Why and how does motor current go up?

[rsdoran]

THis has got to be a REALLY old thread.

I disagree that the answer is "counter emf", that is a bad term.

The term used at the beginning was induction, as has been explained this means that when voltage is applied to a motor...a conductor placed in a magnetic field...emf is produced. As was stated emf opposes the line current that is producing it.

What this means is that when first starting an inductive device, motor, you may see a high current at startup because the forces do oppose each other. How high the current goes will depend on the "load".

If you want to read some really detailed information on motor theory, construction etc then goto this site:
http://www.reliance.com/mtr/

 

[Colossalnut]

Nope. When initially applying voltage to an inductive device you will see a high current because there is absolutely NO opposition present. Before a field is produced you're basically dealing with a short. This, of course, is known as "inrush" current and has nothing to do with the load. As the field strenghtens you then create opposing forces the intensity of which will be determined by the load. Somewhere in there we move from constant torque to constant horsepower, but I don't completely understand everything I know about that! I wouldn't want to expose my ignorance any more than necessary...

 

[elevmike]

Ron, You seem to have such a wide bredth of good working knowledge that I cant believe you said that!! It must have been a typo..

In simple terms.

When the motor starts, there is NO counter emf. As the motor rotor begins to rotate, and comes up to speed, it produces its own internal current somewhat like a generator. That current Oposses the incoming current thereby countering or reducing the incoming current. This phenomenon is known as Couner Electro-Motive Force, or CEMF.

If you were to remove the rotor from the motor and apply voltage to the stator, you will measure current 3 to 4 times higher than the rated working current. This is known as Locked Rotor Amperage. If you conduct this expirement, do it in short order as you will likely burn up the stator.

Lack of Counter Electro-Motive Force (CEMF) is the PRIMARY reason for high current draw at startup.

Please check this link: http://www.tpub.com/content/doe/h1011v2/css/h1011v2_109.htm

Here's another link (PDF file) on AC motor theroy: Page 3 addresses this issue. http://www.nfphampden.com/3phmotor.pdf

 

[STEVESM]

Mike,
Where did you get all this knowledge. Also where do you store it all. I mean you and all the others here who have all these answers are amazing!!!!!!!!!!!!!!

 

[elevmike]

Steve,

It comes from passing way too much of my time on these fourms, instead of paying close attention to my work, kids and wife, (what's her name??).

 

[elevmike]

To all...

Forgive me for my shortcommings. I didnt take the time to realize this thread was more than two years old!! I also Didnt respond properly to ganewton's posted question etc..

However, ganewton, if you check the 2nd link I provided above (AC Motor Theroy) you should find what your looking for.

Best regards, Mike.

 

[rsdoran]

The term used at the beginning was induction, as has been explained this means that when voltage is applied to a motor...a conductor placed in a magnetic field...emf is produced. As was stated emf opposes the line current that is producing it.

This is a fully accurate statement

What this means is that when first starting an inductive device, motor, you will see a high current at startup because the forces do oppose each other. How high the current goes will depend on the "load".

This was phrased improperly maybe, the current draw at start is the product of creating the EMF (electromotive force), which in turn rotates a motor. As the rate changes and the emf increases it opposes the line current...which in turn will lower the current until a balance is obtained, technically I guess this is considered COUNTER EMF. Since work is being produced energy will be used that is proportional. Even at startup there must be an opposition or the amount of power/current used would be unlimited. The constant current used will depend on the load factor...ie how much work is being done.

The startup current is also known as the Locked Rotor Current: Steady state current taken from the line with the rotor at standstill (at rated voltage and frequency). This is the current seen when starting the motor and load.

 

[kamenges]

Some may find this link interesting. It shows the general speed/current/torque curve for a NEMA design B motor. If CEMF were the predominant factor in determining motor current then the current curve in this link would be a straight line from zero speed/max current down to synchronous speed/zero current. But it's not straight. It rises very quickly as you decrease from synchronous speed. We would expect a certain step as you leave synchronous just because of rotor field generation. But that happens almost right away.
I think that transformer action is being discounted a little too much here. An AC solenoid also has a very high current until the armature is pulled into the coil. This is because of the very low inductance without the armature metal in the coil. A transformer with a low mutual inductance will do the same thing. I think that a motor acts pretty much like a transformer with a low mutual inductance until it starts to approach breakdown torque. At that point CEMF takes over as the predominant current limiting factor. Like Ron said, if current was limited predominantly by CEMF, the locked rotor current on a 1HP 480VAC induction motor would be about 200 amps since only armature resistance would oppose current flow.

Keith

 

[elevmike]

The origuoil question at the start of this thread, is Whay is current draw so high at startup.

The short answer is that since the rotor is not yet moving, the motor is experiencing LRA due to the lack of CEMF.

To be honest I think we all have the same answer here, but are attempting to define "CEMF" in different terms.

 

[DickDV]

While for the most part, I am content to deal with a motor based on its torque-speed characteristic curve with its current characteristic superimposed on it, occasionally I do get curious about just what is actually going on inside the motor.

I have studied the arguments for CEMF (in the same sense as CEMF in a DC motor) in an induction motor and I just don't see it. The problem with a CEMF explanation is that an induction motor is not able to generate any current unless it is externally excited with three-phase AC at a frequency slightly lower that the rotor equivalent frequency.

