Unit 4 Synchronous motors
4.1 Principle of working [operation, significance of load angle.
4.2 Torques: starting torque, running torque, pull in torque, pull out torque.
4.3 Synchronous motor on load with constant excitation (numerical), effect of excitation at
constant load (numerical).
4.4 V-Curves and Inverted V -Curves.
4.5 Hunting and Phase swinging.
4.6 Methods of Starting of Synchronous Motor.
4.7 Losses in synchronous motors and efficiency (no numericals).
4.8 Applications areas
4.1 Principle of working [operation, significance of load angle.
Construction of Synchronous Motor:
Construction front, synchronous motor consists of the stator is a stationary part and rotor is
a rotating part. The stator consists of a core and the slots to hold the armature winding
similar to Synchronous generator.
Construction of Synchronous Motor
Usually, its construction is almost similar to that of a 3 phase induction motor, except the
fact that here we supply DC to the rotor, the reason of which we shall explain later. Now,
let us first go through the basic construction of this type of motor. From the above picture,
it is clear that how do we design this type of machine. We apply three phase supply to the
stator and DC supply to the rotor.
Main Features of Synchronous Motors
1. Synchronous motors are inherently not self starting. They require some external
means to bring their speed close to synchronous speed to before they are
synchronized.
2. The speed of operation of is in synchronism with the supply frequency and hence for
constant supply frequency they behave as constant speed motor irrespective of load
condition
This motor has the unique characteristics of operating under any electrical power factor.
This makes it being used in electrical power factor improvement
Stator
The armature winding will be insulated by the varnish or paper. The stator’s core is made of
silicon stamping with the laminated assembly which helps us to reduce the eddy current
losses and hysteresis loss.
Rotor
Rotor poles are projected type and they are mounted on the shaft of the motor. The rotor
winding will be connected to a DC source with the help of slip rings.
Also, the speed of the motor is purely depending on the input supply frequency and poles.
Working Principle of a Synchronous Motor
The stator and the rotor are the two main parts of the synchronous motor. The stator is the stationary
part of the motor and rotor is their rotating part. The stator is excited by the three-phase supply, and
the rotor is excited by the DC supply.
The term excitation means the magnetic field induces in the stator and rotor of the motor. The main
aim of the excitation is to convert the stator and rotor into an electromagnet.
The three-phase supply induces the north and south pole on the stator. The three-phase supply is
sinusoidal. The polarity (positive and negative) of their wave changes after every half cycle and
because of this reason the north and south pole also varies. Thus, we can say that the rotating
magnetic field develops on the stator.
The magnetic field develops on the rotor because of the DC supply. The polarity of the DC supply
becomes fixed, and thus the stationary magnetic field develops on the rotor. The term stationary
means their north and south pole remains fixed.
The speed at which the rotating magnetic field rotates is known as the synchronous speed. The
synchronous speed of the motor depends on the frequency of the supply and the number of poles of
the motor.
NS = 120f/P
When the opposite pole of the stator and rotor face each other, the force of attraction occurs between
them. The attraction force develops the torque in the anti-clockwise direction. The torque is the kind
of force which moves the object in the rotation. Thus, the poles of rotor dragged towards the poles of
the stator.
After every half cycle, the pole on the stator is reversed. The position of the rotor remains same
because of the inertia. The inertia is the tendency of an object to remain fixed in one position.
When the like pole of the stator and rotor face each other, the force of repulsion occurs between
them and the torque develops in the clockwise direction.
Let understand this with the help of the diagram. For simplicity, consider the motor has two poles. In
the below figure, the opposite pole of the stator and rotor face each other. So the attraction force
develops between them.
After the half cycle, the poles on the stator reverse. The same pole of the stator and rotor face each
other, and the force of repulsion develops between them.
The non-unidirectional torque pulsates the rotor only in one place and because of this reason
the synchronous motor is not self-starting.
For starting the motor, the rotor is rotated by some external means. Thus, the polarity of the rotor
also changed along with the stator. The pole of the stator and rotor interlock each other and the
unidirectional torque induces in the motor. The rotor starts rotating at the speed of the rotating
magnetic field, or we can say at synchronous speed.
The speed of the motor is fixed, and the motor continuously rotates at the synchronous speed.
