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Anonymous Poster #1

Voltage Controller in AC Induction Motor

02/08/2018 12:16 AM

I am a College going student and i was having this discussion with a Post graduate student. We were talking about a project on which we are working which involves Ac induction motor 3 phase and variable inputs given to it. For example we have taken a 7.5 Kw Ac induction motor 3 phase and its input value is 240 Vac @ 25 A. the input supply that we are providing is 240V @ 25 A and the motor runs at 1440 rpm giving a torque of 233.33 Nm ( considering 500 W heat loss, just a assumption). Now if we change the source of input and the voltage is 450 V ac and current is 20 A then the rpm and torque of the motor would change. I asked him how can we bring this 450V ac @ 20 A back to the 240V ac @ 25 A. He was having difficulty answering this . SO thats why i am posting this here.

What are the various ways to control the voltage and current to bring it to out desired level? How can i change the 450V @ 20 A to 240V @ 25 A to drive my Ac induction motor at fixed and specified rpm and torque?

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Anonymous Poster #1
#1

Re: Voltage controller In Ac induction motor.

02/08/2018 12:56 AM

I told him to use transformer but he replied its not possible with it . Why?

( sorry forgot to edit the original post)

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#22
In reply to #1

Re: Voltage controller In Ac induction motor.

02/08/2018 2:23 PM

You should find another partner for your project...

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#2

Re: Voltage controller In Ac induction motor.

02/08/2018 12:58 AM

Assuming the motor is not overloaded (and assuming also that it is not a wound rotor motor), then the RPM of the motor is governed by the number of poles and the frequency of the supply. The torque is an inherent design feature dependent on the level of rotor current, rotor interactive flux and power factor.

Increasing the voltage from a rated 240v to 450v (without altering stator coil relationships if that were possible with the motor in question) would create catastrophic failure of the motor and lots of smoke. Your estimated 500W heat would be far exceeded for the very short time that it took the motor to fail.

A step down transformer would be the simplest way to reduce the voltage as desired, it would also increase the available current if the transformer is correctly chosen.

What field is your friend doing their postgraduate degree in??

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Anonymous Poster #1
#5
In reply to #2

Re: Voltage controller In Ac induction motor.

02/08/2018 1:18 AM

I told him to use transformer but he denied and said it does not happen that way. He is doing post graduation in electrical engineering.

So if the source input is changed to 450V @ 20 A and i need it to change it to 240V @ 25 A i will need to use a transformers. I am not going to give the 450V @ 20A to the the motor , that will as you suggest, will damage the motor. I also asked about Variable frequency drive to control the rpm and torque.

Do you think i need both transformer and VFD to control and bring the 450V @ 20 A to 240V @ 25 A ?

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#31
In reply to #5

Re: Voltage controller In Ac induction motor.

02/09/2018 7:58 AM

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#34
In reply to #31

Re: Voltage controller In Ac induction motor.

02/09/2018 3:17 PM

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#29
In reply to #2

Re: Voltage controller In Ac induction motor.

02/09/2018 7:00 AM

The insulators for a 240 V device, be it a motor, transformer, variable transformer or controller are not made for 450 V. The high voltage will cause voltage leak or arcing and not only damage the motor, but could cause an overcurrent situation for the power source, which can cause fire and many other undesirable outcomes. Smoke being one small problem.

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#3

Re: Voltage controller In Ac induction motor.

02/08/2018 1:13 AM

We would have to know the specifications of the motor...

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Anonymous Poster #1
#6
In reply to #3

Re: Voltage controller In Ac induction motor.

02/08/2018 1:21 AM

Here are the specification or reference i have used. Kindly look for 7.5 Kw motor in this. i have a pdf but i cant post it here. or a link but i will get back to you .

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Anonymous Poster #1
#7
In reply to #3

Re: Voltage controller In Ac induction motor.

