DC
Motor Characteristics
As you already know, there are two
electrical elements of a DC motor, the field windings and the
armature. The armature windings are made up of current carrying conductors
that terminate at a commutator.
DC voltage is applied to the
armature windings through carbon brushes which ride on the commutator. In small
DC motors, permanent magnets can be used for the stator. However, in large
motors used in industrial applications the stator is an electromagnet.
When voltage is applied to stator windings an electromagnet with north and south poles is established. The resultant magnetic field is static (non-rotational).
The
field of DC motors can be:
1. Permanent
magnet
(Permanent magnet stator),
2. Electromagnets connected in series (Wound stator),
3. Shunt (Wound stator), or
4. Compound (Wound stator).
Let’s see the
basics of each type as well as their advantages and disadvantages.
1. Permanent Magnet Motors
Permanent Magnet Motor
The permanent
magnet motor uses a magnet
to supply field flux.
Permanent
magnet DC motors have excellent starting torque capability with good speed
regulation.
A disadvantage
of permanent magnet DC motors is they are limited to the amount of load they
can drive. These motors can be found on low horsepower applications.
Another
disadvantage is that torque is usually limited to 150% of rated torque to prevent
demagnetization of the permanent magnets.
2. Series Motors
In a series DC
motor the field is connected in series with the armature.
The field is
wound with a few turns of large wire because it must carry the full armature
current.
A
characteristic of series motors is the motor develops a large
amount of starting torque. However, speed varies widely between no load and full
load. Series motors cannot be used where a constant speed is required under
varying loads.
Additionally, the
speed of a series motor with no load increases to the point
where the motor can become damaged. Some load must always be connected to a
series-connected motor.
Series-connected
motors generally are not suitable for use on most variable speed drive
applications.
3. Shunt Motors
DC Shunt Motor
In a shunt
motor the field is connected in parallel (shunt) with the armature windings.
The shunt-connected motor offers good speed regulation. The field winding can
be separately excited or connected to the same source as the armature.
An advantage to a separately excited shunt
field is the ability of a variable speed drive to provide
independent control of the armature and field.
The
shunt-connected motor offers simplified control for reversing. This is
especially beneficial in regenerative drives.
4. Compound
Motors
Compound
motors have a field connected in series with the armature and a separately
excited shunt field. The series field provides better starting torque and the shunt field
provides better speed
regulation.
However, the series field can cause control problems in variable speed drive applications and is generally not used in four quadrant drives.
We
know that,
There are two types of motors,
namely AC and DC.
· The AC motor uses AC current to transform electrical
energy into mechanical energy.
· The DC motor uses DC current to transform electrical
energy into mechanical energy.
What
is a series motor?
A large wire carrying the full
armature current winds the field with few turns. This kind of motor generates a
large amount of starting torque but the speed cannot be regulated here.
What
is a compound motor?
These have a shunt field which is
separately excited. They have a good starting torque but might face problems in
variable speed applications
What
are the types of compound excited DC motor?
The types of compound excited DC
motor are Short Shunt Motor, Long Shunt Motor, Differential Compound, and
Cumulative Compound
What
are the types of self-excited DC motor?
The types of self-excited DC motors
are Shunt excited, compound excited and series excited DC motor
What
happens when the speed of a DC motor increases?
When the speed of a DC motor
increased, the back emf increases. But there will be a fall in the line
currents. This is because speed is proportional to the back emf. If there is a
series motor, the armature current will be equal to the load current.
Types of DC Motor:
The
commonly used Types of DC Motor are shown in Fig. 5.1. In a separately excited
motor, the field and armature voltages can be controlled independent of each
other. In a shunt motor, field and armature are connected to a common source.
In case of a series motor, field current is same as armature current, and
therefore, field flux is a function of armature current. In a cumulatively
compound motor, the magneto-motive force of the series field is a function of
armature current and is in the same direction as mmf of the shunt field.
The
steady state equivalent circuit of armature of a dc machine is shown in Fig.
5.2. Resistance Ra is the resistance of the armature circuit. For
separately excited and shunt motors, it is equal to the resistance of armature
winding and for series and compound motors it is the sum of armature and
field winding resistances. Basic equations applicable to all Types of DC Motor
are
Where
Φ – Is the flux per pole, Weber;
Ia – the armature current, A;
V – The armature voltage V;
Ra – the resistance of the armature
circuit, ohms;
ωm – the speed of armature, rad/sec;
T – the torque developed by the motor, N-m; and
Ke – the motor constant.
From
Eq. (5.1) to (5.3)
Shunt and Separately Excited Motors:
In
case of shunt and separately excited motors, with a constant field current, the
flux can be assumed to be constant. Let
Then
from Eqs. (5.1), (5.3) and (5.4) to (5.6)
The
speed-torque and torque-current characteristics of a separately excited motor
for rated terminal voltage and full field are shown in Fig. 5.3. The
speed-torque curve is a straight line. The no load speed ωm0 is
determined by the values of armature voltage and field excitation. Speed
decreases as torque increases and speed regulation depends on the armature
circuit resistance (Eq. (5.10)). The usual drop in speed from no load to full
load, in case of a medium size motor,
is
of the order of 5%. Separately excited motors are employed in applications
requiring good speed regulation and adjustable speed.
