inductive vs resistive loads

[ccjersey]

In the discussion about single phase 208 V loads supplied by a 3 phase 120/208 Y, I realized I don't understand why resistive and inductive loads behave so differently when voltage goes up or down.

The "lightbulb" drops amperage and power when the voltage goes down because the resistance is constant.

The "motor" draws more amperage to maintain power when the voltage goes down. Surely resistance is the same in the coils of the motor and current should drop some when voltage goes down, but amperage goes up instead.

Can anyone give a good explanation of this?

Jim

 

[rattus]

Re: inductive vs resistive loads

Originally posted by ccjersey:
In the discussion about single phase 208 V loads supplied by a 3 phase 120/208 Y, I realized I don't understand why resistive and inductive loads behave so differently when voltage goes up or down.

The "lightbulb" drops amperage and power when the voltage goes down because the resistance is constant.

The "motor" draws more amperage to maintain power when the voltage goes down. Surely resistance is the same in the coils of the motor and current should drop some when voltage goes down, but amperage goes up instead.

Can anyone give a good explanation of this?

Jim

First, the resistance of the light bulb is not constant. It goes down as the temperature falls. Still, the current goes down with the voltage.

A motor is far more complicated than a light bulb. It cannot be characterized as a simple impedance.

When the line voltage drops, the motor slows, and the back emf falls causing it to draw more current to try to maintain its speed. If the voltage falls too much, the excess current causes the motor to overheat and fry unless it is thermally protected.

[ February 28, 2005, 11:23 PM: Message edited by: rattus ]

 

[physis]

Re: inductive vs resistive loads

For the moment I only have a couple of things to say.

First, it's an excelent question.

Second, I'm disappointed in Rattus' responce.

And third, I can't answer without giving it a good amount of thought. I'm aware of the question in exactly the way that you phrased it and have never sat down and tried to answer it.

It's a good question and Rattus can do better than that! Ed's gonna beat you to it Rattus.

I'm going to take an unverified stab at it using a commutator motor with brushes, because I have some experience with those.

I'm thinking that maximum efficiency is going to be when the stator field is a little behind the armeture field (These fields of course oppose).

So if the armeture starts to lag in respect to it's intended angle to the stator field, then instead of the stator pushing the armeture forward, it's pushing more "directly into" the armeture.

If you hit a wheel toward the forward direction, it goes foreward.

If you hit a wheel toward the backward direction, it goes backward.

If you slow the armeture down to where the stator field is pushing at 90? to the armeture then you're only making heat.

There are guys here who know far more about how motors behave than I do.

How am I doing, am I close?

 

[Ed MacLaren]

Re: inductive vs resistive loads

The AC "brush-type" motor is a series motor and does not exhibit the behavior under discussion, that is, an increase in current when the voltage decreases.

In fact, the standard method of speed control for these motors is to vary the supply voltage.

A quick overview of the reason why an induction motor draws more current when the voltage is decreased is because it's speed is a function of the line frequency and the motor's designed "slip", and is, up to a point, independant of line voltage.

With the speed and load (power output) basically unchanged, the power input has to be maintained, thus the increase in current.

Ed

 

[physis]

Re: inductive vs resistive loads

Ok.

I'd have to learn how an induction motor operates then.

Are these the type that the stator field induces current into the armature and the current in the armature essentially shorts in a conductive layered arrangement in the armature. And, I think, using the stators own field, flipped 90? twice, to push against itself?

 

[oliver100]

Re: inductive vs resistive loads

To simplify, you can regard the induction motor to be an equivalent of impendence of Z ohms. Thos ohms are in relationship with the slip/speed of the motor. The OHMS are minimum at 0 speed (the inrush max current) and they are maximum at synchronous speed (theoretically 0 current).

Applying the Ohm's law, when the voltage goes down the speed is lower, the slip is higher, the ohms are considerably lower than the percentage of lower voltage and as a result the current is higher. Having the same counter torque, a further voltage reduction will fry the motor.

