TOPIC 3. MOTOR CONROL AND PROTECTION
Protective devices
Protective devices are necessary to pr
otect electrical appliance or equipment against
a)
b)
c)
d)
Short Circuit
Abnormal variations in the supply voltage
Overloading of equipment
To protect operator against accidental contact with the faulty equipment,falling which the
operator may get a severe shock.
Types of Protective Device
Different types of the protective device that are commonly used in electrical and electronic circuit
1. Fuse Wire or Fuse
2 .MCB – Miniature circuit breaker
3 .ELCB – Earth Leakage Circuit Breaker
4. ELCB & MCB
5 .Earthing or Grounding
1. Fuse
Fuse generally means a fuse wire,placed in a fuse holder.It is a safety device,which protects electrical and electronic
circuit against over loads,short circuit and earth faults.
The fuse link or fuse wire is made of low resistivity material and low melting point.
Operation of a fuse
Fuse is a short length of wire designated to melt and separate in case of excessive current. The fuse is connected in the
phase of the supply. It is always connected in series with the circuit / components that need to be protected. When the
current drawn by the circuit exceeds the rated current of the fuse wire,the fuse wire melts and breaks.This disconnects
the supply from the circuit and thus protects the circuit and the components in the circuit.
Rating of Fuse Wire
The maximum current that a fuse can carry,without being burnt,is called the rating of the fuse wire.It is expressed in
Amperes.
Current rating of the fuse ,selected for the circuit, should be equal to the maximum current rating of the machinery,
appliance or components connected in the circuit.
Fuse Carrier and Fuse Channel
Fuse carrier and channel are made of porcelain or Bakelite material.They are used for all domestic,commercial and
industrial application upto 100 A capacity.
Types of fuses
ii.
Cartridge Fuse
This fuse unit is in the form of a cartridge.Its normally manufactured in the range of 2 A to 100 A.Whenever the fuse
blows off,fuse with carrier is replaced by a new one. As it is sealed,it cannot be rewired. Cartridge fuses are used to
protect motors and branch circuit where higher amps or volt ratings are required. They are available in wide variety of
sizes,amp and volt ratings up to 600 Vac and 600 amps.Cartridge fuses are used extensively in commercial,industrial
and agricultural applications as well as residential fuse panels,air conditioning,pumps,appliances and other equipment.
iii.
High rupture capacity fuse (HRC)
High Rupture Capacity fuse unit.It is normally designed for high current.When fuse is blown off,the entire unit is to be
replaced by a new one.It cannot be rewired as it is a sealed one.
Characteristics of a good fuse wire
A good fuse wire should possess the following characteristics
a)Low resistivity
b)Low melting point
C)Low conductivity of the metal vapors formed,when the fuse is blown off.
Advantages of HRC Fuse
1.They require maintenance
2.They are reliable
3.They operate at high speed.
4.They have consistent performance
5.They clear both low and high fault current with equal efficiency.
2. MINIATURE CIRCUIT BREAKER (MCB)
It is safety device which work magneto thermic release principle.It is connected in the phase,between the supply and
load.It is manufactured in standard rating of 6A to 40 A.We can see it on the meter board of each and every house.
When the current drawn by load exceeds the rated value,it acts and trips the circuit,the protecting the
apparatus,operator and appliance.
Advantages of MCB
1.They act and open the circuit in less than 5 milli seconds.
2.Automatic switch off under overload and short circuit condition
3.No fuse to replace or rewire.It needs no repairs.
4.Supply is restored by resetting it again.
3. EARTH LEAKAGE CIRCUIT BREAKER (ELCB)
This is a domestic safety device,which trips the circuit when there is a small leakage to earth or body of the
appliance.Thus it protects the operator from shocks and accidents.This is connected in the circuit of the appliance to be
protected.
There are two types of ELCB
1. Voltage Earth Leakage Circuit Breaker
2. Current Earth Leakage Circuit Breaker
