Why and how does motor current go up?

I have a motor theory problem. This would be an ac induction asynchronous motor. I understand that current goes up in a motor for starting and whenever the load is increased. But why and more specifically how? If there is a rotating magnetic field in the stator why would the current go up if the rotor slows down or from starting? I figure it has something to do with it being an inductive circuit but past that I am confused. All I can find in a book is that it goes up but not actually why it goes up. Another theory that I can think of is that the rotor is basically a spinning magnet and the rotor becomes a magnet through motor slip. When the rotor opposes the rotation (likes repel, opposite poles attract) that this somehow draws more current. I am hoping I can get a simple explanation. Thanks!!

I can do simple. Think of yourself as a motor, is it easier to walk on flat ground or run uphill? Now add a backpack while running uphill.

Energy and work have a relationship. Motors are devices that do work, the amount of work produced by a motor is proportional to the current capacity. So when you increase the load on a motor it has to do more work so needs more energy. Your theory on startup is OK, when induction occurs there is opposition. This applies to motors, on startup there is opposition which creates a high current rate.

A good site to learn more Motors and calculations http://www.lmphotonics.com/home_index.htm

I agree to RSdoran's explanations. A good link was also added. Thanks.

To add on why the motors pull more currrent during start, I can put it in another way. If you check the resistance of a motor by a multimeter, you may find the resistance is very low, probably a couple of ohm. The inductance is only built after an alternating current passes through the coil. So during start, before the inductance is build up current flow is high - i=v/r

Motor Current

I have worked with induction motors for many years and have been very successful in their application to a huge variety of loads. I can give you a detailed explanation of how a motor behaves and what the current does and why from the OUTSIDE of the motor. But.......... if I understand the question correctly, you want to know what goes on INSIDE the motor. I can only offer a few inadequate teasers.

First, I have often heard the analogy of a motor at zero speed as being essentially a transformer with a shorted secondary (the rotor). I think that is fairly understandable in view of the squirrel cage design of the rotor bars and shorting rings.

Second, as the rotor speed matches the spinning field speed, the rotor essentially becomes invisible since no field flux lines are crossing the bars. Thus you get only field amps, more commonly called magnetizing amps, under this no-load condition.

Finally, what happens between these two points I don't understand and have come to the conclusion that I don't have to.

Your curiosity is commendable, however, and I wish you luck in finding your answers. If they can be expressed in reasonably basic terms, I would be very interested in what you find.

To add to what DickDV has stated, the following is an excerpt from "Alternating Current Fundamentals" by John R. Duff and Milton Kaufman, chapter 16 (Three-phase induction motors)

Synchronous Speed and Percent Slip

The field set up by the stator winding cuts the copper bars of the rotor. Voltages induced in the squirrel-cage winding set up currents in the rotor bars. As a result, a field is created on the rotor core. The attraction between the stator field and the rotor field causes the rotor to follow the stator field. The rotor always turns at a speed which is slightly less than that of the stator field (less than synchronous speed). In this way, the stator field cuts the rotor bars and induces the necessary rotor voltages and currents to create a rotor field.

If the rotor is turned at the same speed as the stator field, there will be no relative motion between the rotor bars and the stator field. This means that no torque can be produced. A torque is produced only when the rotor turns at a speed which is less than synchronous speed.

As a mechanical load is applied to the motor shaft, the rotor speed will decrease. The stator field turns at a constant synchronous speed and cuts the rotor bars at a faster rate per second. The voltages and currents induced in the rotor bars increase accordingly, causing a greater induced rotor voltage. The resulting increase in the rotor current causes a larger torque at a slightly lower speed.

The squirrel-cage winding was described as consisting of heavy copper bars welded to two end rings. The impedance of this winding is relatively low. Therefore, a slight decrease in the speed causes a large increase in the currents in the rotor bars. Since the rotor circuit of a squirrel-cage induction motor has a low impedance, the speed regulation of this motor is very good.

Starting Characteristics

At the instant the motor is started, the rotor is not turning and there is 100% slip. The rotor frequency at this moment is equal to the stator frequency.

As the speed of the motor increases, the percent slip and the frequency of the rotor decrease. The decrease in the rotor frequency causes the inductive reactance and the impedance of the rotor to decrease. Thus, the phase angle between the stator and rotor fluxes is reduced. The torque then increases to its maximum value at about 20 percent slip. As the rotor continues to accelerate, the torque decreases until it reaches the value required to turn the mechanical load applied to the motor shaft. The slip at this point is between 2 and 5 percent.

Starting Current

At startup the stator field cuts the rotor bars at a faster rate than when the rotor is turning. The large voltage induced in the rotor causes a large rotor current. As a result, the stator current will also be high at startup. The squirrel-cage induction motor resembles a static transformer during this brief instant. That is, the stator may be viewed as the primary or input winding, and the squirrel-cage rotor winding as the secondary winding.

