inductive vs resistive loads

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Re: inductive vs resistive loads

Actually the speed of the induction motor is dependent on the frequency, the number of poles, and the slip. As voltage goes down, the slip increases, and the motor slows down. As long as the voltage and load is within spec, the variation in speed won't be all that great, but there will be some variation.

For instance, when I reduced the voltage of my fan experiment from 115 down to 105, there "seemed" to be a slight decrease in speed. I say "seemed" because it was a perceptual thing, I did not actually measure it. As I lowered the voltage to 95v, the fan turned even slower. Not tremendously slower by any means, just noticably slower.
 
Re: inductive vs resistive loads

Yes speed is a function of frequency and the # of polls.

N=7200/P

N is speed
P is # Polls
7200 is a constant derived from 60Hz and 120 which 120 is a constant derived from various design characteristics of which believe it or not voltage has an affect.

(OMG you did it, I started using formula's :D
 
Re: inductive vs resistive loads

Here is how I see it:

Actual motor speed in RPM = synchronous speed in RPM - (% slip x synchronous speed in RPM)

Synchronous speed in RPM = 60 x frequency in Hertz / number of pairs of poles

The above formulas work for any frequency.

(note 1: the 60 in the formula is used to convert seconds in Hertz to minutes in RPM)

(note 2: the % slip of any given motor will vary depending on voltage, loading, and possibly other factors)

Boss, your formula gives the synchronous speed at 60 Hertz. The 7200 is a combination of:

1) 60 hertz which is the given frequency

2) 60 that is used to convert the seconds unit in Hertz to the minutes unit of RPM

3) 2 which is used to allow the use of the number of poles instead of pairs of poles

60 x 60 x 2 = 7200

I don't see that it has anything to do with voltage or motor characteristics other than the number of poles.

[ March 03, 2005, 12:23 AM: Message edited by: crossman ]
 
Re: inductive vs resistive loads

Yes the formula is for synchronous speed of the stator field. And I did mention the 60 Hz is part of the 7200. The things that affect synchronous speed are # of Poles and the frequency. That is it, as you said.

Now we are not talking about synchronous speed, we are talking about the speed of the motor. Induction motor will not turn at synchronous speed, it can't.

Slip is a function of speed not a component. You can find the speed using slip in a formula but you have to know the slip. Slip in a motor changes as it's speed changes. The speed does not change as slip changes. Slip does not contribute to the torque it is a result of the load but the slip of the motor will have an effect on the electricity it uses.

Things that produce the torque on the rotor that gives the speed are a whole lot of things. Just about all are variable (especially in fans). Torque applied, skew of the rotor bars, air gap, material motor made of (hysterisis effects), imperfections in laminations.

I guess my main point that I keep trying to explain is everyone keeps trying to use electrical principles to explain this "phenomenon" we keep talking about. Again, it's not electricity that drives the motor it's magnetic principles and those magnetic principles are influence by other rules, and magnetic principles are the things that derive electrical principles. You can't explain what happens in a motor with the electricity it uses, but the way a motor works effects the electricity it uses.

Perhaps we are saying the same things just at different angles. But I just don't see it.
 
Re: inductive vs resistive loads

Boss, we all agree that the induction motor is difficult to model, and we agree that magnetic effects are key to this discussion, but I would differ with you when you say electricity does not drive the motor. It is the electric current which produces the magnetic fields, and this current, and voltage too, must be considered when modeling the motor.

Ideally, we would have a nice, simple formula which could be evaluated easily. Instead, we have complicated, empirical formulas which are messy, but still do the job fairly well.

One point I wish to make though is that you cannot simply say that the motor can be modeled by a "changing impedance". This is meaningless for at least two reasons:

1) To properly use impedance to predict behavior of a circuit or device, we must assume it to be constant. It is a gross simplification to speak of changing impedances.

2) No one knows the formula for this changing impedance. If we did, we would have a model for the motor.

So please, don't irk this old pedagogue by speaking of "changing Ohms, changing impedance, changing inductance, etc." My frail mind can't take it.

[ March 03, 2005, 09:32 AM: Message edited by: rattus ]
 
Re: inductive vs resistive loads

I have been searching the net and books, I cannot find any studies done that compare voltage and current based on real world loads. If any of you find something, please post a link.

Rattus, I know that it is one of your pet peeves about the changing impedance, etc. I see your point on one hand, but I can also see the other folks' point too. If the motor was a "black box" load and we could only take measurements outside of the black box, it seems that a motor load could be mimicked with variable resistors and variable inductors inside the black box.
 
Re: inductive vs resistive loads

I couldn't agree more with what you guy's just said. That is pretty much just my point. No motor black box's no, empirical data that I'm aware of to answer these questions. Many theories, and very involved formulas but they still don't capture what happens.

Thing to note:
In every motor there is generator action.
In every generator there is motor action.
These are two distinct forces that are independent yet exist together. OK I'm making myself dizzy again.

There has just been to many wrong implications put out into the world because people try to simplify it to explain it. What I have found is that makes it very difficult in my field when it comes to trying to explain troubleshooting problems and what's happening in an electrical distribution system to somebody that has an MBA and is running the plant, especially when there is an "engineer" that is just throwing all the nonsense formulas around trying to make himself look smart while my guys are busting their humps to fix the problem.
 
