Okay, be gentle.
I know if I have a drive and motor running a pump and the gearbox locks up, the drive will trip out on high amps. If I reset the drive, it will trip again without turning. I know these things, I've seen it.

But.....
I do not know why.
Why does a motor draw more amps if it has a higher load?

I'm sure Paul will come along with an excellent detailed explanation and I look forward to reading it but here it is in basic terms.

It takes X amount of power to turn the motor. Power is Amps X Voltage or Watts.

1 HP = 745.7 Watts

So Amps = (Hp X 746)/Volts

As the resistance to the motor turning goes up, something has to go up to keep the motor turning. Obviously voltage and our constant of 746 can't go up instantaneously in a normal situation, so the only thing to go up is our hp.

I think this is where you get confused and where I got confused until my latest project. I only have a 3 hp motor and now my equation is calling for 3.5 hp. My motor can't produce but 3 hp. I was talking to a gentleman from Baldor a month or so ago and he explained it well. The hp of a motor is not a rating of how much mechanical power it can produce, it is a measure of how much heat it can shed. A 1 hp motor can turn a 3 hp load for a limited amount of time. In laymans terms a motor is very similar to a fuse. A 100 amp fuse will allow 100 amps through, but it will also allow 150 amps through for a certain amount of time before it will overheat and burn the element in half. A 100 hp motor will handle a 150 hp load for a certain amount of time before it overheats

Hope that helps

Thanks.
That helps.


Ken

Yeah, that is correct. The more load you put on a motor the more work needs to be done, the more you need to increase the Power to do that work. Since P=IxE (or power equals current times voltage) you need to increase something on the other side of the equation to make it true. If P goes up then either I (current) or E (Voltage) needs to increase as well. Usually you can't increase the voltage so the only thing that fluctuates will be the current. And like what was stated before motors are only rated for a certain horsepower, which really means a certain voltage/current combination. You can run a motor at 240 volts with an amp draw of 2 amps and then run the same motor at 480 volts and it will draw somewhere close to an amp (provided you can wire it for different voltages of course :-P) and it will still be the same amount of Power, HP in this case.

I agree, but .......
Why does the amperage go up, what exactly is happening inside the motor to increase the amp draw.
The wiring has not changed, still the same number of windings etc....

Ahhh, well the motor windings are wound in a way that creates magnetic fields that "push" the rotor inside of the motor around in a circle. If you measure resistance of a motor from phase to phase it is typicly very low. The only thing that keeps the phases from shorting together is the inductive reactance that the magnetic fields create. I think, I'm not 100% sure about this (Paul would probably know better), that when the rotor is slowed due to load on the the motor that reactance value changes and basicly lowers the "resistance" of the motor. So, since the effective resistance lowers then the current will go up. It all has to do with the magnetic fields created by the motor windings and how those magnetic fields interact with the rotor.

EDIT: Also, motors are designed to run a specific frequency, in the US it's 60 HZ and in Europe it's 50 HZ, when you run a motor off of an inverter and lower the frequency you will also notice that the current will increase without any additional load on the motor. That's because the magnetic fields are becoming more "static" the less they change. If you put a DC voltage on a motor it would draw very high amps because it would effectivly become a short circuit because there are no magnetic fields being created to limit the current. The lower in frequency you go (away from the native frequency of the motor) the less Inductive reactance you are creating in the motor and it will have the same effect as applying more load at 60 HZ. If you increase the frequency past 60 HZ the current will actually go down a little bit because you're creating more reactance, i.e. the magnetic fields are being created and collapsing at a greater rate.

Basics
a motor is merely a device to turn electrical power into mechanical power
a motor will meet the torque demanded by load
if it cannot it will slow until torque demand = motor torque and will continue to do this until it stalls.
Stalling rapidly burns out motors. Overloads are set in there to turn off motor in stall or other high current conditions to prevent burnout.

KW = I x E x 1.73 we dont care about power factor for this discussion.
HP = 0.746 KW
HP = (RPM x Torque) / 5252

According to equation if I increase torque (more tons of coal on conveyer, more tons on crane hook, bearing going bad etc etc) then HP rises.
HP goes up then KW goes up
and last
E is constant so if KW goes up current (I) goes up.

Get yourself a good motor book it is more complicated than this or better yet get into a community college level course on motors and motor control. AC motors was a full week at Navy eletrician mate A school it is a lot to cover in a forum like this.

Dan Bentler

Ron actually had some pretty good articles on Patchn in the Motors and Drives section. Flip through them.
http://www.patchn.com/index.php?option=com...8&Itemid=74

The number of poles a motor has determines the RPM it will run. You are probably most familiar with a 4 pole motor which is roughly 1800. The magnetic fields of a motor will try to maintain the rated rpm. The slightest lag in the magnetic field will cause it to pull more amps to achieve the rated rpm. Again this is a very basic explaination. Here is a chart of poles vs rpm
http://www.patchn.com/index.php?option=com...2&Itemid=74

I actually have a pretty good book on this I will try to dig up. It is rare for me to have a book on something so it must be pretty good...and have lots of pictures

Thanks to all.
I'm showing my weakness.
Being a self taught controls guy, I've never dived into the theory side of things. Now I have a need to know and I'm slowly absorbing it.