I am much more comfortable with an explanation of the motor being essentially a transformer with a very low resistance secondary, thus the high locked rotor current. As the rotor begins to turn, the magnetic coupling between the rotor and the stator becomes less and less thereby lowering the current. In support of this explanation is the observation that the shape of the torque-speed curve in the dis-synchronous region (from locked rotor thru pull-up torque to breakdown torque) is determined by the magnetic characteristics of the rotor, not the stator. Further, in a wound-rotor motor, when starting with high resistance, the inrush current goes down significantly as in a transformer with higher secondary resistance. I am not aware of any reasonable explanation of higher CEMF due to higher rotor circuit resistance.

Further, since CEMF is a voltage phenonenon, how much voltage can be generated with shorting rings between the rotor bars?

I have never fully understood just what happens when the rotor is driven to overspeed and the motor regenerates. In the application of drives, I know what happens externally and how to deal with the regenerated current but just why and how it happens inside the motor is not clear to me. In fact, I have never seen a motor torque-speed curve that continues over sync speed where positive slip and regen current occur. If anyone knows where such a curve exists, I sure would like to see it. It would be highly useful in the courses I teach on VS drives, too.

I am not trying to trash anyone else's opinion here. I simply what a reasoned explanation for other views and, so far, I haven't seen it.

 

[Peter Nachtwey]

You asked for it. How is your Greek?

occasionally I do get curious about just what is actually going on inside the motor.

Induction motor model

The funny looking p infront of the flux symbols are the author's way in indicating a rate of change or derivative. The pitch fork symbols represent flux. I can make a crude model using these equations in an Excel spread sheet. Each row could represent 10 microseconds of time and 17 columns would be required for each of the 17 formulas.
There are 7 first order differential equations ( the ones with the funny p in front of them ). It takes time and study to understand these equations although you can see each one is really very simple.

I agree with Dick's gut feel than the counter EMF argument. If you look at the forumula's you will see that voltage is not on the left hand side of any equation. This means there is no counter EMF that is building up or any voltage that is changing. There is flux in the rotor building up. I don't think the CEMF terminology is right in THIS ONE CASE of induction motors. That doesn't mean the analogy of DC motor EMF can't be applied to induction motors. If the question was about DC motors then the counter EMF answer would be right.

There is another forum I visit that is about hydraulics. I still haven't conviced the people there that it is force and not flow that makes a piston move. The people there don't understand the hydraulic equations that are equivelent of the 17 equations that are on the IEEE induction motor webpage. I think the two situations are similar. If one understands the equations, one will understand how the system works. I think this is beyond the scope of this forum. It is hard enough to attempt to answer questions about PID. The simulation of and induction motor is much more complicated than the simulations on spread sheet that I have provided so far.

DickDV's ABB drives solve there equations at high rates. Maybe every 125 micronseconds or faster which isn't that hard to do with todays DSP. The ABB drives may even have more formulas or more terms than the 17 formulas in the IEEE web page.

 

[Bitmore]

Not to many people understand the integration of magnetic flux based on the phase angle of the input. It is often refreshing to see such knowledge. This is how the "sensorless flux vector" drive works!
well back to washing the dishes.....

 

[elevmike]

1,

 

[JesperMP]

I have been teached about this when I studied, but I didnt hear the term "CEMF" before. It may be called something else elsewhere in the world. You can consider "CEMF" as a theoretical magnetical field that is required to understandwhat goes on.

Further, since CEMF is a voltage phenonenon, how much voltage can be generated with shorting rings between the rotor bars?

Actually it is the current in the rotor bars that excite a magnetic field that in turn generate a "CEMF" in the stator windings.

But how is the current excited in the rotor bars then ? By the magnetic field from the stator ! Thats where it stats to get confusing. To understand this appearant catch-22 you must realise that the voltage + current + magnetic field + slip etc. will settle on the level where the load (static torque + acelleration of the inertia) on the rotor equals the generated torque.

I have never fully understood just what happens when the rotor is driven to overspeed and the motor regenerates. In the application of drives, I know what happens externally and how to deal with the regenerated current but just why and how it happens inside the motor is not clear to me.

At underspeed, the rotor generates a "CEMF" but not enough to counter the magnetic field from the stator completely.
At same speed, the stator and rotor matches each other (*).
At overspeed, the rotor starts to generate a "CEMF" that is greater than what the stator produces.

*: This is a proof that CEMF is just a virtual thing. As the rotor bars moves with the same speed as the stator magnetic field, no current isinduced in the rotor bars, and no "countering magnetic field" is produced.

In fact, I have never seen a motor torque-speed curve that continues over sync speed where positive slip and regen current occur. If anyone knows where such a curve exists, I sure would like to see it. It would be highly useful in the courses I teach on VS drives, too.

I have such a curve in my e-motor theory book from my engineering school days. I am not at home right now, but shall try to remember to post it when I come back in two weks time. PM me if I forget.

 

[Colossalnut]

On vacation Jesper? And still just can't stop thinking, can you? Hahahahaha...'tis a wonderful life, eh?