This blog is for the new technician
assigned to fix a control panel. We assume you know how use hand tools.
You also need to understand how to check AC voltage, DC voltage, and Continuity
with a Fluke Multimeter.
Talk to the operator and learn what he
knows. When testing the machine, have the operator push buttons for you.
This is very helpful since he knows how the machine is suppose to run.
Entering the control panel.
First, look very closely for anything that is fried.
The first thing you should know is that electrical controls are dangerous. Locking out is always the proper way to start. First check
the main disconnect. It turns the main power off and shuts the machine
down. Most control panels require a screw driver to open the door.
This is to keep unskilled people away. Turn the screw driver and then
rotate the door knob. The door should open. Never touch anything
inside a control panel with your fingers unless the main disconnect is off.
Main disconnect of 230-240 Volts AC, 3 phase control panel. Red-White-Black w/Green
Ground Wire
Remember the wires on top of the Main Disconnect are still hot even when the
disconnect is off!
The main disconnect is in the upper right hand
corner of the control panel. It is a large circuit breaker or
fused disconnect. The first thing to check is the line-to-line voltage on
top of the main disconnect. It is usually 460 to 480 Volts line-to-line.
Some systems are 230 to 240 Volts line-to-line. You should check the
voltage three times. Across phase 1-2. Across phase 2-3.
Across phase 3-1. If you do not have the proper line-to-line voltage call
the power company and have them come over to adjust it. Usually the three
voltage readings are something like 480 V - 479 V - 479V. Think of voltage as
"electrical pressure". If the line-to-line voltage is say 498 volts instead
of 480 volts, you can damage your control panel hardware.
Testing motor starters. Tug wires. Look for loose
connections.
Once you have determined your incoming voltage is
correct, move over to the motor starters. Turn on the main disconnect.
Check the voltage line-to-line on top of each motor starter. It should be
the same as the main disconnect. If the voltage is not correct here you
probably have a blown fuse directly above the motor starter. Note: Some
motor starters have a circuit breaker instead of fuses. Next check the
overloads located underneath the motor starter. Most overloads have a tiny
window with a yellow flag to indicate the the overload is tripped. The
overload can be reset by pressing the button marked "Reset".
Testing the control power.
If the motor starters seem to be good, test the control power. Two legs of
power lead from the main disconnect to a single phase transformer.
The line-to-neutral voltage on the outgoing side of the single phase transformer
is usually 120 Volts AC. Important: Make sure the AC neutral is grounded.
Some control panels use 24Volts DC for control
power. In this situation, a DC power supply is present. Just check
the blue and light blue wires on the downstream side for the correct DC voltage.
Useful: Make sure the DC com (light blue) is NOT grounded. Stray voltage
from other AC power circuits can back feed into DC devices and cause damage.
The case of
the DC power supply should be grounded.
Troubleshooting an electrical
control panel can be high a pressure situation. Just keep in mind that
after you are done, it was always something simple. Pay close attention to
the small details.
Do not randomly swap out circuit boards or other
devices. Only swap out devices you suspect are bad through testing, do not
put swapped devices back on the shelf unless you test them. The easiest
way to test them is to swap them back into the control panel just after you get
the machine running.
If you have control panel problems on a very hot
day, you need to cool it down with a control panel mounted fan or air
conditioner. I suggest a Compact Cabinet Cooler from Noren Products Inc.
Make sure you tell everyone who will listen how
you fixed the control panel. That way your boss will fully understand the
value you provide.
If you really like this article then you would love
a Machine Monitor.
