Class Notes - Year 4 - 2020-2021
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Year 4

Lesson 411

Motor Drives - Accelerating and Decelerating Methods

EMC5 26-1 - Motor Drives

A motor drive is an electronic unit designed to control the speed of motor using solid-state components
May be AC or DC
Can control motor direction, speed, and acceleration/deceleration time
AC are most common
3-phase
Smaller drives can be powered by 1PH 115V or 230V and control a 3PH motor

Drives and Magnetic Motor Starters

Magnetic Starters

Used to turn motors on and provide overload protection
Also provided additional control contacts
Works well with basic on/off

Drives

Provides:
Speed control
Timed acceleration and deceleration
Motor starting boost
Fault monitoring
Programmable set speeds
Differents stopping methods
Amount of control depends on drive used
Basic drives provide on/off, speed control, and overload protection
Most also include:
Display info (voltage, current, frequency, and drive temp)
Motor and circuit fault info
Advanced programming features
Multiple preset speeds
Limiting max and min speeds
Motor braking

Magnetic Starter vs Drive

Cost is about the same for any HP
Drives have more options
Drives may include built in PLC/PLR functions like timers or counters
Level of control for motor depends on application

Magnetic Motor Starter Control Circuits

Connected to starters coil
Normally LV powered by transformer
Standard line diagram to control starter
Starter can turn motor on or off, but cannot accelerate or decelerates
Cannot set speed
Does provide overload protection
Could provide phase loss protection

Motor Drive Control Circuits

Connected to input terminals
Motor drive provides control circuit power (normally 12 VDC or 24 VDC
Drive is wired following control circuit wiring diagram provided by manufacturer
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Also provides overload protection
Offers motor speed control. Methods include
Potentiometer (divides voltage proportionately
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Voltage or current input to control speed
0-10VDC: 5V is 50%
Motor control and drive functions set by switches or programming through keypad
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EMC5 26-2 - Motor Drives

Basic drives can contain 30 parameters
Advanced can contain over 100 parameters

Parameter Menu Formatting

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Display Parameter

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Shows info
Normally includes:
Drive output frequency
Drive output voltage
Drive DC bus voltage
Drive internal temp
Motor fault
Drive elapsed operating time
Motor operating speed

Basic Programming Parameters

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Normally requires motor nameplate info to adjust
Normally includes:
Motor nameplate voltage
Motor nameplate current
Motor nameplate frequency
Desired motor acceleration time
Desired motor deceleration time
Motor stopping mode (ramp, coast, and brake)
Circuit control (drive keypad and external push button operation)

Advanced Programming Parameters

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Normally includes:
Drive output contacts operation
Multiple acceleration and deceleration times
Preset motor speeds
Type and time of braking applied to motor
Preventing a motor from operating in reverse direction
Jog time and speed
Number of times motor drive attempts to restart after fault
Analog inputs

Fault Codes

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Motor drives can detect and display common circuit faults
Common faults include:
Power loss
Overvoltage or undervoltage
Motor overload
High drive operating temp
Ground fault
Loss of phase
Phase to ground short
Drive overload

EMC5 26-3 - DC Motor Drives

Speed of DC motor is proportional to applied DC voltage
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Normally control voltage to change speed from 0 to maximum nameplate voltage

Dynamic and Regenerative Braking

Normally load on shaft determines amount of time for motor to coast to stop
Two methods are dynamic and regenerative
Both methods take advantage of fact that DC motor becomes a DC generator when spinning free

EMC5 26-4 - AC Motor Drives

Referred to as VFDs, AFD, inverter drives, vector drives, direct torque control drives, closed loop drives
Changes incoming voltage and frequency
Can operate 3 phase motor even if drive is designed for single phase
Control speed by varying frequency

AC Motor Drive Construction

3 main sections
Converter
DC bus
Inverter

Converter

Receives incoming AC voltage and changes it to DC voltage
Will step up or step down power depending on motor voltage rating

DC Bus

Filters voltage and maintains the proper DC voltage
May deliver DC to inverter for conversion back to AC

Inverter

Changes DC from DC bus back to AC

EMC5 17-1 - Friction Brakes

Time to stop depends on inertia and friction
Friction brakes consist of two friction surfaces
Similar to brakes on automobile

Solenoid Operation

Solenoid activates brake shoes
Solenoid is energized when motor is running
Solenoid is de-energized when motor is de-energized
Two methods:
Solenoid voltage rating equal to motor rating
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Solenoid rating equal to voltage between L1 and neutral
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In both cases, solenoid should be connected to motor circuit, not control circuit

Brake Shoes

Braking action is applied to rotor, not shaft
Low pressure equally distributed over large area is best

Advantages and Disadvantages of Friction Brakes

Advantages:
Low cost and simple maintenance
Available in AC and DC
Disadvantages:
More maintenance
Replacing shoes and pads

EMC5 17-2 - Plugging

Motor connections are reversed to motor develops counter-torque that acts as braking force
Allows for very rapid stopping

Plugging Switch Operation

Designed to open and close sets of contacts as shaft speed varies
As speed increases, contacts are set to change at given RPM
As speed decreases contacts return to normal condition

Continuous Plugging

Plugging switch may be used to plug motor each time stop is pressed
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Plugging for Emergency Stops

May be used where plugging is required only for emergency stop
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EMC5 17-3 - Electric Braking

DC voltage is applied to stationary windings of motor after AC voltage is removed.
Also known as DC injection braking
Effecient and effective method for most AC motors
Quick and smooth braking for high speed and high inertia loads
Minimal maintenance since no parts come in physical contact

Electric Braking Operating Principles

See text

DC Electric Braking Circuits

EMC5 17-4 - Dynamic Braking

See text

EMC5 18-3 - Motor Loads

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Constant Torque/Variable Horsepower (CT/VH)

Torque requirement remains constant
Any change in operating speed requires change in horsepower
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Examples: conveyors, gear pumps, metal cutting, load lifting, other loads that operate at different speeds
Same torque at low speeds and high speeds

Constant Horsepower/Variable Torque (CH/VT)

Load requires high torque at low speeds, low torque at high speeds
Horsepower remains constant
Speed and torque are inversely proportional in CH/VT loads
Example: center driven winder used to roll and unroll materials
Speed of material must remain constant, so speed of motor must change
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Variable Torque/Variable Horsepower (VT/VH)

Load requires varying torque and HP at different speeds
Both torque and horsepower are increased with increased speed
Examples: fans, blowers, centrifugal pumps, mixers, and agitators
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