What is Load Angle:
Load angle is nothing but an angle different between stator axis and rotor pole axis of the
synchronous motor. For ideal motor, the load angle is zero since the rotor poles aligned with stator
poles, but in practice, this is not possible. The motor has both mechanical and electrical losses, hence
load angle is always present in the synchronous motor.
Load angle
Load angle can be calculated by using the below-mentioned formula,
Refer above, the induced torque is directly proportional to the load angle δ.
It is denoted by δ
It is also called a power angle, torque angle and coupling angle.
Note: Synchronous motor is a constant speed motor which speed does not vary with respect to load.
At No-load:
Fig 1.1 synchronous motor No load and Load Characteristics
Look at fig 1.1, The phase voltage V and back emf Eb has some slight difference. This slight
difference is called the load angle. At no-load condition, the load angle is very small since the losses
in the motor are less.
At load condition:
While raising the load in the synchronous motor, increases the load angle even though the magnitude
of the V and Ebph is same. Look at fig 1.1 the angle difference between V and Eb increases with
respect to armature current Ia.
Significance of load angle:
1. The Torque produced in the synchronous motor is purely depending on the load angle (sin
δ). It is measured by the degree in electrical.
2. Increasing in load angle indicate the decreasing magnetic locking between the rotor poles
and stator poles.
3. When δ = 90 deg. The motor reaches to the full load torque. If you increase the load
further, the motor losses the synchronism. Such a torque is called pull out torque.
4.2 Torques: starting torque, running torque, pull in torque, pull out torque.
Starting torque
It is torque developed by the synchronous motor when name plate voltage is applied to its
armature winding. It is also called as breakdown torque.
It may as low as 10% in the case of centrifugal pump.
Running torque
It is torque developed by the synchronous motor under running condition. It is determined by
horse power and speed of the motor.
P = Tω
Pull in torque
A synchronous motor is started as induction motor during starting condition and its speed
below 2 to 5% of the synchronous speed.
When the excitation ( or DC supply ) to the field winding is applied, the motor pull into
synchronism resulting stator and rotor magnetic field rotates at same speed.
The amount of torque requires for synchronous motor to pull into synchronism is called as
pull in torque.
Pull out torque
It is maximum torque which the synchronous motor can develop without pulling out of step.
When the synchronous motor is loaded, the rotor falls back by some angle is called as load
angle.
The stator and rotor magnetic rotates at synchronous speed in spite of some load is applied on
the rotor.
The synchronous motor developed maximum torque when the rotor falls back by angle 900
.
When the load increases beyond its maximum rating, the rotor steps out of synchronism and
synchronous motor stop.
Starting Torque:
The torque is being developed at the starting time of the motor. It is also called as breakaway torque.
The starting torque of the synchronous motor is purely depending on the method of starting the
motor.
Running Torque:
The full load torque of the motor is called running torque. The running torque is defending on the
motor specifications.
Pull-in Torque:
Let we assume the synchronous motor is started and the speed is nearer to the synchronous speed,
during that time the stator pulls the rotor into synchronism, that torque is called pull-in torque.
Pull out Torque:
Let we assume the motor is running at the maximum torque, beyond that slight increase in load
causes the motor pulls out the synchronism, that maximum torque is called pull out torque. The pull
out torque will be three to four-time of the full load torque of the motor.
Torque Vs Load Angle
Refer to fig 1.1 of torque vs speed characteristic of the Synchronous motor.
At load angle 90 degree the motor produces the maximum torque. Further increasing the loads, the
magnetic locking between the stator and rotor become weak. Then the motor stops. Therefore, the
maximum torque is produced by the motor without loss of synchronism is called pull out Torque.
4.3 Synchronous motor on load with constant excitation (numerical), effect of excitation at
constant load (numerical).
In a d.c. motor, the armature current Ia is determined by dividing the difference between V and Eb
by the armature resistance Ra. Similarly, in a synchronous motor, the stator current (Ia) is
determined by dividing voltage-phasor resultant (Er) between V and Eb by the synchronous
impedance Zs. One of the most important features of a synchronous motor is that by changing the
field excitation, it can be made to operate from lagging to leading power factor. Consider a
synchronous motor having a fixed supply voltage and driving a constant mechanical load. Since the
mechanical load as well as the speed is constant, the power input to the motor (=3 V*Ia *cos Ñ„) is
also constant. This means that the in-phase component Ia cos Ñ„ drawn from the supply will remain
constant. If the field excitation is changed, back e.m.f Eb also changes. This results in the change of
phase position of Ia w.r.t. V and synchronous motor for different values of field excitation. Note that
extremities of current phasor Ia lie on the straight line AB. hence the power factor cos of the motor
changes. Fig: shows the phasor diagram of the synchronous motor.