See link below for some excellent online courses.

http://www3.sea.siemens.com/step/default.html

Simple version: The motor converts electrical to mechanical energy. Generators do the reverse. Some motors, such as permanent magnet motors, can be used in either direction (electrical->mechanical or mechanical->electrical). It does this by coupling the energy via magnetic flux. More load = more slip = more flux = more current.

The following gives a longer winded version of the same thing in more detail:

http://www.reliance.com/mtr/mtrthrmn.htm

Great link, thanks.

Just looking at the article, it never comes out and gives you your answer.

Go back to the basic equation for the force on a current carrying conductor is:

F is proportional to BLI

F=force
B=flux density (the strength of the field that the conductor is in)
L=length of the conductor
I=current in the conductor

This is an EQUALITY. If we invert it, we get:

I is proportional to F/BL

So if the wire is being pulled through a magnetic field, the current on the wire must increase to balance the forces. If it does not increase, then in a motor, the slip will increase and the motor slows down because not enough current is being delivered to the motor to operate at rated speed. In DC and synchronous motors, this isn't a problem because both rotor and stator currents will increase. In an induction motor, the armature current is coupled to the surrounding stator magnetic field. As load increases, the motor slips slightly more until the flux density and current in the armature balance out electromechanically at a new operating point which is a slightly slower speed. If you overload an AC motor beyond the pull-out torque, the motor stalls. The field is rotating at a constant speed, equal to 7200 divided by the number of poles in the motor windings. So a standard 4 pole motor has it's field windings rotating magnetically at 1800 RPM. When the motor is loaded to 100% of it's torque (horsepower) rating, it will run at the name plate rated speed which is usually 1-3% below maximum speed (usually about 1720-1750 RPM). As you reduce the load to 0 (nothing on the shaft), it will gradually increase to some place close to 1800 RPM but never exceed it. For most applications, this speed is close enough that most folks treat it as constant.

OK, getting back to the original situation...

In AC motors, the locked current (and also rated torque) is roughly 150-200% of the rated (FLA or HP) range. Since this is twice the motor's rated cooling capacity, and with an integral fan, there's no air movement, you can't normally operate at this point for very long. Once the motor starts coming up to speed, the rated torque of the motor actually increases even more (the pull in torque), until it gets to rated speed where the force, flux, and current all balance again. So at least for a short time (until the motor overheats), it can pull much higher torques. Once you stop it, you are limited to the locked rotor torque (150-200%), and the heat load shoots up very quickly since the motor isn't moving. So in your theoretical "gear box is shot" situation, you might be able to get it to turn over for just a few seconds before shutting down due to overload, or in some situations, it won't even rotate the slightest bit. Starting currents (as the magnetic fields suck down all the current necessary to charge the fields) is often 17 times rated current, and in energy efficient models has been measured as high as 21 times or more. Don't forget too that this translates into torque internally in the motor...starting currents are the most destructive on rotor bars, insulation, and anything else involved in generating magnetic fields. Even after this is over with, current during stall (locked rotor) conditions surges to roughly 6 times normal full load current. With this in mind, it's obvious why with across-the-line starting, "bumping" is very hard on motors without drives to control the torque/current. Assuming that you deal with all the other problems of using drives (heat loads, potential standing waves, and rotor current buildup), it should be obvious why motors driven by drives often last much longer especially in frequent start/stop conditions than motors driven by across-the-line starters.

If you use a full vector drive, you can manipulate the operating point of the motor. You can change the rotating field speed (frequency) and voltage to allow the pull in torque to occur even at ZERO speed. This gives full vector controlled AC induction motors a higher torque rating (roughly double) than their DC brethren where on a DC motor, torque is fixed from 0 speed all the way to rated (base) speed. Effectively, you can operate a motor with fixed torque, fixed speed, or vary both, just as you can with DC motors. Again, this is of course within the limits of the bearings and cooling capacity of the motor. Adding an external fan cooling system makes almost anything possible.

Unlike a DC motor, AC motor math is ugly because you have the inductive transfer of energy from the fields to the armature. Everything gets very nonlinear and it makes my head hurt trying to follow some of what the motor gurus are talking about. Just remember that the rated speed on an AC motor is "approximate", and that once you throw a drive into the mix, you can achieve a lot more with an induction motor.

Thank you paulengr for yet another great explanation. I was rather dissapointed by your first post in this topic but this one made it all good

1.21 gigawatts of electricity!

He said flux Too many hours programing...