Wednesday, January 7, 2015

Starters and speed controllers : the counter-electromotive force motor controller and the voltage drop acceleration controller (lockout acceleration).

22–8 THE COUNTER-ELECTROMOTIVE FORCE MOTOR CONTROLLER

The counter-emf controller, shown in Figure 22–22, is a commonly used method for the automatic acceleration of a DC motor. First, the line switch is closed. When the start button is pressed, relay coil M is energized. This control circuit remains energized because the sealing contacts are closed, as previously described.

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When coil M in the control circuit is energized, it closes a heavy pair of contactors, M 6–7, in the power circuit. The closing of these contactors establishes a circuit from line 1 through the overload device 1–6. Such an overload device generally operates when excessive heat develops due to overload. This device is commonly known as thermal overload protection. The circuit continues through the M contactors, through the current-limiting resistor in series with the armature, and through the armature windings to the other side of the line. The shunt field is directly across the full-line voltage and ensures maximum starting torque.

At the instant the motor circuit is energized, the counter-emf is 0 and nearly all of the line voltage is expended on the current-limiting resistor in series with the armature. As the armature accelerates, the counter-emf increases in proportion to the speed. With increased counter-emf, more of the line voltage appears across the armature terminals and the accelerating relay coil A. This coil is in parallel with the armature terminals. Relay coil A is calibrated to close its contactors when about 80% of the rated line voltage is applied to the coil. When the voltage across the armature terminals reaches this predetermined value (80% of the line voltage), coil A closes contactors A. These contactors then shunt out the resistor in series with the armature. The armature is now connected directly across the line voltage, and the motor accelerates to its rated speed.

Pressing the stop button breaks the control circuit, and both sets of M contactors open to disconnect the motor from the line. As the armature slows down, coil A cannot hold its contactors closed. With the A contactors open, the current-limiting resistor is again connected in series with the armature. Now the motor is again ready to be started.

Starting protection for this controller, or any DC automatic controller, consists of fuses or circuit breakers rated at 150% of the full-load current of the motor. Running overload protection is provided by the overload heater unit (O.L. 1–6). Overheating of this unit causes a bimetallic strip to trip open the normally closed contactor (O.L. 1–3) in the control circuit. The heater unit is rated at 125% of the full-load current rating of the motor. A continued overload on the motor for 45 to 60 seconds brings it into operation. The 150% starting current surge of 3 to 4 seconds does not produce enough heat to cause the thermal element to open its contactors.

Dynamic Braking

In some installations there is a need for quick stopping and immediate reversal of rotation. Dynamic braking is a method for quickly using up the mechanical energy of motion of the armature and its mechanical load after the armature circuit is opened. Dynamic braking is achieved by connecting a resistor across the armature at the instant the armature circuit is disconnected. This disconnected armature, rotating on its own momentum, is cutting flux and acts as a generator. The generated current quickly dissipates this mechanical energy by heating the resistor; thus, the motor stops quickly.

Figure 22–23 shows how dynamic braking facilities can be added to the previously described counter-emf controller. At starting, the same sequence of events takes place as described before.

Relay coil M and the dynamic braking coil, DBM 6–7, operate on one pivoted armature, as shown in Figure 22–24. When the start button is pressed, coil M closes

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the normally open contactors and pulls open the one set of DBM contactors. As the motor accelerates, relay coil A shunts out the current-limiting resistor as before. Although the DBM coil is now energized by the full line voltage, coil M has already tipped the pivoted armature clockwise to open the DBM contact. Coil DBM is not strong enough to bring the armature back to the position shown in Figure 22–24; thus, the DBM contactors remain open while the motor operates normally.

When the stop button is pressed, coil M releases the pivoted relay armature. Since the shunt field of the motor is still connected to the line, the rotating armature of the motor generates a current that keeps coil DBM energized. Coil DBM is now able to pull contactors DBM to their normally closed position, coil M being de-energized. (Small coiled springs, not shown in Figure 22–24, help make this action more positive.) With DBM contactors closed, the dynamic braking resistor is connected directly across

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the motor armature. Now the armature, acting as a generator, converts its mechanical energy into electrical energy that is quickly dissipated in the DBR resistor, and the motor armature comes to a quick stop. Coil A releases contactors A, reinserting the current-limiting resistor in series with the armature.

In the circuit of Figure 22–22, pressing the stop button disconnected the entire mo- tor circuit from this line. In that case, the collapsing magnetic field of the shunt field winding delivered its energy to the armature circuit. In the circuit of Figure 22–23, the armature is disconnected from the field when the stop button is pressed; therefore, a field discharge resistor (FDR) is connected across the field to dissipate the field energy when the line switch is opened.

