# Motors & electromagnetics

Motors are supposed to rotate inside because of changing magnetic field.. but when looking at high speed fan.. i'm thinking how can the changing magnetic field responds so quickly... is the rotation or changing magnetic/electric field the same speed as the fan?

russ_watters
Mentor
Motors are supposed to rotate inside because of changing magnetic field.. but when looking at high speed fan.. i'm thinking how can the changing magnetic field responds so quickly... is the rotation or changing magnetic/electric field the same speed as the fan?
The answer is yes, but there is a different cause/effect depending on what type of motor it is. 3 phase AC induction motors are essentially sync'd to a multiple of the grid frequency (60x30=1800rpm in the USA) by the alternation of the grid current changing the magnetic field. DC motors or non synchronous AC motors sync their own magnetic fields to their RPMs. The easiest to understand is probably the brushed DC motor, which changes the magnetic field by touching the electrical leads to different parts of the motor as it rotates.

The answer is yes, but there is a different cause/effect depending on what type of motor it is. 3 phase AC induction motors are essentially sync'd to a multiple of the grid frequency (60x30=1800rpm in the USA) by the alternation of the grid current changing the magnetic field. DC motors or non synchronous AC motors sync their own magnetic fields to their RPMs. The easiest to understand is probably the brushed DC motor, which changes the magnetic field by touching the electrical leads to different parts of the motor as it rotates.

Watching several youtube videos now.. I think we can say motors are powered by Fleming's Left hand rule. I though the north pole and south pole just attracted each other in the rotor and stator (casing) and wondered how this could keep rotating. But let's say in a universe that didn't have Fleming's left hand rule.. are there electric motors which can also rotate simply based on the attracting north and south poles only?

Watching several youtube videos now.. I think we can say motors are powered by Fleming's Left hand rule. I though the north pole and south pole just attracted each other in the rotor and stator (casing) and wondered how this could keep rotating. But let's say in a universe that didn't have Fleming's left hand rule.. are there electric motors which can also rotate simply based on the attracting north and south poles only?

So basically one change the speed (or make the motor) go faster by simply increasing the current or by shifting the alternating north and south pole in certain way?

CWatters
Homework Helper
Gold Member
So basically one change the speed (or make the motor) go faster by simply increasing the current or by shifting the alternating north and south pole in certain way?

Not quite. If you connect a motor to a volt meter and spin the shaft you will see it can generate a voltage. This voltage is called the motor Back EMF and it depends on how fast you spin it. When power is applied to such a motor the motor accelerates until the back EMF is roughly equal to the supply voltage. The back EMF also depends on the number of turns on the windings and the strength of the magnets. This relationship between voltage and speed is called the Motor velocity constant or back EMF constant and is given the symbol Kv. It has the units rpm/volt. If the voltage is increased the motor will spin faster.

The current drawn depends mainly on the load on the motor. If the motor was an ideal motor and had no load then once up to speed the current would fall to zero. Adding a load increases the current drawn by the motor. If the motor is an ideal motor the speed would remain constant despite increased load. Real world motors have losses caused by the resistance of the windings, friction and air resistance etc. These all conspire to mean the speed isn't constant and will vary with increasing load. As the load is increased the motor will slow down until at some point it cannot turn the load and it stalls. If you want the motor to maintain speed you will have to increase the voltage.

cnh1995 and Blue Scallop
Not quite. If you connect a motor to a volt meter and spin the shaft you will see it can generate a voltage. This voltage is called the motor Back EMF and it depends on how fast you spin it. When power is applied to such a motor the motor accelerates until the back EMF is roughly equal to the supply voltage. The back EMF also depends on the number of turns on the windings and the strength of the magnets. This relationship between voltage and speed is called the Motor velocity constant or back EMF constant and is given the symbol Kv. It has the units rpm/volt. If the voltage is increased the motor will spin faster.

The current drawn depends mainly on the load on the motor. If the motor was an ideal motor and had no load then once up to speed the current would fall to zero. Adding a load increases the current drawn by the motor. If the motor is an ideal motor the speed would remain constant despite increased load. Real world motors have losses caused by the resistance of the windings, friction and air resistance etc. These all conspire to mean the speed isn't constant and will vary with increasing load. As the load is increased the motor will slow down until at some point it cannot turn the load and it stalls. If you want the motor to maintain speed you will have to increase the voltage.

In turbojets in airplane. Do they use fuel to power the motors directly, how.. maybe they use stroke engine like in cars to rotate the rotors in the turbojets motors or maybe they charge using battery to power the motors? When riding planes.. I always wonder if the motors or turbojet would suddenly stop turning and how fuel power it or via battery.

