Siemens MASTERDRIVES VC Braking Unit Parallel Connection Guide

1. Overview and Application Context

The Siemens SIMOVERT MASTERDRIVES VC (Vector Control) family handles regenerative energy from overhauling loads in two principal ways: through an external braking chopper (braking unit) connected to a power resistor, or by feeding the energy back into the supply through a Regen Rectifier Unit (RRU) or Active Front End (AFE). This reference covers the chopper path - specifically the rules, wiring, and parameter settings required when the regenerative peak power of a single decel event exceeds the P20 (20-second peak) rating of one braking module, and multiple MASTERDRIVES braking units must be connected in parallel to a shared DC bus.

Common triggers for paralleling braking units include: high-inertia decel from a large flywheel, a downslope conveyor with continuous regen, a hoist or elevator drive with heavy overhauling loads, a dynamometer test stand with active braking, and any application where the regenerative energy of a single decel event exceeds the P20 rating of a single chopper. When that rating is exceeded - and the duty cycle still leaves room for the unit to cool between events - paralleling two (or, with caveats, three) units extends the available peak dissipation without redesigning the resistor cabinet.

2. MASTERDRIVES VC Braking Unit Topology

The braking unit is a self-contained chopper module - an IGBT (or bipolar Darlington in older 6SE70 hardware) switching a wire-wound or stainless-steel-grid resistor across the DC bus. Each module contains:

• DC bus input terminals (+DC / -DC, typically X3 on the MASTERDRIVES braking module)
• IGBT chopper with gate driver and freewheeling diode
• Braking resistor output terminals (Rb+ / Rb-)
• DC bus voltage sensing and threshold comparator with hysteresis
• Fault output relay (braking unit OK / fault, Form C contacts)
• Parameter port for setting the turn-on threshold (DIP switches on older units, parameter P740 in the drive on newer modules)

For MASTERDRIVES VC sizes above approximately 75 kW (400 V class), the braking unit is almost always an external 6SE70 chassis-format module or a third-party chopper block built to the Siemens interface specification. For drive sizes below ~ 75 kW, the chopper is often integrated inside the basic unit and the resistor connects to a dedicated terminal pair (X7 on the 6SE7016-... series).

3. P20 Rating: Definition, Duty Cycle, and Heat

P20 is the maximum permitted peak braking power, applied for a maximum of 20 seconds, after which the chopper must be off for at least 70 seconds to allow the resistor and IGBT heat sink to return below their continuous thermal ratings. The P20 value is the number stamped on the resistor cabinet nameplate and used in the Siemens selection tables - it is not the continuous power rating. The 20 s / 70 s ratio is a Siemens convention; some third-party chopper modules use 10 s / 50 s or 30 s / 90 s - always check the unit-specific data sheet.

Calculation example. Two braking units with P20 = 100 kW and P20 = 170 kW in parallel: combined peak = 270 kW for 20 s. If the application requires 250 kW of braking for 15 s, the combined P20 satisfies the peak demand - the 270 kW ceiling is 8% above the 250 kW requirement. The cooldown, however, still applies to each unit independently, and the 100 kW unit reaches its thermal limit before the 170 kW unit does because it is running closer to its own ceiling. The effective system P20 therefore equals the smaller of (combined P20) and (sum of each unit's P20 adjusted for the ratio of actual load to that unit's P20).

4. Sizing Rules for Parallel Braking Units

Three independent constraints govern whether a chosen parallel combination is acceptable. All three must be satisfied simultaneously.

1. Peak power constraint: P20_unit1 + P20_unit2 + ... ≥ P_brake,peak during the worst-case decel. The combined peak must equal or exceed the maximum regen power the application can produce in a single event.
2. Drive thermal constraint: P20_combined < 2.4 × P_drive,rated. Catalog DA65.1 specifies the 2.4× factor as the upper limit to avoid sustained DC bus overvoltage from chopper capacity exceeding the drive's ability to regulate the bus.
3. Duty-cycle constraint: For each chopper, t_on ≤ 20 s and t_off ≥ 70 s. For a moving average over any 90-second window, the equivalent RMS power must stay below the chopper's continuous rating (typically 0.47 × P20).

