fan speed and Voltage 4

[PaulLag]

Hi there,
Second doubt.
I have learned that the rotational speed depends by the number of poles and frequency only.
Why then a fan connected in delta and star connection has a different speed, even though it is the same fan and fed at the same frequency ?
I admit it is a basic question.
Could anybody suggest me a link or a book to buy in order to clarify these doubts ?
thanks !

 

[waross]

Less Volts per turn and less torque when running on the star connection. The speed slips further down the slip curve.
Some fan motors are fairly high slip motors and there will be more of a difference with the start connection than with a standard motor.
Many loads cannot be speed reduced this way without damaging the motor, but the torque fall-off with fans and centrifugal pumps is very forgiving of low voltages.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MikeHalloran]

https://www.google.com/search?q=audel+electric+motors+pdf&ie=utf-8&oe=utf-8

Mike Halloran
Pembroke Pines, FL, USA

 

[Skogsgurra]

Voltage Control is very common for FHP motors with fan loads. A simple Triac is used for speed Control.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[zlatkodo]

I have learned that the rotational speed depends by the number of poles and frequency only.

... and slip ( in case of fan).

Speed regulation of single-phase fans.

 

[rhatcher]

There are certain types of motors that are capable of having two speeds by switching the number of poles when the connection is changed from delta to star. Search 'Dahlander' motor for information about the most common type of two speed, one winding motor. A 'PAM' (pole amplitude modulation) motor is a second, less common, type.

 

[PaulLag]

Hi

many thanks for this link.
This has been absolutely useful.

I have some more question, hoping not to steal time.

1) I understand that there is a square relationship between tension variation and torque variation.
Please, which is the formula that relates this two parameters ?

2)
please, would you be so kind to recommend me some literature concerning the topic of tension, connection as the previous part
Unfortunately I can't find any reference in cowern papers


3) please, I would like to understand following topic.

Let's assume I have a fan labeled @ 400/3/50.

let's suppose I vary the tension.
I understand the current will increase - if the tension decreases - or decrease, if the tension increases.
In one case there is the problem of excess of current, in the other to saturated the magnet, if I have understood correctly

if I vary the frequency, what will happen ?

Why are some fans labeled not only 400/3/50 but also 400/3/60 and 460/3/60 ?

Thanks !

 

[electricpete]

1) I understand that there is a square relationship between tension variation and torque variation.
Please, which is the formula that relates this two parameters ?

T(V) = T(V0) *(V/V0)^2
where w is speed and V is voltage and 0 is the reference condition, usually rated condition.

To predict change in operating point, you'd also need to predict intersection with the fan curve which is often T(w) = T(w0)*(w/w0)^2 excluding the very low speed portion of the curve. Typically you'd do that graphically.

But I'm making an ASSUMPTION that voltage is the only thing changing, not the number of poles which rhatcher has pointed out. It's worth to doublecheck, did you check the nameplate? Or perhaps did you measure the speeds in the two configurations?


=====================================
(2B)+(2B)' ?

 

[zlatkodo]

let's suppose I vary the tension.
I understand the current will increase - if the tension decreases - or decrease, if the tension increases.

I can't agree. The current will decrease - if the voltage decreases, in case of fan application.

You don't have to change the number of poles or frequency in order to change the rotational speed of fan. You can change the slip only as I already wrote.
It is not allowed to exceed the rated voltage.
If you change the frequency (or number of poles) you will change the synchronous speed and consequently the rotational speed of the fan.
If you change the slip ( changing the voltage) the synchronous speed remains the same, but the rotational speed of the fan changes.
Read again a nice explanation from waross (19 Feb).

 

[LionelHutz]

zlatkodo - Sorry, but I can't agree with your posts. Posting about varying the speed via voltage on special high slip capable single phase fan motor has no relevance when talking about a standard 3-phase induction motor. The OP mentioned wye vs delta in the first post which makes it clear that this isn't a single phase application, hence it won't be a special high slip capable single phase fan motor.

