inductive vs resistive loads

[69boss302]

Re: inductive vs resistive loads

Sorry I had to walk away.

Speed Torque curves are used to show you that when you turn on the motor there is very little torque, you need to get some RPM's going and current induced into the rotor which will then induce a CEMF. This CEMF will then bring down the current.

The normal operating speed range of an induction motor is around 75 to 95% rated speed (can not turn at synchronous speed). You notice that the peak of the torque curve is just around or below 75%, because what this means is as you add load to a motor it will slow down, thereby affecting the torque applied and the current drawn. Once you get to the low side of this peak, the motor will stall because as is shown, the torque now drops off even as the current raises. (Note these curves are design curves that are used for picking out a motor to use on a certain load. Nothing is taken into consideration for changing voltage.)

There are other design motors that have different speed torque characteristics to give you better starting torque and a lower pull out torque.

Pull out torque, (breakdown torque) is that tip of the curve where the motor stalls.

 

[69boss302]

Re: inductive vs resistive loads

Crossman, I don't recall saying that a motor wants to produce it's rated horsepower. If that's what I said or how it was understood I apologize. A motor is going to draw what it needs to turn. If the load on it does not require it's rated horsepower output, the motor won't draw it. But as the load increases the motor will produce the horsepower needed until it goes over it's designed rating. Then the temperature will rise due to the increased current and eventually (time depends on size of overload condition), the smoke will come out as it is commonly referred to as.

 

[crossman]

Re: inductive vs resistive loads

Originally posted by 69boss302:
Why do they put UV (Under voltage relay) in motor starters

Give up? I'll tell you why. Because as voltage decreases, current increases and the load needs to be protected. Standard NEC OCPD are not designed to protect this kind of situation.

Hmmm... this is getting fun!

Actually, if current went up, the overloads (heaters) that protect the motor would be what protects the motor. They wouldn't have to worry about under-voltage relays if it were gauranteed that motor current would go up if voltage went down. The heaters would take care of it.

The undervoltage apparatus is included to take care of the situation where voltage goes down, speed goes down, horsepower of driven load goes down, and current goes down, and this lower current doesn't trip the heaters. In this situation, the increased slip will cause an overheating of the motor if it persists for enough time.

DISCLAIMER: This is all speculation on my part! I am going to go do my own undervoltage test RIGHT NOW. Will get back to you with details.

 

[crossman]

Re: inductive vs resistive loads

P.S.

For Ed:

When I did my undervoltage test on the no-load induction motor, I slowly decreased the voltage from 120 down to 50 volts... so I was at 110 for a couple of seconds, then 100, then 80, etc.

 

[crossman]

Re: inductive vs resistive loads

Okay, data hot from the laboratory:

And for real, I am not making this up, I just did it on our lab equipment.

split phase motor - driving a 28" diameter fan via pulleys and belt

Nameplate = 1/3 HP - 120 volt - 6 Amps FLA

Variable voltage supply

Data taken after start-up = motor at full speed

115v = 5.5 amps
105v = 5.2 amps
95v = 5.0 amps
85v = 5.1 amps
75v = 5.5 amps

The original 115v and 5.5 amps indicates the motor is at near full load.

The motor slowed down with each decrease of voltage.

The current reached a minimum of 4.9 to 5 amps between 90-95 volts, then it started increasing as the voltage was lowered further.

I assume the horsepower output to the fan was decreasing as the voltage decreased.

This proves to me that motor output does not remain constant as voltage is lowered. At least when talking about an induction motor driving a fan.

So is the fan a special case and all other loads are "constant horsepower"? I have my doubts. I bet other loads would act in a manner very similar to my experiment.

 

[rattus]

Re: inductive vs resistive loads

Sam, nice diagram, but let me zing you on two points:

1) The CEMF is key to the performance of a motor. Without CEMF, the motor would fry.

2) The diagram you posted looks exactly like the exact equivalent circuit of a xfrmr. No way will this desribe the non linear performance of a motor under varying load conditions.

