Ship Electrical Propulsion

1. AZIPOD (Azimuthing Podded Drive) Propulsion system
Jagabandhu Majumder FIE, Faculty IMTC Mumbai
“The ideal simplicity of the induction motor, its perfect reversibility and other unique qualities render it eminently
suitable for ship propulsion.”
- Inventor of induction motor scientist Nikola Tesla was an early proponent of electric propulsion for ships, wrote
in Feb,1917:
Podded Drive Propulsion:
Azipod, which stands for Azimuthing Podded Drive, represents a revolutionary approach to propulsion, made by
ABB has become an industry standard for ice going vessels with its superior performance in the harshest of ice
conditions,enabling vessels to cross the Northern Sea route independently. In late August 2019, Azipod propulsion
has made history, driving a Norwegian Coast Guard icebreaker all the way to the North Pole.
This hull strength and podded electric propulsion technology unique ability to rotate the vessel 360 degrees with full
torque and thrust in any direction with the combination Arctic’s double-acting ship (DAS) concept made all above
achievement and the year-round journey possible, which gives also the container ships the capability of an icebreaking
vessel and enables them to cut through 1.7 meters of level ice and more than 10 meters of ridged ice with considerably
less installed power (13 megawatts) and lower energy consumption than conventional diesel-driven vessels of the
same weight and hull design.
Figure 1 Double Acting LNG Carrier Propelling Heavy Ice Condition
Operating in temperatures as low as -50°C, a fleet of 15 Arc7 ice-class LNG carriers carrying liquid cargo are
generally referred as double acting tankers with azimuth pod propulsion developed the concept for oil transportation
between the Russian Arctic and Europe and the first double acting tanker is a type of icebreaking ship designed to
run ahead in open water and thin ice, but turn around and proceed astern in heavy ice conditions. In this way, the
ship can operate independently in severe ice conditions without icebreaker assistance but retain better open water
performance than traditional icebreaking vessels
The first ever cargo vessel to sail from Murmansk to Shanghai via the Northern Sea Route – without the assistance
of icebreakers – recently completed its maiden crossing, cutting a 65-day journey on the return leg down to 19 days

2. There are mainly two manufactures on the pod propulsion system. ABB is the biggest producer and has two types of
pod-types, Azipod (5-30 MW) and Compact (0.4-5 MW). Rolls-Royce (RR) is the second biggest company on the
market, producing a pod called Mermaid (5-25 MW).
The latest Azipod M series is equipped with ABB’s fourth-generation permanent magnet motors that draw on proven
Azipod propulsion technologies and have been refined to further increase power and maximize efficiency. The design
simplicity of the system provides increased robustness and reliability, at the same time allowing for ease of
maintenance.
Pod propulsion is type of azimuth
propulsion fitted in the place of the
conventional propeller which is
combination of steering and
propeller system consisting of an
integrated electric motor installed
in a pod which directly drives a
fixed pitch propeller.
The Pod assembly submerged
outside the ship hull, installed in a
gondola under the stern of the ship,
having short shaft line comprises
propeller, seals, bearings and shaft
and the AC motor.
Figure 2 Parts of Pod Propulsion System
The steering module contains mechanical components include electric/ hydraulic steering motors, slewing
transmission, slewing bearing and slewing sealing that enable the steering function of the Azipod. The steering
mechanics are connected to the ship with a mounting block. Each Azipod steering module consists of four steering
motors, each motor transmits power to the main gear rim through planetary gear and overload clutch which is in the
shaft between the reduction gear and steering motor which protects the steering mechanics from external accidental
loads also planetary gear and main gear in case of heavy overload.
The speed controlled AC motor is controlled by a variable-
frequency drive via the slip rings, provides full and smooth
torque in either direction and at low speeds, thus giving
excellent maneuverability, allows to steer large ships in
confined spaces without the help of tugs.
Dynamic positioning of the vessel for cruise ships is possible
with an adequate number of pod propulsion units.
The more effective and safer turning capability of Azipod
propulsion has been verified in the study by full-scale and
full-speed turning circle tests between sister-ships
MS Fantasy, with conventional propulsion, and MS Elation,
with Azipod propulsion, which recorded 38 per cent reduction
in tactical diameter.

