In the first installment of this series, we explained the regenerative-power requirements for the controlled motion of servomotor systems. Here we detail common electrical protection circuitry.

By Hurley Gill • Senior applications and systems engineer | Kollmorgen

Designing a machine with servomotors, drives, and regenerative resistors requires consideration of numerous design factors. These components affect the overall performance of all the design’s axes during normal operation — and during both predicted as well as unforeseen events requiring engagement of stop functions.

Motion profiles in graphical form are useful visual references of axes’ controlled motion to execute machine tasks. These profiles can also be used to determine the energy management and regenerative requirements for a given machine axis or multiple axes. Of course, regenerative resistors aren’t always indicated by initial specifications and calculations, proof-of-concept builds, or prototypes. Even so, it’s often advisable to minimally utilize regen resistors on first build designs — if only to take measurements of the controlled process and identify machine limits for the final design. Additionally, for new designs up and running for the first time, it’s rarely long before someone says, “How fast can we make it go?” It’s wise to err on the side of caution, as regeneration circuits undersized for a given application function can degrade personnel safety, output product quality, and machine throughput.

Without regeneration capabilities beyond the axis drive’s bus capacitance, a DC-bus overvoltage fault will likely be the first problem to arise. That in turn can present significant time lost if it’s the proverbial Friday before an end customer’s Tuesday visit.

For most electronically servocontrolled motion designs, the safest stops are those that bring all controlled motions to a halt in the shortest time — for example, before removal of mains power for an emergency stop (e-stop) …  i.e., the powered servo drive is utilized to stop the axis motion under its control before its DC-bus power source is removed.

Stops ensure safety (but require energy management)

Best practices for machine design utilizing controlled motion dictate that the safest stop is a controlled stop during an event — to bring motion to a stop in the shortest possible time (for a controlled stop of the controlled motion). With few exceptions, that’s a requirement whether the stop involves removing mains power afterwards (as for e-stops) or not (as for priority stops or p-stops). Hence documents such as the NFPA 70 National Electrical Code (NEC) and NFPA 79 — Electrical Standard for Industrial Machinery are “meant to work together to meet the desired end results” and help save lives as well as prevent property loss. After all, protecting people and property from mechanical and electrical hazards is a complex issue for which no single code or standard can accomplish. Closed-loop (servo) controlled motion adds a level of complexity beyond that of open-loop axis or machine control.

So how to quantify the energy associated with such stops? Quantifying stored kinetic energy within each servomotor-controlled mechanism is key to proper regenerative system design and reliable machine operation. For servo-controlled motion, the selected regen resistor is often determined by the stop function requirements. Usually, that demands that all servo-controlled motion must stop before removal of power.

A word of caution regarding red-hot regen resistors: Many regen resistors are mounted atop control cabinets — and glowing red resistors atop cabinets are generally unwanted by end users. Yet pushing the rated wattage of a wound-caged regen resistor (as for speeding up production) can make it glow red. Selecting a regen resistor with 25% more capability than required lets the selected resistor use only 80% of its wattage capability — preventing any unsettling red glow. The wattage capabilities of regen resistors are available in published data sheets or from other manufacturer sources.

Servo drive regen protection circuity

Selecting the best protection for regen circuit protection can seem challenging — especially because of peak currents during normal operation and stops can be so much higher than the circuit’s RMS current required by a given application.

The compactness of today’s power devices means that most servo drives have internal short-circuit protection. Upon detection of a short, these trigger a drive fault and an axis shutdown. Additionally, many drives can also limit regen Irms and Ipeak regen-circuit current to satisfy connected component and application parameters. External regen resistors manufactured to limit this current should have their own internal overload device such as a series-wired NC thermostat. For many applications, such a thermostat is enough. Elsewhere though, it may be better to use fuses or some other device to warn the machine controller when the system is nearing the maximum regen capacity. These other devices may include:

1. A slow-blow fuse in series with the regen resistor selected to handle application’s RMS current up to the regen resistor’s ultimate current limited by its wattage. This fuse should be able to handle the application’s peak requirements without causing nuisance trips — as in systems with frequently blown fuses.

2. A fast-blow fuse in series with the regen resistor selected to handle application’s peak current requirement.

3. A single motor-overload NC relay in series with the regen resistor — often with a fast-blow fuse. Such relays provide an NC contact signal to the servo drive and/or machine controller during normal operation. Then when the NC contacts open, they communicate to the servo drive or higher-level controller that an overheating-regen condition exists. This prompts controls to follow preprogrammed slowdown or shutdown processes. The heater in this setup can be sized to trip the NC thermal-relay circuit quite close to application requirements (or regen current limit) and still handle peak conditions without issue.

Regen resistor calculations

Properly determining regen resistor requirements requires the following information and the calculation of Er(n) = E(k) – E(el) ± E(ext-f) – E(f) — where recovered energy Er is calculated with the motion-profile’s n-segment information for a specific Er(n). For clearer understanding of the energy variables affecting Er(n), consider them grouped by whether they’re kinetic, electrical, frictional, or external in nature.

Kinetic energy E(k) = ½(J_load + J_motor) ωm²   [units in joules]
Electrical energy losses E(el) = 3(I²_dec x Rm/2) t_dec
External energy forces E(ext-f) = (T_ext x Δωm/2) t_dec)
Energy lost to friction E(f) = (T_f x Δωm/2) t_dec — often ignored in hand calculations if T_f is relatively small.