 

[69boss302]

Re: inductive vs resistive loads

I believe I mentioned before that fans are very tricky to use for this example. Change the speed on a fan and the loading can change dramatically some times.

I think if you look at your numbers to you see that the current started to go back up around 75% of rated voltage. HMMMMMM :roll: (does that need a rolley eye)

The best example for this, what we'll call phenomena I guess since we can't seem to explain it, is a three phase induction motor driving a gear box. Torque necessary to drive load stays fairly constant while machine in use anyway, and as I said before motor wants to draw what it needs. Also this is most of my background so that is where you see it the most. Industrial, three phase, table rolls, gear motors, spindles for rolling mills.

It also is more apparent on three phase pump motors but you still have to take into consideration that when it slows down the loading changes on the impeller, but the "phenomenon" still show's up.

 

[crossman]

Re: inductive vs resistive loads



Boss, I would not be surprised if you are correct in your assessment of a motor driving a gearbox which is turning some mechanical apparatus. It may certainly be that a 10 volt reduction of the nominal voltage could actually increase the amperage. On the other hand, perhaps it acts just like my fan experiment? We need more data.

I wish I had another type of load besides a fan to do my test on. Unfortuantely it is all I have.

 

[physis]

Re: inductive vs resistive loads

Take it easy 69boss302, I keep telling you guys I'm trying to learn how these things work.

I'm not making nothing up. I'm checking out different web sites and reading through all this stuff. The curve I posted came from what I think is a pretty decent site. And when I went back to make sure I'm not getting it wrong I found that you're absolutely right. It is a start up curve and it turns out that's what they were using it to describe.

Well anyway, I guess I'll have to read some more.

Edit: You guys got some posts in while I was typing. I'm glad you don't seem as agitated anymore 302. Rattus, I put that schematic in for you.

[ March 02, 2005, 04:44 PM: Message edited by: physis ]

 

[69boss302]

Re: inductive vs resistive loads

Crossman, all I can say is I never collected data, but I have seen this happen over and over again. I have seen motors burn up purely because of low voltage problems. This is also not just a motor problem, it does happen to other types of loads also, however I can not be specific to explain it. You almost have me ready to dig my trunk out of my closet and get out all my old books. I put them away a long time ago and got tired of theory and trying to prove with formula's. I only speak from what I have seen and (no I won't say know because are we ever sure we know anything) understand. I believe what I see when it comes to machinery. Electricity now, what can you see. But I have been in the electrical field all my life.

Something else I would just like to mention because it causes problems understanding a lot of times. A motor get's classified as an electrical device, that it is not.

A motor is an electro mechanical device on it's own. It converts electrical energy to mechanical energy. It is not the electricity that causes the motor to turn.

The mechanical energy (torque) comes from the interaction between the magnetic field generated by the stator coils and the current induced in the rotor (we are just talking induction motors now, don't jump to conclusions with synchronous, or reluctance, or D. C. motors, even though it is magnetic principles that drive them also, not electrical principles).

The electricity that is drawn by the stator is what generates the magnetic field. The strength of that magnetic field is affected by the current it draws. (Only discussing one motor, many other things affect field strength motor to motor.)

Now I refer back to the rules of magnetism stated earlier. Using them you can show how torque is created, CEMF is produced and one more time, how the universe will be swallowed up by a black hole someday, unless the power of anti matter is discovered and the Enterprise can save the day (perhaps a bit much)

 

[69boss302]

Re: inductive vs resistive loads

physis, sorry no offense intended and I much prefer people ask questions to learn that is why I like this site. I have never developed the code knowledge you guys have and have been stuck by to many "engineers" that through these things around to prove themselves and they don't know what they are doing.

It's just you guy's have actually hit a topic I feel very qualified to help in, and there is just not the means on here (for me anyway) to explain myself as well as I would like. I don't have the access to Ed's drawings.

The understanding of how a motor works is what every body seems to avoid yet they want to try and design and install motor controls. And when it comes to troubleshooting :roll: OUCH. People get so way of base it's not funny, just because they can't see or understand what's happening.

