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Pallet Machine Electrical Systems 101
Review of basic electrical troubleshooting for pallet nailing systems, includes practical advice on dealing with routine problems.

By Jim Gookin
Date Posted: 2/1/2012

     Knowing the basics of electrical systems can help you maintain pallet machinery and is important for anyone working on your equipment. This article is meant to help you understand basic machine electrical systems. But it is not intended to be a replacement for a qualified licensed electrician. Working with electrical components can cause serious injury or death. Always turn off the power source, lock out and tag out equipment.

     Pallet equipment built today, can utilize various types of electrical power, all running side by side on a single piece of equipment. AC or DC, single or three phases, and high or low voltage are just some of the terms you will encounter. 


     Although test lights and beep sticks have their place, a good meter is your best bet in troubleshooting electrical issues. There are basically two types of volt meters, digital or an analog meter. I will focus on the digital type because it is the most common. Below is a basic digital meter that I use.

     Although this meter has several features, we are going to concern ourselves with the big three. AC Voltage, DC Voltage and resistance (ohms). The red and black leads are connected (as shown) at the bottom.

     Typical U.S. domestic AC voltages range from 110-220 single and 220-480 three phase power. Most controls use 110 volt single phase while electric motors use either 220 single or 220-480 three phase power. DC voltage is typically 12 or 24 volt. In the pallet industry, DC voltage is used in conjunction with PLC inputs and controls.

AC Voltage

     Below is an example of a 110 volt AC control circuit. This happens to be part of the safety gate circuit for a Viking Duo-Matic. See the symbol chart for symbol identification. This circuit works with the control power switch pulled on and both gates closed; the number 20CR relay is not energized. This will allow the system to operate. But if an operator opens a gate, the 20CR relay is pulled in, the reset light comes on and the system stops. Let’s troubleshoot a bad 5 amp fuse.

     Using our volt meter, we can check this circuit if a problem occurs. For example, let’s say that the machine will not operate and the “GRN” power light is off. Turning on our meter and selecting AC voltage, we would place the black lead on “N” and the red lead on wire 79. Please note that when troubleshooting, I always use a neutral wire to check circuits. Getting a good frame ground sometimes can be difficult, especially on painted surfaces. Wire 79 is located on the power switch. If there is no power, leave the black lead on the “N” and then check wire 125… no power. Now let’s check wire 121… we have power there. So power on 121 but none on 125… a bad fuse.

     After pulling the fuse, switch your meter to ohms-resistance. When the meter leads are not touching anything, the meter should read “OL” for an open circuit. Touch the two leads together and the meter should read at or near “0” (Zero). While the fuse is removed, place a lead on both ends of the fuse. A blown fuse will read “OL” a good fuse “0” (Zero).

     Now let’s look at a simple motor control circuit. This circuit has both 110 volt single phase and high voltage (220-480) three phase.

     The circuit components include fuses (F4, F5 & F6), motor starter or contactor (1CON), overload protection (1 OLR) and a three phase electric motor (MTR1).  Here’s how it works… 1L1, 3L2 & 5L3 bring 440 volts to the motor starter. When the operator presses the motor start button (not shown), 110 volts AC single phase is applied to wire #45. Wire #45 is connected to a motor starter coil terminal and wire # 21 is connected to the other coil terminal. Wire #21 is the neutral. Wires #50 and #103 are part of a holding circuit (not shown) that allows the motor to continue to run after the start button is released. The stop button (not shown) would open up the holding circuit to turn off the motor.

     Once the contactor is pulled in, 440 volts of three phase power is applied to 2T1, 4T2 & 6T3. Those wires are attached to the overload, where power is applied to wires 1T1, 1T2 & 1T3 to make the motor spin. 

     To troubleshoot a non-operating motor, start by checking the fuses. In a three phase circuit, it is best to power down, lock and tag out and pull the fuses out. Then, turning your meter on, switch your meter to ohms-resistance. A blown fuse will read “OL” a good fuse “0”.

     The next steps should be done in a “powered up” condition. A certified electrician should do the remainder of the testing if at all possible. Switch your meter to AC volts, and then carefully place one lead on 1L1 and one lead on 3L2. Your meter should read 440 volts. You checked one phase. Now you must test between 1L1 & 5L3 and between 3L2 & 5L3 for the other two phases. Each time you should read 440 volts. A lower value would be an indication of incoming power issues.

     Checking the three phase power coming out of the motor starter is done in the same way. Check 2T1 & 4T2, 2T1 & 6T3 and 4T2 & 6T3. If all results at the motor starter are zero volts, check the 110 volt single phase on wires #45 & #21. Replace the contactor if needed. Next step is to check power at the motor. Again check 1T1 & 1T2, 1T1 & 1T3 and 1T2 & 1T3. If low voltage, reset or replace the overload. Otherwise the motor could be the problem.

DC Voltage

     In our DC control example below, we have a 110 volt AC (wires 100 & 01) feeding a 24 volt DC power supply. Wire # 500 is the +24 volts (positive) and wire #502 is the -24 volts (negative). Remember that -24 volts is not a ground wire.

     Wires 501 (+DC) and 502 (-DC) feed two encoders and two reset (inductive) proximity sensors. The encoders and the reset proximity sensors are attached to a high speed counter module.  In this example, let’s say you wanted to check out a conveyor reset proximity sensor. When the proximity sensor is activated, -DC power from wire #502 should go to wire #516. The high speed counter module would then pick up that signal. To check the sensor operation, turn your meter to DC voltage. Then place the black lead on wire 516 and the red lead on 501 (+DC). You should have 24 volts. If you were to switch the leads around your reading would be -24 volts. Because of the way the process counts pulses, encoders are not easily checked using a standard digital meter.   

     Note that PLC control wiring can be either PNP or NPN. Sensors, either inductive or reflective, have to match the control circuit. Our example is a NPN circuit because we are switching the negative (-) at the sensor. A PNP control circuit switches a positive (+) at the sensor.

     That’s it for the basics in machine electrical troubleshooting. Regardless of who handles your electrical maintenance, safety must come first because a small mistake can result in injury or death. As I mentioned earlier, always turn off the power source. Follow lock out and tag out procedures for that particular piece of equipment. Make sure that only those who know what they are doing touch the electrical systems of your pallet and lumber machinery. Although there is no substitute for a qualified licensed electrician, by making a few checks you may be able to save time and money by fixing simple issues yourself.

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