02/08/2018 1:40 AM

so the link is this , for reference I have used .

https://www.google.co.in/url?sa=t&source=web&rct=j&url=https://www.rm-electrical.com/wp-content/uploads/2014/11/MOTOR-CURRENT-RM-Technical.pdf&ved=2ahUKEwiz6b2e2pXZAhWMNY8KHaRhBiYQFjACegQIEhAB&usg=AOvVaw1nwygIkh1pPssU3-BMfP3i

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#4

Re: Voltage controller In Ac induction motor.

02/08/2018 1:13 AM

Transformers can do the job if you need only 2 voltage levels and same frequency, though you will need contactors as well.

If you are doing research I believe you can get better results with a VFD where you can tweak voltage and frequency.

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Anonymous Poster #1
#8
In reply to #4

Re: Voltage controller In Ac induction motor.

02/08/2018 1:43 AM

Frequency of 450 V is double the 240 V ( in current terms not in rpm terms ) I need to control the Frequency and bring it back to a desired level so as the motor can run constantly @ 1440 rpm . Do you have any solution to this ?

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#15
In reply to #8

Re: Voltage controller In Ac induction motor.

02/08/2018 3:28 AM

Yep. Look at the technical specifications of the VFD to see whether or not it can do what is wanted. If it cannot, then the wrong piece of equipment has been purchased.

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#25
In reply to #8

Re: Voltage controller In Ac induction motor.

02/08/2018 11:17 PM

I get the impression that you do not understand the meanings of of terms such as Frequency, Voltage, current, and torque, nor do you understand what devices are capable of altering the values of which quantities, nor how the motor load affects most of those values.

A transformer can change voltage, and when the voltage changes, so does the current. A transformer cannot change frequency.

For a given AC induction motor, the speed of rotation is determined almost entirely by the frequency of the supply voltage (with some reduction depending on the load). Changing the voltage will have only a very slight effect on the speed of the motor, as long as the voltage is high enough to provide the required torque. It should be obvious that too much voltage will cause too much current, and therefore too much heat. 450 V on a 240 V motor is too much, regardless of the frequency.

A VFD (Variable Frequency Drive) can be designed to put out a voltage different than its input voltage, but its principal purpose is to provide a constant voltage at variable frequencies, and thus control the speed of rotation of the controlled motor.

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#30
In reply to #8

Re: Voltage controller In Ac induction motor.

02/09/2018 7:06 AM

What do you mean when you post (in current terms not in rpm terms? Do you understand the relationship between input voltage, current and frequency when discussing AC induction motors? If the frequency of the 450 VAC supply is double the 240 VAC supply, then the speed of the motor will be double. RPM is how we discuss speed. Current has nothing to do with the speed of the motor, except when the supply can't provide enough and the load overloads the motor and it turns slower. If you want to control the frequency, there are other devices, but I don't know if this is what you really want. I think what you want is to have a source which can provide enough current to maintain speed, however if your load is too large and the motor is drawing too much current, then you're asking for an overheating problem with burned insulation. You simply need to control the load, so you don't run into a situation where too much current is being drawn.

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#9

Re: Voltage controller In Ac induction motor.

02/08/2018 2:35 AM

Well now you just added the part about the frequency at 450V being higher. You never said that before...

He is correct, a transformer cannot change the frequency, only the voltage. The motor is DESIGNED for a specific voltage and frequency. The relationship, the V/Hz ratio, must remain constant for the motor to produce rated torque. Torque drops at the square of the ratio difference.

So assuming the motor is 240V 50 Hz, that is a V/Hz ratio of 4.8V/Hz. If you apply 450V at 100Hz, the ration will be 4.5V/Hz. That’s a little low, about 7%, so torque would drop to 93% squared or 86% of rated. If the load only required 86% of the motor’s available torque or less, you may never notice the difference. If not, the motor will slow down, pull more current and overload.

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#10
In reply to #9

Re: Voltage controller In Ac induction motor.

02/08/2018 2:46 AM

Well can not I use vfd and transformer both ? Transformer to control voltage and current and vfd to control the Frequency ?

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#11
In reply to #10

Re: Voltage controller In Ac induction motor.

