28 April 2011

Defective Rotor of a SCI Motor

Most of the induction motors that we use are Squirrel Cage Induction (SCI) Motor. This name comes from the shape of the rotor. Some motors copper bars / round conductors are inserted in to the slots and they are short at both ends with end ring. Some other motor rotors are aluminium die-cast type. In both cases the shape of the rotor looks likes that of a Squirrel’s cage.

One of the common complaints that we receive on SCI Rotor is broken copper bars or open die-casting. The main indication of a defective SCI Rotor is that motor does not take load. Most often the no-load current drawn by the motor will be same as that by a good motor. But in Some cases fluctuating current is also seen. There will magnetic noise & excessive vibration while running the rotor. The above figure is of a defective SCI Rotor. This rotor is aluminium die-cast type.

The above picture shows a more clear view of the defective part of rotor. We can see open aluminium casting on slot. Melted aluminium has come out from this portion. We have to re-cast the rotor to make the motor working. It is must to ensure dynamic balance of rotor after re-casting. If the rotor is not dynamically balance then there will be vibration and abnormal noise from the motor. It will also reduce the life of bearings.

23 April 2011

Trident Powercraft - TAVR 20

Trident Powercraft Pvt Limited (TPPL) is a Bangalore based company manufacturing AC Generators (Alternators), DC Motors and Wind Generators. They have two manufacturing units one at Bangalore and the second at Hubly both in Karnataka. Main customer of TPPL is Kirloskar Oil Engines Limited (KOEL). Most of the Kirloskar Green DG sets available are manufactured with KOEL engines and TPPL Alternators. TPPL also do business with Ashok Leyland Limited and Mahindra Powerol In 2009 Emerson acquired TPPL for Leroy Somer Power Generation.

Here I am sharing some informations regarding TAVR 20 used in alternators under 125kVA manufactured by TPPL. TAVR 20 has 6 terminals. This AVR can be used in both single phase and three phase alternators. When used in a single phase alternator short the terminals  N & N1 of AVR. Then connect output phase to U and output neutral to N of AVR. Also connect F1 and F2 leads from the exciter field to the respective terminals of AVR. There will be no connection at V terminal of  AVR. When used in a three phase alternator there is no need to short N and N1 terminals of AVR. Connect output phase U, V and neutral N to the U V and N terminals of AVR and also connect F1 & F2 leads from exciter field to the F1 and F2 terminals of AVR. There will not be any connection at N1 terminal of AVR.

This AVR has three indicating LEDs (clear type, when they glow RED) and they are.
  1. UF (Shows Under Frequency, indicating low speed of prime mover) 
  2. OE - Over Excitation (The excitation voltage/current drawn by alternator is more than maximum value) 
  3. SL - Sensing Lead open (Sensing lead U-V is open circuited). 
AVR also have a F6A, 240V 20mm Glass fuse for protection. There are multy turn POT resistances
  1. OE - To set Excitation Limit 
  2. UF - To set Under frequency cut off 
  3. S - To set the voltage stability 
  4. V - To set the output Voltage. 
All POTs except V are factory set and sealed. They need not be disturbed at site. We can adjust the output voltage by turning the multy turn POT V.  All TPPL alternators are fixed with two permanent magnets (PMGs) on Exciter Field core. So there will not be any voltage built-up failure complaints due to loss of residual magnetism. The normal residual voltage is in between 30 to 40V (between two phases). The following figure shown connection details of TAVR 20.

20 April 2011

Commutator Sparking

Commutator is an essential part of a DC Motor. DC Supply to the Armature of a DC Motor is fed through Carbon Brushes and Commutator. The most common fault observed in DC Motor is sparking of Commutator. The below video shows sparking from a commutator.

The common reason for spark from commutator is improper brush contact. This can be due to 
  1. Defective brush holder, 
  2. Worn out carbon brushes, 
  3. Uneven condition of commutator. 
  4. Defective armature winding
The sparking create pitting of commutator and as we operate the motor spark will increase and will seriously damage commutator.  Spark will make carbon brush burn and it will worn out very fast. The below picture shown damage to the commutator of above motor. Here the segments are worn out at particular (at the place of carbon brushes) part of commutator.  That create sparking. It can be rectified by polishing the commutator. Every time after polishing the commutator it should be ensured that proper undercut is given to the mica between the segments.