Undercurrent Relay: Applications Beyond Dry Run Protection
Most motor protection deals with too much current. But a motor drawing too little current is also a warning sign: it usually means the motor has lost its load. An undercurrent (low current) relay watches for this and trips the motor or raises an alarm.
Why low current signals a problem
A motor’s current rises and falls with the mechanical load on its shaft. If the load disappears suddenly, the current drops towards no-load current. The motor is fine, but something downstream has failed and the process is no longer working, sometimes in a way that damages equipment.
Typical applications
| Application | What causes low current | Why trip |
|---|---|---|
| Pumps (submersible, borewell, booster) | Dry running, air lock, closed suction valve | Seal, impeller and motor damage within minutes |
| Belt-driven fans and blowers | Belt broken or slipping | Loss of ventilation or combustion air goes unnoticed |
| Conveyors | Belt break, coupling failure | Material spills; upstream equipment keeps feeding |
| Agitators and mixers | Shaft or coupling failure | Batch is not mixed; quality loss |
| Cooling tower fans | Gearbox or coupling failure | Cooling water temperature rises |
| Compressors | Unloaded running for long periods | Wasted energy; possible process issue |
Undercurrent relay vs dedicated sensors
Level switches, flow switches and belt-speed sensors detect these problems directly, but each needs field wiring and maintenance. An undercurrent function in the motor protection relay works from the panel with no extra sensors and protects against several failure modes at once. Many plants use both: sensors for the process and undercurrent as a backup.
For pumps specifically, see our detailed guide on dry run protection relays.
Setting the undercurrent function
The EOCR 3DE / 3EZ includes undercurrent protection. It can be set from 0.5 A up to the overcurrent setting, or switched off, with an operation time of 0.5–30 s. To set it:
- Note the normal running current at the lightest real operating condition (the display shows it per phase).
- Find the no-load or lost-load current from the motor or machine data, or by brief supervised observation.
- Set the undercurrent level clearly between the two.
- Set the operation time long enough to ignore short dips, short enough to stop damage.
- Use the start (manoeuvre) delay so the relay does not trip while the machine accelerates or the pump primes.
Common mistakes
- Setting too close to running current: causes nuisance trips when the load varies.
- Using it on lightly loaded motors: if normal running current is close to no-load current, the margin is too small; use a direct sensor.
- Ignoring VFDs: with a drive, current changes with speed; set the threshold for the lowest operating speed or use the drive’s own load monitoring.
Note that analog relays such as the EOCR SS and SSD do not have an undercurrent function. Choose the 3DE / 3EZ where loss-of-load protection matters.
Frequently Asked Questions
What is an undercurrent relay used for?
It detects when a motor loses its load, for example a dry-running pump, a broken fan belt or conveyor belt, or a failed coupling, and trips the motor or raises an alarm.
Can an undercurrent relay replace a level controller?
Not always. A level controller stops the pump before the tank runs dry; an undercurrent relay detects the pump losing load, including a blocked suction. Many sites use both.
Which Concord relays have undercurrent protection?
The EOCR 3DE and 3EZ digital motor protection relays.
Talk to Concord
Concord Automation & Controls has supplied motor protection relays and industrial relays to Indian industry since 1996 and is the exclusive ComatReleco (Switzerland) distributor for India, Bangladesh and Mauritius. Share your motor, pump or panel details and we will recommend the right model: call or WhatsApp +91 96670 40511, email contact@concordrelay.com, or use our contact page.