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March 30th, 2012, 08:58 PM
Most of hybrid vehicles such as Toyota Prius use high voltage
motors which reach voltages beyond 500 V.
Such high voltages are dangerous and require assembly from
many hundreds of batteries.In one place I`ve read that power
electric motor draws is related to volts times amps.
I whish to know why because I thought magnetic field strength
which is created around wire depends on current but not on
voltage.Electric motor mostly works on magnetic field strength.
Volts times current is the formula for power in any electrical circuit. There is no way around that. You can have high voltage or high current, take your pick. There will be more copper loss with the higher current.
Why exactly?There seem to be nothing in Ampere law on voltage.
Strong magnetic fields do not seem to be created near high voltage
power lines.Electric motor is all about strong magnetic field.The stronger
field the higher power.Increasing voltage do not increase magnetic
field.
Last edited by Stanley514; March 30th, 2012 at 11:32 PM.
It's because the electric motor is doing work. It takes power to do that work, and that power needs to come from somewhere.
The power expended in an electric device is voltage × current.
Well, if you build it from 1½-volt batteries, it would require hundreds of batteries; however, electric car makers don't use such low-voltage batteries. Besides electric converters can produce extremely high voltages from much lower voltages. Consider the taser, which produces 10's or 100's of thousands of volts from a couple of household batteries.Originally Posted by Stanley514
The advantage of high-voltage supplies and devices is the low losses in the wiring, etc. These losses are called I²R losses, where I is the current and R is the resistance. This is why electric power distribution lines run in the 10's to 100's of thousands of volts. It keeps the current low, and also the I²R losses.
Such a converter is of course a nice thing.But what is their price,size,and efficiency?For example we need toBesides electric converters can produce extremely high voltages from much lower voltages.
convert 50 V DC to 500 V AC and motor have power 100 kW.What would be the specs?
If we are trying to transmit power on long distance then we need to have high voltage.It allows us to avoid much looses.But the point of electric motor is to create a strong magnetic field.Current is a number of electrons which pass certain distance of conductor at a given time.Moving electric charge creates magnetic field.Therefore strength of magnetic field is proportional to current.High voltage do not create strong magnetic field.Then what is point to take for example 5 V 1 Amp power and convert it into 500 V 10 milliamp power?How could it increase strength of magnetic field rather then decrease it?The advantage of high-voltage supplies and devices is the low losses in the wiring, etc. These losses are called I²R losses, where I is the current and R is the resistance.
It doesn't increase the field strength. It increases the power (available). The motor will provide a varying impedance depending on the load. Therefore a higher voltage will allow more current to be supplied to handle a high load (i.e. high power).
It is also important to note that a modern motor controller will dynamically modulate both current and voltage to achieve the desired speed and torque. Look up "field oriented control" for more info.
[QUOTE=Stanley514;317135]The power consumed/expended by a device in an electrical circuit equals VI, or V²/R, or I²R. To say that high voltage does not cause a strong magnetic field is focusing too narrowly on definitions and cause-and-effect.Current is a number of electrons which pass certain distance of conductor at a given time.Moving electric charge creates magnetic field.Therefore strength of magnetic field is proportional to current. High voltage do not create strong magnetic field.
It's probably not economical (or possible) to build a device that consumes/expends a certain level of power for a low amount of voltage applied to it. Consider an existing voltage/motor. To decrease the voltage by a factor of 10 and maintain the same power consumption/expenditure, you would need to decrease the resistance by a factor of 100. Maybe such a device would cost $100K or $250K to make or just be impossible.
I still do not see here an in-depth explanation of this cause-and-effect.I still do not know how voltage could increase strength of magnetic field.There is no strong magnetic fields near power lines with very high voltage.To check it you need to go to power line and try to manipulate with some metalic object around.There exist such type of motor as an electrostatic one which uses high voltages to create torque but its principle is different from magnetic one.Could you give more details on this question?The power consumed/expended by a device in an electrical circuit equals VI, or V²/R, or I²R. To say that high voltage does not cause a strong magnetic field is focusing too narrowly on definitions and cause-and-effect.
How close did you get to the high voltage power lines? Obviously, not too close, since you're still here
Defenetly,I`m not going to conduct any experiments on myself.Could you give me some link on detailed description ofHow close did you get to the high voltage power lines? Obviously, not too close, since you're still here
law according to which strength of magnetic field depends on voltage of conductor?
Hahahaahhha Very punny! And at no charge
The voltage does not directly increase the strength of the magnetic field. Under no load conditions a certain voltage is required to drive the current to generate a given field strength. Under load, the impedance increases and so a higher voltage will be required to drive the same current and generate the same field.
It is also important to note that a modern motor controller will dynamically modulate both current and voltage to achieve the desired speed and torque. Look up "field oriented control" for more info.
Why do I have a weird sense of deja vu?
