not turn then when you flip the switch the motor is locked into this maximum current situation. Big currents are induced and things stay that way. The normal thing is for this MAX induction situation to only last maybe a second as the rotor is zipped up to speed quickly when you flip the switch. But if the motor is locked up that doesn't happen.

As far as I know that layman explanation is correct but some EE can refine it or debunk it.

 

[PBCampbell]

You're wanting a Physics lesson and that's beyond my capability. Essentially, effectively, etc. the Earth will "****" up ALL available electrons and anything between a source and the earth can be subjugated to do "work". In a way I guess God is saying "More Power". Look at the "Macro" (larger picture) before trying to comprehend the "Micro".

 

[404]

Thank you Snaps1992

Regards,
404

 

[Rossco]

I think you mean volts; don't you?

I don't know what I think anymore. That's my story and Iam sticking to it 15 year since I left mechanics school.

Anyhow I had a minor problem recently with a 250ton CAT loader. Kept popping the 75amp breaker mmm

So anyways, it turned out to be a loose connection leaving the breaker. So in other words : Resistance, Too many Amps passing through the breaker because of the extreme load on the machine during idle.

I think this theory is correct?

I see we have some good electronic guys on here. Will let them crunch the E=MC square

 

[Snaps1992]

Thank you Snaps1992

Regards,
404

No worries.

Always happy to help those who haven't had the privilege to have the education that I have had.

 

[Wakefield]

Induction! Inductive impedance? A whole chapter in physics?
Did a guy named Tesla invent A/C motors and 3 phase?
Say you build an antenna next to a radio station and steal power to run a light in your house-does the radio station's electric bill go up?
Old style EMD locomotive -does it start out from a stop with the traction motors in series with each other (4 of them?) and then the engineer has to throw a switch that puts them all into parallel with each other once the locomotive gets up some speed? (they are series-ed with the generator driven by the Diesel engine)

action at a distance between electric circuits with no direct connection between them because of magnetism,current acceleration/deceleration?

Induction goes both ways?

 

[Verg]

This is the right response, but obviously lacks a few explanations on things you don't understand (yet).

A motor is not a simple resistance - Ohms law (V=I*R only applies to a simple resistance). The coils within the motor act like an inductance in series with a resistance.

The way an inductor works is that any current flowing through it will create a magnetic field through the coils. Due to the laws of physics, this produces a force on any magnetised object (as the object's magnetic field tries to align with the coil's magnetic field).

Now, the opposite effect also happens (and is EXACTLY what a generator is) - when a magnetic object is moved/rotated within a coil, an opposite current is INDUCED (hence the reason a coil is called an inductor) within the wire, and due to that opposite current, a voltage with the opposite polarity will be created on the ends of the coil.

So, what happens is that when you run a current through the motor windings (the inductor), it turns the motor. The motor (the magnet) then speeds up, and causes SOME opposing current to the original current that was used to drive the motor (note that the created/induced current is always less than the original input current). Essentially you have a large amount of current in one direction, and a slightly lesser amount of current in the other direction. The overall current from the addition of these two currents will be a small positive current.

Now, when the motor is not turning, or is loaded too much to turn, the magnetic object (the part that turns, or the 'rotor'), is not turning, and so does not induce any opposite current. The result of this, is that the overall current is MUCH larger.

Hope that answers your question... If anyone's wondering, I'm a final year mechatronics engineering student. I have completed an entire paper on this subject, so that's where the information comes from!

+1

this made me re-remember the right hand rule

 

[larry_g]

This is the right response, but obviously lacks a few explanations on things you don't understand (yet).

A motor is not a simple resistance - Ohms law (V=I*R only applies to a simple resistance). The coils within the motor act like an inductance in series with a resistance.

The way an inductor works is that any current flowing through it will create a magnetic field through the coils. Due to the laws of physics, this produces a force on any magnetised object (as the object's magnetic field tries to align with the coil's magnetic field).

Now, the opposite effect also happens (and is EXACTLY what a generator is) - when a magnetic object is moved/rotated within a coil, an opposite current is INDUCED (hence the reason a coil is called an inductor) within the wire, and due to that opposite current, a voltage with the opposite polarity will be created on the ends of the coil.