Series Motor:
In
series motors, the flux is a function of armature current. In unsaturated
region of magnetization characteristic, Φ can be assumed to be proportional to
Ia. Thus,
Substituting
in Eqs. (5.3), (5.4) and (5.5) gives
Where,
armature circuit resistance Ra is now the sum of armature and field
winding resistances. The speed-torque and torque-current characteristics of a
series motor at rated terminal voltage and full field are shown in Fig. 5.3.
Series motors are suitable for applications requiring high starting torque and
heavy torque overloads. Since torque is proportional to the armature current
squared, for the same increase in torque, increase in motor current is less
compared to that in a separately excited motor where torque is proportional to
armature current. Thus, during heavy torque overloads and starting, power
overload on the source and thermal overloading of the motor are kept limited to
reasonable values. According to Eq. (5.14), as speed varies inversely as the
square root of torque, machine runs at a large speed at light load. Generally,
mechanical strength of a Types of DC Motor permits it to operate up to about
twice rated speed. Hence, the series motor should not be used in those drives
where there is a possibility of the load torque being dropped to the extent
that the speed may exceed twice rated value.
Compound Motor:
Speed-torque and torque-current characteristics of a cumulative compound motor are also shown in Fig. 5.3. The no load speed depends on the strength of shunt field and slope of the characteristic on the strength of series field. Cumulative compound motors are used in those applications where a drooping characteristic similar to that of a series motor is required and at the same time the no load speed must be limited to a safe value; typical examples are lifts and winches. It is also used in intermittent load applications, where the load varies from almost no load to very heavy loads. In these applications a fly-wheel may be mounted on the motor shaft for load equalization. This apart from equalizing load on the supply permits the use of a smaller size motor. Pressing machine is a typical example of this type of application. •
Universal Motor:
The
universal motor can run both on dc and ac supply. It is essentially a dc series
motor, with some differences in construction; which are mainly introduced to
get satisfactory performance on ac. In series motor, torque depends on the
product of armature current and field flux. Reversal of the terminal voltage
reverses both the armature current and field flux. Consequently, torque remains
in the same direction. Therefore, when fed from an ac source, the series motor
produces unidirectional torque. Although the torque fluctuates at a frequency
of 100 Hz between zero and its peak value, its fluctuations are smoothened out
by motor inertia and the motor runs at a uniform speed.
A
simple dc series motor does not operate well on ac. Hysteresis and eddy current
losses that occur in field poles and yoke reduce motor efficiency and increase
thermal loading. The alternating flux produces large induced currents in the
coils that are short circuited by brushes during commutation. This causes
excessive sparking at the commutator. Motor power factor is very poor due to
large inductance of field and armature. Universal motor is specially
constructed to solve these limitations. In addition to the armature, field
poles and yokes are also laminated to reduce eddy current losses. High
permeability silicon steel lamination is used to reduce hysteresis loss. A
compensating winding is used in series with the armature to reduce armature
inductance. The field inductance is lowered by using fewer turns and shallow
pole pieces. In spite of these changes, when fed from ac, commutation is worse
than when fed from dc. Therefore, their power ratings are seldom higher than 1
kW. No load speed is high, but generally not high enough to damage the motor.
Most
universal motors are manufactured for use at speeds in excess of 3000 rpm. This
is the maximum speed of an induction motor when fed from a 50 Hz supply. Below
this speed induction motor is generally preferred. Many universal motors
operate at speeds up to 12,000 rpm and can go up to 20,000 rpm. Because of high
operating speeds, universal motor is much smaller in size compared to an
induction or a low speed dc motor of identical rating. Because of brushes and
commutator, it requires frequent maintenance and has a relatively short
operating life.
Until
recently, universal motor was the cheapest motor capable of running at high
speeds and having relatively very small weight and size. The brushless Types of
DC Motor or a single phase induction motor fed from variable frequency inverter
may become its competitor in near future.
Some
applications of universal motor are fans, electric drills, home appliances etc.
Permanent Magnet Motors:
In
permanent magnet dc motors, field excitation is obtained by suitably mounting
permanent magnets on the stator. Ferrites or rare earth (cobalt samarium)
magnets are employed. Ferrites are commonly used because of lower cost, but the
machine becomes bulky due to low retentivity. Rare earths because of their high
retentivity allow a large reduction in weight and size, but they are very
expensive. The permanent magnet motors are mainly employed in fractional
horsepower range, but they are available up to 5 kW rating.
