Undersize the power supply and the servo drive faults on undervoltage the moment an axis accelerates. Leave regenerative energy with nowhere to go and it faults on overvoltage the moment the axis decelerates.

Between those two failure modes sits a sizing procedure built on four numbers: the continuous current the application actually draws, the peak current it draws in short bursts, the output voltage the motor needs at maximum speed, and the energy that flows back through the drive when the load slows down.

This guide works through all four, with a complete worked example using real hardware.

Objective

Determine the continuous current rating, peak current handling, output voltage, and regenerative capacity a DC power supply needs to reliably run a given servo axis — without paying for headroom the application will never use.

What Information Do you Need Before Selecting the Right Power Supply?

Before sizing anything, gather:

• The servo drive’s rated continuous current, peak current, and DC supply voltage window, from its datasheet.
• The motor’s torque constant (Kt) and back-EMF constant (Ke).
• The application’s motion profile: the torque and duration of each phase — acceleration, constant-velocity run, deceleration, and dwell.
• The total inertia reflected to the motor shaft, including the load, corrected for any gear or belt ratio.
• Whether the axis decelerates significant inertia or an overhauling load. Vertical axes, fast indexing, and large rotating masses make regeneration a primary design factor rather than a footnote.

How the Drive and Supply Divide the Work

Servo drives built around a DC supply voltage input — AMC’s AxCent™, DigiFlex® Performance™, and FlexPro® families among them — do not rectify three-phase line power on board. The power supply is a distinct, separately specified component.

The drive’s datasheet states a DC Supply Voltage window (for example 10–80 VDC for many of AMC’s compact servo drives) and continuous/peak current ratings; the power supply’s job is to hold voltage inside that window while delivering the current the application needs, including whatever comes back during deceleration.

Two supply architectures cover most of this ground, and the choice between them is a real tradeoff.

The steps below determine which one fits.

Step 1 — Determine the Continuous Current Demand

The power supply’s continuous current rating has to cover the RMS current the application draws over a full motion cycle — not the drive’s rated continuous current, which only matches that number if the application runs the drive at its full rating. 

AMC’s own servo motor sizing guidance uses an RMS method to size motor torque; the same method applies directly to current.

IRMS = √[ (I12t1 + I22t2 + … + In2tn) / (t1 + t2 + … + tn) ]

In = current during motion segment n (A); tn = duration of segment n (s)

Consider a single-axis rotary joint driven by a brushless servo motor with Kt = Ke = 0.09 N·m/A (about 9.4 V per kRPM), a total reflected inertia of 2.0×10⁻⁴ kg·m², and a maximum speed of 5,000 RPM (523.6 rad/s). The motion profile: accelerate at 1.4 N·m, run at 0.35 N·m, decelerate at 1.4 N·m, then dwell.

At 1.4 N·m against 2.0×10⁻⁴ kg·m², angular acceleration is 7,000 rad/s², so reaching 523.6 rad/s from rest takes 75 ms — and the same for deceleration. Adding a 150 ms run segment and a 400 ms dwell brings the cycle to 700 ms, or about 1.4 Hz.

Converting torque to current with I = T/Kt: 15.6 A during acceleration and deceleration, 3.9 A during the run segment, 0 A at dwell.

IRMS = √[(15.6²×0.075 + 3.9²×0.15 + 15.6²×0.075) / 0.7] ≈ 7.4 A

That 7.4 A — not the drive’s 10 A continuous rating — is the number the power supply’s continuous rating has to cover.

Step 2 — Determine the Peak Current Demand

Peak current is the highest instantaneous current in the profile — here, the 15.6 A during acceleration and deceleration. Unlike continuous current, the supply doesn’t have to source that peak indefinitely: AMC’s DigiFlex® Performance™ drives limit peak current on a charge basis, permitting it for a bounded interval before folding back toward the continuous rating (see AMC Application Note 003, Peak Current Recovery). What the supply does need is enough headroom that bus voltage doesn’t collapse during that interval — undersized supply impedance or cable runs sag the bus under peak load even when rated current is technically available, and the drive reads a sagging bus as undervoltage.

AMC’s own drive-selection guidance calls for at least 25% more current capacity than the application is expected to draw, applied to both the drive and the supply. Against 7.4 A continuous and 15.6 A peak, a drive rated 10 A / 20 A — AMC’s DigiFlex® Performance™ DPRALTE-020B080 — clears that bar with 35% and 28% margin, respectively.

Step 3 — Select the Output Voltage

Output voltage is set by the motor’s back-EMF at maximum speed, not by the drive’s rated maximum. As the motor spins faster, the voltage it generates rises linearly with speed; the supply has to stay above that voltage by enough margin for the drive’s current loop to keep regulating current, or the axis loses torque and speed authority right where the application usually needs it most.

Vbemf,max = Ke × ωmax

Ke = back-EMF constant, V/(rad/s); ωmax = maximum motor speed, rad/s

For the example motor: Vbemf,max = 0.09 × 523.6 = 47.1 V.

AMC’s drive-selection guidance also calls for a 25% margin between the intended operating voltage and both the undervoltage and overvoltage limits. Applied to the back-EMF floor: Vbus,min = 1.25 × 47.1 V ≈ 58.9 V. A 72 VDC supply — AMC’s PS16H72, an unregulated, transformer-isolated model rated 11 A continuous — clears that floor with 53% headroom above raw back-EMF, and sits comfortably within the DPRALTE-020B080’s 20–80 VDC rated window.