02/08/2018 3:00 AM

Why do you want to control the frequency? In both cases, you say 1440 rpm, so the frequency must be ~50 Hz. (Motor a bit slower on account of slip.)

The current is controlled principally by the load on the motor, not by either the transformer or VFD.

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#13
In reply to #11

Re: Voltage controller In Ac induction motor.

02/08/2018 3:09 AM

I am stating that the source input frequency is 450 V @20 A 100hz . I am controlling this and supplying the 250 V @25 A 50 Hz to the motor. How to control this , that's what I am asking ? How to get from A to B.

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#16
In reply to #13

Re: Voltage controller In Ac induction motor.

02/08/2018 5:11 AM

Go back 3 squares, and tell your graduate friend to go back 5 squares.

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#17
In reply to #16

Re: Voltage controller In Ac induction motor.

02/08/2018 5:31 AM

Um what ?

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#18
In reply to #17

Re: Voltage controller In Ac induction motor.

02/08/2018 5:59 AM

https://en.wikipedia.org/wiki/Snakes_and_Ladders

Hence: "Back to square 1" - Anonymous Poster #0

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#42
In reply to #13

Re: Voltage controller In Ac induction motor.

02/13/2018 4:10 AM

I'm guessing you could replace the motor with one rated for a higher voltage/torque and lower RPM. The higher input frequency input may bring the RPM back up to spec.

You don't need a VFD since the desired output is a constant frequency. In any case, most solid state generators wouldn't care about a higher input frequency. They just rectify and synthesize. The harder part is finding one that can handle your supply voltage since the output and input voltages are so different.

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#12
In reply to #10

Re: Voltage controller In Ac induction motor.

02/08/2018 3:05 AM

There is actually no point. Torque, and therefore current, is something that the mechanical load presents to the motor, and not the other way round. The load will have a characteristic curve between the speed and the torque. All one is doing with a VFD is moving up and down the load's operating line.

The voltage applied is actually of little relevance until the motor approaches its maximum current: increasing the voltage at that point puts the motor into overload, effectively turning it into a convector heater at the least, and sending it into conflagration at the most. Irrespective of the voltage and speed, it is always important to set the overload protection arrangements correctly.

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#14

Re: Voltage controller In Ac induction motor.

02/08/2018 3:27 AM

A VFD "chops up" whatever power is coming into it at its input terminals to provide the voltage and frequency demanded by the motor (provided it is set correctly, of course) connected to its output terminals. So a transformer is not needed unless there is some serious mismatch between the voltage range on the input of the VFD and the incoming supply. And if there is, then the wrong piece of equipment has been purchased.

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#19

Re: Voltage Controller in AC Induction Motor

02/08/2018 10:20 AM

Is this all hypothetical? Because 100Hz is not a frequency that anyone supplies. The problem with your problem is frequency here. The only way to change frequency is with a VFD or maybe a motor/generator set, but that’s all theoretical, because 100Hz as a source is non-existent.

By the way, you can drop the “25A” until you begin talking about load. You don’t “supply” Amps to a circuit, a circuit DRAWS Amps based on the connected load and work performed.

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#26
In reply to #19

Re: Voltage Controller in AC Induction Motor

02/08/2018 11:43 PM

no this is not hypothetical. we have a source which is actually providing 1823.62 V at 120 hz and we have another source which is providing 471.53 V at 60 hz. now i have to maintain these two values and hence i asked the question, of course taking assumption of 50 hz and not 60 hz as is in actual.

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#32
In reply to #26

Re: Voltage Controller in AC Induction Motor

02/09/2018 9:41 AM

Supplies measured to 10mV? How bizarre.

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#35
In reply to #32

Re: Voltage Controller in AC Induction Motor

02/09/2018 3:19 PM

Awww, come on. This is a precision lab they're working in. You know, the Freshman power labs are set up with equipment equivalent to the best at JPL!

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#33
In reply to #26

Re: Voltage Controller in AC Induction Motor

02/09/2018 11:22 AM

The only thing that can give you 1823.62 V at 120Hz is already a Variable Frequency Drive fed by some sort of oddball transformer.