3. The pod propeller assembly revolves around a vertical axis
can rotate 360 degrees ° and is used for steering the vessel,
thus dispensing with the rudder and also easier for ships to
have an ahead and astern to control the direction of motion
without needing for reversing the prime mover motor or using
a controllable pitch propeller. Typical of the evolved POD
propulsion is that the propeller pulls the ship as opposed to
the conventional, less efficient pushing action.
The crash-stop distance was also found to be substantially
reduced as reversal can be performed either by reversing the
propeller speed or by turning the Azipod unit through 180°.
The system eliminates the need for much heavy and bulky
equipment required by conventional propulsion systems,
including long shaft lines, reduction gears, controlled-pitch
propellers and transversal stern thrusters. Construction is
made much easier, the maintenance load considerably less,
and mechanical losses eliminated. Figure 3 Various Maneuvering modes
of Pod Propulsion
Advantages of POD Propulsion:
❖ Up to 20% more energy efficiency with reduced fuel consumption and life cycle costs.
❖ Essential higher reliability and availability by redundant configuration of the propulsion system.
❖ Saving time and money during construction;
❖ Environmentally friendly propulsion system Lower fuel consumption reduces emissions. A minimal need
for lubricants reduces potential leaks. Podded propulsion also allows the use of biodegradable lubricants.
❖ Excellent maneuverability. Safe and maneuverable even the largest vessels can be maneuvered with
decimeter accuracy. Narrow harbors can be entered quickly and safely.
❖ Vibration of the hull is reduced and there is no resonance effect of the propeller.
❖ Cavitation effect is reduced due to the fact that the diameter of the propeller is smaller,
❖ Economical operation as at reduced speed as number of suppling generator can be matched according to the
power demand .
Electrical Power System of Podded Propulsion
A single line diagram of electrical power systems of pod propulsion is shown in the figure 4 which consists of sets
of two pods each with exactly the same configuration independent of each other. Each motor have two stator windings
with frequency converter for redundancy.
Typical Pod Propulsion system have four main generators which are connected to the main switchboard, and the
low voltage switchboard is supplied by ship service transformers. The main switchboard can be divided into two
separate networks by means of the tie breakers to increase the redundancy of the power plant. Pod propulsion power
systems usually consist of High Voltage 11 KV / 6.6 KV gen sets and , switchboards, Step down transformers,
frequency converters are placed in the pod room in the ship and synchronous motors in the pod for propulsion.
Slip ring unit is used to transfer the voltage and frequency controlled power from the converter to the motor in the
freely rotating pod.

4. Figure 4 Typical single line diagram of the onboard Pod Propulsion system
Propulsion Motor and Excitation Components
Three types of propulsion motors are used: synchronous-, permanent magnet- and induction motors.
The most common high efficiency power factor 0.9 in high power range propulsion motor used is the synchronous
motor (SM) because of its great power output . SM rotor runs at synchronous speed, meaning that the rotor is spins
at the same rate as the synchronous speed of the rotating field of stator winding. But the induction motor is making
a great rise in smaller power podded propulsion .
Working Principle of Synchronous Motor ( SM)
Construction of SM is similar to the Alternator .When the
three phase supply is given to the stator, the rotating
magnetic field developed between the stator and rotor gap.
rotor is excited by the DC supply, induces the north and
south poles on the rotor .
DC supply is switched on to magnetize the rotor, when the
rotor starts rotating around synchronous speed by starting
with the help of an external prime mover or starting with the
help of damper windings (SM runs like an induction motor
),or static frequency converter starting .
The north and south poles of the rotor and the stator interlock each other. Thus, the rotor starts rotating at the speed
of the rotating magnetic field and the motor runs at the synchronous speed. The speed of the motor can only be
changed by changing the frequency of the stator supply by the frequency converter.

