Three Wire Control - Overview
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Posts: 46,084
There was an inuiry regarding three wire control when I mentioned it in a
response the other day. Please find below a brief overview.
General Topic: Motor Control Wiring
Subtopic: Systems/Methodology>Start/Stop>Remote controls
Subject: Three Wire Control
Three wire control for remote (or cover mounted) Start Stop "buttons"
encompasses many areas. Those listed below are just a sampling:
1. Design Goal for safety
a. Direct Safety Concerns
1) Personnel, product and system safety, and integrity
2) Prevent unexpected start-ups after unexpected power-down
conditions (power failure)
3) Multiple operators for supervision and operations convenience
4) Ease of implementation; thus reasonable for initial
implemention, present update and future expansion
b. Indirect Concerns
1) OSHA / NIOSHA
2) N.E.C. / U.L. / Regulatory concerns
2. Cost Effectiveness in wiring methods
3. Expandable system
4. Simplified implementation
5. Universal application regardless of manufacturer
Three wire control, when viewed as a wiring method, is simplicity itself.
Since all larger implemetations are merely an expansion of two stations
(one start, one stop), a simple ASCII diagram should more than suffice for
a basic understanding, It is presumed that the reader understands and
accepts the concept of self-latching relays thru circuitry.
Simplified Diagram
self-latching STOP
( - ) coil ( + ) ____
| | ( 1 )
* *
( 3 )
| | N.C.
|
* (1)
| START
|
* ( 2 ) __
| | /
| | ( 1 ) ___/ *_____ ( 3 )
( - ) ( + )
* ( 3 )
control voltage
STOP operators are normally closed (open to stop), and START operators are
normally closed (close to start). Both are generally momentary operators,
except under certain conditions (range control, selector switches, etc. ).
The safety facet occurs when one realizes that manual intervention is
required to START or STOP the controlled (industrial) process. Therein,
should a power failure occur, the latching coil will drop out, and power
will NOT be re-applied to the circuit, until a START operator is activated;
ergo, this is a FAIL-SAFE method and design.
Two wire control will permit re-activation upon re-application of the power.
There are obvious dangers with this sort of system, and these systems are
normally used ONLY under emergency circumstances (overcome a mechancal
stop, or promote slight mechanical movement while supervision is taking
place (see: jogging, plugging, and over-ride systems).
In three wire control expansion, additional STOP operators are placed in
SERIES with the first. Additional START operators are placed in PARALLEL
with the first. Thus a scheme of series and parallal operators provides
unlimited expansion.
This is merely provided as a general overview to a specific wiring method
and safe design practice; therein it is wholly incomplete. Please contact
the manufacturer of the devices employed for specific wiring diagrams and
specific recommendations. See also the National Electic Code (U.S.A) or
documents provided by other authorized and pertinant electrical code and
supervision agencies worlwide. Please remember that the onsite inspector
for all electrical systems has the LAST SAY as to the approval of any
system which is installed in an automated industrial or commercial process.
Regards,
Bruce V. Bates
bvbates@u...
response the other day. Please find below a brief overview.
General Topic: Motor Control Wiring
Subtopic: Systems/Methodology>Start/Stop>Remote controls
Subject: Three Wire Control
Three wire control for remote (or cover mounted) Start Stop "buttons"
encompasses many areas. Those listed below are just a sampling:
1. Design Goal for safety
a. Direct Safety Concerns
1) Personnel, product and system safety, and integrity
2) Prevent unexpected start-ups after unexpected power-down
conditions (power failure)
3) Multiple operators for supervision and operations convenience
4) Ease of implementation; thus reasonable for initial
implemention, present update and future expansion
b. Indirect Concerns
1) OSHA / NIOSHA
2) N.E.C. / U.L. / Regulatory concerns
2. Cost Effectiveness in wiring methods
3. Expandable system
4. Simplified implementation
5. Universal application regardless of manufacturer
Three wire control, when viewed as a wiring method, is simplicity itself.
Since all larger implemetations are merely an expansion of two stations
(one start, one stop), a simple ASCII diagram should more than suffice for
a basic understanding, It is presumed that the reader understands and
accepts the concept of self-latching relays thru circuitry.
Simplified Diagram
self-latching STOP
( - ) coil ( + ) ____
| | ( 1 )
* *
( 3 )
| | N.C.
|
* (1)
| START
|
* ( 2 ) __
| | /
| | ( 1 ) ___/ *_____ ( 3 )
( - ) ( + )
* ( 3 )
control voltage
STOP operators are normally closed (open to stop), and START operators are
normally closed (close to start). Both are generally momentary operators,
except under certain conditions (range control, selector switches, etc. ).
The safety facet occurs when one realizes that manual intervention is
required to START or STOP the controlled (industrial) process. Therein,
should a power failure occur, the latching coil will drop out, and power
will NOT be re-applied to the circuit, until a START operator is activated;
ergo, this is a FAIL-SAFE method and design.