CWatters
Homework Helper
Gold Member
In turbojets in airplane. Do they use fuel to power the motors directly, how.. maybe they use stroke engine like in cars to rotate the rotors in the turbojets motors or maybe they charge using battery to power the motors? When riding planes.. I always wonder if the motors or turbojet would suddenly stop turning and how fuel power it or via battery.
A turbojet is essentially the original form of the jet engine. All the air goes through the engine. Turbojets are still used on military jets but on civilian passenger aircraft the have been mostly replaced by high bypass turbofan and turboprop engines. No electric motors involved ( except for starting and perhaps fuel pumps).

https://en.m.wikipedia.org/wiki/Turbojet

Blue Scallop
A turbojet is essentially the original form of the jet engine. All the air goes through the engine. Turbojets are still used on military jets but on civilian passenger aircraft the have been mostly replaced by high bypass turbofan and turboprop engines. No electric motors involved ( except for starting and perhaps fuel pumps).

https://en.m.wikipedia.org/wiki/Turbojet

Whenever a turbojet powered commercial airplane is powered on.. the fans begin to slowly rotate.. are you saying it is jet fuel that started the turbo jet fan rotations? It is very slowly at first.. it's controlled by fuel flow?

Is there any incident where the fuel flow becomes maximum at beginning so the airplane that is parked suddenly accelerate into the surrounding?

sophiecentaur
Gold Member
2020 Award
. how can the changing magnetic field responds so quickly
Electromagnetic forces can change a lot quicker than mechanical perturbations so the speed of a motor is not limited by the rate at which you can apply a changing field to the mechanical parts. (That's a bit too glib because you would need to design different coils ion order to produce very rapid changes.)
But the bottom line is that EM waves can travel and change faster than mechanical disturbances, which depend on simple electric forces (molecular attraction) and molecular masses (which slow things down)

CWatters
Homework Helper
Gold Member
Whenever a turbojet powered commercial airplane is powered on.. the fans begin to slowly rotate.. are you saying it is jet fuel that started the turbo jet fan rotations? It is very slowly at first.. it's controlled by fuel flow?

No they use an electric motor to spin up the turbine. It has to be going quite fast before they can turn on the fuel and even faster before they can switch off the starter and the turbine becomes self sustaining.

No they use an electric motor to spin up the turbine. It has to be going quite fast before they can turn on the fuel and even faster before they can switch off the starter and the turbine becomes self sustaining.

How do they initially start up the electric motor? Do they use battery or is it directly from fuel to power the electric motor? I think it's the latter. How do you power electric motor directly from fuel like in house power generator? In the airplane case.. it's not gasoline or diesel but directly aviation fuel to power the electric motor?

Drakkith
Staff Emeritus
How do they initially start up the electric motor? Do they use battery or is it directly from fuel to power the electric motor? I think it's the latter. How do you power electric motor directly from fuel like in house power generator? In the airplane case.. it's not gasoline or diesel but directly aviation fuel to power the electric motor?

The have to use either a battery or another source of electricity to power the motor. You can't get electricity directly from any source of fuel without using it to power an engine that drives a generator of some type (except perhaps fuel cells).

The have to use either a battery or another source of electricity to power the motor. You can't get electricity directly from any source of fuel without using it to power an engine that drives a generator of some type (except perhaps fuel cells).

How does home power generator work then? is there piston inside like in car to drive the motor rotors?

Drakkith
Staff Emeritus
How does home power generator work then? is there piston inside like in car to drive the motor rotors?

Yes, there is a combustion engine inside that's connected to the generator portion.

CWatters
Homework Helper
Gold Member
How do they initially start up the electric motor? Do they use battery or is it directly from fuel to power the electric motor? I think it's the latter. How do you power electric motor directly from fuel like in house power generator? In the airplane case.. it's not gasoline or diesel but directly aviation fuel to power the electric motor?

I believe most aircraft use a ground power connection for normal starting but many have a battery as well for when ground power isn't available. You might remember the problem with the Boeing 787 battery..

https://en.wikipedia.org/wiki/Boeing_787_Dreamliner_battery_problems

The battery is charged by generators on the jet engines.

This relationship between voltage and speed is called the Motor velocity constant or back EMF constant and is given the symbol Kv. It has the units rpm/volt.
If the units are rpm/volt then it's a velocity constant (Kω).
Whereas the back EMF constant (Ke) = 1/Kω.

