Driving Multiple Motors with a Single VFD

 
Some applications (fans, pumps, etc.) might benefit from running multiple induction motors at the same time with a single VFD. This post gives an overview of running multiple motors from a central VFD.

Many automation systems employ multiple motors to distribute loads, execute diverse tasks, or enhance overall efficiency. Effective motor management is therefore essential for maximizing performance and reliability. While various multi-motor control strategies exist, using a single Variable Frequency Drive (VFD) to power multiple induction motors presents a simple and often cost-effective solution in suitable applications.

This article explores the advantages, considerations, and setup process for using one VFD to control multiple motors.

Why Use One VFD for Multiple Motors?

Using a single VFD for multiple motors strikes a balance between cost savings and effective control. To understand its benefits, let’s compare it with other control methods.

1. Contactors – Simple On/Off

The first type of system for driving multiple motors is to use contactors to line feed motors. This is the simplest method with the least amount of components. However, this offers no speed  motor control while introducing high inrush currents that cause more wear on system components over time.

• Advantages: Simple setup and minimal components.
• Disadvantages: High inrush currents cause mechanical and electrical strain. Uncontrolled speed leads to greater machine wear.

2. Separate VFDs for Each Motor – Full Control

The next type of motor control setup provides one VFD for every motor in the system. This will be the most expensive option, but it will also give you the most control over speed, torque, and position

• Advantages: Independent speed, torque, or position control for each motor. Precise current measurement/monitoring. Built-in overload protection.
• Disadvantages: High cost, increased complexity, and a larger cabinet footprint.

3. A Single VFD for Multiple Motors

Lastly, we show you the basic system design to power multiple motors with one VFD. By consolidating control into one VFD, machine builders can simplify their systems, reduce costs, and save space.

Advantages of a Single VFD

Utilizing a single variable frequency drive to control multiple motors offers several compelling advantages, impacting both performance and cost. From smoother starts that minimize wear and tear on equipment to a reduction in both component count and required cabinet space, a single VFD can provide significant benefits.

1. Smooth Start → Reduced Wear

A major benefit of a VFD is the ability to gradually ramp motor speed up or down. Unlike direct on/off switching, which generates inrush currents of 300-900% of the motor’s rated current, a VFD limits these currents, reducing mechanical strain and extending the equipment’s lifespan.

2. Fewer Components → Lower Cost

Using one VFD instead of multiple reduces equipment costs significantly. It also cuts down on wiring, cooling requirements, and components like fuses and braking resistors, making it an ideal choice for budget-conscious projects.

3. Reduced Cabinet Size → Spatial Efficiency

Consolidating smaller drives into one larger VFD often results in a smaller overall system. The reduced need for individual drives and associated components frees up valuable space in control cabinets, enabling more compact designs.

Key Considerations

Using a single VFD to drive multiple motors is not suitable for every situation. Below are the key factors to consider:

1. Induction Motors Only

This approach works only with induction motors, as opposed to servomotors. Servomotors require precise position feedback to keep track of the rotor position, which a single VFD cannot provide for multiple motors. Induction motors, however, tolerate some slip and can operate without feedback adjustments. For applications using servomotors, separate drives should always be used for each motor.

2. Correctly Size the VFD

The VFD must handle the combined load of all connected motors. To ensure this, size the VFD for at least the total full-load amps (FLA) of all motors. It’s best practice to add a safety margin of 20-25% to accommodate surges or fluctuations.

3. Individual Overload Protection

A single VFD measures the total current across all motors, so it cannot detect an individual motor’s overload. Each motor requires its own overload relay or protection device to shut down safely if necessary.

Ideal Applications

This setup is ideal for applications where motors need to operate at the same speed without precise control. Common use cases include:

• Conveyor Systems: Motors powering multiple conveyor sections at the same speed.
• Cooling Fans: HVAC systems with multiple fans operating simultaneously.
• Pump Systems: Irrigation or water treatment systems with synchronized pumps.

How to Set Up a Single VFD for Multiple Motors

Configuring a single VFD to control multiple motors requires careful planning and execution to ensure efficient and reliable operation. Follow these guidelines…

1. Match Voltage and Frequency

Ensure all motors share the same rated voltage and frequency. This allows the VFD to distribute a uniform signal to all motors. It is okay for the motors to have different rated speed and rated power.

2. Connect Motor Leads to Drive Output (U, V, W)

Use terminal blocks to connect the VFD’s U, V, and W output terminals to each motor. Ensure all connections are consistent (e.g., U to U, V to V, and W to W).

3. Incorporate Overload Protection

Install individual overload protection for each motor. This step is critical, as it protects individual motors from drawing excessive current and overheating.

4. Program the VFD for Open-Loop (V/Hz) Mode

Set the VFD to open-loop (V/Hz) mode, which uses a fixed voltage-to-frequency ratio for motor control. This mode is suitable for systems where precise feedback isn’t required. The open-loop output will uniformly apply voltage across all motors at the drive output.

5. Test and Fine-Tune

Test run the system, gradually ramping the motors up to speed. Monitor performance and make ramp adjustments to ensure smooth operation.

Three Advantages to Using One VFD to Control Multiple Motors

Smaller cabinet size

One large drive will have a smaller footprint than multiple smaller VFDs. Additionally, that one large drive requires only one branch circuit protection (e.g. fuses), incoming wiring, and braking resistor. The net result is a smaller electrical enclosure.

Cost reduction

There will be some economic savings in using one large drive compared to many smaller drives. There will be similar savings due to line fuses, brake resistors, and a smaller enclosure. There will be added cost for motor overload protection (described later) but this is more than offset by the other factors.

Simple parameter adjustment

One cost often underestimated is the start-up and commissioning time. A centralized drive will have only one set of parameters that need to be configured and set. The parameter adjustment usually entails the setting of the V/Hz curve and required system protection parameters. This can be easily handled in KEB’s free Combivis drive software.

Considerations

Drive size

At a minimum, the drive should be sized to handle the sum of the FLA-rated currents for all connected motors.

A best practice would be to add an additional 20-25% current capacity as a safety factor.

Each motor must have its own individual overload protection

Drives typically include functionality that detects motor overloads and can shut down appropriately. KEB drives comply with the motor overload functionality described in VDE 0660, Part 104.  The functionality allows the motor-rated current to be entered into the drive. Then the drive measures the instantaneous and time-averaged output current to detect an overload situation and shutdown.

If multiple motors are connected to the VFD, the VFD cannot tell how much of its output current is being delivered to each motor. Therefore, it cannot determine a motor overload situation. Furthermore, due to sizing, the drive is typically able to provide much more current than one motor can handle.

This means that each motor should have its own motor overload protection.

Motor shaft performance

The motors will load share but they are not synchronized. This is likely fine for the air handling or pump applications but will not work for winding applications or anything that requires a tight speed regulation.

If this topology does not meet the required speed regulation, then a different drive topology (e.g. master-slave configurations) will need to be implemented.

Conclusion

Driving multiple motors with a single VFD is a practical, cost-effective solution for applications where discrete motor control isn’t required. It provides smooth startups, reduces equipment costs, and saves valuable cabinet space compared to simpler or more advanced configurations.

The simplicity and efficiency of this configuration make it an excellent choice for many industrial applications. With the proper configuration and overload protection, a single VFD can streamline take control of the whole system, improving both system performance and reliability.