Actually, considering that your numbers and conditions jump around from post to post and don't make sense from one to the other, I'm calling troll here.

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#36
In reply to #33

Re: Voltage Controller in AC Induction Motor

02/09/2018 3:24 PM

Buddy, he's not a troll. He's a freshman who knows better than his Grad Student TA.

And you're wrong. He's running 1823.62 VAC at 100 Hz. I checked with NASA and it's a new high tech power source, accurate to .01 VAC. And it's not a VFD, it's a new source of power that came out of some place in New Mexico. The green clouds over the lab were a dead giveaway. Freshman like this are working on the next Manhattan Project. They need precision equipment like this!

FYI, 1823.61 VAC is not acceptable for this Freshman Power Engineering class test.

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#40
In reply to #33

Re: Voltage Controller in AC Induction Motor

02/10/2018 12:02 AM

Well if I or people can solve the posted data here and can tell how , which they have , then I can learn the same methodology and apply it in my problem or question in demand . If I go all data out , it will be not appropriate , because then it would become a lot of mathematics I just wanted some opinions that i get. Not the whole solution. So thank you .

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#43
In reply to #26

Re: Voltage Controller in AC Induction Motor

02/13/2018 5:11 AM

<...have to maintain these two values...> Nonsense. The wiring between the supply and the current flowing in it will have a significant influence on those figures. Nowhere on this planet is a power supply maintained to +/-10mV.

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#20

Re: Voltage Controller in AC Induction Motor

02/08/2018 10:35 AM
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#21

Re: Voltage Controller in AC Induction Motor

02/08/2018 1:49 PM

This sounds like the idea is to experimentally alter the input voltage from 240V three phase to 450V three phase. If this is the case you need either a three phase transformer or a three phase variac (and possibly additional transformer to help step up the voltage). A variac will give you continuous fine adjustment over the voltage where as a transformer is a fixed voltage (which may be why your colleague says it won't work).

Note this will only alter the voltage, not the frequency. Don't get confused with frequency as this really only relates to the variable speed drive output voltage switching frequency (which is a chopped DC waveform made to look like an ac waveform), NOT the input frequency of the voltage feeding the motor by itself or the input to a variable speed drive, which is almost always either 50Hz or 60Hz the main grid line frequency.

https://en.wikipedia.org/wiki/Variable-frequency_drive

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#23
In reply to #21

Re: Voltage Controller in AC Induction Motor

02/08/2018 6:45 PM

For the sake of clarity here, a Variac is a transformer.

Variac is a trade name for a type of adjustable auto transformer where part of the primary also forms the secondary. The pickup for the secondary is similar to that of a potentiometer and wipes across the primary winding to select the required output voltage.

The output of a VFD is not a chopped DC waveform but is an AC waveform going both +ve and -ve alternately. Although not a true sine waveform, it is sufficiently so for most motor applications.

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#24
In reply to #23

Re: Voltage Controller in AC Induction Motor

02/08/2018 9:28 PM

Oops, my clarity mistake.

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#27
In reply to #21

Re: Voltage Controller in AC Induction Motor

02/09/2018 6:28 AM

GA for calling out this "experiment" as a hypothetical what if?

Note that there other variable voltage transformers other than a Variac.

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#28

Re: Voltage Controller in AC Induction Motor

02/09/2018 6:56 AM

It sounds like you have a 240 VAC three phase motor that has a max current of 25A. It's an AC induction motor with 7.5kW rating.

It sound like your question is how to control the voltage and current. And to drop the voltage from 450 to 240? But the underlying question seems to be, your TA (grad student) has told you it can't be done and you don't know why.

