UNIT IV SYNCHRONOUS MOTOR DRIVES

1. Outline the advantages and applications of PMSM.

Synchronous motor operate at constant speed irrespective of the load. It depends only
on the frequency of the supply. Therefore, these kinds of motor finds application in driving
machinery which operates at steady speed for long duration of time period like conveyor belts,
paper-making machines, ball mills steel rolling mills etc.

2. Mention the two modes employed in variable frequency control in synchronous motor.

• Separate or true synchronous mode

• Self controlled mode

3. Outline the advantages of constant margin angle control of synchronous motor drive fed
by an inverter?

• Eliminates the hunting and stability problems

• It does not require commutation circuits

4. What is self control mode of synchronous motors

In self-control mode, the stator supply frequency is changed in proportion to the rotor
speed so that the rotating field produced by the stator always moves at the same speed as that
of the rotor.

5. What is constant margin angle control of synchronous motor drive?

In self-controlled synchronous motor drive, the load side converter is operated as an

inverter and maximizes motor power factor. This control is called constant margin control.

6. Define torque angle.

The angle between excitation emf and supply voltage is called torque angle or load or
power angle. It is denoted by δ.

7. Outline some application of load commutated inverter fed synchronous motor drive.

Some prominent applications of load commutated inverter fed synchronous motor drive are high
speed and high power drives for compressors, blowers, conveyers, steel rolling mills, main-line
traction and aircraft test facilities.

8. What is the separate control mode of synchronous motors?

The motor behaves like conventional synchronous motor (i.e) hunting oscillations exists. The
change in frequency is slow enough for rotor to track the changes. Multi motor operation is
possible here. The open loop ( Separate Control) have stability problem at low speeds.
9. Distinguish self and separate control modes

Self controlled Mode Separate controlled mode

Rotor position encoder is mounted on the Separate oscillator is used to control supply
shaft which is used to control the supply frequency
frequency
No hunting oscillations occur Hunting oscillations occur
Damping windings are not needed Damper winding are needed
Stable behaviour. Posses stability problems at low speeds

10. List out the commonly used synchronous motors.

• Wound field synchronous motor

• Permanent Magnet synchronous Motor
• Synchronous Reluctance Motor
• Hysteresis Motor.

11. Give the uses of a hysteresis synchronous motor.

Hysteresis synchronous motors are used in applications requiring constant speed,

smooth operation, and precise timing, such as in electric clocks, record players, and tape
recorders

12. Explain in short indirect flux control.

13. Show the purpose of inductance and capacitance in the D.C. link circuit.

14. Outline the advantages and disadvantages of rotor resistance control?

15. Infer the features of static scherbius drive.

16. Show where Kramer electrical drive system is used?

17. Outline the advantages of static Kramer drive?

1. Discuss on self-control technique in synchronous motor.

In self controlled mode, the supply frequency is changed so that the synchronous speed is same as
that of the rotor speed. Hence, rotor cannot pull-out of slip and hunting eliminations are eliminated.
For such a mode of operation the motor does not require a damper winding.
Fig shows a synchronous permanent magnet machine with self control.The stator winding of the
machine is fed by an inverter that generates a variable frequency voltage sinusoidal supply. Here
the frequency and phase of the output wave are controlled by an absolute position sensor mounted
on machine shaft, giving it self-control characteristics. Here the pulse train from position sensor
may be delayed by the external command as shown in fig.

In this kind of control, the machine behavior is decided by the torque angle and voltage/ current.
Such a machine can be looked upon as a dc motor having its commutator replaced by a converter
connected to stator. The self-controlled motor run has properties of a dc motor both under steady
state and dynamic conditions and therefore, is called commutator less motor (CLM). These
machines have better stability behavior. Alternatively, the firing pulses for the inverters can also
be obtained from the phase position of stator voltages.

When synchronous motor is over excited they can supply the reactive power required for
commutation thyristors. In such a case the synchronous machine can supply with inverter works
similar to the line commutated inverter where the firing signals are synchronized with line voltages.
Here, the firing signals are synchronized with the machine voltages then these voltages can be used
both for control as well as for commutation. Hence, the frequency of the inverter will be same as
that of the machine voltages. This type of inverters are called load commutated inverter
(LCI).Hence the commutation has simple configurations due to the absence of diodes, capacitors
and auxiliary thyristors. But then this natural commutation its not possible at low speeds upto 10%
of base speed as the machine voltage are insufficient to provide satisfactory commutation. At that
line some forced commutations circuit must be employed.

Self controlled synchronous motor Drive employing load commutated Thyristor Inverter
In fig wound field synchronous motor is used for large power drives. Permanent magnet
synchronous motor is used for medium power drives. This drive consists of two converters. i.e
source side converter and load side converter. The source side converter is a 3 phase 6 pulse line
commutated fully controlled rectifier. When the firing angle range 0≤ǂs≤90º,it acts as a commutated
fully controlled rectifier.

