Subsection 4.4.4 Induction Motors
Although there are dozens of different types of motors, the most important type of motor for industrial applications, by far, is the induction motor.
Induction motors have many advantages over other designs. First they are simple, rugged, reliable and require very little maintenance. Induction motors are relatively cost-effective to manufacture, and are widely available in a broad size range, for all common single and three-phase voltages. The can be quite efficient, especially at full load. Modern designs and technologies, such as variable frequency drives (VFDs), contribute to improved efficiency allow precise control of motor speed and performance.
Induction motors are the subject of this section.
A three-phase induction motor sometimes called a squirrel-cage motor, is a type of electric motor that operates based on the principles of electromagnetic induction.
The stator carries three identical sets of phase windings placed 120 degrees apart in a symmetrical pattern and connected in wye.. The specific pattern of the windings determines the number of magnetic poles. Small motors typically have two or four poles per phase, but the ship’s propulsion motors have ten poles per phase.
When three-phase AC power is applied to the stator windings, a rotating magnetic field develops rotating at the synchronous speed.
The synchronous speed is determined by the frequency of the power supply and the number of motor poles.
\begin{equation*}
n_s =\frac{120 f}{ P}
\end{equation*}
Where:
- \(n_s\) is the synchronous speed in revolutions per minute (RPM).
- \(f\) is the frequency of the power supply in hertz (Hz).
- \(P\) is the number of magnetic poles per phase.
Synchronous speed represents the speed the motor will spin under no load conditions.
The rotor is typically made of a laminated iron core with conductive bars embedded within it. The bars are shorted at the ends, to form closed loop rotor windings.
As the rotating magnetic field passes over the rotor windings, it induces a voltage, according to Faraday’s law of electromagnetic induction. This voltage drives a current through the rotor windings, which, in turn, generates a magnetic field surrounding the rotor windings.
The magnetic field of the rotor interacts with the rotating magnetic field of the stator and magnetic attraction pulls the rotor in the same direction as the rotating magnetic field. As the rotor rotates, it tries to catch up with the rotating magnetic field.
However, the mechanical load on the motor retards the shaft, so the rotor speed \(n\text{,}\) always remains slightly less than the synchronous speed, \(n_s\text{.}\) The difference between the synchronous speed and the actual
operating speed is called slip, often expressed as a percentage of the synchronous speed
\begin{equation*}
\textrm{\% slip}= \left(\frac{n_s-n}{n_s}\right) \times 100\%\text{.}
\end{equation*}
The difference in speed between the rotating magnetic field and the rotor’s rotational speed induces the current, torque, and power that drives the load connected to the motor shaft. Slip is required for the motor to generate torque and perform useful work.
The mechanical power produced by a motor can be calculated using the formula:
\begin{equation*}
P = \tau \omega
\end{equation*}
Where:
- \(P\) is the shaft power in Watt (W).
- \(\tau\) is torque or twisting force generated by the motor, measured in Newton-meters (Nm).
- \(\omega\) is the angular velocity of the motor in radians per second (rad/s). Angular velocity is equal to the shaft rpm \(\times \frac{2\pi}{60}.\)
In practical applications, mechanical power can also be expressed in other units such as kilowatts (kW), Horsepower (HP), or foot-pounds per minute (ft-lb/min).
An increase in load on the motor causes the shaft to slow down slightly, which increases both slip and torque. The power output of the motor automatically adjusts to match the requirements of the load.
Although the motor always has some slip, the rotor speed is generally close to the synchronous speed. For a motor with a fixed number of poles, the rotor speed can be changed by changing the frequency of the power supply with a variable speed drive or a frequency converter.
Although most large motors aboard ship are supplied with three-phase power, smaller motors, such as those driving appliances, must operate from single-phase circuits. Single-phase motors are most often arranged with two sets of stator windings, with the second set supplied from the line connections through a capacitor. The capacitor imposes a phase shift on the line current reaching these windings, to make it look like two-phase AC current. The stator is thus able to develop a rotating magnetic field to drive a rotor.