Most three-phase, squirrel-cage induction motors are started with the rated line voltage applied directly to the motor terminals. This means that the starting surge of current reaches a value as high as three to five times the full load current rating of the motor.

To Kim Gold

I agree with and am aware of the material you presented, Kim, but I think that the original poster likely was looking for a more detailed internal explanation. For example, this or that line of flux cuts across this or that bar inducing current in this direction causing rotor magnetization of this orientation which interacts with the stator field of opposite polarity (my eyes are glazing over) etc. etc. Or, why are torque amps a vector 90 degrees agart from field amps? Or, what happens to the vectors and currents when the motor rotor is overhauled by the load and slip becomes positive instead of negative? These are all things I can't explain. Maybe someone else (likely a college professor somewhere) can do it. Fortunately, I can do my work effectively with what I do know so I run with that!

I see you are in Canada so you may not be dealing with NEMA motors. If you are then, especially in the larger frames, slip is often less than 1% in high efficiency designs. Also, for NEMA motors, starting inrush is generally six to eight times FLA. Either way it's pretty ugly!

Tie together the above responses with your second theory.

Use the static rotor is like a transformer with the secondary shorted idea. In other words- almost zero impedance. What happens at low inpedance? Your current goes up. 100% slip.

At zero slip it follows that it looks like a transformer with the secondary open. High impedance. Almost zero current. Like DickDV said this isn't really true, there is some current used to magetize the field. This is the zero point for torque producing current.

Imagine the motor at near zero slip. Add a load. The slip increases moving more toward the shorted transformer model, lowering the impedance. Current goes up. However the increased current increases the magnetic attraction between the rotor and stator and the motor speeds up, closer to zero slip (open secondary). Current drops. There is a balance between the two trends and the motor reaches steady state.

Make sense?

Dear cdlove23, I'm so happy to see that some people out there ask themselves the real questions. I can ashure you that if you go on like this, you will have great successes.

This question follows the same path has the one about programmers being able to work on any PLC brand. Once you get into the basics and really understand how it works, you can do it all.

Has to the answer to this question... if I can only finish connecting my web dynamic server this week, I'll give it shot!

But thinck about this. The earth has a solid iron center core, surrounded by lava (magma) and the outer layer being more or less solid. The core does not rotate at the same speed as the crust... Well, guess where the earth magnetic feild comes from ?

Second, thinck about when you want to hook some balloons to the wal... passing them fast, close to your air... charging them with static electricity...

Later maybee...

As a mechanical load is applied to the motor shaft, the rotor speed will decrease. The stator field turns at a constant synchronous speed and cuts the rotor bars at a faster rate per second. The voltages and currents induced in the rotor bars increase accordingly, causing a greater induced rotor voltage. The resulting increase in the rotor current causes a larger torque at a slightly lower speed.

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Thanks Kim, Thats the paragraph that had me and made the most sense. Also what Dick had to say about the transformer. It kind of ties back to a transformer. The load is not directly conected to the supply voltage. But when the load on the secondary goes up in current the same must happen on the primary. I was trying to figure out what on the inductive side of this made it all happen. I also talked to an electrical engineer at work and he used the analogy of a generator. He said on the power genertaion side of the wall you just can't turn a generator off like a switch. You have to slow it way down then cut the power. If you just kill the power the rotor will run away on you and you will over RPM the mechanical side of the motor. Because there is nothing holding it back

Thanks for all the responses. I went out for a Promotion and this came up, and for the life of me I just was stumped. It was a tough interview with questions like "tell me how a motor works" Then every reply to my answer was "why"

I also talked to an electrical engineer at work and he used the analogy of a generator. He said on the power genertaion side of the wall you just can't turn a generator off like a switch. You have to slow it way down then cut the power. If you just kill the power the rotor will run away on you and you will over RPM the mechanical side of the motor. Because there is nothing holding it back

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Technically that was a poor analogy, hopefully you obtained something from it tho. BUT generators can be cut off like a switch BUT not to damage equipment the proper way is to shut off the MAIN DISCONNECT then shut down the generator. No generator (if working properly) will/can run away. IF paralleled to other generators and you do NOT disconnect the main and shut it down then it will act like a motor but again it wont run away (it just draws power from the other gen sets and acts like a motor). YOU never want to slow down a generator while its online (especially if parallel to others), this can cause damage.

I worked for several years with Diesel generator sets from 100KW to 1000KW. Caterpillar doesnt want you to idle their gen sets, they want you to shut it down. I have seen what can happen when a gen set goes down because its low on fuel, air breather clogged, low oil pressure, overheating etc etc and it can be devastating in costs.

I have seen diesel gen sets run away but it was because of a fuel injection problem...shot fireballs thru the trailer walls.

I am not as familiar with the real big power plant systems but do know they have multiple redundant safeties to do a shutdown/switch if a problem occurs.

