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DC motors
The development of the DC motor preceded that of the AC motor. Today, even with the preponderance of AC motor applications, notably in traction, the DC motor retains its position due to its unique characteristics. DC motors are used for applications that require a wide range of torque and good speed control.
Types of DC motors
The basic working principle of DC motors has already been dealt with. This section details the types of DC motors. The different types depend on the method of connection of the armature and the field winding and they are classified as below:
- • Separately excited motor
- • Series DC motors
- • Shunt DC motors
- • Compound DC motors
Separately excited motor:
In a separately excited motor, the field supply is provided in a manner other than by an armature supply.
Field Armature Supply:
++++ Separately excited DC motor
Since the field current is taken from a separate supply, field current is independent of the load.
The flux remains essentially constant and does not affect either the speed or the torque. These types of motors are not used generally, as they require a separate field supply and also large variations in field current apart from the armature circuit.
Series DC motor:
The motor has the armature and field winding connected in series, therefore, its name - series motor. Field winding is made up of a relatively few turns of a large diameter wire so that the field resistance remains low.
++++ Series DC motor --Armature Supply Series field
In the series-wound DC motor, the field windings are fixed in the stator frame and the armature windings are placed around the rotor. These are connected in series. The current passing through the armature also passes through the field. In a series-wound motor, any increase in the load results in more current passing through the armature and the field windings.
As the increased current strengthens the field, the motor speed decreases. Conversely, if the load is decreased, the field is weakened and the speed increases. For lighter loads, the speed increase may be excessive and undesirable.
In order to control this, the series-wound DC motors are usually directly connected or geared to the load to prevent runaway.
At times, a series-wound motor designated as a series-shunt wound, is provided with a light shunt field winding, to prevent the dangerously high speeds at light loads. The increase in armature current with the increasing load produces an increased torque, so that the series-wound motor is suited to heavy staring duty. The motor speed can be adjusted with a variable resistance placed in series with the motor, but due to the variation with the load, the speed cannot be maintained at a constant. The series-wound DC motors are used for hoists, cranes, and elevators.
Shunt DC motors:
Shunt motors are used in applications where a good speed regulation is required. In the shunt-wound DC motor, the field winding is connected in parallel with the armature winding.
In this type of motor, the field winding has many turns of small diameter wire to keep the resistance high.
++++ Shunt DC motor -- DC supply, Ia, Ish, Shunt field Armature
The strength of the field is not affected appreciably due to changes in the load. A more or less constant speed is obtainable. Shunt-wound DC motors are used where a more or less constant speed, a low staring torque, and a light overload on the motor are required.
The shunt-wound motor can also work as an adjustable-speed motor by means of the armature control or field control. If a variable resistance is placed in the field circuit, the amount of current in the field windings can be controlled and the speed of the motor can be controlled. As the motor speed increases, the torque decreases proportionately, resulting in an approximately constant horsepower.
If a variable resistance is placed in the armature circuit, the voltage applied to the armature can be reduced, and hence the motor speed can be reduced. With an armature control, speed regulation becomes poorer as the speed is decreased. Since the current in the field remains unchanged, the torque remains constant. Adjustable-speed shunt-wound motors are used on large machines for boring mills, lathes, planners, etc. These are particularly adapted to spindle drive because of the constant horse-power characteristic that permits heavy cuts at low speeds and light cuts at high speeds.
Adjustable-voltage shunt-wound motor drive:
Due to the requirement of DC power, the application of shunt-wound DC motors has been limited. Extensive use of the shunt-wound motors has been made possible by a combination drive that includes a means of converting AC to DC. A self-contained unit may achieve the conversion of AC to DC. This consists of a separately excited DC generator driven by a constant-speed AC motor, connected to the regular AC supply. An electronic rectifier with suitable controls, connected to the regular AC supply, can also achieve this. The conversion of AC to DC with an electronic rectifier has the advantage of causing no vibrations. In an adjustable-speed, shunt-wound motor drive, speed control is affected by varying the voltage applied to the armature while supplying a constant voltage to the field.
In addition to providing for the adjustment of the voltage supplied by the converter, to the armature of the shunt-wound motor, the amount of current passing through the motor field may also be controlled. In fact, a single control may be provided to vary the motor speed, from a minimum base speed, by varying the current flowing through the field.
With such control, the motor operates at a constant torque up to the base speed and at constant horsepower above the base speed.
Adjustable-speed shunt-wound motor drives are also called DC-adjustable voltage drives. These drives are used for milling machines, boring mills, lathes, and other industrial applications where wide, step-less speed control, uniform speed under all operating conditions, a constant torque acceleration and adaptability to automatic operational control are required.
Compound-wound motors:
These types of motors provide desirable characteristics of both series- and shunt-wound motors. They provide high-starting torque as in a series motor, as well as good speed regulation as in a shunt-wound motor.
