HMI Flashcards
AC Drives covert incoming DC to a AC bus? True or False?
False: AC drives convert incoming AC to a
DC bus
What are parts 4 of an AC Drive?
Input 3 phase - Rectifier Circuit - DC bus - Inverter Circuit - Motor
What does the DC bus use to filter current?
Large Electrolytic capacitors
Do AC drives use pulse width modulation?
Yes
What happens is the bleeder resistor blows up/not work?
The capacitors will not discharge? This can mean lethal voltages may be present hours after drive is locked out
What do you need before servicing an AC Drive
-Wear correct PPE,
-Before locking out drive measure and actually see the DC bus voltage go to 0
-lock out at disconnect
What does an AC drive use for its inverter circuit?
Transistors or IGBT devices are used to switch the DC bus on and off to the motor
What is the term “Motoring”?
When a motor is controlling the speed of a load
What is the term “Regeneration”?
When the load causes the rotor to rotate faster than the rotating field
- The motor turns into an alternator and generates voltage back to the drive
What are 2 ways to avoid “Regeneration”?
Oversize the motor/drive combination
or
Dynamic Braking
What is does “Dynamic Brake” consist of?
A sensor to detect a rise in DC bus voltage. The sensor energizes a relay
One or more high wattage resistors that are connected across the DC bus by the relay contacts
How does “Dynamic Brake” work in an AC drive?
If the voltage in a DC bus rises, a sensor picks it up. This energizes a relay that connects high wattage power resistors to limit the current (use excess energy into heat).
Does “Dynamic Brake” only work is the load is slowed down?
No, if load is too fast it drags the rotor faster than the synchronous field rotation and causes regeneration
What stop config is this: Typically used to slow down conveyor type loads
Ramp to stop
- Ramp to 0 following the Decel curve
What stop config is this: Used for fans and pumps
Coast to stop
- turn off power
immediately
What stop config is this: Drive applies a DC voltage and “locks” the rotor
DC Brake
- applies a DC
voltage to the stator
-Can stop the load quickly but may
also overheat the motor
What stop config is this: Heavily loaded conveyors with high inertia
Ramp then DC Brake
Which mode is for safety?
Coast to stop with magnetic clutch
What is the equation for S-Curve time?
Scurve time = Accel or Decel time + S-Curve setting in percentage
What is the equation for total accel time if there is an S-Curve applied?
Total Accel Time = Accel time + Scurve time
What are parameters?
Storage Locations in the memory of an AC Drive
What can parameters be (Data type wise)?
Single bit, a byte, an integer value or a 32 bit value
What are parameters used for?
They match the drive to the incoming
AC
They match the drive to the motor
They match the drive to the load
They provide operating information
such as Load Current Being Drawn By
the Motor
What is accel time parameter?
The parameter that defines how long it will take the drive to ramp from 0 hz to maximum frequency
What is decel time?
how long it will take the drive to slow down to 0 frequency
True or false: The preset frequencies could be
selected using hardwired controls
or through network control
True
Some facts
- The Allen-Bradley 1305 AC drive
had approximately 70 parameters - The AB PowerFlex70 has
approximately 400 parameters
What are encoders?
Encoders are devices which are
coupled to the shaft of a motor (DC or
AC) and generate a series of square
waves proportional to rotational
speed
Encoders typically offer some
variation of a binary count – 512
square waves per 360 degrees
rotation, 1024 and so on
How does an encoder detect direction?
They use “windows” which are mechanically offset by 90 degrees. If the motor is going CCW, light will pass through from LED A before LED B
How many wires does an encoder use? What are they?
5 wires
A, ANOT - 180 degrees out of phase with A
B, BNOT - 180 out of phase with B
Z - homing shaft
VDC
DC COMMON
What speed references do most AC drives allow to be used?
-Built in potentiometer
- External 0 to 10v DC
- External 4-20 mA signal
- Network Command (Ethernet)
- Motor Operated Potentiometer
- Pulse Train
- Preset Frequencies
Analog References examples?
DC voltage or Current
What do majority of conveyor controlled by AC drives use for speed control?
Preset Frequencies
Where are Motor Operated Potentiometers used?
Common in industries where a film like substance (paper, steel, aluminum foil etc) is pulled through motor driven rollers
Offers “Speed Up” and “Slow Down” buttons that change drive
frequency
What can parameter P038 do?
Set which speed reference to use
What can parameter A051 do?
determines what function
digital in 1 causes the drive to do
It can be set to the value shown to
accomplish a different purpose:
1 Follow Accel 2 and Decel 2 curve
2 Use an external hardwired Jog
3 Cause an Aux fault (fault the drive
on purpose)
4 Run at a preset frequency
6 Use Comm port data (Ethernet,
DeviceNet, ControlNet)
7 Digital in 1 will clear a fault
8 Causes a Ramp to Stop
9 Causes a Coast to Stop
10 Causes a DC braking voltage to be
applied to the motor
How does AC drives control speed of AC Motor?
Ac drives vary the frequency applied to a 3 phase AC Motor - Vary the frequency vary the speed
Must also vary voltage or the motor will be destroyed
