Extremely compact multidrives offer an array of benefits to integrators, OEMs, panel builders and end users who are optimizing energy usage through precise control of motors. Since drives are available both in multidrive and single drive configurations, it is useful to define the difference -- and profile the benefits that the next generation of multidrives now provides to users.

A Single Industrial Drive

Adjustable speed drives are used in any application in which mechanical equipment is powered by motors; the drives provide extremely precise electrical motor control, so that motor speeds can be ramped up and down, and maintained, at speeds required; doing so utilizes only the energy required, rather than having a motor run at constant (fixed) speed, using an excess of energy.

Since motors consume a majority of the energy produced, their control, based on load demands, increases in importance, as energy supplies become ever more strained. Additionally, end users of motors can realize 25-70% energy savings via use of motor controllers. (Despite these benefits, the majority of motors continue to be operated without drives.)

Top-of-the-line, single industrial drives are highly flexible ac drives that can be customized to meet the precise needs of a single-motor application. These units convert ac power to dc, and then invert the dc back to an ac output to a motor. These drives cover a wide, full range of powers and voltages. Single industrial drives also feature a wide range of built-in options as standard equipment. They can be installed for most applications right out of the box; and they also can be ordered and manufactured as a customized unit for a particular application.

What Is a Multidrive?

A multidrive is built from industrial drive modules that are connected to a common dc bus bar. The common bus bar is used to supply the drive modules with dc power, and each module then inverts the dc to ac and powers an individual motor. The dc power is derived from a single supply unit (rectifier) that is built into the front end of the same multidrive configuration.

This construction simplifies the total installation and results in many benefits: savings in cabling; reduced line currents and simpler braking arrangements; energy distribution over the common dc bus bar, which can be used for motor-to-motor braking without the need for a braking chopper or a regenerative supply unit; reduced component counts; increased reliability; space savings; and there is no need for a separate Motor Control Center (MCC).

An extremely compact multidrive series from ABB, for example -- the ACS800 -- offers an array of benefits to integrators, OEMs, panel builders, and end users who are optimizing energy usage through precise control of motors.

Where Can Multidrives Be Used?

In general terms, multidrives can be used whenever several drives/motors form part of a single or integrated mechanical process. The common supply of the multidrive enables implementation of overall safety and control functions, and permits the close coordination of individual drive motors. For example, a paper machine has many motors that must be individually controlled as a complete system.

Multidrives offer fast communication of torque and speed signals between the drives, to control the tension in the paper web. Multidrives also can be used where the shafts of the individual drive motors are not tightly coupled; for example, in processes where each drive module can be programmed with a speed profile so that the overall use of energy is minimized. These two examples merely demonstrate the range of applications where multidrives offer substantial benefits over other types of drive constructions.

Multidrive Benefits

Modular configuration of multidrives provides control of multiple motors across a 1-7,000 hp power range, from 380-600 (690) Vac. A single rectifier unit can be combined with a host of inverter units that are connected in parallel, to provide output current required by any given application. Additional major benefits include:

• Encoderless Motor Operation -- Superior dynamic-response performance of Direct Torque Control (DTC) motor control algorithm. Many applications can be run without encoder feedback.
  
• Flexibility -- Only four types of diode rectifiers are used to invert ac 3-phase input power to the common dc bus bar across the entire ACS800 power range (200-4,600 hp). Inverter modules (drives) are available in seven sizes (R2i-R8i frame sizes) and cover the 1-7,000 hp range; single R8i units, or R8i units in parallel, supply the motors.
  
• Compact Design -- Improvements in technology and power components have reduced the total footprint of these multidrives by up to 50%, compared to the generation of multidrives preceding the ACS800 line. This frees up significant wall, panel, and floor space.
  
• Wide Range of Options -- I/O extensions, fieldbuses, and a pulse encoder module all fit inside the inverters.
  
• Adaptive Programming -- This program extends the freely programmable I/O and extensive parameter selections built into the multidrives and accessed via the Start-up Assistant. The Adaptive Programming accesses extensive parameter selections for complete flexibility in precise control of all individual motors; like a mini PLC built in.
  
• Reduced Installation Costs -- Since the modular configuration utilizes a common dc bus, the cabling input power required is a single 3-phase ac connection. Further, the cabling is wired to the rectifier module through a unique plug-in connector, which then powers the entire common dc lineup. And the plug-in connectors for the inverters (132 kW and up) are wired to the motor terminals. Both the rectifiers and the inverters are rolled in on their wheels and seated into the connectors.
  
• Redundancy -- Common spare parts and the ability to keep running at reduced load, if module is disabled.

Overview of Construction

A multidrive is built from several different sections. The key units within these multidrive systems include:

• Drive units -- inverters have built-in capacitors for smoothing the voltage of dc bus bars.
  
• Diode Supply Units (DSU) -- these units automatically adapt to 6- or 12-pulse operation and, during startup, they deploy a built-in control system that automatically charges the inverters' capacitor banks.
  
• IGBT Supply Units (ISU) -- used in regenerative drive systems and applications requiring low harmonics, to convert 3-phase ac voltage to dc voltage.
  
• Thyristor Supply Units (TSU) -- in a regenerative drive, can regenerate power back to the mains.
  
• Dynamic braking units -- in resistor braking, whenever the dc bus voltage of a frequency converter exceeds a certain limit, a braking chopper connects the circuit to a braking resistor.
  
• Control units (optional) -- are equipped with communication interfaces, power supplies, etc., for automation equipment.