Can You Connect Multiple Machines to One Servo Stabilizer?

Why a Single Stabilizer for Multiple Machines Makes Sense

The economics of servo stabilizers improve sharply with scale. A 100 kVA three-phase unit costs approximately ₹1,750/kVA installed; ten individual 10 kVA units protecting the same 100 kVA of load cost approximately ₹3,500/kVA each — nearly double the per-kVA cost, plus ten times the maintenance footprint (ten sets of brushes to track, ten service visits, ten control PCBs that can fail).

Beyond cost, a single incomer-level stabilizer offers something individual machine stabilizers cannot: it corrects feeder voltage variation and phase imbalance for all machines simultaneously, including shared infrastructure loads (compressor, lighting, HVAC) that individual machine stabilizers would miss.

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Standard Multi-Machine Architecture

Servo Stabilizer Architecture Options for Multi-Machine Facilities

Architecture	Description	Best For	Drawback
Single incomer stabilizer	One stabilizer at main LT panel, feeds all machines	Most factories; ≤500 kVA total load	Single point of failure; bypass needed
Zone stabilizers	One stabilizer per production zone (e.g., machining zone, welding zone)	Large facilities with varied power quality needs per zone	Higher cost than single incomer
Individual machine stabilizers	One stabilizer per machine	Critical single machines (medical, lab) or geographically scattered equipment	Highest total cost; most maintenance
Incomer + critical machine individual	Shared incomer stabilizer + dedicated unit for most sensitive machine	Facilities with one ultra-sensitive machine (e.g., CMM) plus general machinery	Balanced cost and protection

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Sizing for Multiple Machines: The Correct Method

The most common sizing mistake is adding all machine nameplate kVA ratings. This double-counts diversity — in practice, not all machines run at full load simultaneously.

Step 1: Determine Simultaneous Demand

Walk through your facility and identify the maximum load scenario: which machines run simultaneously and at what percentage of their rated load during peak production?

Example: 8-machine machining shop

Step 2: Convert to kVA

66.1 kW at average PF 0.85 = 77.8 kVA

Step 3: Add Starting Inrush Margin

Largest single motor: compressor at 22 kW. DOL start inrush ≈ 22 × 6 = 132 kW for 3–5 seconds. Stabilizer must supply this without output voltage collapsing.

For the inrush period, the stabilizer faces: 66.1 – 13.2 (compressor running) + 132 (compressor starting) = 184.9 kW peak for ≤5 seconds.

Servo stabilizers are rated for 125% of rated kVA for up to 30 seconds (IS 9815 overload requirement). So select a unit rated for: 77.8 / 1.0 ≈ 80 kVA running + starting capacity check:

80 kVA × 1.25 = 100 kVA overload capacity → compressor start peak demand 184.9 kW / 0.85 PF ≈ 218 kVA → exceeds 100 kVA overload

Correct size: 160 kVA, which at 1.25× overload = 200 kVA peak capacity → safely handles the 218 kVA starting demand.

Final Selection

160 kVA three-phase servo stabilizer (three-phase independent-phase, ±1% output regulation, ±25% input range, 30 V/sec correction speed)

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Load Sequencing: Managing Starting Inrush

When multiple machines start simultaneously (Monday morning startup, shift changeover), the total inrush can overwhelm even a correctly-sized stabilizer for a brief period.

View data table

Source: Ozmist simulation; 160 kVA independent-phase unit; 8 machines, largest compressor last in sequential sequence

Best practice: Use a programmable logic controller (PLC) or a simple timer relay scheme to sequence machine startups with 15–30 second delays between each. This is especially important on Monday mornings when all machines start cold simultaneously. Sequencing requires no additional hardware beyond the PLC already present in most CNC facilities.

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Cable Design for Multi-Machine Feeds

When one stabilizer feeds multiple machines, the distribution cabling is critical. Voltage drop in the cables between the stabilizer and the machines is not corrected by the stabilizer — the stabilizer only corrects voltage at its own output terminals.

