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]
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)
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.
The 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.
This 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.
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.
The timer is a device that is used to turn on a circuit at a particular time.
This is the part of a circuit the delivers power to the motor from the main 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.
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...