Page 228 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

When the no-volt coil is activated by the start button, the auxiliary contact of the starter should
close to complete the NVC circuit and should remain in the closed position even after the start
button is released.
If the overload relay is not functioning properly i.e. not tripping the motor as per setting of the current rating, then
replace it with a new one as per with the original specification of the manufacturer.

If a humming and chattering noise is observed in the starter then check for the rated voltage. If the voltage is okay,
then check for any gummy material adhered to the pole faces. If found, clean it properly. See whether the shading
ring over the pole faces of the NVC is loose. Tighten it properly and also check the spring tension of NVC housing.

Suppose the starter trips often then, check up the load on the motor. (Might be due to overload or over tension
of the belt) Reduce the load or tension of the belt. Further check up the motor current in each phase. If the motor
takes higher current than specified even though the load is normal, then the fault is with the motor and not with
the starter. After attending to the faults and rectifying them, reassemble the starter, connect it to the motor for
proper functioning.
Starter check - chart given below could be used to locate trouble in a D.O.L. starter.

Maintenance of DOL starters
Trouble Cause Remedy
I Starter check chart
1 Contacts chatter Low voltage, coil is not picking up Correct the voltage condition. In case there is
properly. persistent low voltage,
Broken pole shading ring. check the supply of the transformer tapping.
Poor contact between the pole Replace.
faces of the magnet. Clean the pole faces.
Poor contact between fixed and Clean contacts and adjust, if necessary.
movable contacts.
2 Welding or Low voltage preventing magnet Correct the voltage condition. In case of
overheating. from sealing. persistent low voltage, which is accepted normal
Abnormal in rush current. change the NVC to lower voltage coil. Check
Short circuit in the motor. excessive load current or use larger contactor.
Foreign matter preventing Remove the fault and check to ensure that the
contacts from closing. fuse rating is correct. Clean contacts with
Rapid inching. suitable solvent. Install larger device or caution
the operator not to operate the inch button too
quickly.
3 Short life of contact Weak contact pressure. Adjust or replace contact springs.
points
4 Noisy magnets Broken shading coil. Replace magnet.
Magnet faces not mating. Align or replace magnet assembly.
Dirt or rust on magnet faces. Clean with suitable solvents.

5 Failure to pick up Low voltage. Check system voltage. In case persistent low
and seal the contacts. voltage, change to a lower voltage coil.
Coil open or short-circuited. Replace the coil.
Mechanical obstruction for the Clean and check for free movement of
moving parts. contact assembly.
6 Failure of moving Voltage not removed. Check wiring in the NVC coil circuit.
mechanism to Worn or rusted parts causing Replace parts.
drop out. binding. Residual magnetism due Replace worn out magnet parts
to lack of air gap in magnet path. or demagnetise the parts.
Gummy substance on pole faces Clean with suitable solvent.
causing binding.

215

CITS : Power - Electrician & Wireman - Lesson 93-100 CITS : Power - Electrician & Wireman - Lesson 93-100