(i) Under excitation
The motor is said to be under-excited if the field excitation is such that Eb < V. Under such
conditions, the current Ia lags behind V so that motor power factor is lagging as shown in Fig: (i).
This can . be easily explained. Since Eb < V, the net voltage Er is decreased and turns clockwise. As
angle ( δ δ = 90°) between Er and Ia is constant, therefore, phasor Ia also turns clockwise i.e., current
Ia lags behind the supply voltage. Consequently, the motor has a lagging power factor.
(ii) Normal excitation
The motor is said to be normally excited if the field excitation is such that Eb = V. This is shown in
Fig: 2.28 (ii). Note that the effect of increasing excitation (i.e., increasing Eb) is to turn the phasor Er
and hence Ia in the anti-clockwise direction i.e., Ia phasor has come closer to phasor V.
Therefore, p.f. increases though still lagging. Since input power (=3 V*Ia *cos Ñ„) is unchanged, the
stator current Ia must decrease with increase in p.f.
Suppose the field excitation is increased until the current Ia is in phase with the applied voltage V,
making the p.f. of the synchronous motor unity [See Fig: 2.28 (iii)]. For a given load, at unity p.f. the
resultant Er and, therefore, Ia are minimum.
(iii) Over excitation
The motor is said to be overexcited if the field excitation is such that Eb > V. Under-such conditions,
current Ia leads V and the motor power factor is leading as shown in Fig: 2.28 (iv). Note that Er and
hence Ia further turn anti-clockwise from the normal excitation position. Consequently, Ia leads V.
From the above discussion, it is concluded that if the synchronous motor is under-excited, it has a
lagging power factor. As the excitation is increased, the power factor improves till it becomes unity
at normal excitation. Under such conditions, the current drawn from the supply is minimum. If the
excitation is further increased (i.e., over excitation), the motor power factor becomes leading. Note.
The armature current (Ia) is minimum at unity p.f and increases as the power factor becomes poor,
either leading or lagging.
4.4 V-Curves and Inverted V -Curves.
The graphical representation of armature current Ia vs field current If is called V-curve since the
final view looks like English letter V. At the same time the power factor vs field current is called
inverted V-curve of a synchronous motor.
Experimental setup of synchronous motor
Just you connect, two three-phase wattmeters and an ammeter to the stator winding of the
synchronous motor. Also, connect another ammeter to the field input supply. Continue this
experiment, with different field current as well as various load conditions such as load, half load and
full load.
Here stator side connected ammeter gives you armature current Ia and field-side connected ammeter
gives you field current If,
V curve Ia Vs If:
V curve Take If reading at x-axis and Ia reading at Y-axis. Connect all the reading points, we get a
V-shaped curve for a synchronous motor is shown in Figure 1.1.
At no-load condition, the real power supplied to the machine is zero since Ia=0, at unity power
factor. When you increase the field current above that point, the line current increases with respect to
applied field current, (Here the reactive current flows from the motor to source). in such case, the
synchronous motor can be used as a synchronous capacitor.
At load condition, the motor draws a small amount of real current at unity powe r factor. After
plotting the graph, the same V shape is still maintained.
V curve at load
Inverted V-curve:
Now take the reading of the power factor in the y-axis and field current in the x-axis and plot a
graph. Look at the shape of the graph, it seems like inverted V.
Inverted V curve Power factor vs If
Frequently asked questions:
What is the use of the V curve and Inverted V-curve?
Just understand the characteristics of the synchronous motor both the curves are used.
How to calculate the power factor of the synchronous motor?
In modern days, we can use digital energy meter to calculate power factor, armature current, the
power consumed by the motor etc, but to calculate the pf traditionally, we can use two wattmeter
method with the help of below mentioned formula
4.5 Hunting and Phase swinging.
Function of Damper Winding ( Effect of hunting in the Synchronous Motor )
When the synchronous motor is driven for variable load such as shears, punching press,
compressors etc., its rotor falls back or advance by some angle .