5. Synchronous Motor for Propulsion
There are two methods to provide
excitation for field windings, either by slip
rings and brushes or by using a brushless
excitation machine by which maintenances
are greatly simplified, since there is no
brushes and slip rings.
Brushless excitation system, DC current
was supplied to the rotor of the propulsion
motor from an exciter unit mounted on the
same shaft as the main rotor.
The AC generated in the exciter rotor was
converted to DC via a rotating diode
rectifier unit and supplied to the rotor of
propulsion motor.
Figure 5 POD Synchrnous Motor Brushless excitation of Rotor
Windings
Rotor Excitation by Brush & Slip ring
Applications for variable speed where very fast and
accurate speed or torque control are required, the motor
is generally equipped with brushes and a slip ring unit to
allow excitation and control of the motor from the
frequency converter.
There are two slip rings of the face type located
concentrically which are mounted on the motor shaft with
access via removable inspection covers.
The slip ring unit with brushes consisted of the
installation of a slip ring assembly, diode bridge
assembly, carbon brush assembly and dust removal unit
mounted on the existing exciter rotor and stator hub
assembly.
The excitation of rotor is supplied from an excitation transformer and excitation converter. The exciter converter is a
three phase converter with back to back thyristors (SCR) which can regulate the excitation of Propulsion SM as
shown in the figure 4.
The SM motor can be started with frequency converters by static frequency converter starting, and it’s speed can
be precisely controlled. This is a great advantage when maneuvering through ports and berthing places.
Permanent Magnet Synchronous Motor (PMSM)
The permanent magnet synchronous motor (PMSM) used for high power application, which have the high electrical
efficiency 98%, good power factor and has a robust and reliable design .
Rotor losses is minimized and also eliminates brushes and slip rings or brushless excitation components required for
magnetizing the rotor.

6. Permanent magnets fitted in rotor of
SMPM are made from neodymium
iron boron (NdFeB), is the most
powerful magnetic material
available.
It is also has with high values of flux
density at very high values of
magnetization and also extremely
resistant to demagnetization.
Frequency converter
The purpose of the frequency converter is to control the speed and torque of the motor by changing constant frequency
into variable frequency and voltage . The technical development of semiconductors has been important and made
many different designs of converters possible. A converter has unwanted effects such as harmonic distortion that may
disturb the system.
Figure Power Flow and Efficiency of Pod Electric Propulsion Installation
The most commonly Frequency converter used in pod propulsion drive
1. Direct descendants of DC drive technology and use semiconductors component Silicon Controlled
Rectifier(SCR) or “Thyristors”
Thyristors also known as
a Silicon Controlled
Rectifier (SCR) is turned on
by applying firing pulse,
small gate current when it is in
forward bias and turns off
when the anode current
reaches zero.
Output voltage is varied
by varying the firing or
delay angle of gate pulse
of SCR.
❖ Cyclo-converters (Cyclo) for AC motors, normally for synchronous motors
❖ Current source inverter type (CSI) converters for AC motors (synchronous motors)
2. Latest frequency converter type due to the technical development of power semiconductors differs from the
two above by using high speed controllable switches like Insulated Gate Bipolar Transistor (IGBT ) or

7. Integrated Gate Controlled Thyristors, (IGCT). instead of thyristors. These frequency converter is referred
as VSI-PWM as IGBT/IGCT can be turned on and off at high speed , so the output voltage can be controlled
by Pulse-Width-Modulation (PWM).
❖ Voltage source inverter (VSI) type converters for AC motors, i.e. asynchronous, synchronous and permanent
magnet synchronous motors.
A major difference between the three converters is their switching frequency, where the VSI has the highest
output frequency at about 300Hz. The CSI have a maximum output frequency of approximately 120Hz
while the cyclo only is able to give out 40% of the input frequency, 25Hz at 60Hz input.
Cycloconverter
A Cyclo-converters is an older technology AC drive , AC
to AC frequency converters converts a constant voltage,
constant frequency AC to another AC waveform of a
lower frequency of a supply to a desired motor speed.
These are naturally commutated, direct frequency
converters that use naturally commutated thyristors .
The major advantage of the cyclo is that it can produce a
high torque at a low speed with low torque pulsation and
a good dynamic behavior.
Disadvantage of Cyclo converter is the output frequency
is limited to one-third of the input frequency for
reasonable power output and efficiency.
The Cyclo - converter consists of six
groups of converter circuits where three
groups are called as positive bridge
converter while other three are negative
bridge converter are connected in opposite
direction (back to back), with both bridges
being fed from interpose HV step-down
transformers.
This reduces the motor voltage and its
required insulation level while also
providing additional line impedance to limit
the size of prospective fault current and
harmonic voltage distortion at the main
supply bus-bar.
Figure 7 Cyclo-Converters for Speed Control of Synchronous
Motor

