How to do Electrical Troubleshooting of Electrical Motor Control Circuit

How to do Electrical Troubleshooting of Electrical Motor Control Circuit


This video demonstrates how to locate and
repair a motor circuit fault.
Effective trouble-shooting starts with preparation.
It is important to have all the diagrams and
documentation for the system, as well as an
understanding of how it is designed to operate.
After reviewing the problem description, we’re
ready to begin. We start by observing the
state of the system in its “as found” condition.
The sight gauge tells us that the tank is
full and that the mixture appears about right.
The status of the previous batch was “Timed Out,” which means that the last batch did.
not reach its destination.
All the lights on the control panel are off, and the “Emergency Stop” button is pressed.
The next step is to operate the system and look for signs of abnormal behavior.
Before we can start the cycle, we must first release the ‘Emergency Stop” and drain the tank.
The “Power” and “Drain” lights on the
control panel turn on, and the tank drains
normally. Once the tank is empty, the “Ready” light comes on. When we restart the system,
the “Cycle” and “Intake 1” lights turn on,
and the tank begins to fill with the first fluid.
At the midway point, “Intake 1” stops,
and “Intake 2” starts. Once the tank is full,
the second intake stops.
At this point, the “Agitator” light should turn on,
but it does not, and we do not hear
the agitator start. Inside the power box,
we see that the agitator’s overload is tripped.
Resetting the overload causes the agitator
contactor to pick up, but the overload trips
again after a few seconds. To sum up our observations,
we saw that the cycle operates normally up until the point when the agitator should operate,
at which time its overload trips.
The observations tell us a lot about the problem area.
The control portion of the circuit can
be excluded, because it works as expected.
In the power circuit, we can exclude the two phases, which supply the control circuit.
Also, since the overload trips, we know that there is current in at least one phase of the agitator circuit.
Therefore, the problem area is confined into the agitator power circuit
and the third supply phase.
We now need to consider the possible causes.
Any one of this list could produce the symptoms we are seeing. Now, we consider if there is
an obvious probable cause. Several are more likely, such as an open fuse or problems with the motor,
but there is not one that stands out. Based on this, we need to reduce the problem area by testing.
Because the overload trips, we know that there is excessive current in one or more phases.
Therefore, our first test is to use the amp meter to measure the current in each phase.
We read “locked rotor current” in two phases
but “no current” in the center phase. Therefore, there is an open in the center phase.
In the new problem area, the fuse is now the best place to test, as a blown fuse is an
obvious probable cause. To safely test voltage in the power circuit, we open the breaker
each time we place the leads. The readings all show “full voltage,” indicating the fuses are okay.
We now know that the fault is an open in the center phase,
somewhere between the bottom
of the fuse and the motor start point.
Since there is no obvious probable cause in the new problem area, we will need to perform
tests to reduce it. To find an open in a three-phase motor circuit an ohm meter is the best choice,
so we need to lock out the circuit. Next,
we sectionalize the circuit where it leaves
the power box by disconnecting a wire and
measuring from there to the fuse with the
contactor closed. We read “infinite resistance.”
This means that the open is within the control
box. We split this area in two at the bottom
of the contactor. The “low resistance” reading
tells us the open is above the test point.
Testing at the top of the contactor with it
closed indicates the contact is okay.
Now, the only possible causes are one wire and its connections.
We begin to disconnect
the wire but find a loose terminal.
Since this fits the symptoms, we tighten the terminal to repair the circuit. To verify the repairs,
we test the full path through the motor. The
“low resistance” reading indicates the path
is complete.
Next, we get ready to test-operate the system
to make sure there are no other problems.
Restarting the system, we see the tank begin
to fill with the first fluid and then switch
to the second fluid. Once the tank is full,
we hear the agitator start, indicating that
we have repaired the malfunction. The cycle
now continues, mixing and heating the fluid.
When the proper temperature is reached, the
agitator and heater stop, and the mixture
is pumped to its destination. The batch status
now shows “passed,” so we are done.
The work order shows the total time and cost for the repairs.
An evaluation for our trouble-shooting
process is also shown. We can use this information
to improve our techniques in the future. You can learn and practice your skills on more than fifty faults,
like the one shown in this
video.

10 Replies to “How to do Electrical Troubleshooting of Electrical Motor Control Circuit”

  1. Awesome content. As a student interested in Industrial Maintenance, this helps me see some realistic troubleshooting scenarios. Thanks for the upload!

  2. Nnice video, 3 phase induction motor is working normally when no-load test. But if it is connected to pump its getting trip with a high current. reason for overload. pump also free rotation. can we check this same scenario in this simulation?

  3. Can I have a wiring diagram for two 3phase water pump, using two contoctors with overload relays. Pump must work automatically with enterval of one hour autonative not all motors at a time?

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