Some Question and Answer 3 phase AC motors 01

(a) State the advantages and disadvantages of 3 phase AC motors with wound rotor (slip ring rotors). (5m)

(b) Briefly describe the effect on a 3 phase AC motor if the supply voltage was at a lower frequency than the motor design frequency . (5m)



Answer :-

Advantages wound rotor (slip ring rotor) :
This has the benefit of reducing the starting current surge while providing a high starting torque.
Speed can be regulated, except that rotor external resistance brush is continuously rated.

Disadvantages wound rotor (slip ring rotor) :-
It is inefficient because of power wasted in the resistance for speed control purposes.
The external resistance bank may require additional cooling facility.


At frequency lower than that for which motor is designed, the stator flux increases and magnetising current increased, so the heating on winding and iron will increase. Motor will run hotter, unless reducing in speed, also bring about reduction in load for example centrifugal pump, fan etc.
An increase in magnetising current, power factor will be lower which indicating increasing in air gap. Uneven air gap can cause bearing worn out. The possibility rotor and stator rub each other occur causing overheating which can destroy the coil insulation. Life of bearing will reduce due to unbalanced torque and vibration can occur as a result of increase in magnetic flux. Excessive heat generating to the bearing may cause seizure resulting noise sound in the motor. Directly the damage on the moving part of the pump occur and seizure takes place which reduce efficiency of the pump performance.

With reference to three phase motors state with reasons:

(a)        effect of one supply line becoming open whilst the motor is running,           (4m)

(b)        significance of air gap in induction machines, and                                         (4m)

(c)        types of bearings normally employed.                                                            (2m)

Answer :-

(a)                Effect on supply line becoming open whilst the motor is running :

1)      For star connected motor winding, the phase and line current are equal, so the line connected is correctly sensing the winding current. If the over current setting is exceeded during single phase fault, the motor will trip off.

2)      For delta connected winding, when one line become open circuited, a balanced 3 phase condition is no longer exists since I line = Ö3 I phase.

          L1

                                                      Ia      Ib 

                                          A                               B

   L2                                                     Ic 

                                                         C

    L3                                                                        

        At full load, the line current I line 2 and I line 3 is 2.5 times full load current. The current winding in C, Ic nearly three times its rated current, unless thermal overload disconnect the machine from power line, the excessively high temperature will burn out the winding.

3)      If the motor already running, when open circuit occurs, it will continue to run on single phase supply, with unbalanced distribution of current, motor will be overheated.

4)      If the motor is single phasing while accelerating, it will have increased in slip. Its available torque will be less than half three phase value causing poor acceleration and long run up time.

(b)               Significance of the air gap in the induction motor is made as small as possible to ensure that the strongest possible electromagnetic induction will take place. The size of air gap affects the power factor of the motor because if the air gap width increases, the magnetizing current will increased and power factor is lowered.

Uneven air gap can cause rotor and stator rub each other causing overheating which can destroy the coil insulation. Life of bearing also reduced due to unbalanced torque, resulting in vibration. So seizure bearing takes place which create noise in the motor. Consequently, moving parts of pump side affected and efficiency of pump reduced.

(c)                Types of bearings being used depends on the size of motor and type of services it performs.  Ball bearing applies where high radial load or axial thrust are encountered. Sealed ball bearing are well protected from exterior dirt , thus give many years of service without further lubrication. Its offer lows starting friction and favoured for small precision motors.

Sleeve bearings come in used where loads are moderate and long uninterrupted service without bearing replacement is desired. They may oil ring or externally lubricated depends on speed and size of the motors. Its have to be guarded against oil contamination and ingress of dirt.

They have higher starting friction but once oil film has been established, then friction reduced substantially.     

Modern cargo winches driven by AC motors must be able to meet high cargo handling rates involving frequent starting and stopping.

(a)                Briefly describe how this is accomplished without incurring excessive losses.   (5m)

(b)        Briefly describe the run up operation of such a motor.                                         (5m)

Answer :-

(a)                AC motor suitable for cargo winches to meet high cargo handling rates and involving frequent starting and stopping would be a 3 speed AC cage motor.

It is a specially design motor with dual starter. This would minimise the starting and accelerating losses which are responsible for a large proportion of the overall losses and heat generated.

 

                                                                                     1     1     1     2     2      2       3     3      3

                  Reversing connection                                                     333 sped

                                                                                                         3 spped

 

                                                                                                        3 speed motor       

                                                         brake circuit      

            The 3 speed motor is connected as drawn. It consist of :

a)      a low speed starter with 24 poles wound controlled by contactor.

b)      A medium / high speed starter with 2 independent windings, are wound for 8 poles controlled by contactor. High speed contactor with 4 poles controlled by contactor.

c)      All windings connected and fed y reversing contactor.

(b)               Procedure of running up the motor :

1)      low speed contactor cosed, forward or reversed contactors  are on if energised. Motor will run at low speed.

2)      Electro magnetic brake simultaneously released by another contactor.

3)      As controller is moved to step 2 and 3, motor accelerate first to medium and then to high speed.