Rectifier circuit as a shared DC bus supply: a six-pulse diode or SCR bridge that converts three-phase AC once and feeds the DC links of a group of drives, sized by summed DC input current, precharge and bus capacitance

01

Sizing on the drives' DC input currents: 109.4 A on a 120 A diode supply, 773 A on a 1,000 A SCR supply

Rockwell defines a non-regenerative shared DC bus as a three-phase diode bridge, SCR bridge or a combination, converting AC to a fixed DC bus voltage with power flowing one way, from which one or more drives are fed. The bus supply is sized so that the sum of the connected drives' DC input currents, at their Normal Duty or Heavy Duty rating at 40 °C, does not exceed its continuous DC output rating. [1]

Rockwell notes that its SCR bus supply examples have no overload capability, while the drives' Normal Duty and Heavy Duty ratings include short overloads (110 % for 1 minute and 150 % for 3 seconds, or 150 % for 1 minute and 200 % for 3 seconds). [1] Editorial calculation: the example pairs 540 V DC with a 400 V supply and 650 V DC with 480 V, a ratio of 1.35 in both cases (540 / 400 = 1.35; 650 / 480 ≈ 1.35), which is the DC level a six-pulse bridge delivers from the line-to-line voltage. [1]

Bonitron's M3713 is a stand-alone six-pulse diode bridge built for this job, in 30 to 375 A sizes for 208–240, 460–480 and 575–600 V AC supplies, with or without SCR-bridge precharge (SC and DM versions). Up to three units can run in parallel, and pairs or triples with a phase-shifting transformer form 12- and 18-pulse supplies that Bonitron says let standard drives meet IEEE 519 with proper configuration. [2]

List every drive on the bus with its DC input current at the duty rating that will be used, the DC bus voltage, the supply voltage, and any short-term overload the bus supply has to carry.

02

Who charges the capacitors: a diode bus supply has no precharge, so the drives must provide it

Precharge raises the DC bus voltage gradually so the filter capacitors charge in a controlled way. Rockwell's diode bus supply does not include precharge, so it may only feed drives that have their own precharge circuit; Rockwell warns never to connect drives without a controlled, current-limited precharge (certain Kinetix servo drives) to a diode bus supply, because the uncontrolled inrush damages them. [1]

Inside the drives, Rockwell describes three precharge forms: a resistor in series with the bus that a relay contact bypasses once the precharge level is reached (smaller frames); an SCR rectifier phase-advanced to limit inrush (larger AC-input frames); and resistors and diodes that charge the bus while the front-end SCRs are open, after which the SCRs take over. Drives without an internal precharge circuit must not be connected to an energised DC bus. [1]

Rockwell's SCR bus supply includes precharge, and drives without precharge may then be used only if power is always removed and restored to the whole system and no drive is isolated and reconnected to a live bus. For drives that do precharge, Rockwell recommends the 'Precharge Enable' digital input, coordinated by a PLC or by an auxiliary contact on each drive's disconnect, so that drives of different sizes have settled before precharge closes. [1]

On Bonitron's M3713 the precharge is a model choice: the SC version adds SCR-bridge precharge and the DM version has none; Bonitron's paralleling tables are given for the SC version. [2]

For each drive state whether it has its own precharge, whether any drive will be switched onto a live bus through a disconnect, and which unit (bus supply or drives) controls the precharge sequence.

03

Bus capacitance per amp: Rockwell's 40 µF/A target, a 110 µF minimum, and capacitor banks rated for 300 Hz ripple

All drives on a shared bus put their DC capacitors in parallel, so the bus ripple divides in proportion to each drive's capacitance. Rockwell targets a ratio of 40 µF or more per amp of drive DC input current; below that, an external capacitor bank equal to 40 µF × DC input amps minus the drive's own capacitance is added, as close as possible to the DC terminals of the largest drive. An SCR bus supply also needs at least 110 µF on the bus to commission, or its fault detection reads the bus as shorted. [1]

Editorial calculation: applying Rockwell's own formula to the Frame 6 row gives 40 × 272.1 − 9,200 = 1,684 µF, not the 1,648 µF printed in its table; the 700S rows check exactly (40 × 1,162 − 19,800 = 26,680 µF; 40 × 272 − 8,400 = 2,480 µF). Confirm the figure with the supplier before ordering a bank. [1]

Bonitron's M3612EC filter capacitor banks for shared buses show the scale involved: 28,000 µF at 450 V DC for 230–240 V AC systems and 7,000 µF at 900 V DC for 460–480 V AC, with maximum ripple current at 300 Hz of 385 A and 200 A at 45 °C, falling to 175 A and 90 A at 105 °C; end of life is defined as a 20 % capacitance loss or a 100 % ESR rise. [3] Paralleled rectifiers do not add up in full either: Bonitron lists 270 A for two 150 A M3713 units and 400 A for three. [2] Editorial calculation: 270 / (2 × 150) = 90 % and 400 / (3 × 150) ≈ 89 % of the summed single-unit rating. [2]

Where two or more drives keep their own input bridges and are tied only by their DC buses, circulating currents between the bridges are a risk; Bonitron lists its M3345CBM sharing diodes as preventing circulating currents in parallel bridges, for up to 6 connections, while allowing one regenerative or braking unit to serve more than one drive, and Rockwell sizes back-to-back diodes in shared AC/DC configurations at 125 % of the drive's DC input current. [2] [1]

Ask for the capacitance of each drive, the µF/A check against 40 µF/A, the rating, ripple current at 300 Hz and temperature, and end-of-life criteria of any added bank, and the DC fuses with diagnostic feedback recommended for it.

04

Bus supply details to send

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Supply: [AC voltage; frequency; short-circuit level]
DC bus: [voltage; diode or SCR bridge; 6, 12 or 18 pulse]
Drives: [each drive's DC input current and duty rating; capacitance; own precharge yes/no]
Precharge: [in the bus supply or in the drives; disconnects onto a live bus]
Capacitor bank: [µF; voltage; ripple current at 300 Hz; fusing]
Paralleling: [number of units; sharing or isolation diodes]

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