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Joined: Oct 2000
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Scott35 Offline OP
Broom Pusher and
This Thread has (finally!) been created for the Discussion of the Start / Stop "3-Wire" Motor Control Schematic, located at the Technical Reference Area - per the listing:

Start Stop 3 Wire AC Motor Control

in conjunction with the Thread:
Start/Stop Motor Control within this Forum Area.

Feel free to contribute to this Message via replies.

Per the Schematic:

[Linked Image]
Fig. 1.1: Basic 3 Wire Start / Stop AC Motor Control.

This Schematic details the basic method for Starting & Stopping an AC Motor from One Control Station.

The Control Station incorporates Two Momentary Contact Push Buttons:
(1) Normally Closed "STOP" Switch (N.C.),
(1) Normally Open "START" Switch (N.O.).

*** Note ***
Any number of Start/Stop Stations may be incorporated in this type of Motor Control. Each Station will contain (1) Stop and (1) Start Push Button.
All "Stop" Switches are in Series with the Motor Starter's Coil Circuit, and all "Start" Switches are Parallel to the Coil Circuit.


With the Circuit "At Rest" (Coil is DE-energized, and all Overload Relays Closed), pressing the N.O. Start Switch energizes the Coil of the Magnetic Starter - resulting in the device "Pulling-In", and closing the Motor Contacts.
In addition, the Auxiliary Contact closes, which keeps the Coil energized through the Control Circuit.

Pressing the N.C. Stop Switch temporarily opens the Control Circuit, which DE-energizes the Coil - thereby opening the Motor Contacts and DE-energizing the Motor.
The Auxiliary Contact opens as well, and the device remains "Off" until the Start Button is pressed once again.

If there is an Overload issue, One (or more) of the Overload Relays open, which opens the Control Circuit and drops out the Coil.

This Control will also offer "Under-voltage Protection" when the Coil is driven at the same Voltage of the Motor.
If the Motor Circuit Voltage drops below the threshold of what the Coil can hold the Contacts closed, the Starter drops-out, and the Motor is DE-energized.


A newer set of Start/Stop Drawings may be viewed at the Technical Reference Area, per the Topic:

Basic Start/Stop AC Motor Control

Clicking on the above link (underlined text) will open that page in a new Tab.

--Scott (EE)

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: Jul 2007
Posts: 1,335
Hey Scott, maybe you or someone else can clarify something. I read it somewhere but don't quite remember where or exactly why. It stated that it was critical to route the control power through the stop (NC) buttons first. It had something to do something about something if the control circuit got damaged or something. Kinda like it was a more safer way to wire it. Does that ring any bells with any one? if not, I'll track that done. I am pretty sure It was in one my books in my nerd library...

Last edited by sparkyinak; 11/02/13 02:28 PM.

"Live Awesome!" - Kevin Carosa
Joined: Nov 2005
Posts: 827
I think that the opposite would be true, while not necessarily critical. If you put the "Stop" N.C. button(s)first, then they are hot the whole time that the control power is present. This is based on the right side of the control ladder being grounded. If the "Start" N.O. button and its parallel sealing contact comes first, the "Stop(s)" is/are only hot while the relay or contactor is picked.

Joined: May 2005
Posts: 984
Likes: 1
If the Stop buttons are placed before the rest of the circuit, it make troubleshooting easier.
If you don't have power there that means that there is an open in the stop wiring. If you do have power there that places the problem inside the box.
That's what I was taught at GE's training school.

Joined: Apr 2002
Posts: 7,333
Likes: 7
Looking at the three control circuits via Scotts link above, the control circuit goes to 'stop' first.

Back in the days, I believe that is the method that was widely used.

Joined: Nov 2005
Posts: 827
I think that the main point is that it is NOT critical. I would design a control ladder the better way for a specific application. For instance, if I wanted an Off indication, I would usually put my Start/Run first. Then, one Form c contact set can be my sealing "a" and Off indication "b" function. You could actually build a simple, single phase control circuit, with a single DPDT relay.

Joined: Jan 2005
Posts: 5,370
Likes: 1
Cat Servant
Keep it simple - and watch the obvious!

In any circuit, what happens if you have a short to ground? Current keeps flowing, that's what.

If that short occurs after the load, the load keeps running. If the stop buttons are after the short- AND after the load- the load cannot be disconnected.

So, it's not a matter of the 'stop' coming first, so much as the load coming last.

Placing 'start' contacts before the stop contacts can also create a situation where the 'stop' button doesn't work. The whole purpose of using relay contacts is to 'get around' open contacts, after all.

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