View Full Version : electric motor control problem help needed

03-28-2010, 10:53 AM
I have a motor driven power hammer machine called a pullmax. It has a 4 speed transmission on the back end of the motor and had a large box also controlled the 4 speed transmission with a switch / button interlock control box wired in to the momentary contact “start / stop” buttons. The interlock unit would not allow the motor to start unless the transmission was in a selected gear, nor allow the transmission to be shifted unless the motor was completely stopped.
This is old relay / limit switch technology and one of the shift control motors was burned out causing many of the wires in this unit to melt or burn off completely. . .

I have removed the assembly and intend to simplify the shifting and starting with just a couple momentary contacts witches and manual controlling of the gear changes. The problem im having is with the magnetic motor starter and its wiring. Does the term “latching relay” come into play here ?
I can make the motor start and run using a wooden wedge to lift the relay latch and hold it in place. I can also get the relay latch to make the motor start by momentarily connecting 110 volts to it but when I remove the 110 volts, the latch of course releases and the switch opens up, causing the motor to quit. Somehow the latch must be powered so a momentary contact start button will electrically lift the latch but what or where does the power come from that holds the latch in place until another momentary contact stop button breaks the circuit and will cut the power to the motor ? is that another 110volt wire thru the two momentary contact stop / stat buttons ? these start / stop buttons are n/o and n/c now as they were prior to my removing everything that they controlled.
I have taken a couple pictures of the magnetic motor starter (relay) assembly and the 4 small connections in the center of the photo are the ones in question. The top left hand connection (labeled 2) has a 110 volt supply wire already attached but from there I assume I must run a wire from the right hand connection (labeled 3) to the start button that’s n/o. that will raise the latch and the machine runs until I take my finger off the start button.
The two screw connections with the 110v 60c tag between them (below 2 & 3 )are possibly the holding circuit but I don’t want to even attempt putting power to them until I know I won’t burn up anything.
Can anyone with electrical knowledge tell me if im thinking along the right path ?
Simplified it would be a latching relay, using a combination of a n/o and n/c pushbutton assembly and a bit of simple circuitry between them. . . ? ? ?
Thanks very much.
Davidh (knowing just enuf about this to be dangerous)

Ted Coffey
03-28-2010, 11:17 AM
Your 110 volt control circuit should be as follow:

1. A normally open "Start" button with a normally open relay contact "on the starter somewhere" in parallel that closes when the starter coil is energized.
This relay contact will keep the coil energized when you release the start button.

2. In series with this Start button circuit described above should be a normally closed "Stop" button.
This button will de-energize coil and the starter will turn off.

Hope that helps.


03-28-2010, 11:19 AM
A latching relay will remain in the selected position until power is applied to move it to the other position.

A seal-in relay with remain closed as long as current is flowing through the contacts. This sometimes involves a second coil that operates off the power through the contacts. A momentary switch (often with different voltage) can be used to close the relay.

I've used a seal-in relay to make an inexpensive time-delay switch. A momentary switch closes the relay and charges some capacitors. When the momentary switch is released, the relay stays closed until the power in the capacitors is depleted.

Paul Alciatore
03-28-2010, 02:00 PM
This is a generic, simplified drawing of a standard motor control circuit using a mechanical contactor or relay.


The "relay holding contacts" are just wired in parallel with the Start push button so they take over the job of supplying current to the contactor's coil when the start PB is released. The Stop PB switch is a normally closed version so it normally passes this current, but when it is pressed, it does not and the motor contactor can drop out, stopping the motor.

Not shown in this drawing is the fact that the "relay holding contacts" MUST also be activated by that same current that activates the contactor. This can be via any of several means. They may be on the main contactor and are pulled in when it is. Or they could be on a separate, smaller control relay that is also activated when the main contactor is. Or they could be somehow mechanically part of the motor or of a device it drives, but this is unlikely. The only requirement is that they close when the Start PB is pressed momentarily and that they open when the Stop PB is pressed momentarily.

In your case, these holding contacts may have been in the interlock unit you removed. Or they may be on the contactor as you suggest. You can test those extra terminals on the contactor by first observing the part of the contactor that moves when the start current is applied. Most contactors have external access to this part of the mechanism and it is usually insulated electrically from any of the terminals so it can be manually activated when power is present. After locating this feature, disconnect ALL POWER and use an Ohm meter to check for continuity between these extra contacts and ALL other terminals: check for continuity with the contactor both open and closed (by pressing the part of the mechanism that moved above). If no continuity is found, then check between the two extra contacts to see if they close (show continuity) when the contactor is activated as above and open when it is released. If both of these conditions are met, you can probably use them for the Holding Contacts shown in my drawing.

If this does not work, then you could add a new relay as shown below. This relay would need a coil that is rated at 115 VAC and contacts that are rated for about 2 Amps at 115VAC or so. This is what the circuit would look like:


The dotted line between the added relay coil and the holding contacts indicates they are part of the same divice (the added holding relay).

J. R. Williams
03-28-2010, 03:08 PM
The starter circuit drawings are OK but the numbers do not match the starter in the photo. The coil terminals are adjacent to the panel marked "120V 60 CY". The terminals number 2 and 3 should be the holding contacts.


Weston Bye
03-28-2010, 03:49 PM
I've wired up literally hundreds of those starters over my career. Here is a basic circuit using ladder logic symbols. The top circuit is the power section and the middle drawing is the control section. The drawing at the bottom is a semi-pictorial of the control circuit.


not obvious in the photograph are the terminals for the overload relays 1OL and 2OL with the white D-shaped buttons. The control terminals are way down at the bottom and back of the starter. These relays sense motor current and open the control circuit if the overloads trip.

03-28-2010, 09:08 PM
We always wired all the stops, limits and safeties in series _before the start button. In the real old days we would put all the OL's (contacts) in series on the "other" side of the motor starter coil. The 1 , 2 and 3 numbers were always standard on any MCS wiring you did.

Weston Bye
03-29-2010, 06:54 AM
We always wired all the stops, limits and safeties in series _before the start button. In the real old days we would put all the OL's (contacts) in series on the "other" side of the motor starter coil. The 1 , 2 and 3 numbers were always standard on any MCS wiring you did.

Yes, I've seen a multitude of "standard" wiring schemes for motor starter circuits. As an electrical controls designer and contractor, I was always subject to customer specifications and standards. Every one was different.

The starter in the photo is actually just a little before my time. I started in the controls business in '77 or '78. All the Allen-Bradley starters I used for new equipment were equipped with 3 overloads rather than 2, although I did a lot of repair and rework on equipment that had the older starters.

03-29-2010, 07:38 AM
So basically when you mash the start button it starts,but as soon as you let off it stops right?
If that's the case check your stop button contacts.

03-29-2010, 12:07 PM
If a stop was bad or open it should never start, if its wired correctly. My guess is the holding contacts or 2 & 3 contact is bad.