Re: inductive vs resistive loads

Crossman,

It is a misuse of the concept of impedance to use it in this way. To do so, you would need a formula to predict the change in impedance. It is another way of saying that one really doesn't understand what is happening. Twiddling knobs on a rheostat tells us nothing about the motor's behavior.

I have even seen this explanation used where impedance is not even defined, e.g., a step function response in an RC circuit. It is a cop out.

You could instead spend a few hours in the lab and come up with a set of data which describe the motor's behavior at different loads and different voltages. That would mean something. Then you could spend a few more hours curve fitting equations to the data. That would mean something.

Do it right!
 
Re: inductive vs resistive loads

Crossman,

The diagram that I posted is the work of Edward H. Cowern, P.E. Mr. Cowern is an accomplished technical writer and has answered many motor related questions over a long career with Baldor Motors. He has compiled his answers in a Baldor publication The Cowern Papers.

Take a look at the Low Voltage Section of this Paper titled: ELECTRIC MOTORS AND VOLTAGE.
 
Re: inductive vs resistive loads

Crossman:

You posted:

Rattus, I know that it is one of your pet peeves about the changing impedance, etc. I see your point on one hand, but I can also see the other folks' point too. If the motor was a "black box" load and we could only take measurements outside of the black box, it seems that a motor load could be mimicked with variable resistors and variable inductors inside the black box.
Click to expand...
I agree with the small exception that we also need a changing voltage to represent the counter EMF. The diagram Physis posted gets rid of this voltage by using a new "calculated" input voltage instead of the real input voltage. The "calculated input voltage" varies with the motor slip, even though the real input voltage would remain constant.

Steve
 
Re: inductive vs resistive loads

I wish I had more time right now but I can't even look at all the posts.

You can look at it as a black box, I asked Kirchoff and said he doesn't mind if I you do that.

Crossman there's something more to the inductance side of the impedance. Without proving this I'd suggest that a motor acts like a super duper inductor that doen't conform to the rules of a simple inductor. Some body correct me if I'm wrong but I think after the magnetic field is inflated the super duper inductor goes on from there to, next, start storing energy in inertial mass. Pretty cool really.

And also Crossman, can I talk you into doing ypur motor test again with a different kind of load. I think that propelers are the least helpful in trying to demonstrate increased current with decreased voltage. Stick some real friction in there.

And I haven't found anything much on the effect of undervolting these things either.

[ March 03, 2005, 02:00 PM: Message edited by: physis ]
 
Re: inductive vs resistive loads

I read the pertinent sections of the links that were posted. This is what I discern: There are 2 types of loads on motors. One type has a variable load based on speed, the other type has a constant load regardless of speed. Varying loads would be fans, blowers, and centrifugal pumps. The link says these variable loads comprise a significant portion of motor loads.

With these variable loads, as speed decreases, the power required drops very quickly. Hence the outcome of my experiment where the voltage decrease gave a current decrease (the motor did reach a point where a 30% voltage reduction saw the current start rising again).

What percentage of motors in the world are turning fans and blowers and centrifugal pumps? 30%? 50%? Any guesses?
 
Re: inductive vs resistive loads

Physis, I would love to do the experiment with several different types of loads. Right now, I only have access to a fan motor. Does anyone have any ideas on what I could use for a different type of load?
 
Re: inductive vs resistive loads

I'm not informed enough for my guess to matter much, but I think we move a lot more metal than we do air and water.
 
Re: inductive vs resistive loads

I was thinking of something simple and caveman like. Tighten the belt on that pulley. Tie a rag around the armature and nail it to something. It doesn't have to be a linear load, I don't think most are. The cards in the bicycle tire spokes trick.
 
Re: inductive vs resistive loads

Actually we move a lot more air and water. The numbers I remember are close to 80% fans and centrifugal pumps. Just think every house probably has a fan, or blower for AC and furnace, Well pumps, WWTP pumps, Industrial machines have tons of water cooling pumps, oil pumps, places you never would imagine with centrifugal pumps. Sewer pumping stations, fans on AC units, heat pump, water pumping stations. Non industrial places with nothing but fans, and HVAC, and large units.
 
Re: inductive vs resistive loads

There is something called a Prony brake. A brake shoe is fitted around a flywheel, and the torque is measured on an attached scale. Measure the RPM and you can compute horsepower.
 
Re: inductive vs resistive loads

We have a dynamometer which I can connect as a load to the motor, but what i want to see is the effect of low voltage on real world loads like a conveyor.

If 80% of the loads on motors out there are varying loads that go down in power as speed is reduced, and that a 10 or 15 percent reduction in voltage causes the current to go down as with my fan.... then it seems that the statement "if you lower the voltage you raise the current" would be false in more cases than it is true....
 
Re: inductive vs resistive loads

Maybe something that uses a drum or a disk sander (bench type) that can be belt or direct driven by a squirrel cage motor.

Then run a chunk of wood into it, maybe under a spring pressure. . .couple of screen door springs pulling it into the sander. . .?

Set up the load so the motor is ? to ? of Full Load. Then start decreasing the voltage.
 
Re: inductive vs resistive loads

Also, do a Google search on "Prony brake". You'll get a lot of interesting ideas from the returns.

Here's one: Prony Brake
 
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