Universal read motors
Universal read motors are low-power excitation electric motors with a sectioned winding excitation, thanks to which they can work on both direct and alternating standard voltages with approximately the same properties and characteristics. Such electric motors are used to drive low-power, high-speed devices and many household appliances. They allow simple, wide and smooth speed control.
In terms of design, these engines are different from engines. general purpose DC stator design, a magnetic system which is assembled from mud sheets insulated from each other electrical steel with protruding poles on which two sections of the excitation coil are placed. These sections are connected in series with the armature and located on both sides of the terminals, which reduces radio interference from pricing of the collector under the brushes, which when driving the motor from the mains AC voltage is especially amplified due to a significant deterioration in switching conditions.
Depending on the design of the motor, the excitation winding may be connected to an armature inside the machine or may have independent external clamps, which is more convenient for changing the direction of rotation of the armature by changing the places of the wires suitable for its clamps or for the clamps of the excitation coil. The universal motor armature is designed in the same way as the machine armature. direct current, and its winding is connected to the collector plates, to which brushes are pressed.
These motors are started by direct connection to a DC or AC network that corresponds to the nominal voltage indicated on its nameplate.
Universal speed motor brush armature series excitation is directly proportional to the voltage at its terminals and inversely proportional to the amplitude of the magnetic flux, depending on the load on the motor shaft.
Mechanical characteristics of such electric motors differ depending on what voltage (AC or DC) the electric motor operates, because when it is powered by a constant voltage network there is only a voltage drop created by the resistances of the windings excitation and armature direct current, while when connected to the mains AC voltage, there is still a significant inductive voltage drop across the excitation and armature windings. In addition, with alternating current at low armature speeds, there is a significant phase shift between voltage and current, which sharply reduces the torque on the motor shaft.
To obtain approximately the same mechanical characteristics of AC and DC include a sectionalized field winding DC motor completely, and when switched on for alternating current — partial, for which the engine is connected to the corresponding network with brackets «+» and «-» symbols or brackets with markings «~».
In nominal modes corresponding to the mains supply DC and AC voltage, the nominal speed of the armature is the same. However, if the motor connected to AC voltage is overloaded, the armature speed decreases more strongly, and when unloaded it increases more quickly, than when it is operated from a DC voltage network.
At idle, the armature speed may exceed the rated speed. 2.5 — 4 times and more, and this is not permissible due to significant centrifugal forces that can destroy the anchor. For this reason, idle speed is only permissible for low-rated motors with relatively high mechanical losses limiting armature speed. Motors with negligible mechanical losses must always carry a load of at least 25% nominal.
The speed of the armature is controlled by changing the voltage at the machine terminals, as well as by maneuvering the field winding or armature winding with a resistor. Of these ways, pole regulation, implemented by parallel connection of the excitation coil of the regulated resistor, is the most economical.
The main advantage of universal read motors compared to asynchronous and synchronous motors is that they develop a significant initial torque due to a constant excitation winding and make it possible without the use of a step-up gear to obtain armature speed much higher than synchronous.
The speed of universal read motors limits their size and weight.
The rated efficiency of these machines depends on their rated power, speed and type of current. So, for motors with a rated power of 5 to 100 W, it varies from 0.25 to 0.55, and for machines with a rated power of up to 600 W, its value reaches 0.70 and more, and the operation of the motors is included alternating current is always accompanied by reduced efficiency, which is caused by increased magnetic and electrical losses. The nominal power factor of these engines is 0.70 — 0.90.