22–5 STARTERS FOR SERIES MOTORS
Series motors require a special type of manual starting rheostat called a series motor starter. These starting rheostats serve the same purpose as the three- and four- terminal manual starting rheostats used with shunt and compound motors, which is to limit the surge of starting current and to accelerate the motor in one direction of rotation. However, series motor starters have different internal and external connections. There are two types of series motor starters, one with no-voltage protection and the other with no-load protection.
A series motor starter with no-voltage protection is illustrated in Figure 22–8. The holding coil is connected across the source voltage. This starter is used to accelerate the motor to rated speed. In case of voltage failure, the holding coil no longer acts as an electromagnet. The spring reset then quickly returns the arm to the off position to protect the motor from damage.
The series motor starter shown in Figure 22–9 has no-load protection. The holding coil is in series with the armature. Because of the large current in the armature circuit, the holding coil consists of only a few turns of heavy wire.
The same care is used in starting a motor with this type of starting rheostat as is used with three- and four-terminal starting rheostats. The arm is slowly moved from the off position to the on position, pausing on each contact button for a period of one to two seconds. The arm is held against the tension of the reset spring by means of the holding coil connected in series with the armature. If the load current to the motor drops to a low value, the holding coil weakens and the reset spring returns the arm to the off position.
This is an important protective feature. A series motor can reach a dangerously high speed at light loads; this type of starting rheostat protects the motor from damage caused by excessive speeds.
22–6 DRUM CONTROLLERS
Series and cumulative compound motors are often used on cranes, elevators, machine tools, and other devices where the motor is under the direct control of an operator and where frequent starting, varying speed, stopping, and reversing are necessary. A manually operated controller that is more rugged than a starting rheostat is used in these applications. This starting rheostat is called a drum controller.
A typical drum controller is illustrated in Figure 22–10. Inside the switch is a series of contacts mounted on a movable cylinder. These contacts, insulated from the cylinder and from each other, are the movable contacts. There is another series of contacts, located inside the controller, called stationary contacts. These contacts are arranged to make contact with the movable contacts as the cylinder is rotated. On top of the drum
controller is a handle that is keyed to the shaft for the movable cylinder and contacts. This handle can be moved in either a clockwise or a counterclockwise direction, providing a range of speed control in either direction or rotation. Once set, a roller and notched- wheel arrangement keeps the cylinder and movable contacts stationary until the handle is turned by the operator.
A schematic of a drum controller having two steps of resistance is shown in Figure 22–11. In this wiring diagram, the contacts are shown in a flat position to make it easier to trace connections. For operating in the forward direction, the movable contacts on the right connect with the center stationary contacts. For operation in the reverse direction, the movable contacts on the left touch the stationary contacts in the center.
There are three forward positions and three reverse positions in which the controller handle can be set. In the first forward position, all resistance is in series with the armature. The circuit for the first forward position is traced as follows:
1. Movable fingers A, B, C, and D contact the stationary contacts 7, 5, 4, and 3.
2. The current path is from 7 to A, from A to B, from B to 5, and then to armature terminal A1.
3. From A1, the current path is through the armature winding to terminal A2, then to stationary contact 6, and then to stationary contact 4.
4. From contact 4, the current path is to contact C, to D, and then to 3.
5. From 3, the current path is through the entire armature resistor, through the series field, and then back to the line.
In the second forward position, part of the resistance is cut out by the connection from D to E. The third forward position bypasses all resistance and puts the armature circuit directly across the source voltage.
In the first reverse position, all resistance is again inserted in series with the armature. Figure 22–12 illustrates the first position of the controller for the reverse direction.
The current in the armature circuit is reversed. However, the current direction in the shunt and series fields is the same as for the forward direction. As shown earlier, changing the direction of the current in only the armature changes the direction of rotation. In the second position, part of the resistance circuit is cut out. The third reverse position cuts out all resistance and puts the armature circuit directly across line voltage.
There are more elaborate drum controllers with more positions and a greater control of speed. However, they all use practically the same circuit arrangement.
22–7 MAGNETIC CONTROLLERS
The manual starters and controllers described in the foregoing sections have been increasingly replaced by magnetic starters with push-button or automatic control. This type of equipment is convenient and has the added advantage of reducing damage caused by human misjudgment. Many of these automatic systems control DC motors with wide speed range and excellent torque characteristics.