Er = ½(J_load + J_motor) ωm² – 3(I²_dec x Rm/2) t_dec ± (T_ext x Δωm/2) t_dec) – (Tf x Δωm/2) t_dec.

E(caps) = Energy (additional capacity) = ½ C (VDC² _max(fault) – VDC² _bus)
E(int-reg) = Energy (internal regen resistance) = R_watts x time
E(ext-reg) = Energy (external regen resistance) = R_watts x time

Once the kinetic energy E(k) value is known, at the very least we must subtract the motor’s electrical losses E(el) while in regenerative mode. We must also add or subtract any applied external forces ±E(ext-f) such as gravity that work for [-] or against [+] the axis’s change in velocity. Ideally, these calculations also account for losses due to frictional energy E(f).

The resulting energy needing to be absorbed or dissipated over the total cycle time (t_total) can be calculated from the summed energy Er(n) by subtracting the energy capacity E(caps) of the DC-bus capacitance of the proposed or selected servo drive.

Determining the type of regen resistor needed

If Er(total) – E(caps) > 0, an internal (int-reg) or external (-reg) regen resistor or equivalent is required.

If Er(total) – E(caps) – E(int-reg) > 0, then an external regen resistor or other energy-absorption feature is required. In this case, the total energy-absorption capacity must be greater than Er — where Er is the Er(n), Er(total), Er(wc), or Er(sf) recovered energy under consideration. In addition, the application of an external regen resistor typically removes any internal regen resistance from the drive’s regen circuit. That means for situations necessitating an external regen resistor, all power requirements must be calculated without assuming any internal regen resistor capability of the drive.

Application information required to design for a rotary axis

There are 12 different axis variables needing definition to properly select and size a regen resistor for a rotary servo application.

1. J_load (kg·m²) = Total reflected and directly coupled inertia as seen by the motor.

2. N (RPM)= Motor speed as revolutions per minute just prior to the start of subject deceleration.

3. ωm (rad/sec) = N ÷ 9.55 — subject to motor velocity N.

4. T_f (Nm) = friction torque and assisting deceleration during t_dec (linear deceleration assumed).

5. t_dec (seconds) = Subject deceleration time.

6. T_dec (Nm) = Required constant torque assuming linear deceleration. For nonlinear deceleration, use the RMS torque required over t_dec.

7. I_dec (Arms/Ø) = T_dec (Nm) ÷ Kt (Nm/Arms) = Application’s required deceleration current. We assume this is constant (for linear deceleration); if it’s nonlinear, refer to the note for T_dec on how to account for that fact. Note that I_dec must be less than or equal to the drive capability for t_dec. Otherwise, the selected drive might limit current I_dec = I_peak available for t_dec. Then T_dec for t_dec will need re-evaluation. If limited, t_dec(new) = ((J_load + J_motor) x ΔRPM / 9.55) ÷ (Kt x I_peak + Tf ± T_ext).

8. T_ext (Nm) = ± External torque — here assumed constant. Gravity on a vertical axis extends deceleration time, so this makes for a positive T_ext value.

9. t_on (seconds) = Energy recovery ON time = t_dec.

10. t_total = Total cycle time (start to repeat) of repeating decelerations.

11. V_terminal = Motor’s developed B_emf less voltage IR drop just prior to starting deceleration at N velocity, with V_terminal = √3 KB x N ÷ 1,000 ÷ √3 – I_dec × (Rm/2).

12. I_shunt(Rmax) = Maximum possible regen-resistor shunt current from the DC-bus capacitance is based on the selected resistor’s peak wattage Ppk_resistor.

Motor (proposed or otherwise) information required to design for a rotary axis

There are three different motor variables needing definition to select and size a regen resistor for a rotary servo application.

1. J_motor (kg·m²) = J_motor (kg·cm²) ÷ 10,000 — the motor’s published rotor inertia.

2. Kt (Nm/Arms) = Motor torque constant (listed in the manufacturer’s published data)

3. Kb (Vrms/kRPM) = motor’s Bemf constant for sinewave commutation (published data)

4. Rm (ohms) = line-to-line motor resistance at manufacturer’s published ambient (typical: 25°C); ambient value presents desirable worst-case condition for these calculations.

Drive (proposed or otherwise) information required to design for a rotary axis

1. C (farads) = DC-bus capacitance of the selected drive (manufacturer’s published data).

2. VDC_max(fault) = DC-bus voltage limit, trigging an overvoltage fault by the drive.

3. VDC_max = VDC_max(fault)-1; for maximum normal operation.

4. Vo = Turn-off voltage (VDC_off) of relaxation oscillator Regen circuit [or eq.] or the minimum turn-on voltage for otherwise controlled regen circuit. VDC_max(fault) > Vo > VDC-bus.

5. VDC-bus = Nominal DC bus voltage » crest of sinewave » Vac(source) x √2.

6. I_pk(drive) = Available Peak RMS phase (Ø) current of the proposed Drive amplifier over t_dec time.

7. Ppk(regen)_DriveCapability = Regen peak power output drive capability for one second

8. I_shunt(drive max) = Maximum (1_second) regen current capability of the drive:

About the author: Hurley Gill is senior applications and systems engineer at Kollmorgen located in Radford, VA. He’s a 1978 Engineering Graduate of Virginia Tech who has been engaged in the motion control industry since 1980s. He can be reached at hurley.gill@kollmorgen.com.

Click here for the third part of this series on how to specify a system for a rotary servo axis needing a regenerative resistor.