May 15, 2026

Matching a synchronous motor with a gearbox is not only about connecting two mechanical parts together. The right combination must meet your required speed, torque, load type, duty cycle, installation space, efficiency target, and safety factor. If the motor is too small, the system may fail to start, overheat, lose synchronism, or trip under load. If the gearbox is not selected correctly, it may suffer from high wear, vibration, backlash, oil leakage, or premature gear failure. Synchronous Motor and Gearbox Matching Matching Factor What to Check Why It Matters Output speed Required machine speed Determines gearbox reduction ratio Output torque Load torque demand Ensures the system can drive the load Motor power kW or HP rating Prevents overload and overheating Gear ratio Motor speed ÷ output speed Controls final shaft speed Service factor Load shock and duty cycle Protects gearbox from damage Mounting type Foot, flange, shaft-mounted Ensures mechanical compatibility Shaft size Motor shaft and gearbox input Avoids coupling and alignment issues Efficiency Motor and gearbox losses Affects energy use and heat Backlash Gear clearance Important for positioning accuracy Environment Dust, moisture, temperature Affects sealing, lubrication, and protection Understand the Required Output Speed The first step is to know the speed required by your driven machine. A synchronous motor speed depends on frequency and pole count. The gearbox reduces this motor speed to the working speed needed by the equipment. Basic Formula Gear ratio = Motor speed ÷ Required output speed For example, if your synchronous motor runs at 1500 rpm and your machine needs 100 rpm: Gear ratio = 1500 ÷ 100 = 15:1 This means you need a gearbox with a reduction ratio close to 15:1. Common Motor Speed and Gear Ratio Examples Motor Speed Required Output Speed Approx. Gear Ratio Typical Application 3000 rpm 300 rpm 10:1 Small automation equipment 1500 rpm 150 rpm 10:1 Conveyor drive system 1500 rpm 100 rpm 15:1 Mixer or feeder 1000 rpm 50 rpm 20:1 Heavy-duty conveyor 750 rpm 30 rpm 25:1 Rotary table or slow drive Calculate the Required Output Torque Torque is critical when pairing a synchronous motor with a gearbox. The gearbox reduces speed but increases torque. This allows a smaller high-speed motor to drive a lower-speed, higher-torque load. Basic Torque Formula Torque = 9550 × Power ÷ Speed Where: Torque is in N·m Power is in kW Speed is in rpm For example, if the motor power is 5.5 kW and the gearbox output speed is 100 rpm: Torque = 9550 × 5.5 ÷ 100 = 525.25 N·m This means the gearbox output torque should be higher than 525 N·m after considering efficiency and safety factor. Choose the Correct Motor Power The synchronous motor power should match the load demand. If the motor power is too low, the motor may fail to start, run with high current, lose synchronism, or overheat. If the motor power is too high, the system cost increases and energy efficiency may become poor under light load. When selecting motor power, check: Starting torque requirement Continuous running torque Peak load torque Load inertia Working hours per day Number of starts and stops Shock load level Ambient temperature Cooling condition For stable loads, such as fans or pumps, the required power is usually easier to calculate. For shock loads, such as crushers, mixers, presses, or heavy conveyors, you need a higher safety margin. Select the Right Gearbox Ratio Once motor and output speeds are clear, choose the gearbox ratio. However, the exact standard gearbox ratio may not match your calculation perfectly. In that case, choose the closest standard ratio and check whether the final output speed is acceptable. Example Motor speed: 1500 rpm Required output speed: 120 rpm Required ratio = 1500 ÷ 120 = 12.5:1 If standard gearbox ratios are 10:1, 12:1, 15:1, and 20:1, a 12:1 ratio may be more suitable if the machine can accept a slightly higher output speed. Check Gearbox Output Torque Capacity Do not only check the gear ratio. You must also check whether the gearbox can handle the required torque. A gearbox with the correct ratio but insufficient torque rating may fail quickly. You should compare: Required load torque Gearbox rated output torque Peak torque capacity Service factor Overload capacity Thermal rating For heavy-duty use, the gearbox rated torque should be higher than the calculated torque after applying a service factor. Consider the Service Factor The service factor is a safety multiplier used to protect the gearbox from real working conditions. A machine that runs smoothly for 8 hours per day needs a lower service factor than a machine that works 24 hours per day with shock loads. Suggested Service Factor Reference Working Condition Load Type Working Time Suggested Service Factor Light duty Smooth load Less than 8 hours/day 1.0–1.2 Medium duty Moderate load 8–16 hours/day 1.2–1.5 Heavy duty Shock load 16–24 hours/day 1.5–2.0 Severe duty Frequent impact Continuous operation 2.0+ For example, if your calculated torque is 500 N·m and the service factor is 1.5: Required gearbox torque = 500 × 1.5 = 750 N·m So you should select a gearbox rated for at least 750 N·m. Match the Motor Starting Characteristics Synchronous motors have different starting requirements compared with standard induction motors. Some synchronous motors use damper windings, VFD control, soft starters, or special starting systems. When matching a gearbox, you must make sure the gearbox and driven load do not create excessive starting resistance. Check these points: Can the motor start the load through the gearbox? Is the load too heavy during startup? Is the load inertia too high? Does the motor need unloaded starting? Is a clutch or soft start system needed? Will the gearbox experience high shock during starting? For high-inertia loads, the motor may need a longer acceleration time or a different starting method. Check Load Type and Application Different applications require different gearbox types. A conveyor may use a helical gearbox or shaft-mounted gearbox. A precision automation system may require a planetary gearbox. A lifting system may need a worm gearbox