The type of schematic diagram used to describe motor-control circuitry is often referred to as a ladder diagram or relay ladder logic. The word ladder is derived from the fact that all control components are arranged like the rungs of a ladder between two vertical lines that represent the control voltage.
Standard symbols have been established for control circuit components, such as: relay coils, contactors, push-button stations, overload devices, and limit switches. Such symbols, often known as JIC (or Joint Industrial Council) standards, conform to standards established by the National Electrical Manufacturers’ Association; see Figure A–12.
The following sections will serve as an introduction to the concept of automatic control of DC motors.
A contactor operated by a relay is an important part of any automatic motor controller. A contactor is a switch that is closed or opened by the magnetic pull of an energized relay coil. Figure 22–13 shows a relay coil with contactors. A relay coil connected in series in the circuit is normally represented by a heavy line, as shown in Figure 22–14. A relay coil connected in parallel is represented by a light line. A contactor that is open when the coil is de-energized is known as a normally open (N.O.) contactor and is indicated by two short parallel lines; see Figure 22–14. A contactor that is closed when the coil is de-energized is known as normally closed (N.C.) and is indicated by a diagonal line drawn across two parallel lines; see Figure 22–14. Letters are added to show which contacts are operated by a given coil.
When contactors interrupt a large current, a severe arc forms. This arc can burn the surface of the contactor. To reduce this burning effect, a magnetic blowout coil is added in series with the contactors to extinguish the arc by electromagnetic action, as shown in Figure 22–15. An arc is, after all, nothing but a stream of electrically charged particles (similar to current in a wire) and, therefore, can be deflected by a magnetic field. Figure 22–15 shows how the blowout coil sets up a magnetic field that serves to force the arc off the contacts by deflecting it. An arc chute is provided for the protection of sur- rounding equipment. Figure 22–16 shows a typical relay so equipped, and Figure 22–17 represents its corresponding schematic.
Some relay coils used in DC circuits have tapped-coil arrangements to limit the continuous holding current and prevent overheating of the coil. These arrangements work because it takes less energy to hold the energized plunger in place after it has been pulled up to its closed position.
Consider the coil, with dual windings, shown in Figure 22–18A. Both coils are required to provide sufficient pull-in power. But once the relay plunger has activated, a timer (or similar) contact will disconnect one of the coil sections, reducing current to an appropriate “hold” value.
A different technique can be used to achieve the same result. Instead of a dual- winding coil, a single-coil relay is used, with a current-limiting resistor that is connected in series with it as soon as the coil is energized. This is illustrated in Figure 22–18B, where control relay CR provides a normally closed contact to insert its own current- limiting resistor in series.
Push-button stations, like the ones shown in Figure 22–19, are used to provide control of the motor. The push buttons are really spring-controlled switches, which are classified as being either normally open or normally closed (N.O. or N.C.).
Pressure on a normally open start button closes the switch contacts momentarily. When the button is released, the spring reopens the switch, as shown in Figure 22–20A. By contrast, a stop button is a normally closed switch. Finger pressure opens the contacts, which close again when pressure is released; see Figure 22–20B. Many push buttons have
two sets of momentary contacts, Figure 22–20C, one of which opens when the other set is closed.
These push buttons generally are used in connection with a relay coil, as shown in Figure 22–21. This diagram represents part of an elementary control circuit. When the start button is pressed, closing contacts 2–3, there is a circuit from line L1 through normally closed contacts 1–2, through 2–3, and through relay coil M to supply line L2. The current in relay coil M causes contact M to be held closed. Hence, when the start button is released (opening contacts 2–3), there is still a circuit through the stop button 1–2, through contact M, and through coil M to L2. This arrangement is called a sealing circuit and is a part of the control circuits soon to be described. Momentary pressure on the start button energizes the relay coil. The sealing contact M keeps the coil energized. In control circuits, coil M also closes other contactors as well as the sealing contact.
When the stop button 1–2 is momentarily pressed, the circuit is broken. Coil M loses its magnetic pull and contact M opens. The release of the stop button (closing 1–2)
does not reestablish the circuit. Both contact M and start button 2–3 are open. Consequently, coil M cannot be energized until the start button again closes the circuit.
There are so many types of automatic controllers for special applications that it is impossible to cover all of them in this chapter. Instead, we describe three standard types in some detail: the counter-electromotive force controller, the voltage drop acceleration controller, and the definite time controller.