Torque Control and Master-Slave Operation in Industrial Motor Systems

When Torque Sharing Goes Wrong

Two motors drive the same conveyor belt. One runs hot. The other runs cool. The belt tracks off-center, mechanical wear accelerates, and the maintenance team chases vibration faults every few weeks. The problem is not mechanical. It is torque distribution, and fixing it starts with understanding torque control and master-slave operation in drive systems.

Most multi-motor applications have this problem somewhere. Overhead cranes, winders, extruders, and any system where two or more motors share a mechanical load are all candidates. Without active torque management, small differences in motor parameters, cable lengths, or drive firmware create load imbalances that compound over time.

This article covers how torque control works at the drive level, how master-slave configurations enforce load sharing, and the practical steps for getting both right during commissioning or troubleshooting.

What Torque Control Actually Does

A variable frequency drive (VFD) running in speed control mode regulates shaft RPM. The drive adjusts output voltage and frequency to maintain the speed setpoint, and torque is a byproduct. This works fine for single-motor applications with predictable load profiles.

In torque control mode, the drive regulates output current to maintain a torque setpoint instead. Speed becomes the dependent variable. The motor produces whatever torque is commanded, and the shaft speed responds to the load.

Torque control is not the same as current limiting. Current limiting is a protective function that prevents the drive from exceeding its rated output. Torque control is an active loop that targets a specific torque value, typically derived from a reference signal supplied by a controller or a master drive.

Torque control operates on a fundamentally different control philosophy than speed mode. Understanding that distinction is essential before configuring any multi-drive application.

Torque Reference Sources

The torque reference can come from several places depending on the application architecture. Analog input from a PLC or master drive is the most common. Some drives accept torque references over fieldbus protocols such as PROFINET, EtherNet/IP, or CANopen. Others derive it internally from a speed error signal, which is the basis for droop control.

The source of the torque reference determines the response latency and the level of coordination possible between drives. Analog references are simple but slower. Fieldbus references offer tighter synchronization but require network reliability. Understanding this tradeoff matters when selecting hardware for a new installation.

How Master-Slave Configuration Works

In a master-slave drive configuration, one drive (the master) runs in speed control mode and establishes the system reference. All other drives (slaves) run in torque control mode and follow the torque reference output by the master. The mechanical shaft ties them together, but the torque control loop is what balances the load.

The master drive outputs a torque reference signal derived from its own torque demand. If the master is working at 60% torque to maintain speed, it sends that same 60% reference to every slave. The slaves regulate their output to match, distributing the load evenly across all motors. This is active load sharing, and it is the correct approach for any application where mechanical coupling exists between motors.

Hard vs. Soft Coupling

The coupling type between motors changes how the master-slave loop behaves. Hard-coupled systems, such as a common gearbox or rigid shaft, force the same speed on all motors. Load imbalance here is purely a torque problem. Soft-coupled systems, like conveyor belts or winders with some compliance, allow slight speed differences. In these cases, both torque and speed need to be managed.

For soft-coupled systems, some engineers add a small amount of droop to the slave drives. Droop intentionally allows the slave speed to sag slightly under increasing load, which introduces a self-correcting mechanism without requiring tight loop coordination. The tradeoff is reduced speed accuracy at the slave, which is acceptable in most winder and conveyor applications.

Drive-Level Implementation

The implementation path depends on the drive platform. Most modern VFDs from Siemens, ABB, Danfoss, Rockwell, and Yaskawa support torque control natively. The parameter names differ, but the underlying function is the same.

Analog Torque Reference (Simple Method)

Connect the master drive's analog torque output (typically 0-10V or 4-20mA) to the slave drive's analog input. Configure the slave's control mode to torque control and map the analog input to the torque reference. Set the torque scaling to match the master's output range.

This method is reliable and straightforward, but it introduces an inherent delay. Analog signal propagation, A/D conversion, and loop scan times add up. For most conveyor and pump applications, the latency is acceptable. For high-performance winders or printing machines where torque response is critical, fieldbus is preferable.

Fieldbus Torque Reference (High-Performance Method)

Configure the master drive to publish its torque output value on the fieldbus network (PROFINET, EtherNet/IP, or DeviceNet). Slave drives subscribe to that value and use it as their torque setpoint. The PLC handles the topology, but the torque data moves directly between drives with minimal processing overhead.