During this mode, output voltage Vds and output current Ids is positive. When the firing angle
range is 90º≤ǂs≤180º, it acts as a line commutated inverter. During this mode, output voltage Vds
is negative and output current Ids is positive. When synchronous motor operates at a leading power
factor thyristors of the load side 3φ converter can be commutated (turn off) by the motor induced
voltages in the same way, as thyristors of a 3φ line commutated converter are commutated by
supply voltage Load commutation is defined as commutation of thyristors by induced voltages of
load (here load is synchronous motor).

Triggering angle is measured by comparison of induced voltage in the same way as by the
comparison of supply voltages in a line commutated converter. Load side converter operates as a
rectifier when the firing angle range is 0≤ǂl≤90º.It gives positive Vdl and Id. When the firing angle
range is 90º≤ǂl≤180º,it gives negative Vdl and positive Id.

For 0≤ǂs≤90º, 90º≤ǂl≤180º and with Vds >Vdl,the source side converter works as a line commutated
rectifier and load side converter, causing power flow from ac source to the motor, thus giving
motoring operation. When firing angles are changed such that 90º≤ǂs≤180º and
0º≤ǂl≤90º,the load side converter operates as a rectifier and source side converter operates as an
inverter. In this condition , the power flow reverses and machine operates in regenerative braking.
The magnitude of torque value depends on (Vds – Vdl). Synchronous motor speed can be changed
by control of line side converter firing angles. When working as an inverter, the firing angle has to
be less than 180º to take care of commutation overlap and turn off of thyristors. The commutation
lead angle for load side converter is βl =180º -αl. if commutation overlap is neglected ,the input
ac current of the converter will lag behind input ac voltage by angle ǂl. Here synchronous motor
input current has an opposite phase to converter input current, the motor current will lead its
terminal voltage by a commutation lead angle β. Therefore, the synchronous motor operates at a
leading power factor. The commutation lead angle is low value, due to this higher the motor power
factor and lower the inverter rating.

2. Explain the separate controlled mode of operation of synchronous motor

This is an open loop control mode in which the stator supply frequency is controlled from an
Independent oscillator. Hence the frequency is gradually increased from its initial value to the final
desired value so that the difference between the synchronous and rotor speed is always very small.
This enables the rotor to track the changes in synchronous speed and catch up with out pulling out.
When the desired synchronous speed is reached, the rotor pulls into step, after hunting oscillations.
This method can be used for smooth starting and regenerative braking. This method is best suited
for multiple synchronous reluctance or Permanent magnet (PM) motor drives where close speed
tracking is essential among a number of machines in applications such as fiber spinning mills,
paper and textile mills where accurate speed tracking is required. The block diagram of such an
open loop control system using this separate control method for multiple synchronous motors is
shown in the figure below.

Here all the machines are connected to the same Inverter and they move in response to the
command frequency f*at the input to the Ramp/delay circuit. The Input speed command is given
through a ramp generator with a finite delay to ensure that the rotor gradually picks up speed and
pulls into synchronism with the stator magnetic field and settles at the final synchronous speed.The
frequency command f* after passing through the ramp/delay circuit generates the required V and f
control signals just like in a VSI with a PWM Inverter as shown in the figure. The V control is
applied to the DC converter through a flux control block so as to generate the required Voltage to
generate a constant flux with varying frequency. The Rectifier output then gets applied to the PWM
inverter through L& C filter as required for a VSI type drive. The frequency command is directly
applied to the PWM inverter. The synchronous motor can be built with damper winding to prevent
oscillations.

3. Explain the operation of a ‘power margin control’ based self-controlled synchronous

motor drive.

The operation of the inverter at the minimum safe value of the margin angle gives the highest
power factor and the maximum torque per ampere of the armature current, thus allowing the most
efficient use of both the inverter and motor. The commutation Margin Angle Control of
Synchronous Motors is defined as the angle measured from the end of commutation to the crossing
of the phase voltage which was under commutation (natural firing instant). For satisfactory
operation, without commuta-tion failure, this margin angle must be greater than the turn off angle
(ωtq) of the thyristors. In the constant Margin Angle Control of Synchronous Motors it is always
observed that the margin angle does not go below a minimum value. Referring to Fig. the margin
angle is ψ = γ-u, Where γ is the lead angle of firing and u is the overlap angle.

Fig shows the constant margin angle control for a wound field motor drive employing a rotor
position encoder. This drive has an outer speed loop and an inner current loop. The rotor position
can be sensed by using rotor position encoder. It gives the actual value of speed ωm. This signal is
fed to the comparator. This comparator compares ωm and ωm* (ref value). The output of the
comparator is fed to the speed controller and current limiter. It gives the reference current value
Id*. Id is the DC link current. It is sensed by current sensor and fed to the comparator. The
comparator compares Id and Id*. The output of the comparator is fed to the current controller. It
generates the trigger pulses.
It is fed to the controlled rectifier circuit. In addition, it has an arrangement to produce constant
flux operation and constant margin angle control. From the value of dc link current command Id*,
Is and 0.5u are produced by blocks (1) and (2) respectively. The signal φ is generated from DŽmin
and 0.5u in adder (3). In block (4) If’ is calculated from the known values of Is,φ and Im. Note that
the magnetizing current Im is held constant at its rated value Im to keep the flux constant. If* sets
reference for the closed loop control of the field current IF. Blocks (5) calculates’* from known,
values of φ and If*

