Ch 1-6 Understanding motor Controls

When installing a motor control system, list four major factors to consider the power system

• Power source

• Motor connections

• Controller type

• Environment

Explain the difference between the manual control, semiautomatic control, and automatic control

• Manual control - simple devices that connect the motor directly to the line

• Semi-automatic control - Characterized by the use of push buttons, limit switches, pressure switches, and other sensing devices to control the operation of a magnetic contactor or starter

• Automatic control - pilot sensing devices are employed to operate a magnetic contactor or starter that actually controls the motor

What is the simplest of all starting methods for a motor?

Across-the-line starting: Accomplished by connecting the motor directly to the power line at full voltage

What is Jogging?

Accomplished by momentarily connecting the motor to full line voltage

What is Inching?

Accomplished by momentarily connecting the motor to reduced voltage

What is the most common method of controlling the speed of an alternating current motor?

Variable frequency drive (VFD): A method that adjusts the frequency of the power supplied to the motor, allowing for precise speed control

Ramping

Start a motor at low speed and accelerate to full speed over some period of time

What is the most important function of any control system?

Motor and circuit protection

A manual motor starter controls a single-phase 120 volt motor. The motor is not running, and the switch handle on the starter is found to be in the center position. What does that indicate?

starter tripped the overload and is preventing the motor from starting. the starter must be restarted by flipping the switch into the OFF position first, then ON

A single phase motor operates on 240 volts. Why should a starter that contains two load contacts be used to control this motor?

NEC requires that a disconnection means open all ungrounded supply conductors to the motor for safety during maintenance and to prevent electrical hazards.

What is the simplest way to determine if a motor is overloaded?

Find the motor full load current on the nameplate and then check the running current with an ammeterto see if it exceeds that value.

What are the two basic types of overload relays?

What’s the purpose of fuses and circuit breaker?

Designed to protect the circuit from a direct ground fault or short circuit condition

What are overload relays designed for?

Designed to protect motors from overheating and damage due to excessive current. They detect conditions of prolonged overcurrent and interrupt the power supply to prevent motor burnout.

Overcurrent

Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuits, or ground faults.

Short Circuit

An abnormal connection (including an arc) of relatively low impedance, whether made accidentally or intentionally, between two or more points of different potential, causing excessive current to flow.

Ground Fault

An unintentional, electrically conductive connection between an ungrounded conductor of an electrical circuit and the normally none-current-carrying conductors, metal enclosures, metal raceways, metal equipment, or earth

Overload

Operation of equipment in excess of normal, full-load rating, or a conductor in excess of its ampacity that, when it persists for a sufficient length of time, would cause damage or dangerous overheating.

What are two basic types of overload relays?

Thermal and magnetic

What is the major difference in characteristics between thermal and magnetic type overload relays?

Thermal overload relays operate based on heat generated by the current, while magnetic overload relays use electromagnetic force to trip the circuit when excess current is detected.

What are the two major types of thermal overload relays?

bimetallic and eutectic alloy

what type of thermal overload relay can generally be set for manual or automatic operation

bimetallic thermal overload relay

All overload relays are divided into two sections. What are are these two sections?

• current sensing section

• contact section

What device is used to sense the amount of motor current in an electronic overload relay?

Current transformer

How many overload sensors are required by the NEC to protect a direct-current motor?

one overload sensor device to protect a direct current motor

Explain the difference between clapper-type contacts and bridge-type contacts

Clapper-type contacts use a swinging arm to operate the contacts, while bridge-type contacts utilize a sliding mechanism to connect and disconnect contacts. Both types serve to control electrical circuits in different ways.

what is the advantage of bridge type contacts over clapper type contacts?

The advantage of bridge-type contacts is that they provide a more stable connection and reduced wear on the contacts due to the sliding mechanism, which can enhance the reliability and lifespan of the circuit.

Explain the difference between auxiliary contacts and load contacts?

Auxiliary contacts are used for control and signaling purposes, while load contacts are designed to directly control the power to a load. Auxiliary contacts can operate at lower power levels and are often used in conjunction with load contacts in motor control applications.

what is the purpose of a shading coil?

A shading coil is used in single-phase induction motors to create a rotating magnetic field and improve starting torque. It helps in preventing the motor from experiencing a dead zone during operation.

what type of electronic device is used to connect the load to the line in a solid-state relay used to control an alternating current?

A solid-state switching device such as a thyristor or triac is used to connect the load to the line in a solid-state relay for controlling alternating current.

what is optoisolation and what is its main advantage?

Optoisolation is a technique that uses an opto-isolator to transfer electrical signals between two circuits while keeping them electrically isolated. Its main advantage is protecting sensitive components from high voltages and transients, thereby ensuring safe operation and reliability.

what is the difference between a motor starter and a contactor?

A motor starter is a device that not only starts and stops a motor but also provides overload protection, while a contactor is a switch used to control the electrical supply to the motor without protective features.

what is the operating voltage of most magnetic motor control systems?

120 volts

how many primary windings do most control transformers have?

Most control transformers have one or two primary windings, typically used to convert higher voltages to lower control voltages.

how are the primary windings connected when the transformer is to be operated on a 240 volt system?

The primary windings are typically connected in parallel to handle the higher voltage of the 240 volt system.

how are the primary windings connected when the transformer is to be operated on a 480 volt system?

the primary windings are typically connected in series to manage the higher voltage of the 480 volt system.

why are two of the primary leads crossed on a control system?

Crossing two of the primary leads on a control system helps to achieve a specific phase relationship, ensuring proper voltage and current flow for correct operation.

eddy current

loops of electric current induced within conductors by a changing magnetic field, due to Faraday's law of electromagnetic induction. They can cause energy losses in electrical components and are typically minimized in transformer design.

hysteresis loss

A type of energy loss in magnetic materials due to the lagging of magnetization behind the magnetic field, resulting in heat generation during each magnetic cycle.

molecular friction

is the resistance to motion that occurs when layers of molecules slide past each other, often resulting in energy loss in mechanical systems.