Induction Motor Fundamentals ?? Can any one clear...

Induction motor:
Most of the 3 phase Induction motors found today have star connected
stator. Also most of them are Squirrel Cage induction motors.
Can any one tel me the reason for having Star connected stator then
delta connected. What are the dis-advantages and advantages of it.
Reply to
mayur713
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Do you mean a star connected stator switching over to delta soon after the motor starts? This is to reduce the initial stationary rotor current (switch-on surge), by not applying full stator voltage until the rotor has picked up some speed.
Reply to
Andrew Gabriel
Actually i want to know why are major motors star connected on the secondary side and not delta connected. ......
Andrew Gabriel wrote:
Reply to
mayur713
On 10/22/06 12:48 AM, in article snipped-for-privacy@f16g2000cwb.googlegroups.com, " snipped-for-privacy@gmail.com" wrote:
I am not an expert on this but the star connection ordinrily prevents third harmonic current from flowing in the lines.
Bill -- Fermez le Bush
Reply to
Salmon Egg
Delta connected aren't all that unusual. Many motor designs allow reconfiguration for each connection, depending on desired operating voltage. Some machines that use a special type of controller will start wye connected and then the controller will shift to delta connection (break wye connection before connecting delta). Quite often on large motors, all the leads are brought out from the motor so that they can be configured in a number of ways. This can give multiple speeds as well as multiple operating voltages.
Generators are often connected wye to as to eliminate/block third harmonics. Wye connection also increases terminal voltage for a given mass of 'iron' in the stator.
As to the squirrel cage, there *are* motors that have individually wound rotors. But they are more expensive and only used when the application warrants it. The squirrel cage design is cheapest.
daestrom
Reply to
daestrom
Usually it's the smaller motors that are connected star. This is because they are already using small wire with lots of turns. To wire it delta would mean even smaller wire with more turns, raising manufacturing cost. As the motor size increases, the wire size increases (to support the larger load current) and the number of turns per coil drops (because the loop area increases). At larger motor sizes, the connection tends to switch to delta as the most economical connection. What is "large" depends on the voltage. Figure 100+ HP for a dual voltage 480/240 motor. Figure 2000+ HP for a 4160 motor.
As another poster noted, often star start delta run is used as a reduced voltage start method.
Reply to
Matthew Beasley
"To wire it delta would mean even smaller wire with more turns, raising manufacturing cost"
Sir, Could you explain the above line......
Matthew Beasley wrote:
Reply to
mayur713
Sir, as written below that larger size motor are more delta connected. I fail to understand this due to the following reasons.
1) the 6.6KV motor of 180KW used to run pumps is star connected. 2) The 6.6KV motor of 1.6 MW used in condensate extraction pump is star connected. 3) The 6.6KV motor of 2 MW used to run Boiler Feedback pump is star connetced. 4) The 6.6KV motor of 1.8 MW used in CW pipe line is also star connected.
One reason for having star connection might be protection of motor (Please correct me if i am wrong). in case of single earth fault in the motor connected in delta, the fault will not be detected untill the there is another fault since neutral is absent.
Matthew Beasley wrote:
Reply to
mayur713
Motor laminations have a maximum flux density before they will saturate and large losses will occur in the iron laminations. The Peak flux density B for sine wave excitation is:
E B=------------ 4.44 ANf
Where E equals the RMS excitation, A equals pole area, and N equals the number of turns and f equals the frequency.
For a delta connection, E will be 1.73 times greater. So for the same area of the pole, delta connection requires 1.73 times as many turns with wire .577 times as big.
For small motors, it's more economical to wind fewer turns of larger wire.
Reply to
Matthew Beasley
You missed this line: "What is "large" depends on the voltage. Figure 100+ HP for a dual voltage 480/240 motor. Figure 2000+ HP for a 4160 motor."
For 6.6kV, I would expect to start seeing delta connected motors somewhere around 3 or 4 MW.
Earth fault protection depends on the source. If the source isn't grounded, it would take two faults to trip on ground fault. One fault would be detectable by voltage balance measurement.
Far superior would be to impedance ground the system to provide a detectable but limited ground fault to a manageable amount. (Usually 100's of amps).
A delta connection is actually superior on the account of ground fault detectability. Imagine what would happen on a wye connected motor where there is a fault near the wye point. The voltage isn't high, so fault current may not rise high enough to trip protection. On a delta motor, the voltage on a winding is always above 57% of line to ground voltage for a grounded source.
Reply to
Matthew Beasley
| "To wire it delta | would mean even smaller wire with more turns, raising manufacturing | cost" | | Sir, Could you explain the above line......
For small motors, the cost increases as the smaller finer wire that is more fragile needs to be wound many more times. The lower voltage means fewer winds and larger wire instead. This is good for smaller motors as it lowers the manufacturing costs.
Reply to
phil-news-nospam
A ground in delta connected is not hard to 'relay' for. Run all three line leads through a CT and connect the CT secondary to a protective relay of choice. Unbalanced currents in the motor leads will create a current in the protective relay. Seen it done lots, no neutral required.
A lot of 'large' motors have the connections external. This allows operation at either voltage (XX or XX/sqrt(3) ). It also provides some flexibility in meter/relaying, some CT's on the lines and some at the 'star' point. Also allows connection of 87 device (differential relay). On some fairly large units (~4 MW), depending on how they are started, you may even see 87 with harmonic restraint or starting time-delay. The transient DC offset of starting such a unit 'across the line' can be enough to trip the more conventional 3-phase CT with a 50G device.
But I've seen 'large' motors with external connected delta. Six CT's, two on each phase, one each side of winding, connected to three 87 relays.
Keep looking and you're likely to find just about every combination imaginable.
daestrom
Reply to
daestrom
Sorry for bothering you again.
Is there any effect on the torque speed characteristic of the motor's when connected in star and when connected in delta during starting and normal running.
Matthew Beasley wrote:
Reply to
mayur713
Sorry for bothering you again.
Is there any effect on the torque speed characteristic of the motor's when connected in star and when connected in delta during starting and normal running.
Matthew Beasley wrote:
Reply to
mayur713
Sorry for bothering you again.
Is there any effect on the torque speed characteristic of the motor's when connected in star and when connected in delta during starting and normal running.
Matthew Beasley wrote:
Reply to
mayur713
As long as it's connected to rated voltage, there isn't as far as I know.
The one advantage to delta is that it can be started star for reduced inrush (but with reduced torque) and switched to delta as the speed rises.
Reply to
Matthew Beasley

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