The phase delay circuit suitably shifts the pulses produced by the encoder to produce the desired
value of 0’.This signal is fed to the load commutated inverter. The load commutated inverter drives
are used in medium power, high power and very high power drives, and high speed drives such as
compressors, extractors, induced and forced draft fans, blowers, conveyers, aircraft test facilities,
steel rolling mills, large ship propulsion, main line traction, flywheel energy storage and so on.
This drive also used for the starting of large synchronous machines in gas turbine and pump led
storage plant. High power drives employ rectifiers with higher pulse numbers, to reduce torque
pulsations. The converter voltage ratings are also high so that efficient high voltage motors can be
employed.
4. Write brief notes on different types of permanent magnet synchronous motor drive.

5. Explain the construction and working of permanent magnet synchronous motor drive.

6. Explain open loop V/F speed control of synchronous motor.

7. Explain the closed loop operation of permanent magnet synchronous motor drive.
 UNIT V DESIGN OF CONTROLLERS FOR DRIVES

1. Draw the speed control loop block diagram

2. Outline the types of feedback in DC Drives?

o Voltage feedback
o Current feedback
o Torque Feedback
o Speed feedback

3. Summarize the final transfer function equation of a separately excited DC motor.

4. Outline the advantages of closed loop speed control?

System protection, Greater accuracy, Improved dynamic response and reduced effects of

disturbances such as loading.

5. Outline the advantages of load commutation over forced commutation.

• Load commutation has a number of advantages over forced commutation

• It does not require commutation circuits
• Frequency of operation can be higher
• It can operate at power levels beyond the capability of forced commutation
6. Write the transfer function of power converter.

Converter transfer function is represented by

Where Kc – Converter gain, Tc – Converter Time delay, s – Laplace operator

Converter gain for a maximum control voltage Ecm is given as

7. Summarize the field weakening modes?

The DC motor speed can be varied by varying the field current and armature voltage
is kept constant. The field current can be controlled by using power converter. By using
this method, the motor field flux decreases i.e., field weakening mode. This method only
applicable for above base speed because speed is inversely proportional to flux.

8. Interpret margin angle of commutation?

The difference between the lead angle of firing and the overlap angle is called the
margin angle of commutation. If this angle of the thyristor, commutation failure occurs.
Safe commutation is assured if this angle has a minimum value equal to the turn off angle
f the thyristor.

9. Outline the applications of cyclo-converter?

A cycloconverter drive is attractive for law speed operation and is frequently

employed in large, low speed reversing mils requiring rapid acceleration and deceleration.
Typical applications are large gearless drives, e.g. drives for reversing mills, mine hoists,
etc

10. Compare D.C. link converter and cyclo-converter?

• D.C. link converter is a two stage conversion device which provides a variable voltage,
variable frequency supply.
• Cycloconverter is a single stage conversion device which provides a Variable voltage,
variable frequency supply.

11. Outline the advantages of brushless D.C. motor?

Brushless DC (BLDC) motors offer several advantages over their brushed

counterparts, including higher efficiency, longer lifespan, quieter operation, and precise
 speed control. They also boast a high power-to-weight ratio, lower maintenance
requirements, and reduced electromagnetic interference.

12. Show the characteristics of true synchronous mode operated synchronous motor?

A true synchronous motor in synchronous mode exhibits a constant speed, matching

the supply frequency, from no-load to full-load. It's not inherently self-starting and requires
external means to reach synchronous speed. These motors can operate at various power
factors (leading, lagging, or unity) and are known for their high efficiency and ability to
improve overall system power factor.

13. What are the two types of static scherbius system?

• DC link static Scherbius system

• Cycloconverter Scherbius system
14. What is meant by super synchronous speed operation?

In super synchronous speed, extra slip power is taken out of rotor through the AC-
DC-AC link and fed in to the grid . By doing this, excitation at rotor remains constant and
output at stator is always up to grid standards. Stator is directly connected to grid, and rotor
is connected through AC-DC-AC link.

15. What is Commutator Less Motor (CLM)?

A commutator less DC motor, or brushless DC motor, is an electric motor that uses

electronic means to control the speed and direction of rotation rather than a commutator .
It includes a stator, rotor, and electronic controller.

PART C

1. Develop the transfer function of the closed loop control of the speed controller.
2. Develop the transfer function of the dc motor load system and power converter.
TRANSFER FUNCTION OF POWER CONVERTER
3. Describe the closed loop speed control of separately excited DC motor by proportional
Controller.
4. Explain how the converter power output and the controller characteristics are related.

5. Explain (i) Converter selection and characteristics (ii) Field weakening mode control.

6. Discuss the operation of a closed loop scheme for speed control of a dc motor below and
above the base speed.
7. Explain the armature voltage control of the dc motor with constant field and field weakening
modes.

8. Describe the closed loop speed control of separately excited DC motor by proportional
integral Controller.