In these types of motors, there are two field windings; one is in series with the armature, while the other is in parallel with the armature.
In the compound-wound motor, the speed variation due to load changes is much less than that in the series-wound motor, but greater than that in the shunt-wound motor.
The compound-wound motor has greater starting torque than the shunt-wound motor and is able to withstand heavier loads for narrow adjustable-speed range.
++++ Compound motor short shunt connected Armature Shunt field Series field Supply
++++ Compound motor long shunt connected -- Supply Shunt field Armature Series field
Characteristics of DC motors
• DC motors have good starting torque as well as good speed-regulation capabilities.
• Permanent magnet-type DC motors are used for exact positioning of objects with high-operating torques.
• Series type gives high-starting torque; hence the ability to start with high loads.
• Series motors when operated with no loads can attain high speeds causing harm to motor.
• Shunt-type motors have good speed regulation.
• Direction reversal of DC motor can be done by changing the leads of the armature or the field.
• Speed of DC motor changes either by changing armature voltage or field current.
• If armature voltage is increased, speed increases till base speed and vice versa.
• Similarly, speed can be increased above base speed, by decreasing field current.
• In shunt motors torque is proportional to armature current.
• In series motors torque is proportional to square of armature current.
Example 2:
Consider a 250 V DC shunt motor with armature resistance of 0.5 Ohm and a shunt field resistance of 125 ohm. When it’s running light the motor current taken is 5 A. The efficiency of the motor when taking current of 52 A from the supply is calculated as follows: On no load.
Motor enclosures
Motors are manufactured in standard frame sizes, corresponding to the rated output. The size of the frame naturally increases with the rated output. The frame size standards stipulate various dimensions like the shaft center height, axial distance between shaft end and the nearest pair of mounting holes, axial distance between the sets of mounting holes and other mounting dimensions. This ensures interchangeability between motors of the same frame size manufactured by different vendors. In a given frame size, a number of designs are available to suit various applications, For example, types of mounting like foot-mounting or flange-mounting.
The type of enclosure required, depends upon the conditions under which the motor has to work. It’s therefore selected so as to protect the internal parts against the ingress of dust and water. At the same time the enclosure is expected to protect the surrounding areas from the internal, live, and moving parts of the motor. The type of insulation and the type of cooling required is selected, depending on the temperature rise and the operating temperature limits. The position of the terminal box, which is either on top or on the side, is often required to be specified while ordering a motor. The frame itself is available in a number of designs to suit the requirements of site conditions, duty, etc. Some of the standard designs are as follows:
1. Totally enclosed, non-ventilated type
Such motors are limited to sizes up to 2 or 3 kW. The cooling is by surface radiation as there are no openings for ventilation. The frame is of solid construction.
2. Splash-proof type (washdown)
The frames of such motors, incorporate ventilated openings, so constructed that the liquid drops and dust particles that fall vertically or greater than 10º from the vertical angle, cannot enter the motor directly. They also cannot enter the motor by striking or running along the outer surface. This is the extension of drip-proof motors and is called the hose-proof type.
3. Totally enclosed fan-cooled
Totally enclosed fan-cooled motors, incorporate a fan, mounted on the motor shaft. The fan draws air and forces it between the inner fully enclosed frame and an outer shell. An internal fan carries the heat that is generated internally to the enclosed frame, which is cooled by the air drawn by the fan on the shaft. The enclosing frame protects the motor against corrosive and abrasive effects of dust, moisture, etc.
4. Protected-type
Protected-type motors contain feature-perforated covers for the openings in the end shields. Thus the internal and live parts of the motor are mechanically protected using wire mesh or metal covers without affecting the flow of air.
5. Drip-proof type
The frames in such motors afford protection against liquid drops or dust particles falling on the machine at angles greater than 15° from the vertical. Water drops and particles cannot enter the motor directly or indirectly by either striking or running along the surface.
The ventilated openings are protected by use of a hood.
6. Pipe or duct ventilation type
Sometimes the air surrounding the motor is such that, if it’s passed through the motor winding, it can damage it.
In such cases, clean air can be brought from outside, and by means of a pipe or a duct, it can be used for motor ventilation. For force induction of air, a blower is installed at either the entry side or the exit side of the duct.
Motor nameplate
The motor nameplate gives important information about the motor. It gives among others, information about the following:
• Motor rating
• Motor supply details
• Motor connection details
• Motor frame type and size
• Motor rpm
• Permissible temperature rise
• Motor duty
• Enclosure type
• Number of poles.
The nameplate gives details about the motor at a glance.
Motor terminal identification and connection diagram
Usually, the motor terminal connection diagram is given on the motor. For a three-phase motor, three winding end connections are shown - U1 and U2, V1 and V2, and W1 and W2.