This approach requires all drives to share the same network protocol and have compatible firmware. Check that the master's torque output word maps correctly to the slave's torque reference input. Many commissioning errors here come from word length mismatches or scaling differences between drive families.

Comparing Torque Control Modes

Not all torque control configurations suit every application. The table below summarizes the key tradeoffs to guide selection.

PLC Integration and Communication

Implementing torque control through a PLC adds flexibility but increases the demands on the control program. The PLC reads the master drive's torque output via fieldbus, applies any scaling or filtering, and writes the torque reference to each slave drive. This allows dynamic torque adjustments based on process conditions, which is not possible with direct drive-to-drive communication.

In Siemens TIA Portal, the SINA_SPEED or SINA_POS function blocks handle drive communication. For Allen-Bradley systems using Studio 5000, the Drives AOI library handles parameter mapping for PowerFlex drives. In both cases, the torque reference word must be enabled in the drive's telegram configuration before the PLC write will take effect. Missing this step is one of the most common commissioning errors.

Scan time matters here. A PLC with a 10ms task cycle introducing a 10ms latency into the torque loop may cause hunting or instability in fast-response applications. On slower processes, it is not an issue. Match the PLC scan rate to the application's torque response requirement before finalizing the architecture.

Troubleshooting Scenario: Load Imbalance on a Conveyor

Field experience teaches what documentation does not: small configuration errors in master-slave setups produce symptoms that look mechanical until you check the drive diagnostics.

A 150-meter inclined conveyor uses two identical 45kW motors and two VFDs configured in master-slave. After a firmware update on the slave drive during a scheduled maintenance window, the conveyor starts running hot on the tail-end motor. Belt tension is uneven, and the master drive begins tripping on overcurrent at full load.

The firmware update reset the slave drive's torque scaling parameter to factory default. The master was outputting a 0-10V analog reference scaled to 100% torque at 10V. The slave was now interpreting 10V as 150% torque. The slave motor was over-torquing, the master was compensating by reducing its output, and the imbalance was driving the overcurrent fault.

Diagnosis Path

• Compare live torque feedback values on both drives simultaneously via the HMI or drive diagnostic software.
• Verify that torque scaling parameters on both drives match. Check manufacturer-specific parameters (e.g., Siemens P1520, ABB 1.03).
• Cross-reference any recent firmware updates or parameter backups against current settings.
• Restore scaling, verify at no-load, then run a loaded ramp test before returning to production.

This scenario repeats across plants regularly. Firmware updates, drive replacements, and even temporary parameter changes for testing can silently shift scaling. Always document baseline parameters and compare after any maintenance event involving drive configuration.

Sourcing Considerations

When sourcing drives for a master-slave application, purchasing drives from a single manufacturer within the same product family reduces integration risk significantly. Analog and fieldbus signal scaling, telegram structures, and default parameter sets are consistent, which shortens commissioning time and simplifies troubleshooting.

Mixed-manufacturer setups are common in retrofit projects where the master is already installed. In these cases, verify communication protocol compatibility before ordering. A Siemens G120 master and a Yaskawa A1000 slave can share an analog torque reference, but fieldbus integration requires matching protocol support and careful telegram mapping.

Counterfeit VFDs are a real procurement risk, particularly in online marketplaces and secondary supply chains. A counterfeit drive may pass visual inspection but fail under load or exhibit erratic torque response in closed-loop operation. Always procure from authorized distributors and verify part numbers against manufacturer documentation before installation.

Lead times on specific drive models can run 10 to 16 weeks in constrained market conditions. Identify your drive specification early and confirm stock levels with your distributor before finalizing the project schedule. Substituting an alternative model mid-project to meet a deadline introduces compatibility risk that typically costs more time than the original delay.

Closing

Torque control and master-slave operation solve real problems in multi-motor systems, but only when the configuration is correct and verified. The parameters, scaling, and communication topology need to be confirmed at commissioning and after any maintenance event that touches drive configuration. These are not set-and-forget settings.

If you're specifying drives for a master-slave application or troubleshooting a load-sharing problem in an existing system, Techno Control Corp can assist with component selection, verified sourcing, and technical guidance. Reach out to the team directly or explore our VFD and motor control inventory to find the right hardware for your application.