H. T. MOTORS

H. T. MOTORS: Motor workings with voltage 3.3 KV and above are HT motors. This motor normally used for higher loads as the operating current cab be reduced. Normally these motors are connected. These motors are having some special cooling arrangements like forced air cooling, condenser type cooling in which water is circulated to cool the temperature inside. Instead of a ball bearing, sleeve bearings are also used. For cooling the bearing, separate oil us used.

The following protection normally used;

1	Instantaneous over current	Settled normally 10 times the rated current. Operates only during short circuit.
2	Thermal overload relay	Set normally at 110% rated operates at motor overload.
3	Locked rotor or prolonged start protection	Set normally at 125% the rated operates when the load got jammed. Set at normal starting time + sec.
4	Negative phase sequence	Set normally at 25% the rated. Operates when phase unbalance occurs and single phasing. (one phase failed).
5	Earth fault	Set normally at 20% the rated operates when fault occurs between phase and earth.

Motor protection relay is having all the above set protection.

Motor is provided with the differential relay protection connected between Start side CTs and Phase side CTs as shown.

VIBRATION: Vibration of motor is usually the result of troubles such as unbalance of rotor, misalignment, looseness or bad bearings. Vibration is checked by vibration meter.

VELOCITY AND DISPLACEMENT: Vibration can be measured in terms of how fast the part moves. This is called the Peak Velocity and is measured in inches/seconds or millimeter/seconds. Velocity is a function of both displacement vibrations.

Vibrations can be measured in terms of how far the move back and forth. This is called the peak to peak displacement measured in mils. 1 mil = 25.4 microns

BEARINGS: The bearings used in electrical machineries are ball bearings, Roller Bearings, Tapered Roller Bearings, Thrust Bearings, Sleeve Bearings, Needle Bearings etc, generally on load side of the machine Roller Bearings are used.

                D.E. – Drive and

                N.D.E – Non drive end

DEFINITION FOR BEARING NO 6308

6              –                             Refers deep groove ball bearing

3              –                             Refers medium weight (2 Light weight /4 – Heavy weight)

08           –                             08 × 5 = 40 mm diameter of the shaft

7308 -                                    Angular contact or thrust bearing

C3           -                             Clearance extra for high temperature

Z              -                             Shield bearing

MOUNTING NAD DISMOUNTING PRESSURE

1.    Remove motor pulley with 3 leg puller if the motor is small and medium size. For large pulley Hydraulic puller may be used.

2.    Remove fan cover and fan.

3.    Remove outer bearing cover on both ends and remove end shields with markings.

4.    Take out the rotor from stator.

Now the motor is dismantled totally while assembling reverses the above procedure.