The standard cage-rotor a.c. induction motor operates as an almost constant speed drive over its load range. This feature is satisfactory for most of the ship’s auxiliary services supplying power to ventilation fans and circulating pumps.
Variable speed control is necessary for cranes, winches, windlass, capstans, forced-draught fans etc.
Ship’s electric propulsion with electronic speed control may use d.c. motors or a.c. induction motors for low/medium power applications.
Large power electric propulsion, e.g. for a passenger cruise ship, will use a.c. synchronous motors.
Two main forms of speed change/control are available:
Pole-changing for induction motors to give two or more fixed speeds:
2-speed forced-draught fans
3-speed winches
Continuously variable speed control, e.g. smooth control of deck cranes, winches and electric ship propulsion using variable frequency.
Fixed set speeds can be obtained from a cage-rotor induction motor by using a dual wound stator winding, each winding being designed to create a different number of magnetic poles.
A 3 speed pole-change ship winch motor can be arranged by having two cage rotors mounted on the same drive shaft.
One stator winding (usually 2-pole) gives a low speed while the other is dual wound
to give medium speed (8-pole) and high speed (4-pole) outputs.
Speed control and drive direction are achieved by u set of switching and reversing contactors operated from the winch control pedestal.
Remember that to reverse the rotation of an induction motor it is necessary to switch over two of the supply lines to the stator winding.
An alternative method giving two fixed speeds in a 2:1 ratio from a cage-rotor induction motor is to use a single stator winding which has centre-tap connections available on each phase. This method uses a starter with a set of contactors to switch the phase windings into either single-star (low speed) or double-star (high speed).
Two of the supply lines are interchanged in the double-star connection – this is to maintain the same direction of rotation as in the low speed connection.
A continuously variable speed range of ship’s motor control involves more complication and expense than that required to obtain a couple of set speeds.
Various methods are available which include:
Electro-hydraulic drive.
Wound-rotor resistance control of induction motors
Ward-Leonard d.c. motor drive.
Variable-frequency induction or synchronous motor control.
The electro-hydraulic drive, often used for deck crane control, has a relatively simple electrical section. This is a constant single-speed induction motor supplied from a DOL or star-delta starter.
The motor runs continuously to maintain oil pressure to the variable-speed hydraulic motors.
A crude form of speed control is provided by the wound rotor induction motor.
The rotor has a 3-phasewinding (similar to its stator winding) which is connected to 3 slip rings mounted on the shaft.
An external 3-phase resistor bank is connected to brushes on the rotor slip rings. A set of contactors or a slide wiper (for small motors) varies the amount of resistance added to the rotor circuit.
Increasing the value of external resistance decreases the rotor speed.
Generally, the starters of wound-rotor motors are interlocked to allow start-up only when maximum rotor resistance is in circuit.
This has the benefits of reducing the starting current surge while providing a high starting torque.
The wound-rotor arrangement is more expensive than an equivalent cage-rotor machine. It requires more maintenance on account of the slip rings and the external resistor bank which may require special cooling facilities.
Where continuously variable speed has to be combined with high torque, smooth acceleration, including inching control and regenerative braking, it is necessary to consider the merits of a d.c. motor drive.
Speed and torque control of a d.c. motor is basically simple requiring the variation of armature voltage and field current.
The problem is: where does the necessary d.c. power supply come from on a ship with an a.c. electrical system?
A traditional method for lifts, cranes and winches is found in the Ward-Leonard drive. Here a constant speed induction motor drives a d.c. generator which in turn supplies one or more d.c. motors.
The generator output voltage is controlled by adjusting its small excitation current via the speed regulator. The d.c. motor speed is directly controlled by the generator voltage.
Obviously the motor-generator (M-G) set requires space and maintenance.
An alternative is to replace the rotary M-G set with a static electronic thyristor controller which is supplied with constant a.c. voltage but delivers a variable d.c. output voltage to the drive motor.
Although the Ward-Leonard scheme provides an excellent power drive, practical commutators are limited to about 750 V d.c. maximum which also limits the upper power range.
The commutators on the d.c. machines also demand an increased maintenance requirement.
To eliminate these problems means returning to the simplicity of the cage-rotor induction motor. However, the only way to achieve a continuously variable speed output by electrical control is to vary the supply frequency to the motor.
A static electronic transistor or thyristor (high power) controller can be used to generate such a variable frequency output to directly control the speed of the motor.
In an electronic variable speed drive (VSD), the fixed a.c. input is rectified and smoothed by a capacitor to a steady d.c. link voltage (about 600V d.c. from a 440V rms a.c. supply).
The d.c. voltage is then chopped into variable-width, but constant level, voltage pulses in the computer controlled inverter section using IGBTs (insulated gate bipolar transistors). This process is called pulse width modulation or PWM.
By varying the pulse widths and polarity of the d.c. voltage it is possible to generate an averaged sinusoidal a.c. output over a wide range of frequencies.
Due to the smoothing effect of the motor inductance, the motor currents appear to be approximately sinusoidal in shape.
By directing the currents in sequence into the three stator winding a reversible rotating magnetic field is produced at a frequency set by the PWM modulator.
VSDs, being digitally controlled/ can be easily networked to other computer devices e.g. programmable logic controllers (PLCs) for the overall control of a complex process.
A disadvantage of chopping large currents with such a drive creates harmonic voltage back into the power supply network.
A harmonic voltage waveform is a distorted sinusoidal wave shape.
Such harmonic voltage disturbances caused by current switching can interfere with other equipment connected to the power system.
progressive insulation breakdown due to high voltage spikes
flickering of the lighting
malfunction of low current devices such as electronic computers and instrumentation/control circuits
