Okay, this is a serious situation and needs a thorough investigation. The fact that the overload didn't trip AND the contactor welded closed points to a significant issue that could lead to a fire or equipment damage. Here's a breakdown of potential causes, things to check, and steps to take

Possible Causes and Things to Check (In Order of Likelihood/Impact):

1. Overload Relay Malfunction



Most Likely Culprit

The overload relay is supposed to protect the motor. If it failed to trip, it's either set incorrectly, malfunctioning, or bypassed somehow (though this would be highly dangerous and hopefully not the case).

Testing



Calibration/Settings

Verify the overload relay is set correctly according to the motor's nameplate FLA (Full Load Amps). This is critical. Under-rated settings will trip prematurely, but over-rated settings defeat the protection purpose. Consider the service factor too.

Functionality

Test the overload relay's tripping mechanism. There are often test buttons on the relay. If the test button doesn't trip it, it's faulty. Use a meter to confirm that when the overload is "tripped" the power is actually cut to the contactor coil.

Wiring

Check the wiring to the overload relay. Make sure it's securely connected and not damaged. The wiring in the control panel to and from the overload relay is critical.2.

Motor Failure (Partially Shorted Windings)



Significant Concern

A partially shorted winding in the motor will draw excessive current without necessarily tripping the overload if the short is not severe enough. This is what can cause localized overheating. Phase to phase and phase to ground faults can be very destructive.

Testing



Megger Test (Insulation Resistance Test)

This is the most important test for this scenario. Disconnect the motor leads. Use a megohmmeter to measure the insulation resistance from each phase to ground and from phase to phase. Low resistance readings (typically below 1 Megohm, and often even higher for new motors) indicate damaged insulation and potential short circuits. Compare readings between the phases. If one phase has a significantly lower reading than the others, that's a very strong indication of a winding fault.

Winding Resistance Test

Use a low-range ohmmeter to measure the resistance of each winding. The resistances should be very close to each other. Significant differences indicate a potential short or open winding.3.

Contactor Issues (Beyond Welding Closed)



Sticking Plunger/Armature

If the contactor plunger wasn't moving freely, it could have caused arcing and overheating even before it welded shut.

Weak Spring Pressure

Weak spring pressure can prevent good contact closure, leading to increased resistance and heat.

Arcing/Pitting on Contacts

Check the other contactor's contacts (dispersion side) for signs of arcing, pitting, or damage. This indicates a problem with the contactor's ability to handle the current.4.

Voltage Imbalance/Phase Loss



Less Likely (But Still Important)

A significant voltage imbalance or a phase loss can cause one phase of the motor to draw excessive current, leading to overheating.

Testing



Voltage Measurement

Use a multimeter to measure the voltage on each phase at the disconnect, the electrical panel, and the mixer cabinet (both with the mixer off and while it's running – if possible, given the situation). Look for significant voltage differences between the phases.

Phase Rotation

Verify the phase rotation is correct. Incorrect phase rotation can sometimes cause motor problems (though usually more related to direction).5.

Mechanical Overload on the Planetary Motion



Potential Factor

Is the planetary motion system excessively loaded? Too much product, incorrect mix ratios, or a mechanical binding in the planetary gears could be overloading the motor.

Inspection



Visual Inspection

Carefully inspect the planetary gear system for signs of wear, damage, or binding.

Load Monitoring

(If you can get the mixer running again after addressing the electrical issues) Monitor the motor current draw while the planetary motion is engaged. Compare it to the motor's FLA.6.

Ambient Temperature



Contributing Factor

Is the ambient temperature in the mechanical room excessively high? High ambient temperatures can derate the motor's capacity and make it more susceptible to overheating.7.

Incorrect Motor Sizing



Less Likely, But Worth Checking

Is the motor properly sized for the application? If the motor is consistently overloaded, it will overheat even with a properly functioning overload relay.

Verification

Review the original design specifications and calculations to ensure the motor is adequate for the mixing load.

Immediate Actions & Steps to Take


LOCKOUT/TAGOUT

Ensure the mixer is properly locked out and tagged out before doing any electrical work.2.

Replace the Damaged Components



Contactor

The welded contactor must be replaced.

Overload Relay

Given the fact that it didn't trip, the overload relay must also be replaced.

Damaged Wiring

Replace all the wires that show signs of overheating or damage. Use the correct wire gauge and insulation type.3.

Thorough Inspection (As Outlined Above)

Don't just replace parts and hope for the best. You must determine the root cause.4.

Motor Testing (Megger Test First)

This is crucial to determine if the motor windings are damaged.5.

Review Electrical Drawings

Review the electrical schematics for the mixer to understand the control circuit and ensure there are no modifications or incorrect wiring.6.

Document Everything

Keep detailed records of all your tests, measurements, repairs, and replacements.

Important Considerations



Qualified Personnel

All electrical work must be performed by qualified and licensed electricians.

Safety First

Electrical work is dangerous. Follow all safety precautions.

Root Cause Analysis

Don't just treat the symptoms; find the underlying cause. Otherwise, the problem will likely recur.

Preventive Maintenance

Implement a regular preventive maintenance program for the mixers, including visual inspections, cleaning, lubrication, and electrical testing.

In Summary:

The most likely cause is a combination of a malfunctioning overload relay and a motor that may be failing. The overload relay must be replaced and tested. The motor must be thoroughly tested (especially the megger test). Don't overlook the mechanical aspects of the mixer – a mechanical overload can contribute to the problem.Good luck, and please be careful! Let me know if you have any more questions as you investigate. Flag for review