Many applications use one or more motors running in parallel at the same desired speed. Using a frequency converter to control these multiple motors has many advantages, summarized below.

Advantages of using one drive to control multiple motors:
1. save money
2. Reduce cabinet size, complexity and design costs
3. Can reduce the footprint of the motor and drive load
4. Reduce maintenance time and cost
5. Reduced inventory requirements
6. Reduce the complexity of the control system

This saves money because one high-power rated drive is less expensive than multiple low-power drives. Each frequency converter requires its own circuit protection, so using one frequency converter also reduces costs in this regard.

one inverter drive multiple motors

Drives can be housed in smaller enclosures because one large drive requires less cabinet space than multiple smaller devices, saving space and money. Design costs are also reduced because it is easier to design an enclosure to accommodate one relatively large frequency converter than multiple smaller frequency converters. A single large frequency converter also generates less heat than multiple smaller devices, further simplifying enclosure design and saving energy.

The footprint can often also be reduced in terms of motors and connected loads. For example, it is possible to fit multiple small fans of smaller size into a narrow duct space than one large fan.

Maintenance time and costs are reduced because only one drive needs to be maintained instead of multiple smaller drives, which also reduces inventory requirements. Each motor is relatively small and is usually available as an off-the-shelf standard product. Because many motors are identical, spares can be stockpiled and quickly replaced in the event of a failure.

The entire control system has also become simpler. There is no need to connect many frequency converters to a master controller, typically a PLC, to have them run in sync; only one connection is required. When programming the PLC, only one inverter speed control loop needs to be configured, not multiple.

In general, the use of frequency converters as described above is justified in applications where the cost of using one frequency converter per motor is prohibitive. In this case, the application can benefit from all the advantages of running the motor at a controlled speed including reduced energy costs, longer motor life and better operational performance.

But even with these benefits in mind, most drive installations with multiple motors use one drive per motor. Why is this?

Disadvantages of using one drive to control multiple motors:
1. All motors must run at the same speed
2. The design must consider the inverter as a single point of failure
3. All motors start at the same time

control-multiple-motors

When applying one frequency converter to control multiple motors, several conditions must be met.

1. Each motor must have the same operating speed.

2. Running multiple motors from one inverter creates a single point of failure. If the frequency converter fails, all motors connected to it will be unusable. Various inverter bypass schemes can be used to overcome this limitation but these schemes add cost and complexity to the system. Although the frequency converter becomes a single point of failure, in many applications the reliability of the motor and connected load can actually be improved by using multiple frequency converters. Because now there are multiple smaller motors instead of one big motor. If one motor fails, it can usually continue to operate with the remaining motors. In these cases, the rest of the motors run at the speed controlled by the frequency converter, with reduced power but usually enough to run the entire system.

3. Care must be taken when operating the motor. In order to minimize the number of frequency converters, all motors need to be started simultaneously. The frequency converter brings all the motors up to a controlled speed, minimizing the inrush current required by each motor at start-up. If the motors cannot start at the same time, the frequency converter must increase the power.

What types of applications meet the three specific criteria above?

Applications using dual fans or pumps are good choices. Two devices at the same speed, rather than competing with each other or putting more load on one device than it was designed for. Air handling systems, exhaust/supply fans, makeup air units, recovery wheels and fan arrays are also good candidates for controlling multiple motors with one frequency converter.
Once it is determined that the application falls under the specific conditions required, the next step is detailed design, where attention must be paid to the correct application of the frequency converter and associated components.

Many applications use one or more motors running in parallel at the same desired speed. Using a frequency converter to control these multiple motors has many advantages, summarized below.

Advantages of using one drive to control multiple motors:
1. save money
2. Reduce cabinet size, complexity and design costs
3. Can reduce the footprint of the motor and drive load
4. Reduce maintenance time and cost
5. Reduced inventory requirements
6. Reduce the complexity of the control system

This saves money because one high-power rated drive is less expensive than multiple low-power drives. Each frequency converter requires its own circuit protection, so using one frequency converter also reduces costs in this regard.

one inverter drive multiple motors

Drives can be housed in smaller enclosures because one large drive requires less cabinet space than multiple smaller devices, saving space and money. Design costs are also reduced because it is easier to design an enclosure to accommodate one relatively large frequency converter than multiple smaller frequency converters. A single large frequency converter also generates less heat than multiple smaller devices, further simplifying enclosure design and saving energy.

The footprint can often also be reduced in terms of motors and connected loads. For example, it is possible to fit multiple small fans of smaller size into a narrow duct space than one large fan.

Maintenance time and costs are reduced because only one drive needs to be maintained instead of multiple smaller drives, which also reduces inventory requirements. Each motor is relatively small and is usually available as an off-the-shelf standard product. Because many motors are identical, spares can be stockpiled and quickly replaced in the event of a failure.

The entire control system has also become simpler. There is no need to connect many frequency converters to a master controller, typically a PLC, to have them run in sync; only one connection is required. When programming the PLC, only one inverter speed control loop needs to be configured, not multiple.

In general, the use of frequency converters as described above is justified in applications where the cost of using one frequency converter per motor is prohibitive. In this case, the application can benefit from all the advantages of running the motor at a controlled speed including reduced energy costs, longer motor life and better operational performance.

But even with these benefits in mind, most drive installations with multiple motors use one drive per motor. Why is this?

Disadvantages of using one drive to control multiple motors:
1. All motors must run at the same speed
2. The design must consider the inverter as a single point of failure
3. All motors start at the same time

control-multiple-motors

When applying one frequency converter to control multiple motors, several conditions must be met.

1. Each motor must have the same operating speed.

2. Running multiple motors from one inverter creates a single point of failure. If the frequency converter fails, all motors connected to it will be unusable. Various inverter bypass schemes can be used to overcome this limitation but these schemes add cost and complexity to the system. Although the frequency converter becomes a single point of failure, in many applications the reliability of the motor and connected load can actually be improved by using multiple frequency converters. Because now there are multiple smaller motors instead of one big motor. If one motor fails, it can usually continue to operate with the remaining motors. In these cases, the rest of the motors run at the speed controlled by the frequency converter, with reduced power but usually enough to run the entire system.

3. Care must be taken when operating the motor. In order to minimize the number of frequency converters, all motors need to be started simultaneously. The frequency converter brings all the motors up to a controlled speed, minimizing the inrush current required by each motor at start-up. If the motors cannot start at the same time, the frequency converter must increase the power.

What types of applications meet the three specific criteria above?

Applications using dual fans or pumps are good choices. Two devices at the same speed, rather than competing with each other or putting more load on one device than it was designed for. Air handling systems, exhaust/supply fans, makeup air units, recovery wheels and fan arrays are also good candidates for controlling multiple motors with one frequency converter.
Once it is determined that the application falls under the specific conditions required, the next step is detailed design, where attention must be paid to the correct application of the frequency converter and associated components.

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