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MMR monitoring, level and thermal relays

minia.png The relay measures the given physical quantity by a probe and switches the output contacts based on the values measured.

The MMR-U3 and MMR-X3 voltage monitoring relays monitor line voltage and respond to a failure by switching the output contact depending on the set limits. The basic design MMR-U3 is intended for monitoring of overvoltage (225 ÷ 265 V), undervoltage (180 ÷ 220 V) and failure of phases.


Fig. 1: MMR-U3-001-A230, monitoring of overvoltage and undervoltage


The relay switches the output contact if the set limit is exceeded. Failure signalling delay may be set to 0 ÷ 10 s. After the line parameters return to the set limits the contact switches back with 5 s delay to eliminate oscillation.


Fig. 2: Monitoring failure of phases


Monitoring of overvoltage and undervoltage may be disabled individually. Therefore, in theory the relay may monitor failure of phases only. Red diode on the front panel of the device is used to indicate the failed phase:

  • 1 flash … phase 1 failure
  • 2 flashes … phase 2 failure
  • 3 flashes … phase 3 failure.

These relays may be used in single-phase systems as well. In this case L1, L2 and L3 terminal must be interconnected.

The extended design MMR-X3-001-A230 allows monitoring of overvoltage (225 ÷ 265 V), undervoltage (180 ÷ 220 V), failure of phases, phase sequence and asymmetry.
The function of overvoltage, undervoltage and failure of phases monitoring are identical to MMR-U3-001-A230 including LED indications.

Monitoring of phase sequence and asymmetry is described in the chart below:


Fig. 3: Monitoring of phase sequence


Fig. 4: Monitoring of asymmetry (5 ÷ 20 %)


Fixed delay of 5 s is set for contact switch back to the reset position to eliminate oscillation. Monitoring of asymmetry may be disabled just like monitoring of undervoltage and overvoltage.

MMR-HL level relays are intended for monitoring level of conductive liquids in a container.


Fig. 5: MMR-HL-001-A230


They may be operated in two modes:


  1. Liquid withdrawing
    When the upper probe (max. level) is reached, the relay switches on a pump for example, which withdraws liquid from the container until the lower probe (minimum level) is reached when the pump is switched off to stop.
  2. Liquid refilling
    The pump is switched on when the level reaches the minimum level probe and switched off then the maximum level probe is reached.


If the container is made of conductive material, it may substitute the GND probe. Standard conductors may be used as probes, provided that application of commonly available end pieces is recommended to prevent moistening of the probes and to prolong their service life.

The MMR-T1 is intended solely for monitoring of temperature inside motor winding by probes installed in the motor.


Fig. 6: MMR-T1-001-A230, functional chart


The output contact is switched based on the shown dependence. If the motor heats up above the acceptable limit, the thermistor resistance changes above 3.3 kΩ. The relay opens the make contact. Two conditions must be met concurrently to restart the motor:

  1. The motor winding temperature must drop to a value when the thermistor resistance is below 1.8 kΩ.
  2. After RESET.

RESET may be done using three different methods:

  1. Manually by a button on the front panel of the device
  2. Using a remote rest button (R1-T1)
  3. Using automatic reset (permanent connection of R1-T1)


The MMR-T2 thermal relay performs the function of a double thermostat. The device monitors two independent temperatures with thermistor probes and switches two independent output contacts. Each temperature corresponds to one contact.


Fig. 7: MMR-T2-200-A230, functional chart

The advantage of this device is based on broad temperature range within a single device (-25 ÷ 95 °C).

The MMR-TD is a differential thermostat. The device monitors one or two temperatures and switches the output contacts based on the set function. Both types MMR-T2 and MMR-TD are delivered including two 3m long probes.


Fig. 8: Thermostat probes

The probes may be prolonged or made shorter. Considering the thermistor resistance value (app. 3 kΩ at ambient temperature) there is no significant shift in temperature evaluation.


Switching devices

The Minia switching devices include five groups of products: