Drive Motor Characteristics
Why a single phase induction motor does not self start?
When a single phase supply is fed to the single
phase induction motor. Its stator winding produces a flux which only alternates
along one space axis. It is not a synchronously revolving field, as in the case
of a 2 or 3phase stator winding, fed from 2 or 3 phase supply.
What is meant by plugging?
The plugging operation can be achieved by
changing the polarity of the motor there by reversing the direction of rotation
of the motor. This can be achieved in ac motors by changing the phase sequence
and in dc motors by changing the polarity.
Give some applications of DC motor?
constant speed, lathes, centrifugal pumps, machine tools, blowers and fans, reciprocating pumps
locomotives, rapid transit systems, trolley cars, cranes and hoists, conveyors Compound: elevators, air compressors,
rolling mills, heavy planners.
What are the different types of electric
Dynamic or Rheostatic
Counter current or plugging
What is the effect of variation of armature
voltage on N-T curve and how it can be achieved?
The N-T curve moves towards the right when the
voltage is increased. This can be achieved by means of additional resistance in
the armature circuit or by using thyristor power converter.
Compare electrical and mechanical braking?
Brakes require frequent maintenance
Can be applied to hold the system at any
very little maintenance
cannot produce holding torque.
When does an induction motor behave to run off as a generator?
When the rotor of an induction motor runs
faster than the stator field, the slip becomes negative. Regenerative braking
occurs and the K.E. of the rotating parts is return back to the supply as
electrical energy and thus the machine generates power.
Define synchronous speed?
It is given by Ns = 120f / p rpm.
Where Ns = synchronous speed, p = no.
of stator poles, f = supply frequency in Hz
meant by regenerative braking?
In the regenerative braking operation, the
motor operates as a generator, while it is still connected to the supply here,
the motor speed is grater that the synchronous speed. Mechanical energy is
converter into electrical energy, part of which is returned to the supply and
rest as heat in the winding and bearing.
the disadvantages of inserting resistance in the rotor circuit in slip ring
Losses (I2R) is
increasing and efficiency of the motor is decreased.
Since speed is dependent on
both resistance and load. We can change speed for short periods only.
what condition, the slip in an induction motor is
Greater than one
Slip of an induction motor is
negative when the induction motor is operating in generating mode.
Slip of an induction motor is
greater than one when the induction motor is operating in the braking mode (its
direction is opposite to the direction of rotating magnetic field).
cumulative and differential compound motors?
The orientation of the series flux aids the
series flux opposes shunt flux
Draw the speed torque characteristics of DC shunt motor?
Draw the speed torque characteristics of DC series motor?
Draw the speed torque characteristics of compound motor?
What is meant by mechanical braking?
In mechanical braking, the stored energy of the
rotating parts is dissipated in the form of heat by a brake shoe (or) a break
drum. in electric braking, the stored energy of rotating parts is converted
into electrical energy and dissipated in the resistance in the form of heat or
returned to the supply.
What is back emf?
If the voltage is applied and current is
passing in the armature, the motor starts rotating. At that time armature
conductors also rotates and hence cuts the flux. Due to that an emf is induced
in the armature and the direction is found to be opposite to that of the supply
this is known as back emf.
What are all the types of electrical machines?
Electrical machines are classified as AC
machines and DC machines.
Types of DC machines
Types of AC machines
Single phase (b) three phase
(a)Single phase (b) three
the two types of 3 phase induction motor?
There are two types of 3-phase induction motor
based on the type of rotor used:
Squirrel cage induction
Slip ring induction motor.
What is the principle used in induction motor?
Conversion of electrical power into mechanical
power takes place in the rotating part of an Electric motor. In D.C. motor, the
electrical power is conducted directly to the armature (i.e. rotating part)
through the brushes and Commutator. Hence, in this sense, a D.C. motor can be
called a conduction motor. However, in A.C. motors, the rotor does not receive
electric power by conduction but by induction in exactly the same way as the
secondary of a 2-winding transformer receives its power from primary. That is
why such motors are called induction motors. In fact, induction motors can be
treated as a rotating transformer i.e. in one which primary winding is
stationary but the secondary is free to rotate.
What are the advantages of the slip-ring
induction motor over squirrel cage Induction motor?
It is possible to speed
control by regulating rotor resistance.
High starting torque of 200
to 250% of full load voltage.
Low starting current of the
order of 250 to 300% of the full load current.
Hence slip ring induction motors are used where
one or more of the above requirements are to be met.
the types of single phase induction motors?
The types of single phase induction motors are:
Split phase induction motor.
Capacitor start induction
Capacitor start and capacitor
Shaded pole induction motor.
Why regenerative braking not possible in DC
In DC series motor, regenerative braking is not
possible without necessary modifications, because reversal of Ia would result
in reversal of field and hence of Eb.
What is meant by dynamic braking?
In this method of breaking, the motor is
disconnected from the supply, the field connections are reversed and motor is
connected in series with a variable resistance R