Comparision of 2 Motors with Different Winding Connections

The connection does not make any difference.Connection makes a difference only if the same motor is reconnected from star to delta or delta to star.However 2 electric motors with same rating can have different characteristics both electrical and mechanical depending upon the design.Electrically Power factor, No load current, starting current (aasuming same method of starting), Efficiency at Full and part loads, starting torque, POT and so on.

No general statement, like different current, differenf p.f. different torque

be specific in comparision of values.

Try This

http://cr4.globalspec.com/thread/54100/Motor-Replacement-Torque-Problem

Cost off running motor and inrush current when starting is your main difference if your motor is that big you should have a set of star/delta contactors for starting the motor pulls less current connected as star configuration at startup and is more efficent to run in delta once motor is up to speed. You could also put a vsd on it and ramp it up to start with it wired in delta.

Comparision is to be made when both the motor are started DOL.

No VSD, Soft starter, Star/Delta...

According my opinion, if a motor is delta connected, theire is no third harmonic current in the supply.

So the amount of harmonic distrosion in the distribution grid will be lower.

Can somebody verify this .....

This is Khalid Soomro, working in oil and gas development company Pakistan. As per my observations, when motor of same name plate data, started/connected in star, its starting torque is low, while on the other hand when motor of same name plate data is connected in delta, its starting torque is very high or we can say, its starting torque is maximum, which we get from torque formula, Torque=(5252*Horsepower)/RPM.

Dear Khalid,

Refer to your formula,

Is torque function of connection (Star/Delta) ???????

It is function as said in formula ...HP and speed...not the Star or delta

same name plate rating ,capacity of motors connected either in star or delta will give different starting current and torque ,based on the application of motors with starting

methods,DOL ,STAR-DELTA OR SOFT STARTERS.Based on the design two motors will have different electrical and mechanical characteristics.Both the motors can be connected in star or delta to suite our requirement.

Star connection withstands higher voltage and delta provides higher power output from the same voltage as star. Star/delta switching was often used for starting up, though today we prefer use of frequency inverters for starting as this limits starting current to only a fraction. To get more power you should use delta connection.

I think the first 'difference' is the rpm is double on one, compared to the other.

Why would you say that? The OP says they are the same.

I read that as the rating plate is the same. Don't mean some one hasn't connected it "to suit".

Just me going for obvious checks.

But yep one could be 3 pole and the other 6, so be the same rpm.

Different torque of course, but that's covered above.

No factual comparison yet. Consider same motor once connected in star and once connected in Delta. The driven equipment is important. Consider the same -driving a centrifugal fan -driving a hydraulic vane pump.( A positive displacement pump.) That will show what are the differences and mmake Happy Singh -Happy.

Um - well on that bomb shell - I beginning to think no one above realizes that star and delta result in different rpm.

Or that a star delta starter is a way of accelerating a heavy load to half speed, then full speed.

Or in fact a star delta switch gives two speeds for some machines, like drills, mills, cold saws, ....

The speed of an asynchronous 3 phase squirrel cage motor is determined by the number of magnetic poles, according to the formula speed = frequency x 60 / number of poles pairs.

(2 poles give 3000 rpm at 50 Hz and 3600 rpm at 60 Hz).

When connecting a motor star (wye) or delta (triangle) this does not change the number of poles. So the speed will be the same.

Only during start-up when using a start / delta start equipment, it takes a longer time to reach the nominal motor speed. This is because the accelerating motor torque during the time the motor is connected in star, will be lower. The motor winding "feels" a voltage that is square root three (1.73) times lower than when connected in delta and the motor shaft torque is equal to the power of the applied voltage.

This lower speed during start-up is caused by an increased lag between the magnetic field's speed and the actual rotor speed.

Quite so, there is no benifit in a 415V star winding over a 415v delta winding with respect to the OPs question. Excepting that the 415V star winding could be re wired for 240V or 220V if all conections were brought out.

I'm still trying to get my head around people referring to star / delta starting when the original OP refers to separate motors being either star or delta for rated output. I want to see any ideas as to how this can be done.

You are still referring to star-delta switching, not running a motor in star or delta.

He asked the starter question.

But that motor is running in star or delta.

The OP said one connected in star and one in delta, not either or. Forgive me for reading the OPs question and answering as stated. Speculation as to whether the OP meant somthing other than what he asked can be misleading and confusing as this post seems to have done quite well. If the OP is not satisfied with the answers he can restate the question or add further information which he did in this case.

Lots of misinterpretation going on here, lots of semantics issues as well. Let's stick to the question(s) AS ASKED.

"What is difference in electrical and mechanical characteristic of two L.V. motor of same name plate rating (415Volts, 150 KW, 1485 rpm etc) but different winding connection. i.e. one is connected in Star and other in Delta."

"Comparision is to be made when both the motor are started DOL.

No VSD, Soft starter, Star/Delta." (sic)

The very first anonymous response is correct, there is no difference whatsoever, as stated in the question.