So, what happens is that when you run a current through the motor windings (the inductor), it turns the motor. The motor (the magnet) then speeds up, and causes SOME opposing current to the original current that was used to drive the motor (note that the created/induced current is always less than the original input current). Essentially you have a large amount of current in one direction, and a slightly lesser amount of current in the other direction. The overall current from the addition of these two currents will be a small positive current.

Now, when the motor is not turning, or is loaded too much to turn, the magnetic object (the part that turns, or the 'rotor'), is not turning, and so does not induce any opposite current. The result of this, is that the overall current is MUCH larger.

Hope that answers your question... If anyone's wondering, I'm a final year mechatronics engineering student. I have completed an entire paper on this subject, so that's where the information comes from!

If you got better than a C on that then go hug your teacher as she is perty generous. You induce voltage, not current. You only have current in a completed circuit.

lg
no neat sig line

 

[er3456df]

You're wanting a Physics lesson and that's beyond my capability.

No argument there.

Essentially, effectively, etc. the Earth will "****" up ALL available electrons and anything between a source and the earth can be subjugated to do "work". In a way I guess God is saying "More Power". Look at the "Macro" (larger picture) before trying to comprehend the "Micro".

Sounds complicated. You sure you're not a physicist?

 

[Snaps1992]

If you got better than a C on that then go hug your teacher as she is perty generous. You induce voltage, not current. You only have current in a completed circuit.

lg
no neat sig line

I don't intend to get into an argument in my first 10 posts

Induced current is the generally accepted/commonly used term for what we're talking about - an induced current implies a connected circuit, whereas an induced voltage only applies when there is not a complete circuit, which is not the situation we are talking about.

-Brandon

 

[Ruger_556]

So anyways, it turned out to be a loose connection leaving the breaker. So in other words : Resistance, Too many Amps passing through the breaker because of the extreme load on the machine during idle.

I think this theory is correct?

Voltage drop, the lower the voltage the more amps the load draws to run (in the case of an electric motor that is). I'm sure someone will correct me if I'm not quite right on that...

Some people seem to be a bit confused on ohms law, as resistance go's up amps go down. Not vise versa... On the motor thing, I really don't remember much from that class in school, I'll just let someone else explain

 

[theknurl]

because the smoke can't get out...........

 

[Verg]

If you got better than a C on that then go hug your teacher as she is perty generous. You induce voltage, not current. You only have current in a completed circuit.

lg
no neat sig line

Ahhh if the motor is spinning or stalling its a completed circuit. its the motion of the motor that creates a varying magnetic field generating current in the inductor opposite to the feed current. for this example the magnetic field creates current, the change in field strength as the motor speeds up varies the current generated, the change in current generated determines your voltage. Exactly as Snaps1992 put it.....This thread is already slightly convoluted: Snaps and 404's reply's are correct.

 

[jbs]

There are a couple of good explanations posted so far. For more detail, and the math behind it, read up on Lenz's Law. That is the effect being observed here.

 

[ddawg16]

Induction! Inductive impedance? A whole chapter in physics?
Did a guy named Tesla invent A/C motors and 3 phase?
Say you build an antenna next to a radio station and steal power to run a light in your house-does the radio station's electric bill go up?
Old style EMD locomotive -does it start out from a stop with the traction motors in series with each other (4 of them?) and then the engineer has to throw a switch that puts them all into parallel with each other once the locomotive gets up some speed? (they are series-ed with the generator driven by the Diesel engine)

action at a distance between electric circuits with no direct connection between them because of magnetism,current acceleration/deceleration?

Induction goes both ways?

It took 30 posts to get the magic word.

In other words, that wire wrapped around a metal core creates inductance which acts like a higher resistance than the wire by itself....but only when the voltage is first applied.

In other words, inductance only acts on a changing voltage.

If the motor is not turning...with DC, your only inductance is when the current is first applied. On an AC motor, you have more inductance, but not enough to prevent over current.

Think of inductance as a shock absorber....easy to move if you go slow...go fast and you get considerable more resistance.