Worked example 1 (valid). 250 kW peak braking power for 18 s, then 90 s coast, drive rated at 160 kW. Candidate: 100 kW + 170 kW = 270 kW combined P20.

• Peak constraint: 270 ≥ 250 ✓
• Drive thermal: 270 < 2.4 × 160 = 384 ✓
• Duty cycle: 18 ≤ 20 ✓; 90 ≥ 70 ✓

Combination is acceptable.

Worked example 2 (fails on drive thermal). Same 250 kW peak braking requirement but drive rated at only 90 kW. The 2.4 × 90 = 216 kW ceiling is below the 270 kW combined P20 - this combination violates the drive thermal rule. Lowering to two 100 kW units (combined 200 kW) would fail the peak constraint (200 < 250). The correct fix is to change topology: replace the chopper with an AFE or RRU sized to handle the regen continuously.

Worked example 3 (fails on duty cycle). Same 250 kW peak, 160 kW drive, but the application requires 250 kW of braking every 30 seconds (cycle: 15 s braking, 15 s coast). The 70 s cooldown requirement is violated (15 < 70). The IGBT heat sink never returns to ambient between events and will eventually trip on overtemperature. Either slow the cycle (extend the coast to 70 s) or move to AFE/RRU.

5. Parallel Connection Schematic and Wiring

Parallel braking units share the DC bus of the drive but each retains its own braking resistor. The control wiring is minimal: a common fault-OK signal back to the drive (or to a master supervisory relay), and identical threshold setting on each unit.

5.1 DC bus wiring rules

• Connect +DC of every braking unit to the +DC bus of the drive (typically terminal C/L+ on the MASTERDRIVES basic unit).
• Connect -DC of every braking unit to the -DC bus (terminal D/L-).
• Use symmetrical cable lengths from each unit to the DC bus. A difference of more than 0.5 m between unit A and unit B cables creates an impedance imbalance that biases the current toward the closer unit, leading to thermal asymmetry and one unit tripping first.
• Each DC bus cable must be sized for the chopper's peak current: I_peak = P20 / V_DC,turn-on. For P20 = 170 kW at V_threshold = 760 V DC: I_peak = 170,000 / 760 ≈ 224 A. Use 95 mm² or larger copper, and verify voltage drop < 2% at peak current.
• Twist the +DC and -DC cables together (or run in a trefoil formation) to reduce radiated EMC from the high-di/dt switching edges.

5.2 Braking resistor wiring

Each braking unit has its own chopper resistor. Never parallel the choppers' outputs into a single resistor - the asymmetric turn-on times of the IGBTs would cause one unit to deliver all the current into the shared resistor until thermal runaway. Each unit's Rb+ / Rb- terminals go to a dedicated resistor bank. The resistor value for each unit is selected independently to give that unit's rated P20 at the chosen DC bus turn-on voltage: R = V_DC² / P20. For V_DC = 760 V and P20 = 100 kW: R = 760² / 100,000 = 5.78 Ω. For P20 = 170 kW: R = 3.40 Ω.

5.3 Control and signal wiring

The Fault/OK contacts of the two units can be wired in series to a single drive input if the drive only needs a binary "any-unit-faulted" signal. If the application needs to identify which unit failed (recommended for redundancy), wire them to separate digital inputs and program the PLC to alarm with the specific unit ID.

6. Threshold Voltage and Chopper Parameter Settings

The braking unit monitors the DC bus voltage and turns the IGBT on when V_DC exceeds a programmable threshold. Once the bus falls back below the threshold minus a hysteresis band, the IGBT turns off. Both units must have the same threshold and hysteresis settings; otherwise the unit with the lower threshold turns on first, dissipates its resistor's heat, and leaves the second unit idle until the bus rises further - by which time the first unit may already be at its thermal limit.

On MASTERDRIVES VC parameterizable braking modules the threshold is set via parameter P740 in the drive (Braking Module → Threshold) or via the unit's own keypad. On older 6SE70 units without keypads, the threshold is set by DIP switches inside the unit (SW1 positions 1-4 for threshold, 5-6 for hysteresis). Refer to the unit-specific operating instructions shipped with each module for the exact mapping - the setting table varies between 6SE7090-0XB84-... sub-variants.