I'll give you a short lesson in life. Your TA may not always be right, but he does know more about electrical engineering than you. If you want to succeed in life, you must know who controls your happiness. In this case, your TA has control of your lab grade, thus you don't want to piss him off, right? In the real world, you'll have a senior engineer, project lead, program manager, director, VP, Executive VP, COO, CFO, CEO, Board of Directors and Chairman of the Board to answer to. Any of them can crush you in a heartbeat and as a junior engineer, you will have nothing to say about it, except "I'm sorry, it won't happen again" When you've earned the respect of others, via your problem solving skills, ingenuity and/or you've helped your company cut costs or increase profits, then you can start questioning authority, though I'd still be careful getting the C-Suite and Board upset with you.

So, back to your questions. The way you control voltage is via a variable transformer - you are controlling the source voltage. The way you control current, really isn't controlling current, but it's increasing or decreasing the loan on the motor. The higher loan, the higher current. The lower load, the lower current. Voltage is the potential, where current is the movement of electrons - they're doing the work. Frequency is analogous to the speed of the source current. Higher frequency = higher motor speed and lower frequency = slower motor speed. Torque is the twisting force of the motor shaft. Think of torque as the ability to speed up or slow down the motor. If you want your motor to run at a fixed speed with a fixed torque, you'll need to consider the load - is it fixed or variable. If it fixed, your answer relies on the correct frequency for speed and having a power source large enough to provide enough current to make the motor produce enough torque. If the load is variable, you'll need a large enough power source to keep the torque consistent, which means the current will vary based on the load. You do not control the current, it is drawn by the motor based on the load! Remember this and you'll understand how induction motors work.

By the way, have you taken a course in electronic fields? If so, induction motors are a piece of cake. You should be able to understand how the electricity moves through the coils to create a field which rises, collapses and rises in the opposite direction (AC) and how the changing field is used to induce turning forces in the motor. IF you get it, then you'll understand how higher frequencies cause the fields to rise and collapse faster and how more current is needed ot "push" the motor to turn, or to do work.

That's it. I hope this helps.

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#37

Re: Voltage Controller in AC Induction Motor

02/09/2018 4:50 PM

When you and your friend understand what this is telling you, then you will be prepared to answer your questions. Until then, taking an AC Machines course is your best option, especially since your questions and comments indicate that he hasn't.

Copied from NEMA MG-1. Hint: using the nameplate data as you are doing is not the same as using operational data from the speed-torque curves of both the motor and the load that it is driving.

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#38
In reply to #37

Re: Voltage Controller in AC Induction Motor

02/09/2018 5:14 PM

You make a good point. From a real life perspective, the OP needs to look at the much more than the nameplate data.

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#39

Re: Voltage Controller in AC Induction Motor

02/09/2018 5:24 PM

If you took this project is because there is a response (morceau de gateau):

Bring more leads outside for connections (period).

You have a 4-poles unit with a high sleeve.

Regards.

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#41

Re: Voltage Controller in AC Induction Motor

02/12/2018 3:10 PM

In my opinion..... You're WAYYYYYYY overthinking this. If you need to control a certain motor at a certain speed then purchase a VFD that works on the supplied voltage / HZ and supplies the motor with the correct output voltage / HZ. I've been doing Residential, commercial and industrial electrical for over 40 years and have never run into 18xx.xxVAC ANYWHERE!!!! Especially when it is metered and displayed in hundredths of a volt. someone touching the insulation of the wires you're measuring will change that reading....... I've seen it on 120 VAC circuits in residential electrical and here in Las Vegas, I've seen the electrical voltage supplied from NV Energy vary from 108 VAC to as high as 131 VAC though typically I see 117 - 118 VAC as a supply and when someone turns on a lamp while I'm checking the main leads I will get a dip on my meter briefly.

Good luck with your degree... You're going to need it!

I'm 53 years old and started pulling wire and adding devices in boxes with my father at approx. 10 years old.

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Re: Voltage Controller in AC Induction Motor

02/13/2018 8:22 AM

Get a VFD. We use them here to simulate foreign 380V sources from our in plant 480V and also to use 480VAC motors on projects for Canada where they have 600VAC. The output voltage is programmable on most good quality VFD's. You can also vary the frequency, but that is not your goal.

(you are not yet out in industry and maybe acronym deficient so I add VFD = variable frequency drive)

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