Cycloconverter

A cycloconverter or a cycloinverter converts an alternating current (AC) waveform, such as the mains supply, to another AC waveform of a different frequency, synthesizing the output waveform from segments of the AC supply without an intermediate direct current (DC) link. (DC) link is used to first convert AC signal to DC and then again from DC to AC signal.

The input voltage, v_s is an ac voltage at a frequency, f_{i .} Assume that all the thyristors are fired at α=0° firing angle, i.e. thyristors act like diodes.The firing angles are named as  αP for the positive converter and  αN for the negative converter.

Consider the operation of the cycloconverter to get one-fourth of the input frequency at the output. For the first two cycles of v_s, the positive  converter operates supplying current to the load. It rectifies the input voltage; therefore, the load sees 4 positive half cycles as seen . In the next two cycles, the negative converter operates supplying current to the load in the reverse direction. The current waveforms are not shown  because the resistive load current will have the same waveform as the voltage. It should be kept in mind that when one of the converters operates the other one is disabled, so that there is no current circulating between the two rectifiers.

The frequency of the output voltage, v_o  is 4 times less than that of v_s, the input voltage, i.e. f_o/f_i=1/4. Thus, this is a step-down cycloconverter. On the other hand, cycloconverters that have f_o/f_i>1 frequency relation are called step-up cycloconverters. The  step-down cycloconverters are more widely used than the step-up ones.  The frequency of v_o can be changed by varying the number of cycles the positive and the negative converters work.

Induction Disk Relay

In order to operate, the induction disk relay  torque is produced that acts on a metal disc to make contact, according to the following basic current/torque equation:

                                           

                                             

Where

K – is a constant ϕ₁ and ϕ₂ are the two fluxes θ is the phase angle between the fluxes

The relay's primary winding is supplied from the power systems current transformer via a plug bridge, which is called the plug setting multiplier (psm). Usually seven equally spaced tappings or operating bands determine the relays sensitivity. The primary winding is located on the upper electromagnet. The secondary winding has connections on the upper electromagnet that are energised from the primary winding and connected to the lower electromagnet. Once the upper and lower electromagnets are energised they produce eddy currents that are induced onto the metal disc and flow through the flux paths. This relationship of eddy currents and fluxes creates torque proportional to the input current of the primary winding, due to the two flux paths been out of phase by 90°.

A restraining spring forces the disk to rotate in the direction that opens the trip contacts while  current  creates operating torque to close the contacts.  The net  positive torque closes the contacts.  The IPU  relay setting fixes the value of the pickup current.  When the current applied to the relay equals the pickup current, the contact closing torque just equals the restraining torque and the disk will not move regardless of its position.  If the applied current  increases above the pickup current, the disk will begin to rotate so that the trip contacts come closer together.  

DC series motor

In series motors stator windings and field windings are connected in series with each other. As a result the field current and armature current are equal. Heavy currents flow directly from the supply to the field windings. To carry this huge load, field windings are very thick and have few turns. Usually copper bars form stator windings. These thick copper bars dissipate heat generated by the heavy flow of current very effectively. Note that the stator field windings S1-S2 are in series with the rotating armature A1-A2.In a series motor electric power is supplied between one end of the series field windings and one end of the armature. When voltage is applied, current flows from power supply terminals through the series winding and armature winding. The large conductors present in the armature and field windings provide the only resistance to the flow of this current. Since these conductors are so large, their resistance is very low. This causes the motor to draw a large amount of current from the power supply. When the large current begins to flow through the field and armature windings, the coils reach saturation that results in the production of strongest magnetic field possible.

The strength of these magnetic fields provides the armature shafts with the greatest amount of torque possible. The large torque causes the armature to begin to spin with the maximum amount of power and the armature starts to rotate.

In series motors, a linear relationship exist between the current flowing through the field windings and the amount of torque produced. As heavy currents flow through the very thick series field windings, large torques are produced in series motors. This feature makes series motors to be used as starter motors for industrial applications. Series motors can move comparatively heavier shaft loads. A series motor can start an automobile’s engine by drawing a heavy current of 500A. In a factory series motors can help operate huge cranes by carrying several thousands of amperes. Series motors generally operate for a very less duration, about only a few seconds, just for the starting purpose. Motor speed control is achieved by controlling the voltage applied to the motor. This essentially controls the torque developed by the motor. To increase the speed of a series DC motor a low resistance is placed in parallel with the series field. This shunt resistance lowers the field current, which produces a drop in magnetic flux and an increase in speed. To lower the speed an external resistance is connected in series with the field and the armature. This results in armature voltage reduction and a fall in speed. When the armature speed increases the field current reduces, reducing the induced back emf. This results in further increase in speed (as current is reduced because it is directly proportional to overall voltage) and virtually there is no upper speed limit. So running a series motor with no load is very risky, as it can accelerate to destruction.