Star-Delta Starter

SLaubach

Member
Join Date
Apr 2002
Location
PA
Posts
152
hey guys-
I know this isnt a PLC related question but could somebody help me to understand the science behind using a star delta starter to start a pump motor. this is the fisrt time in my career that i have come across this animal.
thanks Scott
 
To put it simple, you are applying a lower voltage (wye or "Y") across the motor to start it. At some point after the rotor is turning then full voltage is applied (delta) to get to or maintain full speed at load. The lower voltage is partially done to prevent high start currents. These type starters are very common on large air compressors. A wye connection offers approximately 58% less voltage than a delta.

Some sites for electrical/motors:
Electrical and Motors 1

Reduced voltage starter

Allen Bradley animation

If you need more, let me know
 
Star-delta starters are used when you want to reduce motor starting current compared to what it would be if you used a DOL starter. Typically, this would be where you're starting a high-inertia load and the motor would draw high amps for too long to be good for the health of the motor and/or the switchgear. They are also used in situations where starting large motors DOL might be OK except the distribution system lacks capacity to reliably supply the starting current.
 
You would also consider using this when you pay your electrical bill based on the highest demand within a month ...

Don't want those peaks to get to high !!!
 
As a design guide, reduced voltage starters, either autotransformer type or star/delta type, have largely been superceded by reduced voltage solid state starters (RVSS) in all but the largest motor applications. The RVSS are smaller, simpler, much more flexible, and in many cases less expensive than the older designs. They allow you to set a current profile optimized for the application. Many of the new RVSS (like the Cutler Hammer IT or the Allen Bradley SMC) have a contactor that goes across the line after the motor is at speed to minimize heating.
 
Star Delta, the first time you come across it can be confusing. I have had many calls from experienced electricians that do not know how to connect a star/delta motor up.
For that matter I do not think there is a standard method. It all depends how the wires are connected at the contactors.
Some connect at the contactor end A1 B1 C1 A2 B2 C2 and some connect A1 B1 C1 B2 C2 A2.

This is where the confusion sets in. because sometimes if you follow the connections at the contactor side in the motor, it doesn’t always work correctly.
Several things can happen.
On start up the motor starts in star but at changeover to delta the motor slows down and stops. If you use a volt meter at the motor, there are the full 3 phases present but it is not turning.
This is because the 3 windings at the motor have the same phase at each end of the winding.
This is cured by shuffling the 2’s wires down one terminal and the last wire back to the beginning.

The next thing that can happen is that the motor goes in to star and delta ok but is going the wrong way.
This is usually cured by changing two of the line phases around. But, this is not always possible and sometimes you have to change the direction at the motor terminals.
To do this you have to change 4 wires around. For instance A1 and B1 and then A2 and B2.
Not many people know how to change the direction of a star/delta motor at the motor terminals; it is a handy trick to have in your repertoire.

And the final thing that can happen is; star ok then heck of a buzzing noise in delta and usually it trips out.
This is because 2 windings have full voltage and the other one has the same phase at each end.
This is nasty and needs the 6 wires to be belled out correctly.

Then there is the worst scenario; the 6 winding wires are hanging out of the motor with no markings. (I have had this)
You cannot tell which are 1’s and which are 2’s.
You can find the three windings but you cannot tell which side they should go on..
It is down to trial and error and if you get it wrong, 2 windings are trying to go one way and the third is trying to go the other. The motor makes some great noises and if you are not quick, you let the smoke out.
 
Goody - I just wanna say "Thanks" for that post.

That is one of those explanations you can't usually pay to get.

I am difinitely going to print it out and put it in my personal "Book of Knowledge".

Steve
 
Additional on Wye-Delta Starting

A couple of corrections to the info above.

First, the name for this kind of "soft" motor starting is Wye-Delta altho Star-Delta is common.

Second, the motor must be built to do Wye-Delta starting. Many are three lead motors and cannot be started this way.

Third, there is no lower voltage involved. The full line voltage is applied to the motor, first with the motor windings arranged in a Wye configuration, then, after the motor is turning at or near nameplate speed, full line voltage is applied to the coils arranged in a delta configuration.

Since the motor coils present a higher impedance arranged in the Wye configuration, the starting inrush current is less and starting torque is also less. This is the primary benefit to doing this. The main disadvantage is the need to buy a motor wound for this service, a two stage and therefore more complex and expensive starter, and twice as many wires needed between the motor and starter.

The earlier comments about solid state reduced voltage starters are correct and are rapidly displacing other methods (there are several other "old" ways) of soft starting motors including those operating on medium voltages. And, just for the record, Benshaw (Glenshaw PA) and ABB (Wichita Falls TX) also build solid state starters.
 
I am from the UK and I assure you it is always called star delta here.
I have never heard of wye/delta (although I can see why that is so.)
Here in the uk, 3 phase voltage is 415V between phases and 240V single phase.
When a motor is in star, there are 240V across the windings, when in delta 415V across the windings. The lower voltage is due to the star point on the motor. So, although 415V 3 phase is fed to the motor in star, there is only 240V across the windings. (I would say that was a lower voltage)
Nobody mentioned 3 lead motors, we are talking about star/delta 6 lead motors.
 
As a rule of thumb I always look at the motor plate. This will be marked up with the motor voltage, current and importantly connection details.
Usually motors in the UK above 4kw are designed to be delta connected when up and running. Although if you have a word with your re-wind people they will wind your motor anyway you want. Star or Delta run.

I like to say that the motor when connected in star is two windings in series therefore less power, and when in delta the motor windings are individually connected across the supply therefore more power.
When explaining too younger sparks, if you imagine a (6 connection A1,A2 B1,B2 C1,C2) star connected motor with the links on for star. Then the motor connected with the links in delta(6 connection A1,A2 B1,B2 C1,C2). With these two in mind.....
When running a motor on star-delta all you are doing with your external contactors is moving the links outside the motor and into your panel. Hence no links required on the motor terminals. Star for running up, delta for running.
Star-delta is not always necessary, if you are lucky enough to visit a power station or an Aluminum smelting works (somewhere that generates there own electric), they just start their motors direct on line. With no current limitations you can do this. Watch those lights dim though!

However where I used to work, if you started the two main motors both at the same time it used to take the main incomer out. Not good.

I have seen rotor resistance starters, rotor regulator starters, vapourmatic starters, liquid starter starters, soft starters and VSD's all which are common where I work now.

All of which gives you a good grounding in motor starters.
 
Star-Delta

Since the original poster shows as American, I assumed...... (Oh, I know! Shucks!) Sorry. I was thinking USA when this forum is clearly bigger than that. Again, sorry!

And, when I said there was no lower voltages, I was referring to voltages external to the motor. You are correct that each individual coil sees reduced voltage. Of course, that is why (wye!) the current is less, or, as I said, the phase to phase impedance is higher.

You suppose this would be easier to explain with a diagram?!!
 
Oh no not a drawing,

There are so many for S/D starters :( not to mention the number of timers that you can use! Oh yes, sorry in this forum we use the PLC for that...! :D
 

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