Synchronous motors are widely used in pumps, compressors, fans, mills, conveyors, and other industrial drive systems that require constant speed, high efficiency, and stable operation. However, problems can still appear during installation, commissioning, or daily use. Common synchronous motor problems include starting failure, loss of synchronism, overheating, abnormal vibration, excitation faults, high current, poor power factor, bearing damage, and insulation failure. Understanding the symptoms, causes, and solutions helps you reduce downtime, protect equipment, and improve motor reliability. Common Synchronous Motor Problems Problem Common Symptoms Main Causes Quick Solution Motor fails to start No rotation, humming, trip alarm Wrong wiring, low voltage, starter fault Check power supply, wiring, starter, and control circuit Loss of synchronism Motor trips under load, unstable operation Overload, voltage drop, weak excitation Reduce load, check voltage, adjust excitation Overheating High frame temperature, insulation smell Overload, poor cooling, high current Improve ventilation, reduce load, inspect current Abnormal vibration Noise, shaking, loose foundation Misalignment, bearing wear, rotor imbalance Align shaft, inspect bearings, tighten base High current Overload trip, cable heating Mechanical overload, voltage imbalance Check load, power supply, and motor parameters Poor power factor Low efficiency, penalty risk Incorrect excitation setting Adjust excitation current Excitation failure Motor cannot synchronize AVR, exciter, brush, or field circuit fault Inspect excitation system and field winding Bearing problems Grinding noise, high bearing temperature Poor lubrication, contamination, misalignment Lubricate or replace bearings Insulation failure Ground fault, leakage current Moisture, aging, dust, overheating Dry, clean, test, or rewind motor Synchronous Motor Fails to Start A synchronous motor may fail to start if the power supply, starting system, excitation system, or load condition is not correct. In many cases, the motor may produce a humming sound but cannot rotate normally. Common Causes Supply voltage is insufficient. Phase loss or wrong phase sequence. Incorrect wiring connection. Starter, contactor, or protection relay failure. Load is too heavy during starting. Damper winding or starting cage is damaged. Control circuit has loose terminals or faulty sensors. Solutions First, check the incoming voltage and make sure all three phases are stable. Then inspect the motor wiring according to the nameplate and wiring diagram. If the motor uses a soft starter, VFD, or special starting panel, check the parameter settings and protection alarms. You should also make sure the driven equipment can rotate freely before starting. If the load is jammed, even a healthy motor may fail to start. Motor Loses Synchronism Loss of synchronism is one of the most serious problems in synchronous motors. It happens when the motor cannot maintain its magnetic lock with the rotating stator field. The motor may slow down, vibrate, draw high current, or trip suddenly. Common Causes Sudden mechanical overload. Large voltage drop in the power system. Excitation current is too low. Incorrect load angle. Poor power supply stability. Starting process is not completed correctly. Faulty excitation control system. Solutions Reduce the load and restart the motor under proper conditions. Check whether the excitation current reaches the required value after starting. If the motor is used in a heavy-duty application such as a compressor, mill, or large pump, avoid sudden load changes. Also inspect the power supply for voltage dips, phase imbalance, and unstable frequency. Synchronous Motor Overheating Overheating can damage insulation, shorten motor service life, and cause unplanned shutdowns. A motor that runs hotter than normal should be inspected quickly. Common Causes Long-term overload operation. Cooling fan failure or blocked ventilation. Dust buildup inside the motor. High ambient temperature. Voltage imbalance. Excessive current. Bearing friction. Incorrect excitation setting. Solutions Measure the motor current and verify it against the rated current shown on the nameplate. Clean dust from cooling channels, fan covers, and air filters. Make sure the motor has enough space for heat dissipation. If the motor is installed in a dusty, humid, or high-temperature environment, improve the enclosure protection and cooling design. Abnormal Noise and Vibration Noise and vibration usually indicate mechanical or electrical problems. If ignored, they can lead to bearing failure, shaft damage, coupling damage, or rotor-stator rubbing. Common Causes Shaft misalignment. Loose foundation bolts. Damaged bearings. Rotor imbalance. Coupling wear. Air gap unevenness. Driven equipment vibration. Electrical imbalance. Solutions Check the motor base, foundation bolts, and coupling alignment. Inspect the bearing temperature and listen for grinding or knocking sounds. If vibration increases after maintenance, the coupling or rotor may need balancing. You should also check whether the vibration comes from the motor itself or from the connected machine. High Current During Operation High current is a warning sign. It may come from electrical problems, overload, poor excitation, or mechanical resistance. Long-term high current can overheat windings and damage insulation. Common Causes Motor is overloaded. Driven equipment is blocked or jammed. Supply voltage is too low. Voltage imbalance between phases. Incorrect excitation current. Bearing friction increases mechanical load. Motor parameters do not match the application. Solutions Measure the three-phase current and voltage. When one phase shows abnormal current, inspect the power supply and cable connections. If all phases are high, inspect the load condition. For pumps and fans, check whether valves, dampers, or process conditions are causing excessive load. Poor Power Factor One advantage of a synchronous motor is that its power factor can be adjusted by changing excitation current. If the excitation is not set correctly, the motor may operate with a poor power factor. Common Causes Under-excitation. Over-excitation. Incorrect AVR setting. Load changes without excitation adjustment. Fault in field current control. Solutions Adjust the excitation current according to the required power factor. Under-excitation usually causes lagging power factor, while over-excitation can make the motor operate with leading power factor. For plants using synchronous motors for power factor correction, regular monitoring is important. Excitation System Failure The excitation system powers the rotor field with DC. If the excitation system fails, the synchronous motor may not start properly, may fail to pull into synchronism, or may trip during operation. Common Causes AVR failure. Exciter fault. Brush or slip ring wear. Field winding open circuit. Field circuit short circuit. Loose wiring in excitation