Wednesday, January 7, 2015

Starters and speed controllers : trends in motor control, the need for reduced-voltage starting, manual starters and manual speed controllers.

Starters and Speed Controllers
22–1 TRENDS IN MOTOR CONTROL

Until recently, DC motors have been the only choice for applications where accurate control over wide speed ranges and load positioning were required. To satisfy these demands, industry developed a great variety of controllers. In the infancy of motor technology, manual control boxes were built. Later designs incorporated semiautomatic features that gave rise to fully automatic controls.

Historically, such control circuits were based on extensive use of electromagnetic relays. Their schematics came to be known as ladder diagrams, with their vertical lines interconnected with horizontal “rungs,” resembling a ladder.

In recent decades, the innovations of electronic logic and computer circuitry brought about the development of electronic speed controls. The quick proliferation of programmable controllers (PCs) foreshadows the obsolescence of magnetic relay circuits. However, there remains a multitude of such “old-fashioned” equipment in use, and you are advised to study the sections of this book that cover relay ladder logic. They teach important principles that will carry over into future studies of modern electronic control circuitry.

22–2 THE NEED FOR REDUCED-VOLTAGE STARTING

For starting small DC motors (up to 2 horsepower), the motor is simply connected directly to the DC power line. But the sudden connection of a large motor to a DC line would cause unreasonably high current in the line and armature, since, at the moment of starting, no counter-emf exists to limit the current.

For example, in question 19 in Chapter 21, the armature and series field have a total resistance of 0.44 ohm. With no opposing emf at the instant of starting, the armature current is 220/0.44 5 500 amperes. Without the addition of external resistance, this high current puts a great stress on armature windings, burns brushes and commutators, and causes line voltage drop, which can interfere with other machines on the line.

For the gentle starting of large motors, a motor starter is used. It is merely a vari- able resistance placed in series with the armature. Its primary purpose is to limit the armature current to a safe value during the starting and accelerating period. Along with the starting rheostat, there is usually some arrangement for automatically disconnecting the motor (and leaving it disconnected) if the line voltage fails.

The two common types of manual starting rheostats, or starting boxes, used with shunt and compound motors are the three-terminal and four-terminal starting rheostats.

22–3 MANUAL STARTERS

Three-Terminal Starting Rheostat

The three-terminal starting rheostat, shown in Figure 22–1, has a tapped resistor enclosed in a ventilated box. Contact buttons, located on a slate panel mounted on the front of the box, are connected to the tapped resistor. A movable arm K with a spring reset can be moved over the contact buttons to cut out sections of the tapped resistor.

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After the line switch is closed, the arm K is moved to the first contact, A. The shunt field is now connected to the line at full strength. All of the starting resistance is in series with the armature. This resistance, in accepted practice, is calculated to limit the starting current to 150% of the full-load current rating of the motor.

As the motor speeds up, the operator moves the arm gradually toward contact B. The time required depends on the time needed for the machine to build up speed. At B, the armature is connected directly across the source voltage. The magnetic holding coil, M, holds the arm in the full on position. A spring (not shown) tends to return the arm to the off position. If the shunt field current is much reduced while the armature circuit remains connected, the motor races. However, shunt field current reduction is prevented by having the holding coil in series with the shunt field. Reduced current in the holding coil lets the arm fly back to the off position. This protective feature that a three-termi- nal starter provides is called no-field release. The holding coil also releases the arm if the line voltage is interrupted. The motor then has to be restarted when line voltage is restored.

The starting resistance is in series with the shunt field when the arm K is in the on position, at contact B. This additional resistance is so small, when compared with the field resistance, that it has practically no effect on field strength and speed.

Figure 22–2 illustrates the connections for a three-terminal manual starting rheostat used with a cumulative compound motor. Note that the only difference in this circuit and the connections for a shunt motor is the addition of the series field.

Starting rheostats are designed to carry the starting current for only a short time; they are not intended for speed control. An attempt to obtain below-normal speed by holding the arm K on an intermediate contact is likely to burn out the starting resistor.

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The three-terminal starting box is not suited for use where a field rheostat is used to obtain above-normal speeds. The reason is that a reduced field current can release the arm and shut down the motor. With field control, a slightly different arrangement, called a four-terminal starting box, is used.

Four -Terminal Starting Rheostat

Four-terminal manual starting rheostat has two functions in common with three- terminal starting rheostats: (1) to accelerate a motor to rated speed in one direction of rotation and (2) to limit the starting surge of current in the armature to a safe value. However, this starting rheostat can be used along with a field rheostat. The field control is used to obtain above-normal speeds. Figure 22–3 represents a four-terminal starting box connected to a shunt motor.

Note that the holding coil is not connected in series with the shunt field, as it is in the three-terminal starting box. In this four-terminal starter, the holding coil, in series with a resistor, is connected directly across the source voltage. The holding-coil current is independent of field current but still serves as a no-voltage release. If line voltage drops, the attraction of the holding coil is decreased, and a reset spring (not shown) returns the movable arm to the off position.