When the load on the motor increases, its rotor falls back by some angle .
Similarly when the load on the motor decreases, its rotor advances by some angle .
The rotor falls back or advances according to nature of the load. The oscillation of rotor about
its main axis is called as Hunting.
The rotor starts overshoot and pulled back due to variation of load.
The rotor starts oscillating about its new position corresponding to new load.
If the time period of the oscillation is equal to natural time period of the machine, the
mechanical resonance set up.
The amplitude of these oscillations is built up such that machine may out of synchronism.
Function of Damper winding
The rotor oscillation may dampen out by employing damper winding in the faces of field
poles of the motor.
The damper winding is nothing but copper bar embedded in the rotor faces. The copper bars
are short circuited at both ends.
The motion of rotor sets up eddy current in the damper winding.
The direction of this eddy current is such that it suppresses the rotor oscillations.
The damper winding does not prevent completely oscillation but reduce to some extent.
The function of the damper winding in the synchronous generator is to suppress the negative
sequence field and to dampen oscillation whereas it is used to provide starting torque and reduce
effect of hunting to some extent in the synchronous motor.
Causes of hunting
Sudden change of load
Sudden change in field current
Variation of load torque
Loss of Synchronism
Produces mechanical stress on the shaft
Cause temperature rise
Produces resonance condition
Reduction of hunting
Using flywheel
Using damper winding
4.6 Methods of Starting of Synchronous Motor.
Starting Methods of synchronous motor:
1. Reduced frequency method
2. External driver Method
3. Using Induction motor
4. Using a DC machine
5. Using damper winding
6. Slip ring assembly
Variable Frequency or reduced frequency Method:
As you know, the synchronous speed is directly proportional to the supply frequency. By reducing
the supply frequency, we can reduce the speed of the synchronous motor’s stator magnetic fields.
Hence by controlling the speed of the stator magnetic field, you can lock the stator and rotor magnet
easily. This is the basic concept behind the variable frequency starting method.
Generally, variable frequency drives are used to reduce the supply frequency. VFD consists of
rectifier/converter and inverter module. These modules can be used to convert the constant input
frequency to any desired output frequency from a fraction to full rated frequency.
VFD – Starting of synchronous motor
Also, such VFD units are also used to control the motor’s speed to a very low value for starting, and
then raise it up to the desired operating speed for normal running.
While operating a synchronous motor lower than the rated speed, its back emf is generated in the
motor also lower than the normal.
If the back emf is reduced in magnitude, then the applied voltage to the motor must be reduced as
well in order to keep the stator current at safe levels. That’s why VFDs are designed to reduce the
voltage along with its frequency. It means V/F ratio must be constant at any instant.
Motor starting with the external driver circuits:
The second way to starting of a synchronous motor is by coupling an external driver which help the
synchronous motor to reach the full speed.
Here the motor is connected like a generator in the power system. Once the motor is reached to full
speed starting motor will be detached from the synchronous motor.
The synchronous machine can be paralleled with its power system as a generator, and the starting
motor can be detached from the shaft of the machine.
Once the stating motor is disconnected means, the speed of the SM starts to fall down, the rotor
magnetic field Br falls behind stator rotating magnetic field Bs. And the synchronous machine starts
to act as a motor.
Actually we have two kinds of starting motors such as
1. An induction motor or pony motor
Pony Motor starting
Here left side motor indicate – induction motor and right side motor indicate synchronous motor.
A small (also called as a pony) Ac induction motor will be connected with the synchronous motor.
The synchronous motor runs along with the Induction motor initially. If the motor reaches to rated
RPM, the DC excitation will be turned on. If the synchronisms is established the induction motor
will be decoupled from the synchronous motor.
2. DC machine:
The higher rating synchronous motor will be started with the DC machines only. Initia lly the DC
machine act as a motor which drives the synchronous motor from 0 speed to rated speed. Hence the
synchronism will be established. At the same time, the DC machine acts as a DC exciter and which
provide DC supply to the synchronous motor’s rotor.
