Yes, two motors can drive one shaft—but the engineering problem is load sharing, not simply making both motors show the same RPM. A workable system coordinates torque, speed, feedback, protection and fault response. Depending on the motors and mechanics, the right solution may be one larger motor, two matched motors on one approved inverter, two drives with master–follower torque control, or two servo axes using electronic gearing.
What “synchronized” must mean
Several different relationships are often called synchronization:
- Same speed: both motors rotate at the same average speed.
- Same position: the rotors or connected shafts maintain a defined angular relationship.
- Same electrical phase: required when permanent-magnet or brushless motors share one inverter.
- Shared torque: both motors contribute useful torque in the same direction and divide the load.
For a common shaft, shared torque is usually the main objective. A rigid shaft can force equal speed even while the motors produce opposing torque, causing current, heat and vibration.
The torque and power basis
For two motors driving one shaft, the available shaft torque is approximately:
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Tshaft ≈ T1 + T2 − Tloss
During acceleration, required torque is:
Trequired = Jtotalα + Tload + Tfriction
With equal motors and symmetric mechanics, the target is often T1 ≈ T2 ≈ Trequired/2. Real sharing differs because of torque-constant tolerances, winding differences, gearbox efficiency, torsional compliance, backlash, bearing friction, encoder errors, cooling and loop tuning. Size the shaft, couplings, gearboxes, bearings, drives, braking hardware and thermal system for the actual continuous and peak duty—not simply twice one motor’s nameplate.
Four viable system architectures
One larger motor
This is often the simplest and safest first option: one motor, one drive, one feedback system and no torque-sharing loop. It reduces commissioning and failure modes, although a suitably large motor or drive may be harder to source and may require larger cables, protection and cabinet space. Compare complete installed cost and availability, not just motor prices.
Two matched motors on one inverter
A multi-motor drive can be practical for identical induction motors that are rigidly coupled and supported by the inverter manufacturer. SEW-EURODRIVE specifies the same motor type and winding data for its multi-motor arrangement: SEW multi-motor drive guidance. Its asynchronous-motor guidance also addresses rigid coupling, alignment and encoder placement: SEW parallel-motor guidance.
Advantages are one command path and one drive. Limitations include restrictive motor matching, imperfect current sharing, shared fault consequences and poor suitability for arbitrary or precision-servo combinations. Permanent-magnet motors additionally require compatible electrical characteristics and phase alignment; Kollmorgen documents these requirements for two motors on one drive: Kollmorgen parallel-motor guidance.
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Two drives with torque-follower load sharing
This is generally the most flexible industrial arrangement. A master drive regulates common-shaft speed; a follower regulates torque or current and receives a coordinated torque reference. Rockwell describes this speed-master/torque-follower pattern in its load-sharing material: Rockwell load-sharing example.
Each drive can have its own current limits and protection, and the desired split can be equal or deliberately biased. The additional drive, communications, tuning and fault logic increase cost and commissioning effort.
Two servo axes with electronic gearing
Electronic gearing makes a slave follow a master’s commanded position at a defined ratio, such as 1:1. Siemens describes it as a software relationship between servo axes: Siemens servo synchronization overview. Kollmorgen documents master-position following and gear ratios here: Kollmorgen electronic gearing.
This is appropriate for separate rollers, line shafts and coordinated axes. It does not automatically divide torque between two motors rigidly attached to the same shaft. Two aggressive position loops can cross-couple and fight; a rigid common shaft usually needs coordinated torque control instead.
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Choosing the architecture
| Situation | Usually preferred | Important qualification |
|---|---|---|
| Two identical induction motors, rigidly coupled, modest dynamics | One inverter with approved multi-motor function | Follow the drive maker’s motor-matching and protection rules. |
| High power or high torque | Two drives with load sharing | Use a speed master and torque/current follower or a supported equivalent. |
| Separate precision axes | Electronic gearing | Position following is not equal torque sharing. |
| Mismatched motors | Separate coordinated drives, or redesign | Do not parallel them unless the manufacturer explicitly supports it. |
| One suitable motor is available | One larger motor | It often has the lowest control and maintenance risk. |
Recommended load-sharing control
Master speed, follower torque
A common arrangement is:
Common-shaft encoder → master speed loop → master torque command → follower torque reference
The master controls shaft speed; the follower controls torque or current. Set a sharing factor k so that T1,ref = kTtotal and T2,ref = (1−k)Ttotal. For equal motors, k is typically 0.5, but unequal gearboxes, cooling or load paths may justify another value. Monitor speed error, torque difference, encoder validity and communication health.
Other supported methods
- Torque coupling: coordinated torque references and feedback correct unequal contribution.
- Speed override with torque limit: the follower remains speed-compatible but cannot push beyond a defined torque.
- Droop and compensation: a controlled speed offset as torque rises helps the drives settle into a stable load division.
Siemens documents these load-sharing methods and warns that mechanically coupled drives without suitable sharing can oppose one another or oscillate: Siemens load-sharing documentation.
Why independent speed loops can fight
Small differences in speed-loop gain, encoder scaling, torque constant, friction, gear ratio, command timing or deadband create different torque commands. The shaft forces both motors to the same speed, so each controller may try to correct an error caused by the other. Symptoms include circulating torque, excess current, heating, growling, torsional oscillation, poor acceleration and repeated trips. Equal RPM is therefore not proof of equal or useful torque.
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Mechanical and feedback design
Couplings and transmission
- Rigid couplings enforce the angular relationship but transmit torque differences directly and demand accurate alignment.