I understand that current goes up in a motor for starting and whenever the load is increased. But why and more specifically how?
Remembering that we have to supply a 'back emf' (as we used to call it) to 'get the motor energized' with magnetic flux(similar to a transformer) AND we have to provide 'torque' in excess of what it would take to 'stall' the motor...

I(emf) + I(stall)+... divide by the Resitance (add a couple sauteed onions on the rotor side (windage, wire/connecting point losses) and you have a very familiar equation....

I(total)/R(total) = Voltage supplied (guess 'who's' constant here)

Given the INFINITE BUSS of power that shows up at the door (unlike the recent outages of this last year), you have the capability to
supply appropriate 'phase relations' as well (for 3 phase circuits).

Given the V-line and I-line characteristics as 'changing per unit time', you have both Watts and Vars to look out for.
{P cos(i,v)[watts] + S sin(i,v)[vars]}= Total Power needed.

Some of what you experience with 'pull down' or 'startup' torque is filling the 'energy bucket up' so you have 'available' energy to 'run the motor'. Once 'energized' or 'energized and rotating' you need less and less 'Energy(Power)' to keep it running because your losses are less and less (typically run at the less than full 'synchronous speed'). You might see 80% of your energy being used to 'keep the motor running' or 20%, depending on 'what you're doing' with the torque you're providing to what.

You try to stop this motor in any ONE of FIVE typical ways and you will 'reverse the power equations' to maintain the 'energy conservation' laws. You give OFF 'heat' and 'noise' when the 3 phase motor reverses it's (single phase reversal usually) physical (mechanical) rotation CCW(counterclockwise) to CW(clockwise).

If you need more information, please ask. Tons and tons of fun with motors and motor controls (the SAY books from England are helpful). If you're in Europe you might want to briefly review the DelToro book as it's a standard.

conditions that are required to have a motor:
A current carrying conductor in a magnetic field

A rotating magnetic field is supplied to the motor which rotates around the stator at synchronous speed therefore having relative motion to the squirrel cage conductor (I'm assuming squirrel cage here). This sets up current flow through the squirrel cage that is proportional to the difference in the synchronous speed of the field and the rotor speed (this is referred to as slip speed). This develops a magnetic field for the rotor which interracts with the stator field. As the speed of the rotor increases toward synchronous speed the current in the goes down. When the load is increased the speed of the rotor slows while the rotating field stays the same, causing the rotor current to increase

Another aspect to consider:

What are the requirements of a generator?
A magnetic field
A conductor
Relative motion between the field and the conductor

When a motor is turning you have these three conditions. This causes the motor to generate a voltage that opposes the force that created it, meaning the motor generates a voltage opposite to the voltage you supply it to operate (counter emf).

This also applies to generators, although it generates counter torque. That's why when a generator is loaded it tends to slow down until some regulator action speeds it back up.

Is this thread still alive?
I'm trying to figure out induction motor and generator theory as well...
The squirrel cage rotates along with the rotating magnetic field produced by the phasing stator coils, ...that is at no slip..
When a slip is caused by either a positive or negative torque on the cage then there is a dB/dt w.r.t. loops in the cage, which then induces currents in the cage loops in accordance with Lenz's law. The cage parts are shorted and have very low resistance which causes heavy current in the cage loops, which in turn creates a magnetic dipole which in turn makes a torque coupling with the rotating stator field..
My question is this;
Consider taking the cage out of the motor and putting a crank on it and put it in a tabletop configuration with a strong STATIC magnetic field produced by electromagnets. The cage sitting still in this field would be equivalent to the cage rotating with the rotating field of the motor. Or is it? Of course there would be no torque on the cage if it were not rotating. Now turn the crank some... This would seem to be equivalent to a slip frequency. Would there be a resistance torque to turning the cage? Would the back torque increase in proportion to the speed i try to turn the crank? If so, would my work be translated into a back emf in the electromagnet? And if so too,.. if there were permanent magnets would there be a torque as well? ..and where would my work go in this case? I would try the experiment myself if I had materials and a place to do it at.
Cheers to all viewers and replyers.

Labotomi said:

When a motor is turning you have these three conditions. This causes the motor to generate a voltage that opposes the force that created it, meaning the motor generates a voltage opposite to the voltage you supply it to operate (counter emf).

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I'm simply amazed that it only took 12 posts for someone to finaly answer the question... Good Job Labotomi!!

No offence ment to all the others, but nobody else hit on the keyword: Counter Electro Motive Force.

Hi,
Induction motor operate on Fradays laws of electromegnetisum

first law state that when ever a conductor placed in megnetic field, an emf is induce in it. (This emf opposes the line cureent)

And second law state that emf induced is directly proportional to the rate of change of flux (speed)

As per eddy current law. emf induced in a conductor always opposes the current which induces it.(as stated above)

So at the starting of motor when there is no speed, there is no emf induced, so there is no oposition to main current so motor draw high current, but as it catch up speed more and more back emf induced, current decreases. At full speed it come to normal current.