Product: Electric Chain Hoists

Models: SE, LE, SP

Subject: One vs. Two-Button On/Off

By: Bill Anthis, Electrical Engineer

No. UL-002

Date: 4/3/91

My customer wants a two-button on/off on his pendent. How do I tell him that a one-button will do the job?

One Button On/Off  (IR Method)
	The mainline disconnect option for the IR electric chain hoists is achieved by  routing the incoming power to the hoist through an electro-magnetic  contactor. The contactor is operated by a single pushbutton on the  pendent. The button alternately opens and closes a contact each time it is depressed. The button's contact is in the control circuit of the  hoist. When the contact closes, the mainline contactors operating  coil is energized, it's contacts close, and power becomes available  to operate the hoist. The operator must still depress one of the motion  pendents to operate the hoist.
Two Button  On/Off
	The two-button on/off  control circuit uses one-button (on) to energize the mainline contactor  and the other (off) to de-energize the contactor.
What's the difference?
	The difference between  the two methods of operation has to do with what happens if the  power supply to the hoist is interrupted. With one-button on/off,  the mainline contactor returns to its condition prior to the power  interruption. If the two-button on/off is used, the mainline contactor  is always de-energized after a power interruption. The two-button on/off  is most often used in applications where motor driven equipment  runs unattended. If it is desired or required that the equipment  not automatically start on power restoration after an outage, a  two-button on/off control system is used. The two-button on/off  control system is often referred to as "three wire control". Since hoists are operated  with momentary (dead man) pushbuttons, it does not make any difference  which kind of mainline disconnect is used since the hoist cannot  run without human intervention.
One  final note:
	Without the  use of pilot lights or multi-position switches, it is not possible  with either method of control to determine whether or not there is  power to the hoist by merely looking at the button(s) on the pendent.  The one advantage of two-button on/off is that if there is doubt,  the two-button method permits turning power on or off by depressing  the appropriate button. With the one-button method, it is necessary  to attempt to run the hoist to see if the power is on or off.
Even though  the 1990 and 1991 Industrial Users Price Lists for LE Accessories  shows only one-button on/off for control stations, orders are still  being received for two-button on/off. Allowing customers to specify  the two-button on/off will add a potential 72 new wiring diagrams  to our documentation. Inasmuch as the two-button on/off does nothing  that the one-button can accomplish, I strongly urge that all people  placing orders discontinue taking orders for two-button on-off switches.  A technical explanation of the two methods of providing on/off control  is attached so that those concerned can explain to customers that  they are getting the control function they want with the one-button  version.

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