The control system was designed using industrial control practices wherever practical. The use of DIN rail components, ferrules, branch circuit protection, grounding, and a dedicated safety chain improves maintainability, reliability, and operator safety.
This chapter describes the wiring of the 240 VAC power circuit, the 24 VAC control circuit, the alarm system, and the safety chain. It also explains the operation of the start and seal-in circuit used to control the contactor.
\section{Wiring the 240 VAC Power Circuit}
Power enters the control cabinet through the rear wall of the enclosure using 10 AWG four-conductor cable. Conductors L1 and L2 are connected to a two-pole 25 amp circuit breaker. The equipment grounding conductor is bonded to the control cabinet, while the neutral conductor is not used and is insulated with a wire nut inside the enclosure.
See Figure \ref{fig:furnacepower} to view a schematic representation of the power circuit.
The primary 240 VAC heater circuit is wired with 12 AWG THHN copper conductors rated for 90 \dg C service and 30 ampacity. Solid copper wire is used between the circuit breaker, contactor, and SSR. Flexible 10 AWG stranded high-temperature wire with fiberglass and mica insulation is used between the SSR and the heating elements. The stranded conductors are connected to the heating element terminals using stainless steel screws, washers, and nuts.
Ferrules are crimped onto the ends of stranded conductors wherever connections are made to the SSR, contactor, or lever-type wire connectors. The ferrules prevent strand damage, improve connection reliability, and reduce the likelihood of loose electrical connections.
The remaining 240 VAC branch circuits serving the control power transformer, PID controller, cooling fans, and volt-ohm meter are wired using 16 AWG stranded conductors. Since none of these loads exceed one ampere, this conductor size provides ample capacity.
A combination fuse holder and distribution header is used to distribute power to the branch circuits and simplify wiring modifications and maintenance.
The circuit breaker, contactor, SSR, and power distribution header are mounted on 35 mm DIN rail attached to a mounting plate in the rear of the control cabinet.
\begin{description}
\item[25-Amp Double Breaker] A two-pole 25 amp 400 volt circuit breaker used for
circuit protection and isolation. CNC Model YNB7-63N.
\item[3-Pole Contactor with Auxiliary Contact] A three-pole normally open switch rated for 30 amps and 600 volts. One normally open auxiliary contact is used for coil seal-in and indication. 24 VAC coil. The contactor provides positive isolation from line voltage and is remotely closed by the control circuit. Model CJX2-1810.
\item[Solid-State Relay (on L2 only)] DIN rail mounted solid-state relay with heat sink. 3-32 VDC input. 24-480 VAC output. Rated for 40 amps. LCLCTC Model LCDS4048ZD3. Only Conductor L2 is switched by the SSR. Positive isolation of both line conductors is provided by the contactor when the furnace is shut down.
\item[Heater Elements] Kanthal alloy (FeCrAl) wire diameter: 1.2mm (0.047"); coil diameter: 7.2mm (0.28"); length: 800mm (31.45") each. Rated voltage: AC220V, Rated Power: 3000W; Resistance: 15-16ohm. Withstands temperatures up to 2100°F. Two are used in series in this project.
\end{description}
Although each heater element is sold as a 220 VAC, 3000 watt heater, the elements are operated in series in this design. The elements were shortened and adjusted to achieve the desired furnace resistance and power rating.
\section{Wiring the 24 VAC Control Circuit}
The control power circuit begins at the 240-VAC-to-24-VAC control power transformer. The transformer provides a reduced-voltage supply for the furnace control and safety systems. Using 24 VAC for control functions reduces the voltage present at switches and indicators that may be handled by the operator during normal operation and maintenance.
The 24 VAC supply is distributed to the safety chain, indicator lamp, and alarm circuit.
The 24 VAC control circuit is wired with 16 AWG stranded wire with ferrules crimped onto the ends for connection to terminals. The ferrules prevent strand damage, improve connection reliability, and reduce the likelihood of loose electrical connections.