4. MCB & ELCB
It is the combination of both MCB and ELCB palced in one unit.It acts on both the occasion of earth leakage and
overload and protect the circuit,appliance and the operator.
5.EARTHING OR GROUNDING
Connecting the metal body of an electrical appliance,machinery or an electrical installation to earth,through a low
resistance wire,is called Earthing or Grounding.
Necessity of Earthing
Earthing is necessary for all domestic,commercial and industrial installation to safeguard the operator,tall buildings
and machinery against lightning. Metal body of all the electrical appliances,equipment and machinery,the earth points
of all three-pin sockets and the body of the energy meter are connected to earth through a thick G.I. wire.
Whenever a live wire comes in contact with the body of the appliance,it is directly connected to earth the grounding
wire and hence the body voltage comes to zero.Therefor the operator does not get any shock,when he comes in contact
with body of the appliance.The high voltage included during lightning is discharged to earth through grounding wire
and thereby building and machinery are protected.
MOTOR SPEED CONTROL
Speed Of A DC Motor
Back emf Eb of a DC motor is nothing but the induced emf in armature conductors due to rotation of the armature in magnetic
field. Thus, the magnitude of Eb can be given by EMF equation of a DC generator.
/
Eb = PØNZ 60A
(where, P = no. of poles, Ø = flux/pole, N = speed in rpm, Z = no. of armature conductors, A = parallel paths)
Eb can also be given as,
Eb = V- IaRa
thus, from the above equations
/
N = Eb 60A PØZ
but, for a DC motor A, P and Z are constants
Therefore, N
∝ K Eb/Ø
(where, K=constant)
This shows the speed of a dc motor is directly proportional to the back emf and inversely proportional to the flux per pole.
Speed Control Methods Of DC Motor
Speed Control Of Shunt Motor
1. Flux Control Method
It is already explained above that the speed of a dc motor is inversely proportional to the flux per pole. Thus by decreasing the
flux, speed can be increased and vice versa.
To control the flux, a rheostat is added in series with the field winding, as shown in the circuit diagram. Adding more resistance in
series with the field winding will increase the speed as it decreases the flux. In shunt motors, as field current is relatively very
small, Ish2R loss is small. Therefore, this method is quite efficient. Though speed can be increased above the rated value by reducing
flux with this method, it puts a limit to maximum speed as weakening of field flux beyond a limit will adversely affect the
commutation.
2. Armature Control Method
Speed of a dc motor is directly proportional to the back emf Eb and Eb = V - IaRa. That means, when supply voltage V and the
armature resistance Ra are kept constant, then the speed is directly proportional to armature current Ia. Thus, if we add resistance
in series with the armature, Ia decreases and, hence, the speed also decreases. Greater the resistance in series with the armature,
greater the decrease in speed.
3. Voltage Control Method
a) Multiple voltage control:
In this method, the shunt field is connected to a fixed exciting voltage and armature is supplied with different voltages.
Voltage across armature is changed with the help of suitable switchgear. The speed is approximately proportional to the
voltage across the armature.
b) Ward-Leonard System:
This system is used where very sensitive speed control of motor is required (e.g electric excavators, elevators etc.). The
arrangement of this system is as shown in the figure at right.
M2 is the motor to which speed control is required. M1 may be any AC motor or DC motor with constant speed.
G is a generator directly coupled to M1.
In this method, the output from generator G is fed to the armature of the motor M2 whose speed is to be controlled. The
output voltage of generator G can be varied from zero to its maximum value by means of its field regulator and, hence,
the armature voltage of the motor M2 is varied very smoothly. Hence, very smooth speed control of the dc motor can be
obtained by this method.
Speed Control Of Series Motor
1. Flux Control Method