The hysteresis setting affects the switching frequency of the chopper. With 30 V hysteresis on a 400 V class bus, a chopper switching at ~ 1 kHz is typical; this is well within the safe switching frequency range for the IGBT and the resistor. Do not lower hysteresis below 15 V - the chopper will switch at audio frequencies and the resistor cabinet will sing.

7. Operating Principle of Parallel Choppers

When the drive's DC link voltage rises above the threshold (e.g., during a rapid decel from 1500 rpm to 0 rpm with high inertia), both choppers begin to switch on. Because the IGBTs are independent and not synchronized, the chopper with the lower actual threshold (even within tolerance) turns on microseconds earlier. As that unit's resistor drops the bus voltage, the second unit may turn on as well - or may oscillate at the boundary.

Steady-state behavior once both units are engaged: the bus voltage sits at the threshold (minus a small IR drop across the resistor cables), and both choppers share the load roughly proportional to their P20 ratings. With a 100 kW + 170 kW combination, the 170 kW unit absorbs ~ 63% of the total power and the 100 kW unit absorbs ~ 37%. This is natural load sharing - no master/slave wiring is required because both units regulate the same bus voltage. The IGBT with the higher instantaneous current capability (the 170 kW unit, by design) carries the larger share.

Unbalanced sharing (one unit running hot, the other cool) almost always indicates one of three problems:

1. Threshold mismatch between the two units (> 5 V difference).
2. DC bus cable length mismatch (> 1 m difference, or different cross-section).
3. One resistor bank has lost a parallel path (open weld, blown fuse on a resistor section, broken resistor grid).

The chopper switching behavior also affects the DC bus ripple. Two units switching at slightly different times partially cancel the high-frequency ripple current, giving a smoother bus than a single chopper would. This is one of the secondary benefits of paralleling and is sometimes the reason to parallel even when peak power is not the limiting factor.

8. Connecting Three or More Braking Units in Parallel

Siemens Catalog DA65.1 explicitly describes parallel connection of two braking units. For three or more units, the topology is mechanically the same - all +DC tied to the drive's +DC bus, all -DC tied to -DC bus, each with its own resistor - but the threshold-tolerance problem compounds.

With N units in parallel, the chance that one unit's threshold is far enough below the others to do most of the work increases. For N = 2, a typical ± 2 V setting tolerance is fine. For N = 3, the chance of one unit being 5 V low is non-trivial and that unit will run hot. For N = 4 and above, the thermal imbalance becomes a reliability concern and the cost of additional units typically justifies moving to an AFE.

If three units are unavoidable, follow these steps:

1. Order all units from the same manufacturing batch (cross-check serial numbers) for closer-tolerance components.
2. Set the threshold to the highest end of the acceptable range (e.g., 770 V instead of 750 V for 400 V class) to give hysteresis some headroom against the lowest-threshold unit.
3. Use identical cable cross-section and identical cable lengths (within 10%) for all DC bus drops.
4. Install thermal monitoring on each resistor cabinet (PTC thermistor + monitoring relay), not just on one.
5. During commissioning, log the current in each resistor with a clamp meter at full braking and confirm the sharing is within ± 15% of the theoretical ratio.
6. Schedule re-checks of the threshold settings every 6 months - component aging can drift the comparator reference up to 1-2 V per year in older 6SE70 hardware.

9. Alternatives for Continuous High-Power Braking: RRU and AFE

When the application demands high braking power for longer than 20 seconds - or the duty cycle has the chopper on for more than ~ 25% of the time - parallel braking units become impractical. The heat dissipation requirements for the resistor bank exceed what is reasonable for an industrial cabinet, and the duty-cycle bookkeeping (20 s on / 70 s off) becomes impossible to guarantee.