Two wire control will permit re-activation upon re-application of the power.
There are obvious dangers with this sort of system, and these systems are
normally used ONLY under emergency circumstances (overcome a mechancal
stop, or promote slight mechanical movement while supervision is taking
place (see: jogging, plugging, and over-ride systems).
In three wire control expansion, additional STOP operators are placed in
SERIES with the first. Additional START operators are placed in PARALLEL
with the first. Thus a scheme of series and parallal operators provides
unlimited expansion.
This is merely provided as a general overview to a specific wiring method
and safe design practice; therein it is wholly incomplete. Please contact
the manufacturer of the devices employed for specific wiring diagrams and
specific recommendations. See also the National Electic Code (U.S.A) or
documents provided by other authorized and pertinant electrical code and
supervision agencies worlwide. Please remember that the onsite inspector
for all electrical systems has the LAST SAY as to the approval of any
system which is installed in an automated industrial or commercial process.
Regards,
Bruce V. Bates
bvbates@u...
Comments
Original Message
From: Bruce Bates <bvbates@u...>
To: <basicstamps@egroups.com>
Sent: Friday, July 14, 2000 3:17 PM
Subject: [noparse][[/noparse]basicstamps] Three Wire Control - Overview
> There was an inuiry regarding three wire control when I mentioned it in a
> response the other day. Please find below a brief overview.
>
> General Topic: Motor Control Wiring
> Subtopic: Systems/Methodology>Start/Stop>Remote controls
> Subject: Three Wire Control
>
> Three wire control for remote (or cover mounted) Start Stop "buttons"
> encompasses many areas. Those listed below are just a sampling:
>
> 1. Design Goal for safety
> a. Direct Safety Concerns
> 1) Personnel, product and system safety, and integrity
> 2) Prevent unexpected start-ups after unexpected power-down
> conditions (power failure)
> 3) Multiple operators for supervision and operations convenience
> 4) Ease of implementation; thus reasonable for initial
> implemention, present update and future expansion
> b. Indirect Concerns
> 1) OSHA / NIOSHA
> 2) N.E.C. / U.L. / Regulatory concerns
> 2. Cost Effectiveness in wiring methods
> 3. Expandable system
> 4. Simplified implementation
> 5. Universal application regardless of manufacturer
>
> Three wire control, when viewed as a wiring method, is simplicity itself.
> Since all larger implemetations are merely an expansion of two stations
> (one start, one stop), a simple ASCII diagram should more than suffice for
> a basic understanding, It is presumed that the reader understands and
> accepts the concept of self-latching relays thru circuitry.
>
> Simplified Diagram
>
> self-latching STOP
> ( - ) coil ( + ) ____
> | | ( 1 )
* *
( 3 )
> | | N.C.
>
> |
* (1)
> | START
> |
* ( 2 ) __
> | | /
> | | ( 1 ) ___/ *_____ ( 3 )
> ( - ) ( + )
* ( 3 )
> control voltage
>
> STOP operators are normally closed (open to stop), and START operators are
> normally closed (close to start). Both are generally momentary operators,
> except under certain conditions (range control, selector switches, etc. ).
>
Wrong. In a three wire control theN.C. STOP button is wired in series with
and The N.O. START button and a control or motor relay. The holding contact
(N.O.) is held closed electrically when the START button is pressed and
power is flowing thru the coil.
L1 NO| Start L2
| NC| Stop --- CR |
|
o---o
o o
oOo
|
|
| |----|
CR
> The safety facet occurs when one realizes that manual intervention is
> required to START or STOP the controlled (industrial) process. Therein,
> should a power failure occur, the latching coil will drop out, and power
> will NOT be re-applied to the circuit, until a START operator is
activated;
> ergo, this is a FAIL-SAFE method and design.
>
> Two wire control will permit re-activation upon re-application of the
power.
> There are obvious dangers with this sort of system, and these systems are
> normally used ONLY under emergency circumstances (overcome a mechancal
> stop, or promote slight mechanical movement while supervision is taking
> place (see: jogging, plugging, and over-ride systems).
>
> In three wire control expansion, additional STOP operators are placed in
> SERIES with the first. Additional START operators are placed in PARALLEL
> with the first. Thus a scheme of series and parallal operators provides
> unlimited expansion.
>
> This is merely provided as a general overview to a specific wiring method
> and safe design practice; therein it is wholly incomplete. Please contact
> the manufacturer of the devices employed for specific wiring diagrams and
> specific recommendations. See also the National Electic Code (U.S.A) or
> documents provided by other authorized and pertinant electrical code and
> supervision agencies worlwide. Please remember that the onsite inspector
> for all electrical systems has the LAST SAY as to the approval of any
> system which is installed in an automated industrial or commercial
process.
>
> Regards,
>
> Bruce V. Bates
> bvbates@u...
>
>
>
>
>
>