The product developed on latest IC based technology. Seco automatic motor protector is a solid state designed using closed tolerance components for adequate protection to the motor. The factors adversely affecting the operation of three phase induction motors are highlighted in brief.

Single Phasing:

Single phasing is an extreme case of unbalancing which is caused by loss of one phase in a three phase system blowing off the supply fuse breaking in conductor or poor contact in contactors are the common cause of single phasing. If single phasing occurs which the motor is running, it keeps on rotating as single-phase induction motor and draws heavy current which energies the overload relay of the starters. However, if the motor is lightly loaded, the motor current may not exceed full load value and over load relay may not operate. Further more, when the overload relay is not operated, the coil of motor starter remains energized through the lines when fault is on the line side. Thus the single phasing is the main cause of burning out of the motor.

Unbalanced supply:

Unbalance voltage give rise to negative sequence voltage. Motor draws negative sequence current over and above the positive seq. current, negative sequence current is of double the line frequency core losses and rotor copper losses due to skin effect are increased at higher frequency. Thus undetected heating, due to increased losses, may damage the rotor and also stator windings due to convection of head.

Torque produced by negative sequence current is in opposition to that of positive sequence current. Hence the net torque produced by the motor is reduced.  As a result motor draws more current to meet the load torque. More ever, under unbalanced supply, there is reduction in motor efficiency and rise in noise, vibration and motor slip. If the voltage unbalance exceeds the allowable value, need arises of the means like ‘Seco Automatic Motor Protector’ to trip-off the starter of motor to avert such disaster. It should be clear from the discussion that the conventional starters are disable to provide enough protection.

High Voltage:

Higher voltage increases the core losses due to increase in core flux and over stresses the motor insulation. It is desirable to switch off the motor, when the applied voltage exceeds the allowable value.

Low Voltage:

The torque produced by the motor is proportional to the square of the applied voltage. Therefore, the motor torque reduces considerably at reduced supply voltage. And the motor draws large current due to increase in slip. It is desirable to switch-off the motor, if voltage falls below permissible value.

Seco automatic motor protector having an automatic ‘switch-off’ at dangerously low/high voltage and an automatic ‘switch-on’ when voltage return in healthy conditions. Pilot lamps gets ‘switch-off’ when single phasing, excessive unbalance, reverse phasing low/high voltage. The unit provides protection up to motor terminals.

 

Principle of operation	Negative sequence voltage sensing
System Voltage and Frequency	415 AC +- 15% 3 phase, 50Hz +- 5%
Contact Rating	1 Amp. At 415 AC, 6 Amp. At 240 V AC.
Trip Time Delay	2 to 3 seconds
Switch-on time delay	60 seconds approx to ensure the healthy
	supply and suppression of transients
Output Contact	1 NO + 1 NC
Temperature range	-5 degree to 50 degree C
Humidity	Up to 95% R.H.
H.V. Test	2 KV AC (RMS), 50 Hz for one minute
	between the live parts and body
Mode of operation	Auto/SPP/Bypass
Indictor Lamps	Power On, Delay