Symbols used in motor control circuits (Figure 162) Start button Stop button Normally open contact Normally closed contact [pic] [pic] [pic] [pic] Overload Contactor coil Reverse coil Forward coil [pic] [pic] [pic] [pic] Timer [pic] Fig 162 Wiring diagram of a motor control circuit (Figure 163) with power circuit (Figure 164) [pic] [pic]Fig 163 Control circuit Fig 164 Power circuit Control circuit and power circuit (Figure 165) [pic] Fig 165 Stop button The stop button is usually connected in series with the control circuit. In this way, when the stop button is pressed, it cut off the current flow from the entire control circuit causing a break in the circuit. The stop button is a normally closed contact. When it is pressed a spring usually returns it to its normally closed position.
Start buttonThe start button is usually connected in parallel and is used to provide a momentary current flow through the control circuit in order for starting to occur. A start button is a normally open contact. It usually uses a spring to return it to its normally open position after it has been pressed. Normally open contact This is a contact that is normally open. It only closes whenever the coil is activated.
Normally closes contact This is a contact that is closed in its normal position. It only opens whenever the coil is activated. OverloadThis is a protective device that is used to protect the motor circuits whenever the motor starts to draw excessive current.
The overload is usually in the form of a relay or heaters. The heaters will burn or the relay will trip, breaking the circuit and stopping the flow of current. Coil This is usually used to open the normally closed circuits and close the normally open circuits whenever a current passes through it and is magnetized. Reverse and forward coil These two coils are used in an arrangement so that they will allow the motor to run in the forward or reverse direction.Timer The timer is a device that is used to turn on a circuit at a particular time.
Power circuit This is the part of a circuit the delivers power to the motor from the main circuit. Control circuit This is the part of the circuit that controls the power circuit. The control circuit and power circuit has to work together. Sequence control circuit This is a type of circuit that will allow only one motor to be started at any given time. This is done because during starting a motor could use up to 5 times the current required to run it under normal condition.It more than one motor should start one time it could cause the breaker to trip frequently and eventually damage it. This is why sequence control is important.
Figure 166 represents a sequence control circuit. [pic] Fig. 166 Sequence control circuit Operation of the sequence control circuit Current from L1 passes through the stop, because it is a normally closed push button, and flow to the first start button. It cannot pass through the start because it is a normally open push button.Also it cannot pass through any of the M1 because they are normally open contacts. When the first start button is pressed, current can now flow through the start to M1 (in the circle) through the normally closed contact (usually an overload) and to L2. Since M1 (in the circle) is a coil, it will energize and close both normally open M1. When the start button is released, it will return to the normally open position, but the current will still flow through M1 (in the circle) to L2 because the normally open M1 still remain closed.
Therefore current will flow through M1 which is now closed to the coil M1 through the overload and to L2. Since the second normally open M1 is now closed, the second start button can now be pressed to energize the coil M2. Based on the design of this circuit, the coil M1 has to be energized before the coil M2 can be energized. Forward and reverse control circuit (Figure 167) [pic] Figure 167 RC2 – normally closed reverse contact FC2 – normally closed forward contact FC1 – normally open forward contactRC1 – Normally open reverse contact Operation (forward rotation) When the first start button is pressed, current flows from L1 through the stop, through the start, through RC2, because it is normally closed, through the coil FC, through the overloads and to L2. Since there is a direct path from L1 through the coil FC to L2, coil FC will be energized. Since the normally closed FC2 is controlled by the coil FC, when coil FC is energized it will open the normally closed FC2 and close the normally open FC1.FC2 now becomes open to ensure that when the motor is running in the forward direction, if the start for the reverse is pressed it will not be able to reverse the direction of the motor because the power is cut off at FC2 that now becomes open. Reverse rotation In order to reverse the motor if it is already running in the forward direction, the stop button will have to be pressed.
