What Is a Submersible Pump Control Relay? Types and Selection Criteria
A submersible pump control relay monitors the pump motor current and mains voltage and stops the motor on dry running, overload and voltage faults.
A submersible pump control relay is a protection and control device that monitors the current drawn by a submersible pump motor and the mains voltage, and stops the motor on dry running, overload and voltage faults. It is used for filling tanks from boreholes and for irrigation. Because the motor sits at the bottom of the well, protection is done in the panel on the surface by measuring the motor current.
Dry running is when the pump spins without water after the well level drops. The motor current then falls below its normal running value; the relay detects this drop and stops the motor. When the pump jams or is overloaded, the current rises, and the relay trips on that too.
Types
| Type | Housing | Models in the catalog |
|---|---|---|
| Single-phase (MDK) | DIN rail, DIN case, 72 x 72 mm panel type | MDK-24 (2–40 A, DIN rail), MDK-03 (1–30 A, DIN case), MDK-02 (72 x 72 mm) |
| Three-phase, rail mounted (TDK) | DIN rail, DIN case | TDK-24 (2–40 A), TDK-24YU (5–250 A), TDK-30 (DIN case) |
| Three-phase, panel door (TDK) | 96 x 96 and 144 x 144 mm | TDK-96 (1–250 A), TDK-01 (1–120 A), TDK-02 (1–300 A) |
DIN rail and DIN case models mount on the rail inside the panel. The 72 x 72, 96 x 96 and 144 x 144 mm models fit a square cut-out in the panel door, with the front of the device outside the panel. The manufacturer states that the TDK-30 has a 1–30 A setting range and a built-in contactor; with this model, compare the contactor rating with the motor.
Selection criteria
- Phase: Choose the MDK series for a single-phase pump and the TDK series for a three-phase pump.
- Current range: The rated current on the motor nameplate must fall inside the relay's setting range. Do not estimate the current from motor power; use the nameplate value.
- Measuring connection: On high-current models the current is measured through a current transformer. Whether the relay works with one or three current transformers, and which transformer to use, is stated on the product page.
- Protection settings: Set the low-current (dry-run) limit slightly below the current the motor draws when pumping water, and the high-current limit slightly above the nameplate current. Choose the voltage limits according to how much the mains fluctuates.
- Control: The relay output drives the contactor coil, and the contactor switches the motor. Unless you use a model with a built-in contactor, size the contactor separately for the motor power.
- Level control: If the pump must stop when the tank is full or run according to the well level, check the relay's float switch or electrode input in the datasheet. A device that only switches on level is a liquid level relay; it does not protect the motor against current and voltage faults.
Common mistakes
- Setting the low-current limit too low: During dry running the current never falls below the limit, the relay does not see the fault and the pump keeps running dry. Set the limit from the current measured at first start-up.
- Setting the low-current limit too high: The relay trips during normal operation and the pump stops for no reason.
- Leaving a current transformer secondary open: An open secondary on a loaded current transformer develops a dangerous voltage. Short the transformer secondary before removing the relay.
Models in the catalog
The Submersible Pump Control Relays category has 9 active products, all from Tense: 3 single-phase (MDK) and 6 three-phase (TDK). For level electrodes and float switches, see Liquid Level Control Accessories.
Frequently asked questions
What is the difference between a submersible pump control relay and a liquid level relay?
A liquid level relay only switches according to level. A submersible pump control relay monitors the motor current and mains voltage, and stops the motor on dry running, overload and voltage faults.
Does dry-run detection need an electrode in the well?
Not with a relay that detects by current: when the pump runs dry the motor current falls and the relay sees it. Electrodes or float switches are used to start and stop the pump according to level.
Which current range should I choose?
The one that covers the rated current on the motor nameplate. For example, a three-phase motor with a nameplate current of 25 A suits the 2–40 A range of the TDK-24; for motors above 100 A, look at wide-range models such as the TDK-01, TDK-02, TDK-24YU or TDK-96.