Use
of permanent magnets for excitation eliminates field copper loss and need for
field supply. Compared to the field wound motors, they are more efficient,
reliable, sturdy and compact. The field flux remains constant for all loads giving
a more linear speed torque characteristic. In a separately excited motor,
failure of field supply can lead to runaway condition. This does not happen in
permanent magnet motors. As the flux is constant in these motors, speed cannot
be controlled above base speed. Such motors have applications in electric
vehicles like mopeds, forklift trucks, wheel chairs etc.
DC Servo Motors:
There
is no sharp dividing line between servo and conventional field wound and
permanent magnet dc motors. Servo motors are intended to be used in closed loop
speed and positional control systems where performance requirements are such
that they cannot be achieved by a normal dc motor. A normal Types of DC Motor
is designed to achieve good full load performance with minimum cost. It fails
to provide good dynamic response and steady state accuracy when employed in a
closed-loop drive. The servo motor on the other hand is designed to achieve
good dynamic performance and steady state accuracy. It is designed to achieve
the same performance in both directions of rotation, high torque to inertia ratio;
low friction and smooth ripple free torque. In a Types of DC Motor, the
armature inertia is proportional to length and diameter squared. In some servo
motors, inertia is reduced by reducing diameter and increasing length for the
same rating. In low power servo drives, where current control is not
incorporated, the current during transient operation can be even higher than
ten times the rated current. Commutator is designed to obtain spark-free
commutation even at such large currents, which will not be possible in a common
dc motor. Because of these exacting requirements servo motors are much more
expensive than common dc motors. Their ratings can be from few watts (in
instrument servos) to mega watts (steel rolling mills). Small servo motors are
usually permanent magnet type.
Moving Coil Motors:
Some
applications require acceleration much higher than what can be achieved in a
conventional dc servo motor. Armatures of moving coil dc motors have special
constructions which allow a substantial reduction in armature inertia and
inductance, permitting very high accelerations. Two types of moving coil motors
are shell and disc type.
Shell Type:
In
order to maximize acceleration, armature inertia must be minimized. In a
conventional dc motor, the armature consists of a winding housed in slots
provided on a cylinder of magnetic material, which is provided mainly to give
low reluctance path for the stator field, and rotates with armature winding.
Consequently, armature has high inertia. In a shell type moving coil motor
(Fig. 5.4(a)), the rotor consists of only armature winding. Hence it has very
low inertia. Low reluctance path for the stator field is provided by a
stationary magnetic material cylinder. Armature winding consists of conductors
assembled to form a thin walled cylinder. The commutator may have a cylindrical
construction as in conventional dc motors or disc type construction.
Tiny
motors (with diameters around 1 cm), known as micro motors, have armature
winding consisting of simply varnished wires arranged in cylindrical form and a
disc type commutator. Such motors are widely used in cameras, card readers,
video systems etc. In bigger size motors, the armature winding is made by
bonding conductors together using polymer resins and fiberglass to provide
adequate mechanical strength.
Disc or Pancake Type:
The
construction details are shown in Fig. 5.4(b). Armature is made in disc or
pancake form, and armature conductors resemble spokes on a wheel. Armature
winding is formed by stamping conductors from a sheet of copper, welding them
together and placing them on a light weight disc. Conductor segments are then
joined with a commutator at the centre of the disc. Note that the direction of
flux is axial and armature current is radial. This is just opposite to shell
type (or conventional) motors where the current is axial and flux is radial.
The principle of operation is same as that of a conventional dc motor.
Disc
type moving coil motors are more robust and available in sized up to few
kilowatts. They find applications where axial space is at a premium such as
machine tools, disc drives etc.
Moving
coil motors can be provided with large number of conductors (few hundred).
Therefore, torque remains almost constant as the rotor turns. This allows them
to produce very smooth rotation at low speeds. The absence of iron in armature
of disc type motor eliminates the associated core losses, making it more
efficient than conventional dc motors. As already stated, low inertia and low
armature inductance gives moving coil motors an excellent
dynamic response.
Torque Motors:
DC
motors designed to run for long periods in a stalled or a low speed condition
is known as torque motors. A normal Types of DC Motor is designed to optimize
full speed performance. In small ratings, the stalled or low speed current in
normal dc motors can be 5-10 times the rated current. If these motors are
allowed to run at low speed (or standstill), armature winding will get burnt by
overheating and the commutator will get damaged due to heavy sparking. In case
of torque motors, because of special design, the stalled and low speed current
remains below safe value. Some torque motors are designed to operate at low
speeds intermittently.
Applications of torque motors can be divided into three categories:
1.
Where the motor is required to operate in
stalled condition. Here, purpose of the motor is to develop required tension or
pressure on a material, similar to spring. Machine tools, spooling come under
this category.
2.
In second category torque motor is required to
move through only a few revolutions or degrees of revolution. Opening of
valves, switches and clamping devices are some examples.
3.
This category of application involves
continuous movement of the motor at low speed, e.g. reel drive.