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Home > Archive > Electrical Engineering > October 2006 > Induction Motor Fundamentals ?? Can any one clear...
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Induction Motor Fundamentals ?? Can any one clear...
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| mayur713@gmail.com 2006-10-22, 3:25 am |
| 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.
| |
| Andrew Gabriel 2006-10-22, 9:25 am |
| In article <1161503290.152565.292350@f16g2000cwb.googlegroups.com>,
mayur713@gmail.com writes:
> 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.
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.
--
Andrew Gabriel
| |
| mayur713@gmail.com 2006-10-22, 9:25 am |
| Actually i want to know why are major motors star connected on the
secondary side and not delta connected. ......
Andrew Gabriel wrote:
> In article <1161503290.152565.292350@f16g2000cwb.googlegroups.com>,
> mayur713@gmail.com writes:
>
> 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.
>
> --
> Andrew Gabriel
| |
| Salmon Egg 2006-10-22, 1:25 pm |
| On 10/22/06 12:48 AM, in article
1161503290.152565.292350@f16g2000cwb.googlegroups.com, "mayur713@gmail.com"
<mayur713@gmail.com> wrote:
> 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.
>
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
| |
| daestrom 2006-10-22, 5:25 pm |
|
<mayur713@gmail.com> wrote in message
news:1161503290.152565.292350@f16g2000cwb.googlegroups.com...
> 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.
>
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
| |
| Matthew Beasley 2006-10-23, 1:25 pm |
|
<mayur713@gmail.com> wrote in message
news:1161503290.152565.292350@f16g2000cwb.googlegroups.com...
> 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.
>
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.
| |
| mayur713@gmail.com 2006-10-24, 3:25 am |
| "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:
> <mayur713@gmail.com> wrote in message
> news:1161503290.152565.292350@f16g2000cwb.googlegroups.com...
>
> 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.
| |
| mayur713@gmail.com 2006-10-24, 3:25 am |
| 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:
> <mayur713@gmail.com> wrote in message
> news:1161503290.152565.292350@f16g2000cwb.googlegroups.com...
>
> 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.
| |
| Matthew Beasley 2006-10-24, 1:25 pm |
|
<mayur713@gmail.com> wrote in message
news:1161670632.959094.241760@k70g2000cwa.googlegroups.com...
> "To wire it delta
> would mean even smaller wire with more turns, raising manufacturing
> cost"
>
> Sir, Could you explain the above line......
>
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.
| |
| Matthew Beasley 2006-10-24, 1:25 pm |
|
<mayur713@gmail.com> wrote in message
news:1161672661.276136.118720@m73g2000cwd.googlegroups.com...
> 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.
>
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.
>
> 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.
>
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.
| |
| phil-news-nospam@ipal.net 2006-10-24, 1:25 pm |
| On 23 Oct 2006 23:17:13 -0700 mayur713@gmail.com wrote:
| "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.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2006-10-24-1110@ipal.net |
|------------------------------------/-------------------------------------|
| |
| daestrom 2006-10-24, 5:25 pm |
|
<mayur713@gmail.com> wrote in message
news:1161672661.276136.118720@m73g2000cwd.googlegroups.com...
> 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.
>
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
| |
| mayur713@gmail.com 2006-10-25, 3:25 am |
| 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:
> <mayur713@gmail.com> wrote in message
> news:1161672661.276136.118720@m73g2000cwd.googlegroups.com...
>
> 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.
| |
| mayur713@gmail.com 2006-10-25, 3:25 am |
| 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:
> <mayur713@gmail.com> wrote in message
> news:1161672661.276136.118720@m73g2000cwd.googlegroups.com...
>
> 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.
| |
| mayur713@gmail.com 2006-10-25, 3:25 am |
| 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:
> <mayur713@gmail.com> wrote in message
> news:1161672661.276136.118720@m73g2000cwd.googlegroups.com...
>
> 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.
| |
| Matthew Beasley 2006-10-25, 1:25 pm |
|
<mayur713@gmail.com> wrote in message
news:1161761231.669457.194170@b28g2000cwb.googlegroups.com...
> 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.
>
>
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.
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