View data table

Source: IS 732:2019 cable voltage drop tables; copper conductor resistivity 0.0175 Ω·mm²/m

Rule of thumb: Keep voltage drop between stabilizer output and the most distant machine below 2.5% of rated voltage (i.e., ≤10 V on a 415 V system). If longer runs are necessary, upsize the cable cross-section.

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When Individual Machine Stabilizers Are Better

Despite the economics favouring a single incomer unit, individual machine stabilizers are justified when:

1. Machines are in different buildings: A stabilizer in Building A cannot correct cable voltage drop to Building B 200 m away.

2. One machine has much tighter voltage tolerance than others: A coordinate measuring machine (CMM) requiring ±0.5% output cannot share a stabilizer with a welding shop requiring only ±3%. Give the CMM its own dedicated unit.

3. Shift pattern requires one machine to run when others are off: A stabilizer sized for the full 8-machine load has poor efficiency at <20% load. A small dedicated unit for the night-shift machine operates more efficiently.

4. Regulatory or quality system requirement: Some pharmaceutical and medical device GMP frameworks require dedicated, independently monitored power supplies for critical equipment.

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Bypass and Redundancy Planning

A single incomer stabilizer is a single point of failure. If it goes for repair, all connected machines lose voltage protection. Planning for this:

Option 1: Manual bypass switch Every servo stabilizer should have a bypass switch that routes raw mains to all machines. During stabilizer maintenance, machines operate on unregulated mains. Acceptable for planned maintenance periods; train operators to check supply voltage manually when running on bypass.

Option 2: Redundant pair (N+1) For facilities that cannot tolerate even brief unregulated operation, install two stabilizers: primary and standby, with an automatic transfer switch. If primary fails, standby takes over. Cost: approximately 1.8× a single unit of the same rating.

Option 3: Zone isolation For very large facilities, divide into 2–3 zones with one stabilizer per zone. If one stabilizer fails, only one zone is affected — the rest continue in protection.

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Frequently Asked Questions

Can I add machines to my facility after the stabilizer is installed? Yes, if the existing stabilizer has capacity headroom. Calculate the new total simultaneous load and compare to the stabilizer's rated kVA at 80% utilisation. If the new total exceeds 80% of rated, add a supplementary stabilizer for the new machines rather than overloading the existing unit.

Does connecting more machines degrade the stabilizer's output voltage regulation? No — as long as the total load stays within the stabilizer's rated capacity. The servo control loop adjusts independently of how many machines share the output. What matters is total kVA drawn, not the number of machines.

Should the stabilizer be installed at the main panel or at the machine-level distribution board? At the main panel (or zone distribution panel), not at individual machine points. Installing at the main panel means the stabilizer corrects voltage for the entire panel, including feeder cable runs that cause voltage drop. Installing at machine level means each machine needs its own unit, which defeats the economic advantage.

My workshop has a DG set for power backup. Should the stabilizer be before or after the DG set automatic transfer switch (ATS)? After the ATS — the servo stabilizer should see whichever supply (DISCOM or DG) is currently active and regulate it. Placing the stabilizer before the ATS means it only regulates DISCOM supply and the DG output is unregulated. Most ATS panels are designed with the stabilizer on the output side.

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References

1. Bureau of Indian Standards. IS 9815:1981 (Reaffirmed 2019). Specification for AC Voltage Stabilizers. (Overload capacity requirements, Section 6.4)
2. Bureau of Indian Standards. IS 732:2019. Code of Practice for Electrical Wiring Installations. (Cable sizing, voltage drop calculations)
3. Central Electricity Authority. CEA (Measures Relating to Safety and Electric Supply) Regulations, 2010. (Installation standards for voltage regulating equipment)
4. Bureau of Indian Standards. IS 3043:2018. Code of Practice for Earthing. (Earthing of stabilizer output)
5. National Electrical Manufacturers Association. NEMA ICS 1-2000: Industrial Control and Systems: General Standards. (Motor starting inrush for sizing purposes)
6. Bureau of Indian Standards. IS 325:1996. Three-Phase Induction Motors — Specification. (Motor starting current multipliers)
7. Ozmist Engineering. Installation Design Guide IDG-SS-001: Multi-Machine Servo Stabilizer Architecture. Internal document, Rev. 2, 2024.