Counter-emf Controller with Reversing and Dynamic Braking

In many applications, it is necessary not only to bring a motor to a quick stop but also to reverse the direction of rotation immediately. This is usually done by reversal of armature connections, as shown in the circuit in Figure 22–25.

When the forward button is pressed, the normally closed contact 4–7 opens and the normally open contact 4–5 closes. Relay coils 1F and 2F become energized, closing the 1F sealing contact 4–5 and the armature current contactors 1F and 2F. These normally closed contactors (1F) DB2 are held open by a relay like that of Figure 22–12. The (1F) DB2 contactors are held open so that the dynamic braking resistor is disconnected when the motor is energized. The normally open contactors DB2 (1F) at points 9–11 close and connect the accelerating relay coil A across the armature. As the motor accelerates,

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relay coil A closes contactors A. Closing contactors A puts full-line voltage on the armature so the motor operates at rated speed.

Pressing the stop button opens the control circuit. The contactors 1F and 2F then open and disconnect the armature from the line. Contactors DB2 (1F) also open, de- energizing relay coil A, which opens contactors A. At the same time, the normally closed (1F) DB2 recloses and connects the dynamic braking resistor across the armature so that the motor stops quickly.

When the reverse button is pressed, the direction of current in the armature is reversed. The motor accelerates in the reverse direction, and when the armature emf is high enough, coil A closes contactors A and the motor operates at rated speed in the reverse direction. Use of the stop button opens contactors A and inserts the dynamic braking resistor into the circuit as before.

In the circuit in Figure 22–25, each forward and reverse push button has a nor- mally closed contact and also a normally open contact. This circuit arrangement makes it impossible to energize the reverse relays 1R and 2R until the forward relays 1F and 2F are de-energized. For example, if the reverse button is pressed, it first breaks contact at points 5–6 and de-energizes coils 1F and 2F before closing across points 7–8 and energizing relay coils 1R and 2R. The same protection exists if relay coils 1R and 2R are energized and the forward button is pressed. This type of connection arrangement, called electrical interlocking, is often used in control circuitry so that when one set of devices is operating, the circuit to a second set of devices cannot be energized at the same time.

22–9 THE VOLTAGE DROP ACCELERATION CONTROLLER (LOCKOUT ACCELERATION)

Large DC motors require controlled steps of acceleration. A series of resistors connected to lockout relays provide the means for smooth and uniform motor acceleration.

Like the counter-emf controller, the voltage drop acceleration controller makes use of these facts:

1. At the instant of starting, armature current is high and voltage across the armature is low. Voltage losses across each of the current-limiting resistors, connected in series, are high.

2. As the motor accelerates, the counter-emf increases and the armature current de- creases; therefore, the voltage drop across the current-limiting resistors, in series with the armature, decreases. Relays, connected across these resistors, are calibrated to operate and shunt out the starting resistors in a definite sequence as the armature speed increases.

The sketch in Figure 22–26 shows a typical lockout relay. Two coils affect the one pivoted armature. The normally open contacts can be held open by the lockout coil, even if the pull-in coil is energized. With reduced current in the lockout coil, the energized pull-in coil can tip the armature and close the contacts. Each of the three relays in the schematic diagram, Figure 22–27, is the same as in Figure 22–26. Relay coils marked 1A, 2A, and 3A are pull-in coils. Relay coils marked 1LA, 2LA, and 3LA are lockout coils.

Figure 22–27 shows a voltage drop acceleration controller connected to a cumulative compound motor. Since there are three resistors, there are three steps of acceleration. After the line switch is closed, pressure on the start button energizes relay coil M in the control circuit. Coil M closes main contactors M 9–10, which both closes the armature circuit and connects the shunt field across the line.

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The initial current through the starting resistors R1, R2, and R3 produces a relatively large voltage drop across each section of starting resistance; therefore, the lockout coil (LA) of each relay has a relatively high voltage across it and can hold the accelerating contactors 1A, 2A, and 3A open. At the instant of starting, coil M also closes the sealing contactors M 3–6 and M 6–4. The short time interval required for closing these contacts ensures that the pull-in coils 1A, 2A, and 3A become energized no sooner than the lockout coils.

The lockout relays are calibrated to operate and shunt out sections of starting resistance in a definite sequence as the armature accelerates. As current through the series resistors decreases during acceleration, less voltage is impressed on lockout coil 1 LA. Its pull on the movable contactor becomes less than that of pull-in coil 1A; therefore, pull-in coil 1A can close contactors 1A and shunt out resistor R1. As R1 is cut out of the circuit, current increases but decreases again as the motor continues to accelerate. Soon the voltage across 2 LA is low enough to allow pull-in coil 2A to close contactors 2A and shunt out R2. Then R3 is cut out in the same manner as R1 and R2. Thus, the motor is accelerated to rated speed in three steps.

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