Motor Controls Section 1
How many overload sensors are required by the NEC to protect a direct current motor?
1
an 11 pin control relay contains 3 sets of single pole double throw contacts. List the pin numbers by pairs that can be used as normally open contacts
1 and 3, 7 and 6, 11 and 9
When installing a motor control system, list 4 major factors to consider concerning the power system
1. Is it single or 3-phase 2. What is the horsepower 3. What amount of current will run to it 4. Will the motor need a reduced voltage starter
What is the operating voltage of most magnetic control systems?
120 volts
How many primary windings do control transformers have?
2
What pin numbers are connected to the coil of an 8 pin relay?
2 and 7
How many overload sensors are needed for a 3 phase motor?
3
What is the simplest of all starting methods for a motor?
Across-the-line starter
Explain the difference between auxiliary contacts and load contacts
Auxiliary contacts are smaller and load contacts are larger
What is the simplest way to determine if a motor is overloaded?
Check current with an ammeter and compare it to the current on the nameplate
Explain the difference between clapper type contacts and bridge type contacts
Clappers are used for low voltage applications and bridge type are used for high voltage applications
When reading a schematic diagram, are the control components shown as they should be when the machine is turned off or de-energized, or are they shown as they should when the machine is in operation?
Components are shown de-energized
All overload relays are divided into 2 sections. What are these 2 sections?
Current sensing and Contact
What is meant by the term line voltage on some pilot sensing devices?
Directly operate the motor
What action is necessary to restart a motor after it has been overloaded?
Flip the switch off and then all the way back on
A single-phase motor operates on 240 volts. Why should a starter that contains 2 load contacts be used to control this motor?
If it didn't have 2 contacts there would be a hazardous condition as the line is constantly connected to the motor
How are the primary windings connected when the transformer is to be operated on a 240 volt system?
In parallel
How are the primary windings connected when the transformer is to be operated on a 480 volt system?
In series
Explain the difference between jogging and inching
Jogging momentarily connects the motor to full line voltage and inching momentarily connects to reduced voltage
Explain the difference between manual motor starters that provide low voltage release and those that do not
Low voltage release will not restart after a power failure without being reset
What is the major difference in characteristics between thermal type and magnetic type overload relays?
Magnetic type are not sensitive to ambient temperature
Explain the difference between manual control, semiautomatic control, and automatic control
Manual: devices that connect to the motor in-line Semiautomatic: use of pushbuttons and switches Automatic: devices operate a magnetic contactor that controls the motor
Overload relays are designed to protect the _________
Motor
What is the difference between a motor starter and a contactor?
Motor starters are contractors with overload relays
Is the National Electric Code (NEC) a law?
No, unless made so by local authorities
What agency requires employers to provide a workplace free of recognized hazards for its employees?
OSHA
When energizing or de-energizing a combination starter what safety precaution should always be taken?
Position yourself to the side
A pushbutton manual starter has tripped on overload. Explain how to reset the starter so the motor can be restarted
Press the stop pushbutton with more than normal force
What is the purpose of coil clearing contacts?
Prevent continuous power from being supplied to the coil after energization
What is the most important function of any control system?
Provide protection for circuit components and motor
A _________ shows components in their electrical sequence without regard for physical location
Schematic
Why is a circle most often used to represent a coil in a motor control schematic?
So numbers and letters can be written in the circle to help identify it
What are the 2 major types of thermal overload relays?
Solderpot and bimetal strip
What is meant by the term ramping?
Start the motor at a slow speed and accelerate to full speed over a period of time
What is the most important function of any control system?
Strategy function
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 this indicate?
The motor has overloaded
Where is the best place to look to find specific information about a motor, such as horsepower, voltage, full load current, service factor, and full load speed?
The nameplate
What are the 2 basic types of overload relays?
Thermal and magnetic
What is the advantage of bridge type contacts over clapper type contacts?
They can handle a higher voltage because it connects in 2 places instead of just 1
Why is it necessary to permit a solder melting type of overload relay to cool for 2 to 3 minutes after it had tripped?
To allow solder to harden again after being melted
Why are 2 of the primary leads crossed on a control transformer?
To make connection of the primary winding easier
What is the purpose of the shading coil?
To prevent chattering
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 load?
Triac
What 2 factors determine the time setting for a dashpot timer?
Type and viscosity of oil, and setting the opening orifices in the piston
What is the most common method of controlling the speed of an alternating current motor?
Variable frequency control
Does a manual starter provide on/off function and overload protection?
Yes
What type of thermal overload relay can generally be set for manual or automatic operation?
bimetal strip