A motor with a four-terminal starter is started in the same manner as with a three- terminal starter. Any desired above-normal speed of the motor is obtained by adjustment of the field rheostat in series with the shunt field.

When the motor is to be stopped, all resistance in the field rheostat should be cut out, so that motor speed decreases to its normal value. Then the line switch should be opened. This procedure ensures that the next time the motor is started, it has a strong field and resultant strong starting torque.

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22–4 MANUAL SPEED CONTROLLERS

It is often necessary to vary the speed of DC motors. As pointed out in Section 21–8, above-normal rating speeds are obtained by adding resistance to the shunt field circuit. Below-normal rating speeds are obtained by adding resistance to the armature circuit.

Two types of manual speed controllers are used with shunt and cumulative compound motors, above-normal speed controllers and above-and-below-normal speed controllers.

The National Electrical Manufacturers’ Association (NEMA) defines a manual speed controller as a device for accelerating a motor to normal speed with the additional function of varying speed. (Manual speed controllers must not be confused with manual starting rheostats, which simply accelerate a motor to normal speed.)

Above-Normal Speed Controller

This controller combines the functions of a starter and a field rheostat. The starting resistance is used in the armature circuit only during the starting period. This limits the armature current while the motor accelerates to normal speed. The field control circuit is effective only after the motor is brought up to normal speed. After normal speed, insertion of resistance weakens the field and produces higher speed. The controller illustrated in Figure 22–4, then, has three functions.

1. To accelerate the motor to rated speed by reducing the resistance in the armature circuit

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2. To limit the current surge in the armature circuit to a safe value

3. To obtain above-normal speed control by varying the resistance in series with the shunt field

Two rows of contacts are mounted on a slate panel, as shown in Figure 22–4. The top row of small contact buttons connects to a tapped resistor, which is the field rheostat. The bottom row of larger contacts connects to a tapped resistor in series with the armature. The control arm K connects to both sets of contacts.

In the start position, arm B bypasses the field rheostat; thus, the full-line voltage is applied to the shunt field. Arm K, when moved clockwise, cuts out starting resistance as the motor accelerates. When arm K approaches the normal-run position, pin C pushes arm B counterclockwise until it is secured against the holding coil. The motor is now accelerated to normal speed.

In Figure 22–5 note that arm B is removed from the field circuit; thus, it no longer short circuits the field rheostat. Instead, arm B now bypasses the starting resistance, providing a direct path from the supply line to the armature.

If it is necessary to increase the speed of the motor to some value above normal, arm K is moved counterclockwise. This has no effect now on armature current, but it

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does result in resistance being inserted in the shunt field circuit. Motor speed now in- creases. Arm K can be left in any intermediate position to obtain desired above-normal speed.

When the line switch is opened, the holding coil releases arm B, which is returned to its original on position by a spring. Pin C is now released and permits arm K to return to the off position. K is returned by a reset spring.

This type of controller can be used with either a shunt or a compound motor.

Above-and-Below-Normal Speed Controller

In some motor installations it is necessary to have a wide range of speed control, including both above-normal and below-normal speeds. A typical above-and-below- normal controller is illustrated in Figures 22–6 and 22–7. The movable arm K connects to two rows of contacts. The lower row of contacts connects to taps on the armature circuit resistor, and the upper row connects to taps on the field resistor. The contacts are mounted on the front of a slate panel, while the armature and field resistors are housed in a ventilated box in back of the panel. Continued clockwise movement of the arm results in continued increase of speed. This increase is accomplished first by removing armature circuit resistance and then by inserting resistance in the field circuit.

In the position shown in Figure 22–6, there is considerable resistance in series with the armature. The arm K also contacts the radial conductor D, which connects full-line

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voltage to the shunt field. With the arm in this position, the speed is below normal. Once the movable arm is set on any contact point, it locks in that position until moved to some other point. This is done by a unique gear-and-latch system operated with the aid of the holding coil.

When a motor is operating under heavy load at slow speed, there is considerable current in the armature circuit. This large current requires the armature resistors to be of large size in order to radiate the heat produced by the large current. Large resistors make the physical size of this controller larger, for a given horsepower rating, than an ordinary manual starting rheostat.

As the arm is slowly moved clockwise to the upper end of the armature rheostat, it still contacts conductor D (at point B). The arm K also comes in contact with the curved conducting strip marked A. This is the normal speed position. Full-line voltage is applied to both the armature and the shunt field.

In the above-normal speed position, full-line voltage is still applied to the armature through strip A–E. The outer end of the control arm K now contacts a point on the field rheostat; thus, the resistance between the arm and point B is inserted into the field circuit. If the arm is moved to point C, all of the field rheostat is in use, producing maximum speed by field weakening. When the line switch is opened, the holding coil releases the latch, and the reset spring returns the arm to the off position.

This type of controller can be used with either a shunt or a compound motor. Connections for a shunt motor differ only by the omission of the series field.