Lets just look at an example using more realistic typical 3-phase motor data. Start with this data.
At 100% speed the fan requires rated motor torque.
At 50% speed and rated voltage, the motor produces 100% torque and draws 500% current.

So, you want to run this fan at 50% speed. The required fan torque = 100% motor rated torque * (0.5/1.0)^2 = 25% motor rated torque

Now, at 50% speed, the motor started at 100% rated torque with rated voltage, and we need to reduce this torque to 25%. Well, the motor torque is the ratio of the current reduction squared. So, we need to reduce the current to 50%, since 100% torque x 0.5^2 = 25% toque. The problem is that the motor started out drawing 500% current, so reducing the current by 50% only drops the current to 250%. That's a big problem. The motor will quickly overheat and fail if you attempt to operate it at these conditions. You simply can't vary the speed of a standard induction motor via a voltage reduction alone.

 

[PaulLag]

Hello everybody

first of all many thanks for your kind answers.
In a second hand, I just got back to one of my first questions, that were: could you recommend me a suitable literature or web reference where these topics are presented ?
I mean, there are lots of books about electrical motors, nonetheless I was not able to find one about these specific topics.

Is audel the right one ?
are there others ?

Many thanks !

 

[zlatkodo]

Hi, PaulLag,

I can not recommend a particular book that will answer your specific question, because this is part of the basic theory of electrical machines.
Try to find something about the power, torque and slip depending on various types of loads (constant torque vs constant power and specially variable torque application).

LionelHutz:

Everything I wrote in the last post refers to the three-phase fan motors. And not just to fan motors.
The same principle is widely used as a three-speed, three-phase winding of circulating heating pumps.
Note, this is the same kind of load.
This is achieved by adding the turns ( less or more) in each phase winding for middle and low speed, which is equivalent to voltage reducing.
Otherwise, how could you explain this case of three-speed motor ( see the nameplate photo below)?
Obviously, it is 2 pole, 50 Hz, three-phase winding for each of three speeds.
The number of poles and frequency is the same for each speed, but the slip is not.
As you can see Amps decreases with adding the turns ( ie. reducing the volts per turn).

More at:
Overhaul-of-heating-pumps

 

[LionelHutz]

Otherwise, how could you explain this case of three-speed motor

Very simply, if the motor speed can be significantly varied by varying the voltage then it's a specially designed motor, PERIOD!

Voltage reduction for speed control DOES NOT work on standard SQIM's. Claiming that since it works with a specially built motor that it'll work for any motor is idiotic. Claiming that the motor can be speed controlled via voltage just because the OP posted that it's a fan application is also idiotic.

Going back to the first post, I expect the delta and star connection mentioned ACTUALLY changes the poles of the motor because it's a 2-speed motor. BOTH a star-delta motor starter and a Dahlander motor use 3 contactors. If you didn't know what to look for then you wouldn't know the difference.

 

[zlatkodo]

LionelHutz
There is no need for harsh words.
You better try to learn something that is obviously new for you . I would be glad if I inspired you to do that.
Or , you think you already know everything ?

The first question ( and my answer too) is related to three phase FAN motor.

Why then a fan connected in delta and star connection has a different speed, even though it is the same fan and fed at the same frequency ?

An answer is so simple: because of change in slip caused by change of voltage, PERIOD.

I expect the delta and star connection mentioned ACTUALLY changes the poles of the motor because it's a 2-speed motor.

There is no need for two speed Dahlander motor for mentioned application. This is a winding with one number of poles, designed for delta connection at rated voltage.
You do not have to agree with me, but I expect arguments, not any aggressive attitude.

Tech Support in Winding Design and Rewind

 

[Skogsgurra]

Zlatkodo
The speed for an asynchronous motor is given by number of pole pairs and frequency. There is also a small load dependancy so that more load creates more slip.

For a normal motor, the slip is small, simply because slip represents losses and you do not want losses in a normal system. That is why high-efficiency motors are used. They have very low slip.