Motor terminals and connection diagrams for Autotransformer or DOL starting with three- or six- leads connection.
Terminal connection diagram DOL or autotransformer starting with three or six leads.
Number of leads connection
++++13 Typical circuit diagram (a) DOL starter; (b) Delta starter -- To starter.
Motor rating and insulation types
Motor rating
Motor rating is defined, as the output of a motor under prescribed working conditions, with a temperature rise below specified limits.
Motors suffer various losses like core loss, stator loss, rotor loss, winding loss, and friction loss. All these losses result in the production of heat.
As the load on the motor increases, the heat generated also increases. To maintain a healthy state of motor, the heat generated has to equal the heat dissipated.
There are as such two major types of motor ratings:
- • Continuous
- • Intermittent
Continuous duty motors, are motors, which are meant to give the continuous rated load specified on the nameplate. Such motors can be operated at these load continuously, without causing overheating.
Intermittent duty motors are meant for taking loads above the maximum continuous rating, for a short duration, such as for one hour or so. This allows the motor, to get enough time for dissipating the heat generated, in the time intervals when the motor is not running.
Motor insulation
The insulation utilized should withstand the voltage fluctuations of the motor under varying operating conditions. Depending on the load and its surrounding conditions, there could be a rise in the temperature of the motor. The insulation should withstand such temperature rises also.
The hot-spot temperature in any part of the motor should not exceed the permissible limit of the insulation used.
In case of insulating materials, their thermal characteristics are more sensitive than their dielectric characteristics, i.e., the failure of an insulating material is more due to thermal limitations than due to voltage limitations.
In most cases, the temperature rise or the rise in load does not produce a fault in the winding of the conductor itself. The rise of load current or greater fault current, when it’s excessive, causes a thermal breakdown in the insulation covering the conductor. This is what creates a fault in the winding. Thus, the maximum permissible temperature rise, in electrical motors, must be in tune with the type of insulation used and the type of motor.
The main characteristics, of insulating materials used in electrical machines are:
• Dielectric strength
• Thermal strength
The insulating material used for the electrical machines should satisfy the following requirements:
• High dielectric strength, high specific resistance, and minimum loss in alternating electric field
• High mechanical strength and elasticity of material
• Thermal strength of insulation; the insulating material should preserve its insulation and mechanical properties when subjected to the operating temperatures of the windings for a long time
• The material should remain unaffected by chemical influences
The temperature rise permissible can be determined, by deducting the ambient temperature, from the maximum permissible temperature.
For electrical machines, the following, are the types of insulating material that have been classified and standardized as follows:
• Class A insulation: Cotton, silk, paper, and similar organic materials, impregnated or immersed in oil, and enamel applied on enameled wires. The limiting hot-spot temperature for Class A insulation is 105 °C.
• Class E insulation: An intermediate class of insulating materials between Class A and Class B insulation materials.
• Class B insulation: Mica, asbestos, glass fiber, and similar inorganic materials, in built-up form with organic binding substances. The limiting hot spot temperature for Class B Insulation is 130 °C.
• Class F insulation: Includes insulation having mica, asbestos, or glass fiber base with a silicone or a similar high-temperature-resistant binding material.
The limiting hot-spot temperature for Class F insulation is 155 °C.
• Class H insulation: Includes insulation having mica, asbestos, or glass fiber base with a silicone or a similar high-temperature-resistant binding material.
The limiting hot-spot temperature for Class H insulation is 180 °C. 4.7.3 AC motor connections
(a) Multispeed motor
A three-phase induction motor is a constant-speed machine. The speed control of induction motors can be achieved by:
• Changing the applied voltage
• Changing the applied frequency
• Changing the number of poles.
The first two methods, however, are rarely used because of the problems associated with reducing the voltage and frequency. The last method is well suited for squirrel-cage motors, as the squirrel-cage rotor adapts itself to any reasonable number of stator poles.
The change in the number of stator poles is achieved by providing two or more entirely independent stator windings. Each winding gives a different number of poles and hence a different speed.
Each of the windings is terminated on a different set of terminals, which can be connected up and switched, to connect the winding to the supply. Only one winding is used at a time, the other, being entirely disconnected. This method finds application in elevators, traction, and small motor-driven machine tools.
(b) Dual-voltage motor
A dual-voltage motor is a single-phase induction motor. It can be operated from two AC voltages, either 110 or 220 V. Such motors, have two main windings and one starting winding. A suitable number of leads are brought out to permit changeover from one voltage to another.
When the motor is to operate on a lower voltage, the two main windings are connected in parallel. On higher voltage, they are connected in series. The starting winding is always operated on the low voltage mode, for which purpose it’s connected across one of the main windings.
DC motor connection
DC motors, may be of a series, shunt, or compound type. Depending on the type of motor, it will have field-winding, armature-winding, or series-winding terminal connections.