 

[Ed MacLaren]

Re: inductive vs resistive loads

Are these the type that the stator field induces current into the armature

Yes, the stator field (blue in sketch) is rotating at syncronous speed (a speed determined by the line frequency and number of stator winding poles).

It "cuts" the rotor bars (conductors) and induces a voltage into them.

The rotor bar current produces a flux (red) that interacts with the stator field to produce rotor torque in a direction determined by the Right Hand Motor Rule.

Ed

 

[al hildenbrand]

Re: inductive vs resistive loads

Jim,

When I sort out the differences in my mind, the differences between a light bulb's light output and a motor's power to the turning shaft, I look at the constants in the governing formulas. </font>

• <font size="2" face="Verdana, Helvetica, sans-serif">A bulb filament, being resistive, is described nicely by Ohm's Law. R = Volts ? Amps. R is constant. Light output varies.</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">For the induction motor, HP = Volts x Amps. Horsepower (HP) is constant. When volts go down, amps have to go up to maintain the constant.</font>

<font size="2" face="Verdana, Helvetica, sans-serif">Now, admittedly, this is a gross simplification, but I think it is a great starting place.

 

[ccjersey]

Re: inductive vs resistive loads

Thanks everyone!

 

[physis]

Re: inductive vs resistive loads

Similar to a transformer.

No load on the scondary and there's only the impedance of the primary as a load.

Put a load on the secondary and and the primary current increases.

 

[physis]

Re: inductive vs resistive loads

If the armature spins at the same rate as the stator fields then the armature windings aren't cutting the magnetic lines and there's no current induced.

Would I be correct if I assumed that if the armature falls behind it doesn't have to catch back up? I mean the armature doesn't have to have any particular orientation to the stator field. It just tries to rotate with it.

 

[69boss302]

Re: inductive vs resistive loads

The rotor in an induction motor never catches up to the stator field because of what you just said. No induced current, no torque. There is always some slip. To reach synchronous speed you have to use a different type rotor. One with field polls that lock into the rotating magnetic stator field and stay with it. Even this type motor is started as an induction motor and then once up to about 95% of rated speed then the field polls are kicked in to bring it in to synchronous speed.

 

[Ed MacLaren]

Re: inductive vs resistive loads

Would I be correct if I assumed that if the armature falls behind it doesn't have to catch back up?

Correct. In the induction motor the rotor always rotates slower than the rotating field.

The difference is called "slip". At no-load the slip is very low. The rotor only needs to turn a few RPM slower than the rotating field in order to-
-induce sufficient voltage into the rotor bars
-to cause sufficient current in the rotor bars
-to produce enough torque
-to overcome friction and windage losses.

When load is applied, the slip must increase (rotor turns slower) in order to-
-induce sufficient voltage into the rotor bars
-to cause sufficient current in the rotor bars
-to produce enough torque
-to supply the load and losses.

If load is increased, the slip must increase, etc, etc.

It is in the synchronous motor that the rotor "locks onto" the rotating field and rotates at syncronous speed.

Ed

PS - Boss302 types fast. (The Boss 302 was a fast car too. )

[ March 01, 2005, 11:09 AM: Message edited by: Ed MacLaren ]

 

[crossman]

Re: inductive vs resistive loads

Good discussion for sure. I have always had a question though...

It is always said that reducing voltage to a motor will cause the amperage to go up. I understand the concept here, basically we are saying the horsepower is remaining constant.

But does the horsepower output necessarily remain constant? Is this something inherent in an induction motor? Or is it because of the type of load?

If we reduce voltage to a fan motor, does it continue to push the same amount of air = same horsepower and the current goes up? Or does it push less air = less horsepower and maybe the current goes down?

And why does reduced voltage starting work to lower the amperage through the starting cycle?