8. During each positive half cycle, positive
group carries the current and during
negative half cycle, negative group carries
the current. The duration for conduction of
each group of thyristor determines the
desired output frequency.
Average value of output voltage is varied
by varying the firing or delay angle of
SCRs conduction whereas the output
frequency can be varied by changing the
sequence of firing the SCRs.
Current Source Inverter (CSI) & Voltage Source Inverter (VSI)
The two most common types of inverters are the current source inverter (CSI) and the voltage source inverter (VSI).
As their names imply, current source inverters are fed with constant current, while voltage source inverters are fed
with constant voltage. Consequently, the output of a CSI drive is adjustable, three-phase AC current, while a VSI
drive produces three-phase AC voltage with adjustable magnitude and frequency.
A key difference between CSI drives and VSI drives is their energy storage method. CSI drives use inductive energy
storage—that is, they use inductors in their DC link to store DC energy and regulate current ripple between the
converter and the inverter. Conversely, VSI drives use capacitive storage, with capacitors in their DC link, which
both stores and smooths the DC voltage for the inverter.
Figure 8 Variable Speed Control of Propulsion Motor for VSI and CSI type Frequency Converters
Current Source Inverter (Synchroconverter ) Working Principle
A synchroconverter has controlled rectifier termed as source/line side converter and inverter stages termed as load
side converter which both rely on natural turn-off or line commutation for the thyristors by the three-phase a.c.
voltages at either end of the converter.

9. Figure 9 Synchronous Motor Drive employing a load commutated thyristor inverter (CSI)
The line /side converter is a 12-pulse line-commutated thyristor converter takes power from supply of constant
frequency 60 Hz supply transformer star & delta multi secondary winding to give 30 degree phase shift for 12 pulse
rectification to reduce effect of harmonics due to switching phenomenon of the thyristor bridge.
Current flow in the line side converter is controlled by adjusting the firing angle of the input bridge thyristors offline
side converter and by natural commutation of the AC supply line.
The DC link inductor or reactor is used to smooth the DC current which effectively turns the line side converter into
a current source converter for the machine side converter. As a result of the action of the link inductor L, such an
inverter is frequently termed a naturally commutated current source inverter (CSI) occasionally referred to as a load-
commutated inverter (LCI) or Synchro.
Figure 10 Phase Current Waveforms from Current Source Inverter
The thyristors of the output bridge (machine side converter) are fired in step with the rotation of the motor and act
as an electronic commutator by controlling the thyristors of the inverter stage commutation of thyristor T1-T6, which
are fired with a phase difference of 60º in the sequence of their number.
Each phase current waveform of phase difference 120 degree of 6- step is obtained as shown in the figure 10,, resulting
in motor harmonics and torque ripples. The CSI requires a certain counter-induced voltage (EMF) from the motor to
perform commutation. Hence, it is mainly used in synchronous motor drives in which the motor can be run with
capacitive power factor.

