Voltage-based motor protection is the dividing line between a basic thermal relay and a High Performance relay: undervoltage (ANSI 27), phase voltage imbalance, and phase voltage loss are only present in the top tier of the Schneider Electric Electrical Installation Guide classification, alongside the more common current-based functions [S4].
ANSI 27 prevents a motor from re-accelerating while still drawing 6–8x rated current after a system voltage dip, and ANSI 46 trips on the negative-sequence current that voltage unbalance forces into the rotor, where heating is roughly proportional to the square of the unbalance percentage [S2][S5].
Where Undervoltage and Voltage Unbalance Sit in the Protection Hierarchy
The Electrical Installation Guide splits motor protection into four tiers: Conventional, Advanced, Advanced Plus, and High Performance. Undervoltage, phase voltage imbalance, phase voltage loss, and phase voltage reversal are all exclusive to the High Performance tier, which is also the only tier that uses voltage inputs rather than current alone [S4]. Practical consequence: a relay that only sees CT inputs physically cannot execute ANSI 27, even if it has current-unbalance (ANSI 46) logic based on computed I2. The hierarchy of motor protection functions is laid out clearly in the conventional-to-high-performance table, and a motor protection relay without VTs wired to its terminals is limited to the lower three tiers [S4].
Why Undervoltage Trips a Running Motor, Not Just an Incoming Feeder
Undervoltage protection on a motor branch is engineered to clear a bus after a complete loss of voltage and to disconnect the motor before it stalls on a sustained sag, but it must be coordinated so it does not trip on the normal 0.5–1.5 s voltage dip that occurs during across-the-line starting of large motors [S1]. Per the SEL Protection Basics text, "undervoltage protection should not trip motors because of voltage dips caused" by the motor's own start, so the ANSI 27 element is armed only after the start window closes [S1]. The functional description in the SmartElec motor protection reference reinforces the same logic: ANSI 27 "prevents the motor from drawing excessive current to maintain torque during low voltage conditions" and "prevents line current surges from simultaneous re-acceleration of multiple motors after voltage recovery" [S2]. Without that block, every bus disturbance would trip every running motor and collapse the process.
Voltage Unbalance, Negative-Sequence Current, and the I-squared-R Heating Law
Voltage unbalance on the supply side does not just produce a small extra current: it produces negative-sequence current, which rotates against the rotor at roughly twice synchronous speed and induces slip-frequency currents in the rotor cage with very low skin-effect resistance. The result is severe rotor I²R heating disproportionate to the stator-side unbalance [S2][S5]. A common rule of thumb, traceable through motor-protection reference material, is that a 3.5% supply voltage unbalance can produce roughly 20–25% negative-sequence current in the stator and several hundred percent of rated heating in the rotor, which is why ANSI 46 is implemented in nearly every modern relay even at the Conventional tier via the simpler current-unbalance path [S2][S5]. When voltage transformers are also wired in, the relay can compute the true voltage unbalance ratio (max deviation from mean / mean) and trip with finer discrimination than current unbalance alone, which is what the High Performance tier adds [S4].
Typical Setpoints, Time Delays, and Coordinated Curves
Field practice for ANSI 27 on a motor relay is a pickup of 80–90% of nominal line voltage (often 88% for 400 V systems, 85% for 4.16 kV systems) with a definite time delay of 0.5–10.0 s, long enough to ride through autotransformer or soft-starter transfer events but short enough to clear before the motor stalls. A useful cross-reference for engineers sizing these time-overcurrent elements is the SEL Protection Basics damage-curve discussion, where minimum-melt, total-clear, and mechanical-damage thresholds on a time-current plot frame how short an inverse curve can run before the conductor or winding is mechanically compromised [S1].
Comparison: Current-Only vs Voltage-Supervised Motor Protection
The decision between a current-only motor protection relay and a voltage-supervised one is a decision between ANSI 46 derived from current imbalance and a true ANSI 27 plus computed voltage unbalance. Four criteria separate them: sensing inputs (CTs only vs CTs + VTs), unbalance discrimination (current-derived I2 vs computed voltage unbalance V2/V1), ability to prevent re-acceleration surges after a bus disturbance (no vs yes), and tier cost (Conventional/Advanced vs High Performance) [S4]. A Conventional-tier relay with current unbalance will catch a single-phased motor by sensing the sudden jump in negative-sequence current, but it cannot distinguish a 2% supply unbalance from a winding fault, nor can it block re-start on a sag; the High Performance relay with VT inputs adds both capabilities and a microcomputer protection platform running the calculations every cycle [S4].
Standards Anchors, Selection Criteria, and Failure Modes
ANSI/IEEE C37.2 device numbers govern the function numbering: 27 for undervoltage, 46 for negative-sequence (current or voltage) unbalance, 47 for combined undervoltage and phase-sequence, 59 for overvoltage, and 50/51 for the instantaneous and time-overcurrent elements that the undervoltage function must coordinate with [S1][S3]. Per-phase VT fusing, VT secondary wiring integrity, and correct phase-rotation of the VT secondary are the most common failure modes: a blown VT fuse will emulate an undervoltage event and trip the motor on ANSI 27, which is why most relays include a "VT fuse failure" or "loss-of-potential" (ANSI 60) element that blocks the 27/47 trip output [S3]. Engineers working through these coordination choices typically reference the protection relay function-number cross-reference and treat any voltage-element change as a coordinated event with the bus's checking and testing apparatus at commissioning.
For motors with VFD supply or soft-starters, the relay's VT connection point must be on the line side of the VFD input contactor, not the motor side, because the VFD output waveform will corrupt the voltage-unbalance calculation and produce false ANSI 27/46 trips; the same caution applies to the relay's separate high voltage tester used for insulation proof-testing, which must be locked out before the relay is re-energized. Thermal memory behaviour across the undervoltage trip-and-reclose cycle is a separate, often-missitemized parameter and is covered in Motor Protection Relay Thermal Memory Retention After Power Cycle.