I believe most aircraft use a ground power connection for normal starting but many have a battery as well for when ground power isn't available. You might remember the problem with the Boeing 787 battery..

https://en.wikipedia.org/wiki/Boeing_787_Dreamliner_battery_problems

The battery is charged by generators on the jet engines.

"by generators on the jet engines" you mean there is a small combustion engine inside the jet engines that would convert the aviation fuel to power the rotors in the generators? But it can't be on the jet engines themselves or the combustion engine would melt from the heat.. maybe just beside the wing? but aviation fuel is not like gasoline or diesel.. won't the piston just explode from the pressure? What kind of combustion engine can use aviation fuel?

Drakkith
Staff Emeritus
"by generators on the jet engines" you mean there is a small combustion engine inside the jet engines that would convert the aviation fuel to power the rotors in the generators?

No, the generators are powered by the jet engine itself, though large aircraft are typically equipped with one or more auxiliary power units (APU's) in case of engine failure.

No, the generators are powered by the jet engine itself, though large aircraft are typically equipped with one or more auxiliary power units (APU's) in case of engine failure.

How can the jet engine power the generators? The jet engine is pure flame and after burner.. how does it power the generator? what kind of connection does it use?

Drakkith
Staff Emeritus
How can the jet engine power the generators? The jet engine is pure flame and after burner.. how does it power the generator? what kind of connection does it use?

I don't know the exact connections, but I assume the shaft running down the middle of the engine is connected to a gearbox which is then connected to the generator. So the shaft turns, turning the generator, producing electricity. The F107 jet engine in the missiles I used to perform maintenance on works in this manner:

Accessories. The accessory drive gearbox is driven by the compressor shaft. Gearbox outputs drive the airframe and engine ignition generators, lube pump, and fuel control. Hydromechanical fuel control is provided to control engine speed and acceleration/deceleration, along with a positive fuel shutoff. The self-contained lube system contains 1.3 pints (0.61 liters) in a reservoir.

From here: https://www.forecastinternational.com/archive/disp_pdf.cfm?DACH_RECNO=1047

Drakkith
Staff Emeritus
The jet engine is pure flame and after burner..

Also, this isn't true. The entire engine prior to the combustion chamber is relatively cool since it is pulling in ambient air. The compression of the incoming air increases its temperature somewhat, but nowhere close to the temperature of the combustion chamber and other parts "downwind" of this. See the following picture:

Also, this isn't true. The entire engine prior to the combustion chamber is relatively cool since it is pulling in ambient air. The compression of the incoming air increases its temperature somewhat, but nowhere close to the temperature of the combustion chamber and other parts "downwind" of this. See the following picture:

View attachment 196551

Whenever I ride commercial airliner. I always have 2 concerns.. what would happen if the Pilot accidentally just pull the throttle to less than half and the airplane stalls and fall.. would it maintain balance if the throttle is pushed again.. I remember this from my flight similar game in the PC once.

Second. I always wonder if the flame in the engine can suddenly go backward and the make the fuel tank explode. Wonder what is the protection mechanism to make this not possible.. and lastly if the fuel line tube becomes clogged, would the engine just shutdown. Anyone has a diagram of the fuel line connections. Is this classified or publicly available.

These thoughts always occurs when i'm in a commercial plane.. and many times I avoid riding one concerned of this accident.

russ_watters
Mentor
How can the jet engine power the generators? The jet engine is pure flame and after burner.. how does it power the generator? what kind of connection does it use?
A jet engine has a compressor and turbine on a shaft -- the shaft also turns a generator.

russ_watters
Mentor
Whenever I ride commercial airliner. I always have 2 concerns.. what would happen if the Pilot accidentally just pull the throttle to less than half and the airplane stalls and fall.. would it maintain balance if the throttle is pushed again.. I remember this from my flight similar game in the PC once.
A good flight simulator would show that pulling the throttles to idle doesn't do much of anything -- the pilot would just have to keep the plane's nose pointed down slightly to keep the plane flying. It isn't as easy as you think to make a plane fall out of the sky.
Second. I always wonder if the flame in the engine can suddenly go backward and the make the fuel tank explode. Wonder what is the protection mechanism to make this not possible..
Fire requires fuel and oxygen. A fuel line only has fuel in it.

Do fuel lines in airliners need motor pumps or does the fuel flow occur by gravity?

If motor pumps.. don't they get out of order in mid-air?

If an airplane is hit with an EMP (let's say from atmospheric nuclear detonations).. I think the turbojet can still turn but won't the motor pump be affected?