You might want to check out Biot-Savart Law. It's a followup to Ampere's Law.
Besides, you don't need high voltage for electricity to be dangerous. Try short-circuiting a typical 12 volt car battery.
Both high voltage and high current motors should put out the same power, there could be some advantages using one or the other which I am not familiar with. The difference between the two is that the high voltage motor will have a thinner wire making up the windings of the coil as there is not as much current flowing through it. The high voltage motor because of the thinner wire will have more turns of the coil in the same space. The magnetic field of both windings should be the same since the amount of current flowing around the core of the winding is the same. Low voltage/high current = large wire/less turns in the coil. High voltage/low current = thin wire/more turns in the coil.
Magnetic field intensity depends on intensity of current flow: amperage. All other things inchanged, if voltage increases, then amperage also increases, thereby causing a more intense magnetic field. The field may be of constant intensity, or varying intensity, depending on the type of current flow.
Power lines use very high voltages, in order to transmit large amounts of power (wattage) without high current being present. This can be observed when a high-voltage switch opens a circuit, in a switchyard, for example, an enormous arc many feet long is created, but the ends of the switch arms do not become seriously burned, since the current is not high enough to cause sufficient heating in a short time.
I have access to a stupendously eye-opening video of such an event. If I can locate it, and figure out how to present it here, I will do so tomorrow. jocular
http://www.youtube.com/watch?v=C-UTY...yer_detailpage
Last edited by jocular; February 5th, 2013 at 10:50 PM.
Are you sure you don't mean the super capacity caps that make of the battery? The motor is a motor, and a generator. But I think you are some what off base here because these systems are way more complex that what you are suggesting. They are using 2.5 a gate driven optocouplersand a whole host of other thing to make these car go.
I think it all comes down to wire size. Higher voltages use smaller diameter wire to get the same power. Just check out the wires that connect your 12v car battery. They're as big around as your thumb, because to get the required power to the starter @12v, you need several hundred amps. Larger wires are heavier. Can you imagine trying to wind the coils of a motor with thumb-sized wire? It be huge and would weight a ton.
Just another note: Current doesn't do work by itself. Power does. Think of a car's internal combustion engine....the power is measured in horsepower. While there are alot of different things that effect horsepower, it comes down to two main things....Engine displacement, and the amount of fuel and air you dump into it. If you asked me " Hey Mac...that's a nice car, how much power does it make?" and I answered "It's a 5 liter engine"...that would be an incorrect response, as displacement is only one aspect of HP. You could take an 8.2 liter big block from a '78 Caddy that's low compression and get 300 HP....or you could use a 2.0 liter high compression 4 banger and get the same 300 HP. Works the same way with electrical power.
The Prius uses an 80 horsepower electric motor...converted to watts, that's 60,000 watts! If the batteries produce 500 volts, you'd need 120 amps to get 60kw. If you lowered the voltage to something that wasn't as dangerous...say like 50...you need 1200 amps. That would require wires the size of you arm.
Magnetic field strength is a function of "AMPERE TURNS" A motor with more wire turns on the field & armature windings will have higher resistance and inductance and need more voltage but less current to create the magnetic fields that provide torque. The smaller wire size makes the motor easier to construct & the slip rings or brushes can be smaller because of smaller currents. This is the reason American cars switched from 6 volt to 12 volt systems in 1956.
simple to explain
you have wires going to the unit
those wires have a resistance as well as the unit
lets say your wire has 10 ohms and the unit also
then you have a voltage divide and the power will be split
50 50 to the wire and the unit. increasing the operating voltage
of the unit will increase its resistance under same power condition.
lets say now it has 100 ohms. in order to get the same power for the unit
you would need to increase the voltage respectively. back to the equation
before we have now 10 ohms wire and 100ohms of the unit
now you loos only 10% on the wire not 50.
thats why the power plants bringing the power to you over
high voltage lines not low voltage to avoid loss. in high current
motors etc the wire from the power source to the unit plays a major
role.
Let's say you want to increase magnetic field strength. You can do that one of two ways:
1) More current
2) More turns
So you start with more current. You keep increasing current until you get to (say) 150 amps. At that point the project manager says "woah! We can't afford the copper to get the power to the motor any more!" And the safety guy says "if you go any higher in current then we're going to have a very hard time getting protection devices that can pass 200 amps but still break a fault."
So you go to more turns. Works OK but the inductance goes up - which means your motor goes slower (smaller motor constant) and is able to deliver less power. How do you get it to go faster? More voltage.
Eventually you are going to come up with an ideal turns/current/voltage tradeoff. For electric bikes the "sweet spot" is around 36-48 volts at 20-30 amps. For EV's the sweet spot will be 300-600 volts at 50-100 amps.
Of course there is. You can't avoid magnetic fields where current flows. But you can, of course, reduce them with higher voltages.There is no strong magnetic fields near power lines with very high voltage.
V=ir
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