In the hundred years since motors and generators were invented, a vast range of electrical
machine types have been created. In many cases, different companies called the same type of
machine or the same component by completely different names. Therefore, to avoid confusion,
before a detailed description of motor and generator insulation systems can be given, it
is prudent to identify and describe the types of electrical machines that are discussed in this
book. The main components in a machine, as well as the winding subcomponents, are identified
and their purposes described.
Although this book concentrates on machines rated at 1 kW or more, much of the information
on insulation system design, failure, and testing can be applied to smaller machines,
linear motors, servomotors, etc. However, these latter machines types will not be discussed
explicitly.
machine types have been created. In many cases, different companies called the same type of
machine or the same component by completely different names. Therefore, to avoid confusion,
before a detailed description of motor and generator insulation systems can be given, it
is prudent to identify and describe the types of electrical machines that are discussed in this
book. The main components in a machine, as well as the winding subcomponents, are identified
and their purposes described.
Although this book concentrates on machines rated at 1 kW or more, much of the information
on insulation system design, failure, and testing can be applied to smaller machines,
linear motors, servomotors, etc. However, these latter machines types will not be discussed
explicitly.
1.1 TYPES OF ROTATING MACHINES
Electrical machines rated at about 1 HP or 1 kW and above are classified into two broad categories:
(1) motors, which convert electrical energy into mechanical energy (usually rotating
torque) and (2) generators (also called alternators), which convert mechanical energy into
electrical energy. In addition, there is another machine called a synchronous condenser that is
a specialized generator/motor generating reactive power. Consult any general book on electrical
machines for a more extensive description of machines and how they work [1.1, 1.2,
1.3].
Motors or generators can be either AC or DC, that is, they can use/produce alternating
current or direct current. In a motor, the DC machine has the advantage that its output rotational
speed can be easily changed. Thus, DC motors and generators were widely used in industry
in the past. However, with variable speed motors now easily made by combining an
AC motor with an electronic “inverter-fed drive” (IFD), DC motors in the 100’s of kW range and above are becoming less common.
Machines are also classified according to the type of cooling used. They can be directly or indirectly cooled, using air, hydrogen, and/or water as a cooling medium.
This blog concentrates on AC induction and synchronous motors, as well as synchronous generators. Other types of machines exist, but these motors and generators constitute the vast majority of electrical machines rated more than 1 kW presently used around the world.
1.1.1 AC Motors
Nearly all AC motors have a single-phase (for motors less than about 1 kW) or three-phase stator winding through which the input current flows. For AC motors, the stator is also called the armature. AC motors are usually classified according to the type of rotor winding. The
rotor winding is also known as a field winding in most types of machines. A discussion of
each type of AC motor follows.
Squirrel Cage Induction (SCI) Motor (Figure 1.1). The rotor produces a magnetic field
by transformer-like AC induction from the stator (armature) winding. This is by far the most common type of AC motor made, with millions manufactured every year. SCI motors can
range in size from a fraction of a horsepower motor (< 1 kW) to tens of thousands of horsepower (greater than 30 MW). The predominance of the squirrel cage induction motor is attributed to the simplicity and ruggedness of the rotor. In an SCI motor, the speed of the rotor is usually 1% or so slower than the “synchronous” speed of the rotating magnetic field in the air gap created by the stator winding. Thus, the rotor speed “slips” behind the speed of the air
gap magnetic flux [1.1, 1.2]. The SCI motor is used for almost every conceivable application,
including fluid pumping, fans, conveyor systems, grinding, mixing, and power tool operation.
Wound Rotor Induction Motor. The rotor is wound with insulated wire and the leads
are brought off the rotor via slip rings. In operation, a current is induced into the rotor from
the stator, just as for an SCI motor. However, in the wound rotor machine it is possible to
limit the current in the rotor winding by means of an external resistance or slip-energy recovery
system. This permits some control of the rotor speed. Wound rotor induction motors are
relatively rare due to the extra maintenance required for the slip rings. IFD SCI motors are
often a more reliable, cheaper alternative.
Synchronous Motor. This motor has a direct current flowing through the rotor (field)
winding. The current creates a DC magnetic field, which interacts with the rotating magnetic
field from the stator, causing the rotor to spin. The speed of the rotor is exactly related
to the frequency of the AC current supplied to the stator winding (50 or 60 Hz). There
is no “slip.” The speed of the rotor depends on the number of rotor pole pairs ( a pole pair
contains one north and one south pole) times the AC frequency. There are two main ways
of obtaining a DC current in the rotor. The oldest method, still popular, is to feed current
onto the rotor by means of two slip rings (one positive, one negative). Alternatively, the
“brushless” method uses a DC winding mounted on the stator to induce a current in an auxiliary
three-phase winding mounted on the rotor to generate AC current, which is rectified
(by “rotating” diodes) to DC. Synchronous motors require a small “pony motor” to run the
rotor up to near synchronous speed. Alternatively, an SCI type of winding on the rotor can
be used to drive the motor up to speed, before DC current is permitted to flow in the main rotor winding.
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| Figure 1.1. Photograph of a SCI rotor being lowered into the squirrel cage induction motor stator. |
This winding is referred to as an amortisseur or damper winding. Because of
the more complicated rotor and additional components, synchronous motors tend to be restricted
to very large motors today (greater than 10 MW) or very slow speed motors. The
advantage of a synchronous motor is that it usually requires less “inrush” current on startup
in comparison to a SCI motor, and the speed is more constant. Also, the operating energy
costs are lower since, by adjusting the rotor DC current, one can improve the power
factor of the motor, reducing the need for reactive power and thus the AC supply current.
Refer to the section on synchronous generators below for further subdivision of the types
of synchronous motor rotors. Two-pole synchronous motors use round rotors, as described in Section 1.1.2.
the more complicated rotor and additional components, synchronous motors tend to be restricted
to very large motors today (greater than 10 MW) or very slow speed motors. The
advantage of a synchronous motor is that it usually requires less “inrush” current on startup
in comparison to a SCI motor, and the speed is more constant. Also, the operating energy
costs are lower since, by adjusting the rotor DC current, one can improve the power
factor of the motor, reducing the need for reactive power and thus the AC supply current.
Refer to the section on synchronous generators below for further subdivision of the types
of synchronous motor rotors. Two-pole synchronous motors use round rotors, as described in Section 1.1.2.
Permanent Magnet Motors. These motors have rotors made of a special permanently
magnetized material. That is, no DC or AC current flows in the rotor, and there is no rotor
winding. In the past, such motors were always rated at < 50 HP, since they can be hard to
magnetized material. That is, no DC or AC current flows in the rotor, and there is no rotor
winding. In the past, such motors were always rated at < 50 HP, since they can be hard to
shut down. However, some large permanent magnet motors have been recently used in marine
applications, due to their simplicity.
applications, due to their simplicity.
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