If the NAMEPLATE of the motor says it is 150kW 415V 1485RPM Star connected, or 150kW 415V 1485RPM Delta connected, the Star and Delta references are strictly referring to the internal wiring connections of the motor, not the external starting methods. So if starting DOL as the OP said, the internal wiring of the stator windings makes no difference in the stated issues of electrical and mechanical specifications. Those have ALREADY been accounted for in the nameplate rating being 150kW 415V 1425RPM. You cannot read anything else into that nameplate statement without making assumptions (which comes from the root Ass-U-Me).

There MIGHT be a slight difference in efficiency, i.e. absorbed electrical power vs mechanical power output and the efficiency would (should) be listed on the nameplates as well, because it's virtually impossible to get everything exactly the same in two identical motors, let alone two with different construction. But the differences, if any, will be dependent only on the magnet wire used, the steel used in core lamination, the cooling method used, the bearings used etc. etc. If it were anything that affected the torque, it would show up as a difference in slip and the RPM would be different. If the slip and torque were different, the kW output would be different.

All the issues regarding starting torque, speed (although that one is totally incorrect) and connected load are not germane to the question AS ASKED.

Now, all that said, if you (Happy singh) were not meaning what you typed, i.e. if you take the SAME MOTOR and connect it Star or Delta EXTERNALLY and run it via DOL start (without switching from Star to Delta), that is a totally different issue. In that case, the motor kW will NOT REMAIN THE SAME, so your question, AS ASKED, would not be valid.

When you take a motor designed for a specific voltage rating and with all of the leads brought out so that you are ABLE to connect it in Star or Delta, then when connected in Star the effective voltage across the windings is going to be reduced by the square root of 3, so divided by 1.732 (or 58%). Since torque is a function of the square of the applied winding voltage, the torque will be reduced to 33% (.58 x .58) of normal. But since SPEED is a function of the applied frequency and the number of poles in the windings, neither of which has changed, and kW output is a function of torque and speed, 33% torque at the same speed is going to be 33% kW. So your 150kW motor becomes, at best, a 50kW motor if connected in Star.

If then the LOAD remains the same in both cases, the lower shaft kW will result in increased slip, which will draw MORE current. If the added current fails to develop enough torque to accelerate the load or maintain the speed, you fall behind the torque/speed curve and enter a stall condition, it eventually stops, the inductance goes away and the motor experiences locked rotor conditions. Even though the Locked Rotor Amps will be reduced to 33% because of being connected in Star, that is still 33% of 600%, so 200% FLA and the motor is overloading.

So I'll now go out on a limb here. If the nature of your question was based on this common false belief that connecting a motor in Star will "save energy", you are mistaken. It only works if the LOAD is reduced to 33% of normal, in which case if you were to do this permanently, you would be better off just buying an energy efficient 50kW motor, the motor losses will be much less.

This is what I asked, Thanks J, good understanding.

But still your answer is general, no specific detail like starting current, Starting torque etc

OK sorry, I thought you could infer the rest.

If you take a motor that is intended to be connected in Delta and connect it in Star, everything is altered by the effects of the voltage being the Sq. rt of 3. So at 58% effective voltage, all torque values are reduced to 33% of normal, as is current. So compared to the same motor connected in Delta, you get 1/3 torque, both starting and running (assuming it will run at all), and because current is related to torque, 1/3 current both starting and running (nominally because some of the motor losses would remain fixed, therefore they will appear to be an increase of the total current percentage, meaning your efficiency is less).

Does that answer your question?

And in case THAT needs further explanation, lets look at your 150kW motor running a 50kW load* using typical Design B characteristics and nameplate values, started DOL:

If Delta connected:

• Starting current 600% FLA
• Running current at 50kW load = probably around 41% FLA (33% FLA at 80% efficiency)
  
• Starting torque = 160% FLT
• Breakdown torque = 220% FLT
• FLT = 100% rated
  

If Star connected:

• Starting current 33% of 600% FLA, so net 200% FLA
  
• Running current at 50kW load = probably around 39% FLA (slightly less I²R losses because current is less, so assuming 85% eff by comparison)
  
• Starting torque = 33% of 160% FLT, so net 53% FLT
  
• Breakdown torque = 33% of 220% FLT, so net 66% FLT
• FLT = net 33% of rated

*Any more than 50kW of load and the motor will stall, so all values are moot.

And now the big finale!

Relating this back to my earlier comment that if you have only 50kW in load, it's better to use a 50kW motor. This is because running a 150kW motor that is maybe 95% efficient at Full Load, but using only 33% capacity, you are going to lose efficiency no matter what. Even if you connect it in Star to reduce the I²R losses, you are still at maybe 85% efficiency because you still have all of the magnetic losses and windage, friction and stray losses associated with the 150kW motor frame. If you use a fully loaded 50kW motor with 95% eff., it will cost you less to operate it.

Thanks JR. You have explained well. I will be also Happy Singh , if you can also explain the impact for having the motor connected to a. a Centrifugal fan b. a reciprocating compressor While for Compressor we naturally need to go to Delta, but to save energy due to lower fan load requirement we some time connect in star. Say some time instead of fan exhaust required is 50000cu.m in place of say 100000 cu.m per hour. Will it not save energy in star?