10. As the supply and machine bridges are identical and are both connected to a three-phase a.c. voltage source, their
roles can be switched into reverse. This is useful to allow the regeneration of motor power back into the mains power
supply which provides an electric braking torque during a crash stop of the ship
Voltage Source Inverters (VSI) Working Principle
The voltage source inverter drives (VSI) use forced commutated high speed power switches utilizing PWM (Pulse
Width Modulated). A wide range of forced commutated power switches are commonly used are Gate turn- off
thyristor (GTO) , Insulated Gate Bipolar Transistors - IGBTs and Integrated Gate Commutated Thyristors - IGCTs .
Gate Turn-off Thyristor (GTO)
A Gate Turn-off Thyristor (GTO) is a special type of thyristor, which is a high
power semiconductor device. It was invented at General Electric GTOs, as opposed to
normal thyristors, are fully controllable switches which can be turned on and off by their
third lead, the Gate lead.
Insulated Gate Bipolar Transistors – IGBTs
The insulated-gate bipolar transistor (IGBT) is a three-terminal power semiconductor device switching device that
can be used for fast switching with high efficiency mostly used for switching/processing complex wave patters with
pulse width modulation (PWM).
IGBT is a fusion between a Bipolar Junction Transistor (BJT) and
Metal oxide Semiconductor Field Effect Transistor (MOSFET) ,the
input side represents a MOSFET with a Gate terminal and the output
side represents a BJT with Collector and Emitter.
The Collector and the Emitter are the conduction terminals and
the gate is the control terminal with which the switching operation is
controlled.
An IGBT behaves like a switch when a small positive voltage is
applied to the gate the IGBT will turn on. Current will flow between
the collector and emitter. IGBT can be turned off by removing the
positive voltage from the gate.
The freewheeling diode prevent inductive current interruption which provides protection against transient
overvoltage, which may cause reverse breakdown of the IGBT and MOSFET switches.
Integrated Gate Commutated Thyristors - IGCTs
The IGCT is new power semiconductor jointly developed by Mitsubishi and ABB . It is a
gate-controlled turn-off switch which turns off like a transistor but conducts like a thyristor
with the lowest conduction losses .
The IGCT produces negligible turn-on losses as an optical fiber on/off control can be used
which needs only connect to 28 – 40 V power supply. The IGCT's much faster turn-off times
allows it to operate at higher frequencies
This, together with its low conduction losses, enables operation at higher frequencies than
formerly obtained by Thyristors or IGBTS.
Method to change frequency
By changing the period of the voltage pulses which induce the current in the motor phases the resulting output
current waveform frequency can be changed as explained bellow.

11. The frequency changes by
changing the period to turn ON and
OFF the IGBT switches S1 to S4.
For example, if the switches S1 and
S4 are turned ON for 1 second and
S2 and S3 for 1second and this
operation is repeated, the AC with
one alternation per second, i.e., the
AC with a frequency of 2 Hz is
created.
Speed control propulsion motor by changing frequency also will need to control voltage in proportion with
frequency to keep speed and torque characteristic constant at all speed.
Pulse Width Modulation (PWM) for Voltage Control
The goal of the PWM control is to create a sine wave current waveform output to produce torque in the motor.PWM
is a way to control analog devices with a digital (On/Off) output which is used for controlling the amplitude of fixed
DC voltage signals by cycling the on-and-off phases of a DC voltage signal quickly and varying the width of the
"on" phase or duty cycle as seen in the figure 11 .
Figure 11 Voltage Variation by Pulse Width Modulation & Sinewave Current Waveform in Motor Coil
Figure 12 PWM Control to Generate Eqiuvalent Sine Wave at High / Low Speed

12. Figure 13 Motor Speed Control by IGBT switching PWM Voltage Source Inverter
The PWM (Pulse Width Modulated) drive, often also referred to as VSI (Voltage Source Inverter) is characterized
by its DC voltage link which is fed from the power system by a diode rectifier. A capacitor bank is used to smooth
the DC link voltage and to minimize the effect of harmonic distortion from the PWM inverter output on the supply
to the motor . Power factor around 0.955 is maintained at a constant level at all motor speeds since a diode bridge is
used to produce DC voltage, the PWM drive draws almost unity power factor current from the supply source.
To produce three phase 120 degree phase shift between the phases in the VSI, the IGBT/IGCT / GTO switches are
turned on and off at regular intervals to deliver rectangular pulses of voltage to each phase.
Figure 15 shows the line-to-line voltages of the inverter, VAB, VBC, and VCA. The line voltages are formed by
connecting the line terminals of the motor to either the high or the low side of the DC bus voltage.
Three of the inverter switches as shown in the figure 13 will always be closed and, at any point in time, one of the
applied line voltages is zero because the three phase motor terminals must be connected to the two DC terminals,.
Every 60° one switch turns on while another turns off, resulting in the line voltages shown in Figure
Figure 14 Switching Action of IGBT Switch every 30
degree time interval
Figure 15 Voltage source inverter 3 Phase Line Voltage Waveforms