This will cut-off the power to the entire control circuit de-energizing the forward coil (FC). When this is done FC2 will now return to its normally closed position.By pressing the start button for the reverse direction, current pass through the normally closed contact FC2, through the coil RC to L2. The reverse coil RC is energized and open the normally closed contact RC2 and close the normally open contact RC1. RC2 is open to ensure that the forward coil cannot be activated when the motor is running in the reverse direction. RC1 is closed to ensure that the motor keeps running even when the start button return to its normal position.
Interlocking circuit The forward reverse control circuit consists of an interlocking device.The purpose of this interlocking device is to ensure that the forward and reverse coils cannot be activated at the same time. RC2 and RC1 acts as the interlocking device on the forward reverse control circuit. Jogging or inching control circuit Jogging is the frequent stopping and starting of a motor. It is mainly used when aligning a belt on a motor. Figure 168 illustrates a jogging control circuit. [pic] Fig. 168 Jogging circuit Operation M and the start button is a normally open contact.
Therefore, no current can flow to the coil unless the start button is pressed.The start and stop button is a two in one push button. Whenever the start button is pressed, the stop button is automatically opened (signified by the dotted line).
When the start button is pressed and the stop button open, the current flows from L1 through the start button to the coil and to L2. This energizes the coil and closes normally open M, and start the motor. Whenever the start button is released back to its normally open position, M becomes open again and the stop closes. Since the current path from L1 to L2 through the coil is broken, the motor will stop.
Therefore, each time the start button is pressed, and released, the motor will start and stop. Overload protection Overload protection is generally achieved by connecting small electric heaters in series with the motor (Figure 165). When the motor current exceed a safe value, the current through the heaters generates enough heat to activate the thermal device, which opens the normally closed contacts in the control circuit (Figure 163). The overload device is sensitive to current and time.
Small currents take longer to open the contacts than do large currents.It is important to realize that the overload device does not provide short-circuit protection. Short circuit protection must be accomplished through the use of fuses, circuit breakers, or other devices. Each heating element has a specific rating.
To provide adequate protection, the heater must be matched to the motor. The manufacturer specifies the rating according to the full load running current of the motor. Reduced-voltage starting of AC motors (Using autotransformers) The starter shown in Figure 169 uses autotransformers to provide reduce-voltage starting of a squirrel-cage induction motor.The motor (1) is connected to the three lines L1, L2, and L3 by means of the movable lever (2). When the lever is moved from the middle, or off position, to the start position, the autotransformers (3) are connected in the circuit by means of the oil-immersed contacts (4). The low-voltage taps from the autotransformer are connected to the motor and the motor starts. When the motor is up to speed, the lever is thrown from the start position to run position, disconnecting the autotransformer and connecting the supply directly to the motor by the contacts (5).
The lever is held closed by the holding coil (6), which is connected across the phase of the supply through the relay contacts (7) of the relay and the stop button. The relays are built with dashpots, which delay the operation of the relay during the starting period and for momentary overload. Operation of the relays, or of the stop pushbutton, open the holding coil circuit and allow the lever to come to the off position, breaking the circuit made by the relay contacts (7). [pic] Fig. 169 Autotransformers used to start a three-phase induction motor Resistance magnetic startersA resistance type starter is shown in Figure 170 as used with a three-phase squirrel-cage induction motor. The motor is started by pressing the start button to energize the coil C, which closes the three line contactors C1, C2, and C3 and the interlocking C4 to maintain power on the coil after the start button is released. When the contact is closed, the timing relay TR becomes energized, and this relay immediately starts to measure the time for which it is adjusted.
At the end of the timing period, the contacts TR close to energize the coil A of the accelerating contactor A. hen the three-pole contactor A closes, the starting resistors will be short-circuited and the motor will be connected directly to the AC power lines. [pic] Fig.
170 Resistance start used for squirrel-cage induction motor Wye-delta The wye-delta controller is a device which changes the stator connection from wye to delta. The controller provides a reduced voltage for starting when in the wye position and a normal operating voltage when in the delta position. This is done as a means of protection to protect the motor from the high starting voltage.