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Rulmeca operates a Service centre fully dedicated to the repair and assembly of Motorized Pulleys
Rulmeca Corporation
The Rulmeca motorized pulley is a hermetically sealed, internally powered conveyor drive built around a three-phase, squirrel-cage AC induction motor. Designed for demanding industrial environments where space is limited and operator safety is critical, the motorized pulley system integrates motor, gearbox, and drive pulley into a single compact unit. To size the drive correctly, designers must first calculate effective tension (Te) — the belt pull required to move the load — and then derive the required power. Required power (ft-lbs/min) = Te (lbs) × belt speed (fpm), where 1 HP = 33,000 ft-lbs/min. The conveyor belt motor and gearbox are directly coupled: the motor stator remains fixed while the rotor turns on two internal bearings, transferring torque through a two- or three-stage gearbox into the pulley shell. An oil bath inside the unit continuously lubricates all moving parts and draws heat away from the stator, dissipating it into the atmosphere via the belt surface that wraps around the pulley face.
The motorized pulley system operates on standard three-phase AC induction principles. When current flows through the copper windings of the stator, it generates a rotating magnetic field. This field induces a current in the rotor, which in turn produces the torque that drives the pulley shell — and, through friction, the conveyor belt itself.
The conveyor drive motor output is directly coupled to the gearbox, which transmits torque to the pulley shell via the end housing spur gear. There is a direct relationship between full load current draw and rated horsepower: a 20 HP unit draws approximately 23.6 A at full load, a 15 HP unit draws under 20 A, and a 25 HP unit exceeds 30 A. Understanding this relationship is essential for correct system design and fault diagnosis.
A three-phase AC motor cannot self-regulate its current draw: if the load demands more power than the motor is rated for, it will attempt to deliver it until damage occurs. Proper protection is therefore essential in any motorized pulley system.
Three-phase power connects to the motor terminals U, V, and W and is protected by an upstream overcurrent device (fuses or circuit breaker). Additionally, Rulmeca Motorized Pulleys include thermal protection switches built into the stator winding. These bimetallic switches are wired in series and connected to terminals T1 and T2, which must be integrated into a normally-closed control circuit. If the motor overheats, the switches open, triggering the control circuit to cut power through a three-pole starter.
The control circuit typically includes belt misalignment switches, emergency pull cord switches, plugged chute switches, and an on/off switch — all forming a closed loop that ensures the drive shuts down safely under fault conditions.
Before using a multimeter, technicians must be able to read the power and control circuit diagrams for the conveyor drive. Troubleshooting should always be carried out by trained personnel wearing appropriate personal protective equipment.
The first step is to check for control circuit trips. A normally-closed circuit is considered tripped when any safety switch — E-stop, belt misalignment, zero speed, pull cord, or chute tilt — opens. If no trip is present, the multimeter can be used to perform the following checks in sequence: supply voltage measurement, motor current draw measurement, winding resistance measurement, thermal protection switch resistance measurement, and motor insulation continuity measurement.
Supply voltage is measured between terminals U–V, V–W, and U–W; the three readings should be balanced within 5% of each other and within 5% of the rated voltage shown on the data plate. Current is measured with a clamp meter around each phase conductor; readings should be balanced within 10% and must not exceed the full load amps (FLA) stated on the data plate.
With the conveyor drive motor de-energized and all supply wires disconnected, resistance measurements can be taken as follows. Set the probes between U and V, V and W, and U and W to check the resistance of each phase. The three values must be balanced within 10%. Any significant deviation indicates a winding imbalance; a reading near zero indicates a phase-to-phase short circuit.
To check the thermal protection switches, place the probes across T1 and T2 and look for near-zero resistance (full continuity). Then check T1 and T2 individually against the ground lug; both should read infinite resistance (open circuit).
Finally, check motor insulation by measuring resistance between each power terminal (U, V, W) and the ground lug. All three readings should be in the megohm range or higher. A low reading indicates that the winding insulation has broken down and the phase is shorting to ground.
Understanding failure modes helps maintenance teams prevent recurrence. The five most common causes of damage to a conveyor belt motor and gearbox are: motor overload, overvoltage, voltage surge, single phasing, and phase imbalance.
Overload produces uniform burning across the entire stator — a clear sign that the motor was operated beyond its rated power for an extended period. The corrective action is to verify that the installed motor is rated for the actual load, and to confirm that overcurrent protection is correctly set.