The correct alternative in these cases is a regenerative topology that returns energy to the supply instead of burning it in resistors:

For the 250 kW continuous braking case in the original question, an AFE (e.g., the Siemens 6SE70 AFE module sized 1:1 with the drive) or an RRU is the correct selection. The MASTERDRIVES VC drive itself is agnostic - it sees the same DC bus whether the front end is a diode bridge, an AFE, or an RRU. Only the line-side hardware changes. The drive's parameter set for regen-mode operation (P100 / P101 source selection) does not change when the front end is replaced.

Key decision points:

• Use AFE when line harmonics are limited (utility requirement, hospital, semiconductor fab, generator-supplied site, or shared bus with sensitive loads).
• Use RRU when harmonics are acceptable, the load requires 100% continuous regen, and capital cost is more important than harmonic performance.
• Stay with parallel choppers only when braking events are short (≤ 20 s) and infrequent (≥ 70 s between events).
• Consider hybrid chopper + AFE for unusual profiles where peak regen is high but most of the time the regen is low - the AFE handles the average and the chopper absorbs the peaks.

For the Siemens MASTERDRIVES VC family, the AFE and RRU options are documented in the SIMOVERT MASTERDRIVES Compendium and the operating instructions for the line-side converter module. Reference these manuals before specifying.

10. Commissioning and Verification Procedure

Once the parallel braking units are physically wired and parameterized, follow this sequence to verify operation before running the process:

1. Visual and continuity check. With the drive isolated and locked out (LOTO), verify each +DC and -DC cable with a continuity tester. Confirm no cross-wiring between +DC and -DC. Verify resistor ohmic value is within ± 10% of nameplate (cold measurement).
2. Threshold verification. Power the drive's auxiliary control (24 V DC), keep the line contactor open. Use a calibrated multimeter at each braking unit's threshold test pins. Slowly raise the DC bus voltage using the drive's own pre-charge (manual mode if available, e.g., via parameter P700 = 0 on MASTERDRIVES). Record the actual turn-on voltage of each unit. Acceptable spread: ≤ 2 V.
3. Insulation test. Megger each resistor bank to ground at 500 V (or per resistor nameplate). Resistance to ground should be > 1 MΩ. A lower value indicates moisture ingress or contamination and the resistor must be dried or replaced.
4. First energization with no load. Run the drive in no-load mode (motor disconnected if practical). Run a slow decel from nominal speed to zero with a long ramp. Monitor each resistor bank with an IR thermometer - each should warm slightly, but none should exceed 100 °C surface.
5. Loaded test - rated braking. Reconnect the motor. Run the application profile at 50% load and trigger a worst-case braking event. Clamp-meter each resistor cable and log the current waveform. Sharing should be within ± 15% of the theoretical ratio (100 kW unit ≈ 37%, 170 kW unit ≈ 63%).
6. Loaded test - peak braking. Run at 100% load with a worst-case decel (e.g., full speed to zero in minimum ramp time). Monitor DC bus voltage on the drive's parameter view (r026 / r027 on MASTERDRIVES). Verify the bus peaks below the F002 threshold (typically 820 V for 400 V class) and recovers to nominal within 2 seconds of stop.
7. Fault injection. Disconnect the enable wire to one braking unit. Trigger a braking event. The drive should either trip on F002 (overvoltage) within ~ 200 ms (acceptable single-fault behavior) or the second unit should absorb the load without fault (acceptable if sharing has been verified). Document the chosen behavior.
8. Thermal soak. Run the process through several full cycles back-to-back. Measure each resistor cabinet surface temperature at the end of the cooldown (70 s after each event). Verify temperatures are within 15 °C of each other - indicates even sharing.
9. Documentation. Record the final parameter values, threshold readings, cable lengths, resistor ohmic measurements, and clamp-meter current readings in the commissioning report. This becomes the baseline for future troubleshooting.

11. Troubleshooting Matrix

12. Frequently Asked Questions

Reference documentation: Siemens SIMOVERT MASTERDRIVES VC Compendium (Catalog DA65.1, §6.49), MASTERDRIVES VC Operating Instructions, and the braking module-specific data sheet shipped with each unit. Verify all ratings, thresholds, and parameter numbers against the current revision of the manufacturer documentation for your specific drive and braking-unit variants before commissioning.