Jogging, AC Drives

Are you talking about a control function? If so, jogging usually refers to an input or control which can be used to make small moves by pressing buttons (e.g. for aligning a tool or work-piece before starting to machine).

Jogging, or "inching" as it has also been called, is the starting and running of a motor as long as the control pilot device, usually a pushbutton, is maintained in the actuated position.

In the pre-electronic motor drive days, a jogged motor would accelerate to its normal running speed, or to the speed the control system commanded it to if the motor was a multi-speed unit.

Since electronic speed controls came along, the jogging of a motor is usually done at a much lower speed then what the motor is capable of as jogging is usually used for fine adjustments of a machine's position in some way.

For example, a "end of travel" limit switch that actuates a reversing function on a sliding bed could be viewed as a jogging function as the tables is jogged in the reverse direction when the end of travel limit switch is actuated and only runs while the limit switch is closed.

Rick...

If you feed your drives too much power anddo not exercise then enough, they get fat nd lazy (like me). Forcing them to Jog evey day can prevent that from happening. The downside is, when they reach middle age, their knees will be shot and they will need surgery.

when you start a motor, using a control circuit involving a contactor, the contactor normally latches on using auxilliary contacts. Jogging bypasses the latching element so the contactor is energized as long as the switch (push button) is depressed. As has already been said this is for incremental movements of machinery. There is normally an additional start button which latches conventionally.

To get fit! You could try jogging in park or even jogging on the spot if you are short of space!

Aside from "inching" the motor or drive, jogging can also be used to verify if the motor is wired for the correct direction of rotation.

Typically a drive...VFDs and many DC drives... have a seperate input switch for jog. This input typically has a seperate programmed speed independent of the run reference.

It's used ...control wise...as descibed by others above.

The function is as described by several posts above. The purpose is primarily for maintenance. It allows the technician to locally operate the driven component, and acts as a "dead-man" switch. The switch is spring-return to stop, and will only allow movement of the motor while the switch is physically held in place by a person. As soon as the switch is released, the motor stops.

Jogging provides a convenient way to:

• get a coupling in the correct position for disassembly
• test the direction of rotation
• run a belt or chain a few turns to stretch so you can re-check tension
• check freedom of rotation after cleaning a jam on a conveyor
• any other procedure which might require short movements of the shaft.

Unfortunately, too many maintenance personnel jog the machine, then put their hands right back into the drive train without locking out the supply. I've seen injuries to 2 people from this, and had to stop countless others who thought the equipment was safe, even when it would only take a careless nearby person to lean on a switch to start the motor and cause serious injury. For that reason, most of the new drives we receive do not have jog capability.

If the system is correctly designed, there is no reason why the jog function should present any greater hazard than any of the normal running functions.

If the system is configured such that the jog function by-passes the Safety circuits, then the system is inherently unsafe.

This is not a problem with the drives, but how they are being used.

JohnDG,

I agree entirely. The most common problem I've seen is an individual who goes through an entire lockout procedure to disconnect motor from pump. After rebuilding the unit he reassembles it and removes the lockout to test. If a problem is encountered, he will proceed to disassemble or adjust the unit as necessary, placing his hands directly in the rotating components, with only the jog button to protect him. (This is the same person who spent half a shift locking it out the first time).

The first injury I reviewed was caused by a maintenance tech reaching for a wrench while holding a loose shaft coupling in place with his other hand. The toolbox was sitting on a ledge next to the jog button. In grabbing the wrench, he pushed the toolbox into the button, starting the motor. The toolbox was heavy enough that it held the button in, keeping the motor running as the coupling wrapped his shirt sleeve (and the arm it contained) twice around the shaft before the cloth tore free.

There's an old saying: You can't make anything idiot-proof; idiots are too darned ingenious!

Jog can be used in a number of ways and can be a normal mode of operation of a machine. One example is machines like a slitter where jog can be used to move a splice into position or thread up an empty machine. The difference in the drive as mentioned in an earlier post is that the VFD will run at some speed that is preset as the jog speed which is independent of the run speed. So in the example above the slitter may be set up to jog at 25FPM irregardless of the line speed setpoint.

Shawn