Multi-motor drives and their coupling
Multi-motor drives in this context represent applications where multiple motors work together to drive specific load system. Unlike individual drives, there is certain coupling between the motors.
Hard and soft coupling
The motors can be coupled through the power part. That is sort of “hard” coupling. Alternatively, the motors are “soft” coupled through the control system (such as e.g. master-follower or overriding control distributing the process reference among the motor drives).
- Coupling through the power part
- Coupling through the control system
Coupling in multidrives
Specific category among the multi-motor drives are VFD multidrives with multiple inverters supplied from a common dc link. Multi-motor drives with a common frequency converter can have following coupling:
- mechanical coupling
- multi-motor traction vehicles
- roller element conveyor systems
Type of mechanical coupling
Mechanical coupling inside a multi-motor drive can be:
- rigid mechanical coupling (coupling, gear)
- flexible mechanical coupling (friction, adhesion)
Technological coupling
Quite often, the motors are coupled through the technological process itself. Examples of technological coupling include:
- steel rolling mills
- paper production
- fibre production
Advantages of multi-drives with individual inverters on a common dc link
VFD multidrives often wide range of benefits, as reported in depth in our previous posts. Some of the are listed below.
- Individual drives share their input power at dc bus level, i.e. closer to the motor.
It results in:- better overall efficiency (input power part of higher power rating has lower losses
than the sum of multiple smaller units of the same total power) - The motor group does not require special equipment for occasional active braking
- All motor drives can share one common resistive braking equipment (chopper)
- All motor drives can use one common regenerative braking arrangement
- Economically easier minimization of grid harmonics, e.g. by using a common multi-pulse rectifier
- Savings due to less power cables, cheaper common rectifier and dc bus, less switching elements (circuit breakers)
- better overall efficiency (input power part of higher power rating has lower losses
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References
[1] VFD dimensioning: Multidrives and their benefits, MB Drive Services, September 2020, available online, https://mb-drive-services.com/vfd-dimensioning-multidrives-and-their-benefits/
[2] VFD multidrives: Improved energy efficiency, MB Drive Services, September 2020, available online, https://mb-drive-services.com/vfd-multidrives-improved-energy-efficiency/
[3] VFD multidrives: Reduced footprint, MB Drive Services, October 2020, available online, https://mb-drive-services.com/vfd-multidrive-reduced-footprint/
[4] VFD multidrives: Grid harmonics, MB Drive Services, October 2020, available online, https://mb-drive-services.com/vfd-multidrives-grid-harmonics/
[5] VFD multidrives: Investment cost, MB Drive Services, October 2020, available online, https://mb-drive-services.com/vfd-multidrive-investment-cost/
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