Field diverter: A variable resistance is connected parallel to the series field as shown in fig (a). This variable resistor is called
as a diverter, as the desired amount of current can be diverted through this resistor and, hence, current through field coil can
be decreased. Thus, flux can be decreased to the desired amount and speed can be increased.
Armature
diverter:
Diverter
is
connected
across
the
armature
as
shown
in
fig
(b).
For a given constant load torque, if armature current is reduced then the flux must increase, as Ta ∝ ØIa
This will result in an increase in current taken from the supply and hence flux Ø will increase and subsequently speed of the
motor will decrease.


Tapped field control: As shown in fig (c) field coil is tapped dividing number of turns. Thus we can select different value of
Ø by selecting different number of turns.
Paralleling field coils: In this method, several speeds can be obtained by regrouping coils as shown in fig (d).
2. Variable Resistance In Series With Armature
By introducing resistance in series with the armature, voltage across the armature can be reduced. And, hence, speed reduces in
proportion with it.
3. Series-Parallel Control
This system is widely used in electric traction, where two or more mechanically coupled series motors are employed. For low
speeds, the motors are connected in series, and for higher speeds, the motors are connected in parallel.
When in series, the motors have the same current passing through them, although voltage across each motor is divided. When in
parallel, the voltage across each motor is same although the current gets divided.
SPEED CONTROL OF THREE PHASE INDUCTION MOTOR
A three phase induction motor is basically a constant speed motor so it’s somewhat difficult to control its speed.
The speed control of induction motor is done at the cost of decrease in efficiency and low electrical power factor.
Before discussing the methods to control the speed of three phase induction motor one should
know the basic formulas of speed and torque of three phase induction motor as the methods of speed control depends
upon these formulas. Synchronous speed
NS =synchronous speed
F=frequency
P= number of poles
The torque produced by three phase induction motor is given by
When rotor is at sand-still slip, s is one. So the equation of torque is,
Where E2 is the rotor emf
Ns =is the synchronous speed
R2= is the rotor resistance
X2 =is the rotor inductive
The Speed of Induction Motor is changed from Both Stator and Rotor Side
The speed control of three phase induction
motor from stator side are further classified
as:
1. V / f control or frequency control
2. Changing the number of stator poles
3. Controlling supply voltage
4. Adding rheostat in the stator circuit
The speed controls of three phase induction motor from rotor side are further classified
as:
1. Adding external resistance on rotor side
2. Cascade control method
3. Injecting slip frequency e.m.f into rotor side
Speed Control from Stator Side
1. V / f control or frequency control Whenever three phase supply is given to three phase induction motor rotating magnetic field is produced which rotates
at synchronous speed given by
In three phase induction motor emf is induced by induction similar to that of transformer which is given by
Where K is the winding constant, T is the number of turns per phase and f is frequency. Now if we change frequency
synchronous speed changes but with decrease in frequency flux will increase and this change in value of flux causes
saturation of rotor and stator cores which will further cause increase in no load current of the motor . So, its important
to maintain flux, φ constant and it is only possible if we change voltage i.e if we decrease frequency flux increases but
at the same time if we decrease voltage flux will also decease causing no change in flux and hence it remains constant.
So, here we are keeping the ratio of V/ f as constant. Hence its name is V/ f method. For controlling the speed of three
phase induction motor by V/ f method we have to supply variable voltage and frequency which is easily obtained by
using converter and inverter set.
2. Controlling supply voltage:
The torque produced by running three phase induction motor is given by;
In low slip region (sX)2 is very very small as compared to R2 . So, it can be neglected. So torque becomes;
Since rotor resistance, R2 is constant so the equation of torque further reduces to
We know that rotor induced emf E2 ∝ V.
So, T ∝ sV2.
From the equation above it is clear that if we decrease supply voltage torque will also decrease. But for supplying the
same load, the torque must remains the same and it is only possible if we increase the slip and if the slip increases the
motor will run at reduced speed . This method of speed control is rarely used because small change in speed requires
large reduction in voltage, and hence the current drawn by motor increases, which cause over heating of induction
motor.
1. Changing the number of stator poles :
The stator poles can be changed by two methods

Multiple stator winding method

Pole amplitude modulation method (PAM)
Speed Control from Rotor Side
1. Adding external resistance on rotor
side – In this method of speed control of three phase induction motor external resistance are added on rotor side. The
equation of torque for three phase induction motor is
The three phase induction motor operates in low slip region .In low slip region term (sX)2 becomes very very small as
compared to R2. So, it can be neglected . and also E2 is constant. So the equation of torque after simplification
becomes,
Now if we increase rotor resistance, R2 torque decreases but to supply the same load torque must remains constant. So,
we increase slip, which will further results in decrease in rotor speed. Thus by adding additional resistance in rotor
circuit we can decrease the speed of three phase induction
motor.
The main advantage of this method is that with addition of external resistance starting torque increases but this method
of speed control of three phase induction motor also suffers from some disadvantages:




The speed above the normal value is not
possible
Large speed change requires large value of resistance and if such large value of resistance is added in the
circuit it will cause large copper loss and hence reduction in efficiency
Presence of resistance causes more losses
This method cannot be used for squirrel cage induction motor
2. Cascade control method –
In this method of speed control of three phase induction motor, the two three phase induction motor are
connected on common shaft and hence called cascaded motor. One motor is the called the main motor and
another motor is called the auxiliary motor.
The three phase supply is given to the stator of the main motor while the auxiliary motor is derived at a slip
frequency from the slip ring of main motor.
Let
- NS1
be the synchronous speed of main motor;
- NS2 be the synchronous speed of auxiliary motor;
P1
be the number of poles of the main motor;
P2
be the number of poles of the auxiliary motor;
F
is the supply frequency;
F1
is the frequency of rotor induced emf of main motor
N
is the speed of set and it remains same for both the main and auxiliary motor as both the motors
are mounted on common shaft
S1
is the slip of main motor;
The auxiliary motor is supplied with same frequency as the main motor i.e
Now put the value of
Now at no load , the speed of auxiliary rotor is almost same as its synchronous speed i.e;
Now rearrange the above equation and find out the value of N, we get,
This cascaded set of two motors will now run at new speed having number of poles (P1 + P2). In the above method the
torque produced by the main and auxiliary motor will act in same direction, resulting in number of poles (P1 + P2).
Such type of cascading is called cumulative cascading.
INDUSTRIAL CONTROL PANEL
Tests procedures for a control panels.
Earth fault.
Continuity test.
Half split test
Insulation test’