Technology / Zero-Break
Why 3 Milliseconds Changes Everything
One AC cycle at 50Hz lasts 20 milliseconds. An ERD switches from grid to battery in 3ms — well under that single cycle. The elevator motor never loses synchronisation. Passengers feel nothing. Here is the complete engineering story.
The Core Problem with ARD
The Automatic Rescue Device (ARD) has been the lift industry’s answer to power failure for decades. When the grid drops, the ARD waits for the lift to coast to a stop, then uses a small battery to creep the car to the nearest floor, open the doors, and release the passengers. The problem: that process takes 20–90 seconds depending on where the car stops.
During those seconds, passengers are in the dark, in a confined space, with no indication of what is happening. Claustrophobia, panic, and medical emergencies are well-documented consequences — particularly for elderly passengers, children, and those with heart conditions.
ARD also does not keep the lift running. It rescues trapped passengers once, then the lift is offline until the grid returns. If power cuts are frequent, the lift becomes unreliable.
- Grid fails
- Relay contacts open → 200–500ms delay
- Lift decelerates and stops with jerk
- Lights go out
- ARD battery connects after 20–30 seconds
- Lift creeps to nearest floor at low speed
- Doors open → passengers exit
- Lift offline until grid returns
- Grid fails
- IGBT detects voltage drop in <1ms
- ERD pre-charges output bus
- IGBT switches to battery in 3ms
- Motor never loses power — continues at rated speed
- Passengers are unaware of the transition
- Lift continues normal operation on ERD battery
- Runs until battery is depleted or grid returns
The Physics: Why Sub-Cycle Switching Matters
A three-phase induction motor — the type used in virtually all modern lifts — operates on the principle of a rotating magnetic field. The stator generates a magnetic field that rotates at the supply frequency (50Hz in India). The rotor follows this field with a slight lag (slip).
When power is interrupted, even briefly, the rotating field collapses. The rotor continues spinning due to inertia, but the field and rotor are now out of phase. When power returns, the field re-establishes at a random phase angle. If the angle is large, the motor sees an effective short circuit — a massive inrush current that can be 6–10x the rated current. This inrush causes a violent mechanical jerk, stresses the motor windings, and can damage the lift controller and VVVF drive.
At 50Hz, one AC cycle lasts exactly 20ms. If the switchover happens within one cycle (3ms), the magnetic field in the motor stator never fully collapses. The rotor never loses phase synchronisation. No inrush. No jerk. No damage. The motor literally cannot tell that the supply just switched from grid to battery.
How IGBT Makes 3ms Possible
Traditional relay-based switching — used in early ARD systems and standby generators — is mechanically limited. A relay contact must physically open and close, which takes 10–50ms in the best case. That is already longer than one AC cycle.
IGBT (Insulated Gate Bipolar Transistor) switching is electronic — there are no moving parts. The gate signal changes in microseconds. In an ERD, the IGBT bridge continuously monitors the input supply voltage. The moment voltage drops below the threshold (typically within 1ms), the controller fires the gate signals to switch the output bus to battery-backed inverter output.
Critically, the ERD’s inverter output is already running in parallel with the grid supply before the failure. This "pre-positioned" architecture means there is no start-up delay. The battery-backed output is always live and phase-synchronised with the motor. The switch is purely electronic — just changing which source feeds the output.
The result is a seamless handoff in under 3ms — 7× faster than the 20ms AC cycle, and roughly 10,000× faster than a typical ARD.
For a detailed technical breakdown of the IGBT architecture used in Su-vastika ERDs, see our IGBT Architecture deep-dive and the Su-vastika engineering blog on zero-break ERD switching.
Real-World Consequences of Zero-Break
90% Reduction in Maintenance Calls
Most lift maintenance calls are caused by power quality issues — surges, sags, and frequency variations that the motor controller misinterprets as faults. Pure sine wave ERD output at exactly 50Hz eliminates these. Lift controllers see a better supply quality than the grid provides.
Medical Emergencies Prevented
The American Journal of Psychology documents that even 30 seconds of entrapment can trigger life-threatening events in elderly, cardiac, and claustrophobic passengers. Zero-break switching eliminates entrapment entirely — there is no stoppage to escape from.
Motor Lifespan Extended
Repeated inrush events degrade motor winding insulation. A lift with frequent power cuts using ARD technology can suffer motor failure within 3–5 years. With ERD, the motor sees no inrush events. Field reports show motor lifespan increases by 40–60%.
Full Continuous Operation
Unlike ARD, which takes the lift offline after one rescue cycle, ERD keeps the lift running for the duration of its battery backup — typically 1–4 hours depending on battery size. In areas with frequent power cuts, this is the difference between a usable building and an inaccessible one.
Regulatory Context: Why States Are Moving to ERD
Haryana was the first Indian state to mandate ERD technology for all new lift installations under the Haryana Lifts and Escalators Act. The mandate explicitly requires zero-break operation — not just the ability to rescue passengers, but the ability to prevent them from needing rescue in the first place.
Other states are reviewing similar mandates. The BIS (Bureau of Indian Standards) has issued IS/IEC standards for lift emergency backup that align with the technical requirements that only ERD can meet — specifically sub-cycle switchover and continuous operation capability.
Su-vastika Systems, the manufacturer that pioneered ERD technology in India, holds the relevant BIS certifications and technology patents for zero-break switching. See the Su-vastika patents and certifications page for full details.
Size Your ERD
Use the LiftInverter.com calculator to find the right ERD KVA rating for your specific lift — motor power, speed, capacity, and backup duration.