Using Damper winding:
The lower rating synchronous motor’s rotor field winding is designed along with the separate copper
bars and whose end is short-circuited with the help of end rings. This additional winding is called
damper winding and it is the most popular methods of starting synchronous motor.
Synchronous motor rotor with damper winding
Damper winding of the synchronous motor rotor
Consider you turn on the three-phase voltage to the stator of the synchronous motor, the motor starts
rotating at sub synchronous speed. Here synchronous act as an induction motor.
If you turned on the field supply means, the stator pulls the rotor to its synchronous speed and the
rotor start rotating at synchronous speed.
During synchronous speed, the relative motor between the stator rotating magnetic field and damper
winding is zero, hence the synchronous motor cannot produce induced emf in the damper winding.
The damper winding is active at the time of stating only, after reaching synchronous speed, the
damper winding act as dummy winding.
The major disadvantage of this starting method of a synchronous motor is, the motor draws high
starting current like an induction motor. Hence different types of starting method such as star-delta
starter, autotransformer starter need to be employed.
Slip ring Induction motor:
In above starting method, the damper winding/induction motor does not provide the high starting
torque to the synchronous motor, that’s why slip ring assembly is introduced in the field winding of
the synchronous motor’s rotor.
Here the end winding of the damper bar will be connected to the resistance circuit. The rotor acts as
a slip ring rotors of the induction motor. as you know the rotor resistance is directly proportional to
the starting torque.
At starting high resistance circuit will be added and the resistance will be cut off with respect to the
speed. When the motor reaches to sub-synchronous speed, the resistance across the rotor will be
reduced.
You have to turn on the DC excitation at sub synchronous speed, then the stator pulls the rotor to
synchronous speed. The motor rotates at synchronous speed continuously.
By this method of starting, we can achieve high starting torque.
However, you have to follow some certain procedure to avoid electrical and mechanical damage to
the motor.
How to start synchronous motor:
Step1: Turn on the input three-phase voltage to the stator and the same produce RMF at
synchronous speed.
Step3: Start the external driver motors with the direction of the rotating magnetic field. Run the
driver circuit to reach the rated speed.
Step2: Now turn on the DC excitation to the rotor. Now you see, the stator pulls the rotor to
synchronous speed. The motor runs at synchronous speed.
Step4: Stop the driver circuit.
4.7 Losses in synchronous motors and efficiency (no numericals).
Unlike the induction motor, the synchronous machine also has power input to the field windings.
The power flow diagrams are discussed in more detail below, but first, we will consider the losses in
the synchronous machine.
FIGURE 1: Power flow for a synchronous motor.
Types of Losses in Synchronous Machines
The losses in the synchronous machine are similar to those of the transformer and other types
of rotating machines. Like all electrical machines, synchronous machines have copper, steel,
rotational, and stray losses. Whether the machine operates as a motor or as a generator, the losses
can be summed as
Ploss=Pscl+Prl+Pfw+Pcore+Pstray(2)Ploss=Pscl+Prl+Pfw+Pcore+Pstray(2)
Where
Pscl = the copper loss in the stator (armature) windings
Pri = losses in the rotor, which includes the copper losses of the field windings, the losses in the
excitation system, and copper losses in the damper windings
Pfw = friction and windage losses
Pcore = hysteresis and eddy current losses, primarily in the stator iron
Pstray = stray losses not otherwise accounted for in the calculation of the other losses
Copper Losses
Copper losses are found in all the windings in the machine. By convention, they are computed using
the DC resistance of the winding at 75°C. The actual resistance depends on the operating frequency
and flux conditions. Any difference between the actual and computed copper loss is accounted for
under the stray loss category. Brush losses for machines with slip rings are normally neglected and
accounted for under the stray loss category.
Mechanical Losses
The mechanical or friction and windage losses are due to the friction in the bearings and the energy
that is dissipated in turning the rotor through the air inside the machine.
The rotational losses can be determined by driving the machine at rated speed with no load or
excitation. Rotational losses are frequently lumped with core loss and determined at the same time.
Open-Circuit or No-Load Core Losses
The core loss due to hysteresis and eddy currents is measured at no load, and when combined with
the mechanical losses, they constitute the no-load rotational loss. The difference between the
measured and actual core loss is put in the stray loss category.
Stray Loss
Stray losses include the difference between the actual losses and their calculated values, as well as
losses that are not specifically calculated, such as the brush contact loss.