I applaud your desire to learn more.

 

[crossman]

Re: inductive vs resistive loads

Originally posted by al hildenbrand:
I dunno Bob. The data is pretty hard on this.

Boss... I hang around here for what I can learn too! Believe me, I thought I knew alot before I found this site, but man did these guys set me straight!

[ March 02, 2005, 04:56 PM: Message edited by: crossman ]

 

[steve66]

Re: inductive vs resistive loads

Sam:

Your diagram is a correct (simplified) diagram for an inductance motor. The only problem is that some of the parameters (including the rotor losses and the applied voltage) vary with the slip. So its a good diagram for looking what happens at one operating point, but not so good for trying to understand what happens for a changing voltage.

Steve

[ March 02, 2005, 05:20 PM: Message edited by: steve66 ]

 

[jwelectric]

Re: inductive vs resistive loads

Gentlemen, this has been a very interesting thread and I have learned much. I thank you. Just to recap a little on the knowledge that I have gained. When motors are turning, let?s say in a counter clockwise direction there is also a counter force of some sort at work. This being so complicated that it would take quiet a bit of time to explain with a mathematical formula. This explains why on Sunday afternoon the man says, ?Gentleman start you engines? instead of, ?boys crank them motors.?

 

[physis]

Re: inductive vs resistive loads

So now I'm not clear again.

Will that stator current curve follow the same path back if the supply voltage is reduced lowering the rotor speed? (Minus the obvious effect of lowering the voltage)

No offence taken 302. You have to get worked up when someone tries to pull a fast one.

 

[iwire]

Re: inductive vs resistive loads

Originally posted by al hildenbrand:
I dunno Bob. The data is pretty hard on this.

Al, that chart assumes the motor is 'mechaniclly loaded' as much as posable.

When you reduce the voltage to a fan, the fan slows, the HP output drops and the amps decrease.


Try it.

 

[peter d]

Re: inductive vs resistive loads

Originally posted by iwire:
Al, that chart assumes the motor is 'mechaniclly loaded' as much as posable.

When you reduce the voltage to a fan, the fan slows, the HP output drops and the amps decrease.


Try it.

Ok, you got me . If you stall a motor or slow it down, back EMF drops to zero or decreases, and then current is only limited by the resistance of the windings (which is very low) and some inductive reactance. Won't current increase rather than decrease?

[ March 02, 2005, 07:03 PM: Message edited by: peter d ]

 

[crossman]

Re: inductive vs resistive loads

Originally posted by 69boss302:
Crossman, all I can say is I never collected data, but I have seen this happen over and over again. I have seen motors burn up purely because of low voltage problems.

One thing that I don't understand: If a motor is operating at near full load, and the voltage goes down, and the current goes up enough to cause enough heat to burn it up, wouldn't the overloads have tripped first? I mean, just like if the motor was overloaded and drawing too much current?

So I am thinking that the "higher current" caused by low voltage isn't really the culprit for the burned up motors. I have read that increased slip causes heat to build up, I guess something to do with the magnetism, hysterisis, whatever, and that it is this heat that is destroying the motors. Otherwise, why wouldn't the overloads have tripped and saved the motor?

I'm not being obstinate. I feel this is a good question.

 

[crossman]

Re: inductive vs resistive loads

Ok, you got me . If you stall a motor or slow it down, back EMF drops to zero or decreases, and then current is only limited by the resistance of the windings (which is very low) and some inductive reactance. Won't current increase rather than decrease?

But don't forget, we are also lowering the voltage. If the opposition of the motor decreases at a rate less than what the voltage is decreasing, then the current will also decrease.

[ March 02, 2005, 07:16 PM: Message edited by: crossman ]

 

[peter d]

Re: inductive vs resistive loads

Ok, here's another one. Now I'm really

Just how much will a motor slow down if you reduce the voltage?

Isn't AC motor speed solely determined by frequency and number of poles?

A 1725 RPM motor will run on 24 volts. It just won't have any torque.

[ March 02, 2005, 07:33 PM: Message edited by: peter d ]