Actuators: Solenoids, Contactors, and Motor Starters

Between the PLC output and the moving machine sits an actuator —
often a solenoid valve or an electromechanical contactor. These are
robust but not immune, and because they carry real power they fail in
ways a small signal never would.

Solenoid valves

A solenoid valve converts an electrical signal into pneumatic or
hydraulic action. When one does not shift, confirm the coil is receiving
voltage (a meter across the coil terminals), then confirm the coil is
not open (resistance check with power removed), then consider the
mechanical side — a stuck spool, no air supply, or a blocked port. Many
solenoid valves have a manual override you can use to prove the
mechanical path independently of the electrical command.

Contactors and motor starters

A contactor is a heavy-duty relay that switches motor power; a motor
starter adds overload protection. When a motor will not run, the fault
may be the PLC output not energizing the coil, the coil itself, the
contactor’s power contacts, or a tripped overload. Check in that order:
is the coil commanded and energized, does the contactor pull in, are the
power contacts passing voltage through, and has the overload tripped? A
tripped overload that keeps re-tripping is telling you about the motor
or its load, not lying to you — investigate the cause rather than just
resetting it.

Using the manual override

Many solenoid valves include a manual override — a small button or
screw that shifts the valve mechanically without any electrical signal.
This is a superb diagnostic. If a valve will not actuate electrically,
operate the manual override: if the valve shifts and the machine
function works, you have proven the entire pneumatic or hydraulic path
healthy and localized the fault to the electrical side — coil, wiring,
or output. If the manual override also fails to produce the function,
the fault is mechanical or in the fluid supply — a stuck spool, no air,
a blocked line — and no amount of electrical troubleshooting will help.
The override splits the actuator into its electrical and mechanical
halves with a single action.

Contactor troubleshooting in sequence

A motor that will not run has a chain of possible failure points
between the PLC output and the motor, and checking them in physical
sequence is efficient. Is the PLC commanding the contactor coil, and is
voltage reaching that coil? If not, the fault is upstream in the output
or wiring. If the coil is energized, does the contactor physically pull
in — can you hear and see it close? If it will not pull in with voltage
at the coil, the coil has failed. If it pulls in, do the main power
contacts pass voltage through to the motor? Worn or pitted contacts can
fail to conduct even while the contactor mechanically closes. And has
the overload tripped, opening the control circuit? Following this
sequence — coil command, coil energized, contactor closed, contacts
conducting, overload healthy — walks the power path from control to
motor and finds the break wherever it lies.

The overload is trying to tell you something

An overload relay protects the motor by tripping when current stays
too high for too long, and a tripping overload is almost always
reporting a real problem rather than being faulty itself. Resetting it
and walking away is a mistake that ranges from wasteful to dangerous.
When an overload trips, measure the motor’s running current with a clamp
meter and compare against the nameplate. High current points at a
mechanical overload on the motor — a jammed or binding load, a failing
bearing, a driven machine under too much resistance — or at an
electrical problem such as a failing motor winding or a lost phase. A
motor running on two of three phases draws high current and overheats,
and the overload catching that is doing exactly its job. Find why the
current is high; the overload is the messenger, not the problem.

A case file: the contactor that chattered

A contactor buzzes and chatters rather than pulling in cleanly, and
the motor it controls starts and stops erratically. Chatter — the
contactor rapidly making and breaking — points at a coil that is not
getting a stable, adequate voltage to hold itself closed. Metering the
coil voltage during the chatter shows it sagging well below rating
whenever the contactor tries to pull in. The control circuit feeding the
coil has a high-resistance fault — a loose terminal — that drops the
voltage as soon as the coil draws its pull-in current, so the contactor
starts to close, the voltage sags, it drops out, the current falls, the
voltage recovers, and the cycle repeats as a buzz. Finding and
tightening the loose connection restores a solid, stable coil voltage
and the chatter stops. Contactor chatter is a classic signature of
inadequate or unstable coil voltage, usually from a high-resistance
connection in the control circuit, and it is diagnosed by watching the
coil voltage under the load of trying to pull in.