- Flexible couplings tolerate some alignment error but add torsional compliance and possible resonance.
- Gears require closely matched ratios; backlash and efficiency differences alter load division.
- Belts and chains can stretch, slip and develop unequal tension.
- Differential or summing gearboxes combine inputs mechanically but add backlash, lubrication, cost and maintenance.
Encoder location
Feedback may be taken from one motor, the common load shaft, or each motor. A load-side encoder better represents actual output when gearbox backlash or shaft torsion is significant. In a specific asynchronous multi-motor configuration, SEW recommends placing the encoder on the gearmotor with the greatest clearance or elasticity relative to the load inertia: SEW encoder guidance. ABB discusses load-side and line-shaft feedback arrangements for master-follower systems: ABB servo catalog.
For coupled servo motors, use a drive platform that explicitly supports the topology. Siemens’ SERVCOUP documentation covers multiple mechanically coupled drive objects and shared-encoder arrangements: Siemens SERVCOUP manual.
Design and commissioning procedure
- Document the mechanics: record whether the motors share a rigid shaft or connect through gears, belts or chains; note backlash, slip, compliance and whether one motor can rotate while the other is disabled.
- Calculate duty: determine continuous and peak load torque, acceleration, speed, inertia reflected through transmissions, friction, regenerative energy, duty cycle and thermal environment.
- Select the topology: choose an approved one-inverter arrangement, torque-sharing drives, electronic gearing or a simpler single-motor design.
- Match components: verify motor type, voltage, pole count, winding data, ratings, gearbox ratios, encoder compatibility and drive firmware.
- Configure limits: set current and torque limits, ramps, regeneration limits, sharing bias, follower speed window, communication timeout, overspeed and encoder-loss responses.
- Verify direction and feedback: check phase order, encoder polarity, scaling and physical rotation with low-speed jogs.
- Run unloaded: observe each motor’s current and torque, then add load gradually while checking sharing and temperature.
- Exercise the machine: test acceleration, deceleration, reversing, stall and emergency stop under realistic duty.
- Test faults: test master or follower failure, encoder disconnection, communication loss, one motor disabled, coupling problems, overtemperature, jam, overspeed and regenerative overvoltage. The machine must enter a defined safe state rather than leaving one drive pushing a locked shaft.
Failure symptoms and likely causes
| Symptom | Likely causes |
|---|---|
| One motor draws much more current | Unequal parameters, ratio error, misalignment, preload, brake not releasing, encoder or torque scaling error, different limits. |
| Motors oscillate or growl | Independent speed loops on a rigid shaft, excessive gain, torsional resonance, wrong sign or encoder polarity, follower bandwidth or communication delay. |
| Shaft turns but overheats | Opposing torque, poor sharing, one motor carrying nearly all load, inadequate low-speed cooling, excessive duty or incorrect motor data. |
| One drive trips during acceleration | Peak limit, excessive inertia, jam, wrong ramp, reversed torque sign, follower not tracking or DC-bus limitation. |
| Electronic-gear slave loses synchronization | Slave acceleration or speed limit, feedback scaling, gear ratio, command/update rate, synchronization mode or position-error limit. |
Kollmorgen notes that an electronic-gear slave must reach and follow the master within its configured velocity and acceleration limits: Kollmorgen gearing limits.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Industrial platform examples and buying criteria
Examples are architecture references, not universal recommendations:
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- Siemens SINAMICS G: documented torque coupling, speed override and droop/compensation for mechanically coupled drives.
- Siemens SINAMICS SERVCOUP: coordinated mechanically coupled servo drive objects.
- SEW-EURODRIVE MOVI-C: integrated gearmotor and supported multi-motor systems.
- Rockwell PowerFlex 755: torque-follower arrangements for Rockwell-based plants; product information is at PowerFlex 755.
- Kollmorgen AKD/AKD2G: precision servo control and electronic gearing; see AKD and S200/AKD2G information.
- ABB servo systems: line-shaft and master-follower feedback arrangements.
Industrial systems are generally sold through configuration, distributors or quotation rather than fixed public pricing. Compare complete quotations including motors, drives, gearboxes, encoders, coupling, cabinet, braking, safety functions, programming, commissioning, service and replacement availability.
When two motors are the wrong answer
Choose one larger motor, a gearbox, a different transmission or a redesigned load path when it provides the required torque with fewer controllers, sensors, couplings and fault combinations. Two motors are justified when packaging, availability, redundancy, power level or independent mechanical load paths outweigh the added control complexity.
Frequently Asked Questions
Can I connect two different motors to one drive?
Usually not without explicit drive-manufacturer support. One-inverter multi-motor systems commonly require matched motor type and winding data; mismatched motors need a separately engineered, coordinated-drive solution.
Does electronic gearing guarantee equal torque?
No. Electronic gearing coordinates commanded position or speed. Equal torque requires a load-sharing strategy with suitable torque/current control and feedback.
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No. One encoder can be sufficient in specific supported topologies, but backlash, torsional compliance, safety requirements and diagnostic needs may justify load-side or per-motor feedback.
The Bottom Line
For two motors rigidly driving one shaft, start with torque sharing: use one approved multi-motor inverter only with properly matched motors, or use two coordinated drives with a speed-regulating master and torque-regulating follower. Reserve electronic gearing for axes that need a precise motion relationship, and choose one larger motor whenever it delivers the required duty with materially less system risk.
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