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stc midterm 2014
| Question | Answer |
|---|---|
| If it is not the voltage that kills, what does? | Current. |
| How much current does it take to kill? | Less than 1A, probably nearer to 200-300 milliamperes. |
| What is ventricular fibrillation? | When the heart races out of control, usually caused by making contact with electricity. |
| How us a dual-element fuse sized? | By ampere rating. |
| What is grounding? What is a system ground? | Grounding is the connection of equipment or system to the earth or ground. There are two systems of grounding system grounding and equipment grounding. |
| Why are wiring connections and terminal markings useful to the electrician? | Because it is almost impossible to remember each wire when reassembly is needed. |
| Describe a ladder diagram. | A ladder diagram is a simple elementary diagram if a switch or other device in a completed circuit. |
| What is the difference between a two-wire and a three-wire control system? | Usually a 2-wire is a ladder-connected on-off switch manually operated. A 3-wire is used to indicate a device is on or off. Also it can be made so that a push-to-test pilot light arrangement aids troubleshooting the light bulb filament. |
| What is locked rotor current? | The speed before achieving full speed |
| Why are fuses need in control circuits? | To prevent overheating of motors |
| How is a drum switch converted from maintained to momentary operation? | by turning the shaft 180̊ |
| What do some dc motors use to suppress brush arcing? | inter-poles (commutating windings) |
| What is the purpose of an interlock switch? | to stop a motor before reversing. |
| What is one of the most commonly used switches for motor control? | The single pole, single throw, on-off switch is most commonly used. |
| When was magnetism discovered? | About 2600 BC. |
| What is a solenoid? How does it operate to control switching action? | A solenoid is a current-carrying coil used as a magnet and a movable iron piece tends to be pulled to the center of the solenoid when it is energized. The moving iron can be fitted with a switch contacts and it will serve as an on-off switch by closing th |
| What is inductive reactance? In what unit is it measured? | By varying the coil’s current or inductance it produces an opposition to current flow. Inductive reactance is measured in ohms |
| What five conditions may cause excessive hum in a magnetic assembly? | (1) broken shading coil; (2) operating voltage to low; (3) wrong coil; (4) misalignment between armature and magnetic assembly; (5) dirt, rust, etc.; (6) jamming or binding of moving parts; and (7) incorrect mounting of controller. |
| Describe a relay? | A device designed for remote control of another device. |
| What are two types if solid-state relays? | Solid-state relays can be NPN or PNP transistors, or an SCR (silicon controlled rectifier). |
| What is the difference between an SCR and a triac? | A SCR can handle higher currents than a triac. |
| What are the two types of thermal overload? | Thermal overloads relays are bimetallic and melting alloy. |
| What does inductive rating mean? | It’s the load it must handle |
| What does resistive rating mean? | Load capacity when switching resistive (resistor) loads. |
| What does continuous rating mean? | The rating for current relay contacts handling when holding a circuit closed for some time. |
| What is the right-hand rule of a motor? | Use the right hand with the thumb in the direction of the motor rotation and the index finger pointing in the flux direction, now the middle finger indicates current flow direction. |
| What is counter EMF? | Electromotive force, or voltage, induced in a motor that opposes the applied voltage, |
| What effect does the load have on a motor? | Load usually causes a motor to slow down as it (the load) increases. |
| What are the three types of ac motors? | Series, synchronous, and induction. |
| What type of motor can use both ac and dc? | Universal. |
| What is the main advantage of a synchronous motor? | Constant speed. |
| How did the squirrel-cage motor get its name? | Shape of rotor is similar to a squirrel cage. |
| Name the three categories of timers. | Dashpot, synchronous clock, electronic timers. |
| Explain the terms on-delay, off-delay, and times out. | On-delay ̶ period of time before relay closes. Off-delay ̶ has NO and NC contacts. Remain activated after turn off for a pre-set time. Times-out ̶ time it takes the relay to close. |
| What is a dielectric? Where is the term useful? | Dielectric is the material (air or vacuum or other substance) that separates the two plates of a capacitor. |
| How does the thermistor operate? | A thermistor produces a decrease in resistance when heated. |
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vjambriz
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