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EMEC 1250
Motors and Control Logic Final - CSCC
| Question | Answer |
|---|---|
| What is the difference between a wiring diagram and a ladder diagram? | A wiring diagram is used to show the real-world wiring of a circuit, while a ladder diagram is used to show electrical flow and logic in an organized fashion. |
| What would be the advantage of a magnetic motor starter over a manual motor starter? | A magnetic starter can be used with automatic control circuits or memory-lock in circuits. A manual motor starter always requires an operator. |
| What is the difference between a contactor and a motor starter? | A contactor is a control relay designed to handle high current loads. A motor starter is the same thing but fitted with overload protection for a motor. Not all contactors can be used as motor starters, but all mechanical motor starters are contactors. |
| Explain how a single-phase generator works with a rotating conductor. | A permanent magnet is fixed with both poles opposite the center conductor loop. The loop rotates in the magnetic fields so that at 0 degrees and 180 degrees, voltage is zero, while at 90 and 270 degrees, voltage is at positive and negative peaks. |
| How does a standard transformer work? | A transformer uses two coils of differing numbers of coil loops. The first coil contains a set voltage and current, which induces a voltage and current in the second coil. The second coil can either maintain, step-up, or step-down those values. |
| What is transformer turns ratio? | The turns ratio is the number of turns on the first coil divided by the number of turns on the second coil. |
| What is a center tap? | A center tap is a wire taken from a middle location of the primary or secondary coil that has a reduced voltage from the full voltage of the coil. At 50%, this value will be 50% of the coil voltage. Similar to the lab transformers that have 6V at the tap. |
| What are conductive motors (DC/Universal) and inductive motors (single/three-phase)? | Conductive motors are those which require a brushed delivery of current to the rotor. Inductive motors have no connection of the coils to the rotor and supply current via a magnetic field. |
| How does a single-phase motor work? | Unlike a three-phase motor, a single phase motor only has one running coil and cannot self-start. It requires a starting coil to bring it up to speed before being reduced to only the running coil via a capacitor, a timer, or a centrifugal switch. |
| For an AC motor, what two factors determine synchronous speed? | Pole count and frequency. |
| What are the two main failure points for AC motors? | Deterioration of insulation on coil windings and failure of bearings. |
| What is service factor? | Service factor is the built-in safety margin for how much overload a motor can take in short bursts. Extended usage of the motor in the service factor threshold can destroy it. |
| How to calculate the service factor overage amount from SF and FLA? | Multiply the two together to get the overage value. |
| What are the types of single-phase motor? | Shaded pole, split-phase, and capacitor start. |
| What is required by NEC for all motors to have? | A motor starter with thermal overload. |
| How to reverse direction on single-phase motors? | Switch the start winding leads. |
| When to use FLA vs FLC from NEC? | FLA is used to size thermal overload protections while FLC is used to size wiring and branch circuit protection (either fuses or circuit protectors). |
| How to reverse direction on three-phase motors? | Switch any two leads or use a dual-magnetic starter configuration or use a VFD |
| How do you safely use a dual-magnetic starter configuration to reverse a motor? | Use two separate contactors wired through to one overload. If they can be mechanically interlocked, do so, but always include an electrical interlock using the auxiliaries. |
| What do you think of for friction brakes on a motor? | Think a clamp on either side of the motor shaft or the disc break on a car. |
| What do you think of for electric brakes on a motor? | Applying a DC current to the leads of the motor through an additional braking circuit and a rectifier. |
| What do you think of for dynamic brakes on a motor? | Motor is effectively turned into a generator and slows down gradually. The generated remaining electricity is burned off in a resistor or handled in the VFD's DC bus. |
| What does across the line starting do? | Instantly supplies full voltage to the motor. Only turns it on or off and has no speed control. |
| What does a VFD do? | A VFD acts as a sold-state controller for a motor, which turns it on and off, regulates ramping and controls speed. It uses analog control signals. |
| What is a stepper motor? | A stepper motor controls the increments of a motor's positioning and movement, but has no feedback response for the motor's positioning. |
| What is a servo motor? | A servo motor controls the increments of a motors positioning and uses an encoder to give feedback on positioning, allowing it to adjust for overshooting a position if it happens at all. |
| What is deadband? | The amount of a measured value on an automatic control device that needs to be removed before it returns to default status and another cycle can happen. |
| What are the two most common timers in modern industrial controls? | Solid-state and solid-state programmable. Solid state is an internal circuit board while solid-state programmable typically refers to PLC. |
| What is delay on make? | Delay on make makes a device power after receiving the relay coil energizing signal. |
| What is delay on break? | Delay on break keeps a device powered or locked out for a set time after the relay coil is deenergized. |
| What is one-shot timing? | A one-shot timer is one that activates a delay and closing/opening cycle once per energization of the relay coil. In order for it to happen again, the relay coil needs to been shut off. |
| What is recycling timing? | A recycling timer is one that repeats the delay and closing and opening of contacts for as long as the relay coil is energized. |
| What is a retentive timer? | A retentive timer is one where the energizing to the coil can be lost and the timer retains (or pauses) the current time being counted down. |
| What is a non-retentive timer? | A non-retentive timer is one where the energizing to the coil can be lost and the timer delay is reset. |
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griffr311
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