Power Flow in Machines
Motor Power Flow
Motors receive electrical power at the armature as their inputs and deliver mechanical power at the
shaft of the machine. For the power flow diagram, shown in Figure 2, the electrical power is input at
the left. The copper losses are subtracted, leaving the developed power in the middle.
The developed power is represented in the equivalent circuit of the synchronous motor by the power
in the controlled voltage source and is calculated from equation 3. From the developed power, we
subtract the friction and windage, core, and stray losses to find the output mechanical power.
4.8 Applications areas
Application of Synchronous Motors
1. Synchronous motor having no load connected to its shaft is used for power
factor improvement. Owing to its characteristics to behave at any electrical power factor, it is
used in power system in situations where static capacitors are expensive.
2. Synchronous motor finds application where operating speed is less (around 500 rpm) and
high power is required. For power requirement from 35 kW to 2500 KW, the size, weight and
cost of the corresponding three phase induction motor is very high. Hence these motors are
preferably used. Ex- Reciprocating pump, compressor, rolling mills etc.
Synchronous Motors Advantage:
1. The speed of the motor is constant irrespective of the load changes.
2. The overexcited synchronous motor [leading power factor] can be used as a reactive power
generator, the same principle is used in the transmission and distribution.
3. The efficiency of the Synchronous motor is high.
4. They are mechanically stable irrespective of the air gap.
5. Can be operated at variable power factor.
Synchronous Motors Disadvantages:
1. They are not self-starting motors
2. High cost and cost vs Kw is higher than the three-phase IM. i.e Siemens make 2HP 3 Phase
IM costs around Rs. 5,000 but the same capacity motor in SM will be 20,000 rupees,
almost 4 times of the IM.
3. Field winding needs a DC external source.
4. Slip rings are connected the excitation circuit with the rotor circuit. It causes additional
maintenance cost.
5. Hunting
6. If the load torque is higher than the motoring torque means, the motor comes to standstill
position, again we need to restart the motor.
Synchronous Motor is Not Self Starting:
Why synchronous motor is not self-starting? Do you know the real reason? Do know, actually the
net torque developed in the motor is zero at starting of the motor, that’s why the motor cannot rotate
the rotor shaft at the time of starting. Let see why the torque is zero.
For simple understanding, we take a single electrical cycle of 50Hz. Here, Bs is the magnetic field
developed in the stator and it is rotating magnetic field. Br is the magnetic field developed in the
rotor and it is a steady magnetic field. Tind is the Torque developed in the motor.
Let we consider complete cycle with a different instance.
Fig 1.1 Input voltage
At t=0:
Fig 1.2 The stator and rotor Position at t=0
Refer the picture 1.2, at t=0, the stator magnetic field Bs and rotor magnetic field Br is alleged with
each other. They are magnetically locked.
The torque developer in the AC machine is
Hence the net torque produced in the motor is zero Tst=0
At t=1/200 s:
Fig 1.3 The stator and rotor Position at t=1/200 s
Refer to fig 1.1 and 1.3 At this time t=1/200 s, the rotor slightly moves but the stator magnetic field
now points to the left. By the induced-torque equation, the torque produced in the rotor will be the
counter-clockwise direction.
At t=1/100 s:
Fig 1.4 The stator and rotor Position at t=1/100 s
At this instance (t=1/100 s), the rotor magnetic field Br and stator magnetic field Bs pointing in
opposite directions, and hence the torque produced in the rotor is zero.
t=3/200 s:
Fig 1.5 The stator and rotor Position at t=3/200 s
The stator magnetic field Bs now points to the right, and the resulting torque is clockwise. Hence the
rotor shaft moves clockwise direction.
t = 1/50 s:
Fig 1.5 The stator and rotor Position at t=1/50 s
Br and Bs are lined up, hence the torque produced in the rotor become zero.
You observe that, during a complete cycle, first the torque produced in the rotor shaft is counterclockwise and second is clockwise. Therefore, the net torque developed in the rotor is zero.
Meanwhile due to the rotor slight movement causes vibration in the motor for each electrical cycle
and finally overheats.
Now you understand that how the net torque developed in the motor is zero. Due to this zero torque,
why synchronous motor cannot start itself.
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