See Figure \ref{fig:furnacesafety} to view a schematic representation of this circuit.
\subsection{24 VAC Safety Chain}
The safety chain is a series circuit consisting of five normally closed safety switches and one normally open momentary start pushbutton used to energize the contactor coil. When the contactor coil is energized, the contactor closes and permits power to reach the heating elements. If any normally closed safety device opens, current flow to the contactor coil is interrupted and spring pressure causes the contactor to open, removing power from the heating elements.
The safety devices included in the chain are:
\begin{description}
\item[Emergency Stop Button] This is a large red normally closed momentary push button mounted on the top of the control cabinet. When the operator presses this button the safety chain opens and de-energizes the contactor coil causing the contactor to open and isolate the electric heating elements.
\item[Cabinet Over-Temperature Switch] A small normally closed bimetallic snap-action thermal switch mounted to the control cabinet wall set to automatically open at 90 \dg C. When the switch opens the safety chain opens and the furnace shuts down. A 90 \dg C switch was selected to protect wire insulation and components in the cabinet.
\item[Lid Limit Switch] A normally closed switch that provides a lid safety interlock. Opening the furnace lid causes the switch to open, interrupting the safety chain and de-energizing the contactor. This removes power from the heating elements before the operator can access the furnace interior. The lid limit switch is installed inside the furnace body shell.
\item[Dump or Anti-Tilt Switches] Two normally closed tilt switches are wired in series, and will open whenever their tilt is more than approximately 15\dg \ from vertical. This causes the furnace to shut down if the furnace is accidentally tipped or overturned. The dump or anti-tilt switches are mounted inside the furnace body at locations 90\dg \ from each other.
\item[Start Push Button] A normally open momentary push button mounted on the front panel used to remotely energize the contactor coil and initially start the furnace.
\item[Indicator Lamp] A panel-mounted green indicator lamp is connected in parallel with the contactor coil. The lamp illuminates whenever the contactor coil is energized, providing the operator with a visual indication that the safety chain is intact and the heating elements are capable of being energized.
\end{description}
\subsection{24 VAC Alarm Circuit}
A combination flashing LED indicator and audible alarm module is mounted in the top of the control cabinet. The alarm is controlled by a relay internal to the PID controller that closes when a programmed alarm setpoint is reached.
See Figure \ref{fig:PID-SSR} and Figure \ref{fig:furnacesafety} for schematic representations of the alarm circuit.
Control power is routed through a cutout toggle switch and connected to Terminal 12 of the PID controller. The alarm device is connected to Terminal 11, which provides a normally open relay contact. When the alarm relay closes, power is supplied to the alarm module.
A momentary pushbutton is wired in parallel with the PID alarm relay contact to permit alarm testing before furnace start-up. This feature allows the operator to verify proper alarm operation without requiring the furnace to reach the alarm setpoint.
\section{Starting and Seal-In}
The starting sequence will be explained now because it may not be apparent to readers how a momentary push button can start the furnace and permit continuous operation.
When all switches in the safety chain are closed (or reset) the furnace is able to be started. The operator presses the normally open momentary switch on the front panel. This completes the 24 VAC circuit to the contactor coil. When the contactor coil energizes it closes the contactor. The three main contacts close and the normally open auxiliary contact closes. 24 VAC is connected from the upstream side of the Start button to Terminal 13 of the aux contact. Terminal 14 of the aux contact is connected to the contactor coil.
While the start button is pressed, 24 VAC has two paths to the contactor coil -- through the Start button, and through the auxiliary contact. When the Start button is released the aux contact continues to provide a current path to the contactor coil. The contactor coil will remain energized and hold the contactor closed until a switch in the safety chain opens and removes the supply of current to the coil.
Following a shutdown event, the furnace will remain de-energized until all safety chain devices have returned to their normal condition and the Start Pushbutton is pressed again. This arrangement helps protect the operator from exposure to energized heating elements and prevents the furnace from automatically restarting after a power interruption.