I do know that if I connect a 1/4 horsepower 1 phase 120 volt split phase motor on our lab volt equipment that has a variable power supply, under no-load conditions, as I reduce the voltage the current goes down and the motor also slows down. I know this because I just finished doing it.

Yes, this is under no load. But at what point would a load become a constant horsepower load? What if the motor is loaded only 1/8 load? 1/4? 1/2? When does this "lower voltage = higher current" take effect?

I don't think that "lower the voltage to a motor and the current will go up" is always a true statement. Also, it isn't necessarily the amperage that overheats a motor. Increased slip also causes excess heat... and doesn't necessarily draw more current.

 

[69boss302]

Re: inductive vs resistive loads

Ed. I don't type fast just been away from the teaching mode and don't have all the fancy drawings or take the time to explain things as well as I should anymore. Your post's are terrific, all the drawings bring back many memories.

Crossman, you have asked some terrific questions, motors are very interesting machines. What happens inside of them is not what a lot of people try to put across. I would love to answer your questions but I would need a lot of Ed's drawings and you also give me the impression you want to see formula's. Another thing I have lost track of. I can't remember all of them and there are so many different constants in one given motor and believe it or not with one motor not that many variables. One motor to the next thought there are lots of variables. Any way I am failing to answer your question so I will leave this one for Ed.

 

[laidman]

Re: inductive vs resistive loads

I believe the question was about resistance and current in a motor. DC resistance does not play a major role in the current that a motor draws. It is based on inductive reactance of the motor windings and cannot be calculated with simple ohms law. I just demonstrated to my class this point.

The windings of a 120V motor read a resistance of 3 Ohms by ohms law the current should have been 40A but the name plate said FLA 2.4A.

Don't get caught in the ohms law trap with motors.

 

[rattus]

Re: inductive vs resistive loads

Laidman,

Here, here! I see a tendency to characterize complex devices such as motors as impedances. This is a meaningless simplification. A motor, under fixed conditions, does present a load impedance, but this impedance cannot predict its behavior.

The motor is also a generator. Synchronous motors can be used as alternators. Even the induction motor produces a back EMF which limits the current.

In summary, I say that the resistance and inductance of motor windings are parameters of the motor, but neither limits the current when the motor is running under load. It is a complex matter involving, load, slip, resistance, inductance, etc., etc., etc.

The guy who invented the induction motor must have been some kind of genius!

 

[laidman]

Re: inductive vs resistive loads

I agree!! But not being a genius I just stick to simple ideas I just meant you cannot predict motor currents using winding resistance alone.

 

[69boss302]

Re: inductive vs resistive loads

laidman, that is absolutely true. Even in a D. C. motor you can not just use ohms law. Magnetic lines of flux are moving and as soon as you have movement between a conductor and lines of flux s___ happens. That is why it takes a lot more explaining with drawings and formulas if you want to understand. I like to stick to simple explanations also anymore, but crossman seems to want to understand what is happening inside.

It all really boils down to magnetic characteristics and rules of magnetism. Three basic rules I adhere to that explain it. Now real basic and you have to build from there;

Magnetic lines of flux travel from North to South, Outside a magnet (Remember OUTSIDE the magnet)

Magnetic lines of flux take the path of least reluctance.

Magnetic lines of flux do not (well lets say like to) cross over each other.

With those three rules your on your way to explaining how torque in a motor is produced.
Electrons are pushed in some direction.
And that some day a black hole from space will swallow up the universe.

There's just a lot of science, formulas, and stuff in between to prove those things.

Crossman, have you ever heard of the American Electricians Handbook, or Rosenburg Electric Motor Repair. Two great resources, for motor info.

 

[crossman]

Re: inductive vs resistive loads

Well, I am not looking for all the mathematical equations and nth degree of understanding... It is just that I have heard all my career that with a motor, "if the voltage starts decreasing, the current will start increasing."

I am certain this is not always true. Does anyone have anything to say about that?