That leaves only about 10% margin below the drive’s 80 V ceiling, though, and back-EMF isn’t the only thing that pushes bus voltage up. Deceleration does too — which is the next problem to size for.

Step 4 — Size for Regenerative Energy

Every deceleration event drives the load’s kinetic energy back through the drive and onto the DC bus. An unregulated supply has no path to absorb it — it isn’t built to sink reverse current — so without somewhere for that energy to go, bus voltage rises until the drive faults on overvoltage or, in the worst case, the bus capacitors are overstressed.

Eregen = 0.5 × Jtotal × (ωi2 − ωf2)

Jtotal = inertia reflected to the motor shaft, kg·m²; ωi, ωf = angular velocity before and after deceleration, rad/s

For the example axis, decelerating from 523.6 rad/s to rest: Eregen = 0.5 × 2.0×10⁻⁴ × 523.6² ≈ 27.4 J per event.

At the profile’s 1.4 Hz cycle rate, that works out to an average regenerative power of about 39 W (27.4 J × 1.4 events/s). A shunt regulator sized for that average — not just the instantaneous peak — is what a resistor-based solution actually needs to survive continuously.

AMC’s SRST80 shunt regulator clamps at 80 VDC — matched to the DPRALTE-020B080’s rated maximum — and is rated for 95 W of continuous dissipation. The example’s 39 W average load uses well under half that capacity, leaving margin for faster cycling or higher inertia than the nominal design case.

This is also the real defense against the Step 3 voltage-headroom shortfall: the shunt regulator clamps the bus at a known voltage before it reaches the drive’s overvoltage limit, instead of relying on headroom alone.

Step 5 — Match Supply Regulation to the Application

An unregulated supply plus an external shunt regulator — the PS16H72/SRST80 pairing above — is a common, cost-effective combination for single- or few-axis machines with a bounded, predictable regen profile. Its output does move with load and with input line voltage, though: a 10% high line pushes the bus 10% higher before any regeneration is even considered, and that has to be folded into the voltage-headroom check in Step 3, not treated as a separate margin.

A regulated supply, such as AMC’s PFC2400W375, holds output voltage essentially flat across load and line variation, and its built-in shunt regulator and power factor correction absorb regen and reduce input current harmonics without adding a separate component. That capability costs more and adds complexity most single-axis applications don’t need. It earns its keep when line variation is wide, when multiple axes share one bus and their regen events can overlap, or when harmonic content matters to the facility’s power quality.

Cable runs between supply and drive add their own voltage drop under peak current; long or undersized leads erode the margins computed above just as directly as an undersized supply does.

Step 6 — Check Inrush and Shared-Bus Loading

DC bus capacitance presents a low-impedance load at power-up; confirm the supply’s inrush handling — or add a pre-charge circuit — before the first startup, not after a tripped breaker on the factory floor.

When multiple drives share one supply, size continuous current as the sum of each axis’s RMS demand — that part doesn’t have a shortcut. Peak current is more application-specific: size for the sum of peaks unless verified motion sequencing rules out simultaneous acceleration, in which case some designs apply a diversity factor — a judgment call, not a default assumption.

Verification

Safety: Standard bus-discharge and lockout/tagout precautions apply when probing a live DC bus. Confirm bus voltage is within safe limits before contact, per the drive’s hardware manual.

Confirm the sizing on the bench before it goes into production. Measure DC bus voltage at the drive terminals during the worst-case peak current event — it should stay above the drive’s minimum rated voltage with margin. Measure it again at maximum commanded speed under load, to confirm back-EMF headroom holds up in practice, not just on paper. If a shunt regulator is in the design, monitor its duty cycle or case temperature during representative continuous operation; a regulator running hot or near-continuous duty signals the regen calculation needs revisiting.

Troubleshooting Undersized or Mismatched Supplies

Fault signatures point back to a specific sizing step:

Matching AMC Hardware to the Numbers

The worked example above uses three real, compatible AMC parts: a DigiFlex® Performance™ DPRALTE-020B080 servo drive (10 A continuous / 20 A peak, 20–80 VDC), an unregulated PS16H72 power supply (72 VDC, 11 A continuous, transformer-isolated), and an SRST80 shunt regulator (80 VDC clamp, 95 W continuous dissipation). Every number in the calculation maps to a specification on a real datasheet — the sizing procedure is only as reliable as that verification.

For higher-power or multi-axis systems where line variation or overlapping regen events are a genuine concern, AMC’s PFC2400W375 — a regulated, power-factor-corrected supply with a built-in shunt regulator, rated 375 VDC at 2.4 kW continuous — covers the case where an unregulated supply’s tradeoffs stop being acceptable. Choosing between the two comes down to which set of tradeoffs from Step 5 fits the application in front of you.

Conclusion

Power supply sizing for a servo drive comes down to four checks — continuous current, peak current, voltage headroom over back-EMF, and where regenerative energy goes — run against real datasheet numbers rather than the drive’s rated maximums. Skip any one of the four, and the supply becomes the thing that fails first.