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Putting Instructors in Control of the Classroom Workshop

Putting Instructors in Control of the Classroom Workshop
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Sprecher + Schuh has seen several different types of requirements for school workshops over the years. Some just want an E-stop on a simple starter and that is the reason we developed the KwikStarter with an E-Stop. Others need to meet strict state or local safety requirements, and we have helped them too. One inquiry for a keyed switch presented an interesting challenge.

 
Sprecher & Schuh Control panel with Emergency Stop push button and controlled key locking device used in control panel for woodworking classroom in Canada
 

Specifications

The instructor of a student workshop wanted the ability to enable all the machines (like saws, lathes, and drills) with a Keyed Selector Switch. The Instructor wanted a pilot light to show that the machine was enabled. After the Instructor enables the machine and puts the key back in his pocket, the student should then be able to Start and Stop the machine multiple times during any session. Additionally, the Instructor wanted a Mushroom Stop to disable the machine when pushed, and require the Instructor’s key to enable it again. This same Mushroom Stop would be used to disable the machine at the end of the day.

CS7-22E
Control Relay

Sprecher & Schuh CV7-11 Mechanical Latch

CV7-11
Mechanical latch

D7P-KR21
Keyed Selector Switch

D7P-MM64
60mm Mushroom Operator

Solution

Tech Support developed the idea of using a Keyed Switch with spring return in combination with a mechanical latching relay. The schematic in Figure 1 shows the solution we developed. The control relay “R” could be CS7-22E-120 combined with the mechanical latch “L” CV7-11-120. The Keyed switch selected would be a D7P-KR21PX10 two position spring return from right keyed switch, combined with a D7P-MM64PX10 red 60mm momentary mushroom head operator with one normally open contact. The start/stop represents any basic 3-wire control circuit.

Sprecher & Schuh Schematic for EStop used in control panel for woodworking classroom in Canada

Figure 1

 

Operation

The Instructor uses his key to enable the machine, which causes the control relay “R” coil (A1-A2) to close the relay and the mechanical latch “L” CV7 locks the “R” moveable contacts into closed position as the coil clearing “L” contact 65-66 removes the voltage from being applied to the coil of “R” (A1-A2). Also the normally open latch “L” contact 57-58 closes, which empowers the student to start and stop the machine multiple times as required. Note that “R“ contact 13-14 closes while latch contact 65-66 stays open as long as the latch is engaged with no voltage on either latch or the base relay coils. Additionally “R” relay contact 23-24 closes, turning on the red pilot light (could be green if one also wanted a red run light when the machine motor is running). The latch will hold the relay closed until someone hits the Disable momentary mushroom head button which causes the Latch solenoid at terminals E1-E2 to unlatch the relay “R” and now the Instructor’s key is required to enable again.

If the motor is running and the school loses power (or brownout), when power resumes the enable light will come back on. More importantly, the motor will not turn back on automatically due to the 3-wire control circuit. Because the Disable mushroom button was not pushed, the relay is still latched by “L” in the closed position. The Instructor’s key is not needed. The students simply hit the start button to run the motor to finish their work.

For additional information please contact your nearest authorized distributor, sales representative, or call our customer service or technical support lines.

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