Overloads, and reading the motor’s current

The overload relay is a diagnostic instrument in its own right if you
listen to it. It trips on the integral of excess current over time, so a
trip is a statement that the motor drew more current than it should for
long enough to matter. The clamp meter turns that statement into a
number: measure the running current and compare it against the
nameplate. Modestly elevated current across all phases suggests the
motor is working too hard — a mechanical overload, a binding or
over-heavy driven load, a developing bearing failure. Severely
unbalanced current between phases, or one phase far lower than the
others, suggests an electrical fault such as a lost phase or a failing
winding. Normal current with the overload still tripping suggests the
overload itself or its setting. Reading the current against the
nameplate turns a bare trip into a directed investigation, and it
enforces the cardinal rule: find why the current was high before you
reset, because the overload tripped for a reason that has not gone away
just because you cleared it.

Pneumatic and hydraulic context for solenoid
faults

A solenoid valve sits at the boundary between the electrical control
system and a pneumatic or hydraulic system, and solenoid faults often
lie on the fluid side rather than the electrical side, which is why
understanding that context matters. When a solenoid valve does not
produce its expected action, the electrical side — coil voltage, coil
integrity — is only half the picture; the fluid side can fail
independently. No air or hydraulic pressure supply means the valve
shifts electrically but nothing happens because there is no fluid power
to deliver. A blocked or restricted port, a stuck spool, or a failed
seal means the valve cannot pass fluid even when it shifts. This is
precisely why the manual override is so valuable: operating it tests the
entire fluid path independently of the electrical command, so that if
the override produces the action, the fluid side is proven healthy and
the fault is electrical, while if the override also fails, the fault is
on the fluid side. Approaching a solenoid fault as potentially
electrical or fluid, and using the manual override to separate the two,
prevents the error of endlessly checking coil voltage on a valve whose
real problem is a lost air supply or a jammed spool.

A case file: the nuisance-tripping starter

A motor starter trips its overload every few hours, and the easy
response — resetting it and moving on — has been repeated many times
without resolution, which is itself the problem. An overload that trips
repeatedly is protecting the motor from a real condition, and each reset
without investigation simply lets that condition continue damaging the
motor toward eventual failure. Taking the overload seriously as a
messenger, the technician measures the running current with a clamp
meter and finds it elevated across all three phases, above the
nameplate. The motor is being overloaded mechanically — investigation of
the driven equipment reveals a bearing beginning to seize, adding load
that raises the current until the overload does its job and trips. The
real fault was never electrical; the overload was accurately reporting a
mechanical problem in the driven machine, and every reset had ignored
that report. Replacing the failing bearing returns the current to normal
and the tripping stops. The principle, stated once more because it is so
often violated: a repeatedly tripping overload is telling the truth
about an excessive current, and the professional response is to measure
that current and find its cause, not to reset the messenger and let the
underlying problem advance toward a far more expensive failure.

A case file: the valve with no air

A pneumatic solenoid valve will not actuate its cylinder, and the
technician begins, reasonably, on the electrical side — confirming the
coil has voltage and is not open, both of which check out fine. With the
electrical side proven healthy yet the valve still not producing motion,
the manual override becomes the decisive test: operating it should shift
the valve mechanically and, if the pneumatics are sound, move the
cylinder. The technician operates the override and the cylinder still
does not move, which immediately shifts the entire diagnosis from the
electrical side to the fluid side, because a valve that will not produce
motion even under manual override has a problem past the electrical
command entirely. Checking the air supply, the technician finds it lost
— a closed supply valve upstream, cutting off the pneumatic power the
solenoid valve needs to do anything. The solenoid and its coil were
healthy throughout; the fault was simply no air. Restoring the air
supply, the valve works normally. The case demonstrates the power of the
manual override to split a solenoid valve fault into its electrical and
fluid halves: with the coil proven healthy electrically, the override
tested the fluid path independently, and its failure to produce motion
pointed unmistakably at the fluid side, where a lost air supply was the
simple cause. Without the override, the technician might have continued
down the electrical side, endlessly rechecking a healthy coil while the
real problem — no air — sat unexamined. Using the manual override early,
especially once the electrical side checks out, tests the entire
mechanical and fluid path in one action and directs the diagnosis to the
right half of the valve’s world.

Part V — Communications