For certain low power applications and where load torque is speed dependent (torque proportional to speed squared) it is common to use simple voltage control of the motors. Kitchen fans and such appplications come to mind. They are usually FHP motors and the losses at half speed, where torque is around 25% of nominal torque, is tolerable if compared to the losses in a VFD.

These truths were valid for many years, but it will probably change when (constant) losses in the auxiliary Control system gets lower or when matrix inverters are being introduced also in the lower power ranges. But, for the time being, these truths are still valid.

To sum up:
Normal ASIM's speed is almost purely frequency dependent and can not be speed controlled with voltage change. That is why VFD:s are used.
Small and special built ASIM:s (high rotor resistance, high and load dependent slip) *can* be speed controlled via voltage (and load) change.

I can very well understand that your opponents get angry when you deny these well-known facts. It is you that need to learn basics before you try to educate others.



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[zlatkodo]

I do not want to repeat the same thing over and over again.
I think my explanation is sufficient and I do not care whether someone's vanity is affected or not. It is not my problem if someone rejects something he don't know anything about.
My explanation is addressed to answer the first question:

Why then a fan connected in delta and star connection has a different speed, even though it is the same fan and fed at the same frequency ?

which confirms that the fan has different speeds at different voltages ie. at Y and D connection . Quite enough.
Quote any of my statements to be inaccurate.
The photo below shows an external wiring from the terminal box of similar fan motor: high speed in Delta and low speed in Wye ( not Dahlander, YY-D connection).

The only thing more dangerous than ignorance is arrogance. (Albert Einstein)

 

[LionelHutz]

zlatkodo - You are the one that has no clue what you are posting about. Your posts are misleading. PERIOD.

Gunner is EXTREMELY knowledgeable about motors. I've been working with induction motors for a long time too. For me it's not vanity, but rather annoyance at seeing such poor information being posted as fact and leading the person asking the original questions completely astray. Your explanations only work on very small motors specifically designed to allow high slip operation. Standard 3-phase induction motors WILL NOT operate that way.

My example was based on REAL motor data for a 75hp motor that I was looking at yesterday. This REAL motor would ABSOLUTELY require 250% current to run a fan at 50% speed.

I gave rhatcher the star because what appears to be a star-delta motor starter is most likely controlling a Dahlander motor. It's the most plausible answer based on the question.

PaulLag - There is a simple reason that you don't find information on reducing voltage to reduce speed when researching industrial standard induction motors - It can't be done.

 

[zlatkodo]

LionelHutz

I do not want to be so rude as you are and I'm glad to inspire you to search little bit more on this topic:

Your explanations only work on very small motors specifically designed to allow high slip operation.

You begin to realize what it is about. It is OK for beginning. It is a big advance for you. Just keep continue.
It is a proof that my post is not useless.
By the way, I am talking about three phase fan that is connected in Delta connection for high speed and Wye connection for low speed, because, that was an initial question.
Also, looking at the picture above ( example of external wiring for this type of connection) , it is clear to everyone (even to beginner), it can not be Dahlander winding. You are wrong again and your advice is misleading.

Tech Support in Motor Rewind

I am patient with lack of knowledge but not with those who are proud of it. (TZ)

 

[Skogsgurra]

I am saving this thread for two puposes:
1. It shows how someone, who thinks that he knows a subject without understanding it, sometimes acts.
2. To remind myself not to behave like this if I ever find myself in a similar situation.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

zlatkodo - You keep claiming we know nothing and you understand all, so prove yourself right. Post detailed data on a typical standard (not Dahlander) 100hp or 75kW SCIM that can be speed controlled via voltage reduction. Post the nameplate data, speed vs torque and speed vs current curves and then explain using the data how the speed can be successfully reduced to say 80% and 60% speed by simply reducing the applied voltage. Use a typical squared type variable torque fan load that applies rated load to the motor shaft at rated motor speed. If you actually know what you are talking about and what you are claiming is true then this is an easy task for you.

PaulLag - Why don't you post details of your motor so everyone knows what motor you are actually dealing with?