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 is 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 havebeen 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.
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 is 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.