Motor Control Flashcards
In which circuit are the pilot devices?
Control circuit.
What are two controller designations?
National electrical manufacturing association. And international electrotechinical commission.
What are the different setups for contractors?
Separate enclosures, combination starter, and an mcc.
Latter two via control txr
What are the required functions of motor control?
1) safety
2) starting
3) stopping
4) jogging or inching
5) speed control
6) reversing
7) surge protection
8) motor circuit protection via overloads.
What is nema’s range?
What about iec?
00 to 8 (which is 900hp)
A to z (which is 671.4 kw).
In a 2 wire, what kind of contact causes run? Also what are its characteristics?
A maintained contact causes it to run.
It has low voltage release, and auto restart.
What does low voltage release mean vs low voltage protection?
Low voltage release means there is only a release from load when there’s no power (ie when powers back, load turns on automatically).
Low voltage protection means it doesn’t turn on automatically.
How does a low voltage protection work in 3 wire control?
A momentary contact starts circuit. Power loss cuts the circuit.
What is a good way of seeing if a schematic symbol is normally open or closed?
Use gravity.
Limit sw symbol.
Triangular.
Pressure switch symbol.
Upside down D.
Do overload heating elements ever open?
No. They open ol contacts in controller.
Where to hook up alarm light to overload?
parallel to entire control cct through no ol contactsz.
All pilot devices require what?
Variable acting upon it.
How are M coil contacts linked to power circuit?
Via one armature movement. The 3 power contacts and the no contact and the nc contact of the m coil. All 5 adjoined via one armature movement.
Describe forward and reverse controller connections.
In forward controller, normal connections.
In reverse, bottom of c connects to bottom of reverse controller contact to the top of a (by top and below I mean above and below contacts of the forward controller).
How to get more contacts?
Add another relay aka aux contacts.
If there isn’t a control transformer stepping down voltage, then the coil must be…
Rated for line voltage.
In cold settings, which timer’s settings are changed? And which way?
Hydraulic. Time is extended. Oil
Moves slower.
Arrow up is TDOE or TDOD
Time to energize; on delay.
Plugging has what that other braking systems don’t.
Highest stopping force.
Describe plugging, in ac and dc.
Stator field is reversed in ac. Frequency doubles in rotor. (Remember rpm is proportional to freq).
Voltage goes up in rotor (due to field velocity difference being high as it’s so fast it’s going opposite direction).
In dc, the armature current is reversed.
What are the downsides of plugging?
-Stresses the bearings
-stresses the mechanical connections
-winding insulation is stressed
-current excess in rotor and stator gives heat
Why is a zero speed switch better than a timer for plugging?
Timer might reverse direction. Zero speed interlocks until rpm is zero and stops.
Anti plugging can be done with a timer; how is time determined?
According to how long it takes motor to coast to a stop.
But if the load takes longer to coast, it can cause plugging.
Plugging is reversing the motor as it’s running forward in order to stop it; t or f.
T. It’s done to stop it.
Out of the rvs, only 2 can be open. Which ones?
Auto txr.
Or wye/delta.
Primary r, z, and solid state soft and vfd are all closed.
Zero speed switch, in the Reverse contact that’s designated, does it open or close when running reverse if it’s connected to forward coil and we want to plug?
It’ll be open in reverse so forward can’t be energized when it’s running in reverse (aka it’ll be a normally closed).
Normally open will provide plugging as it’ll close when that direction that is designated by the contact is active or energized.
Of rvs or fvs, which is
Cheaper, simple, higher starting torque (maximum actually), higher in rush, higher line disturbance, and torque is not sensitive to the load?
Fvs.
Starting torque is ALWAYS equal to what in terms of voltage change?
( new voltage/ old or full voltage) times the starting torque at 100%.
Or (voltage ratio squared) times (starting torque at 100)
Line disturbance is increased with…
More open transitions.
Kva left side of the auto equals the right side.
T.
In a eye delta, 2s has resistors to avoid…
A bolted short with 2 m.
What are benefits of a vfd?
Has speed control,
Ramp starting,
Ramp stopping,
Constant torque,
And constant hp
Does soft start have speed control?
No. Only voltage control.
Describe dc induction braking in ac.
A bridge rectifier receives ac. The dc output goes to stator windings. The stator field is now static. The rotor slows as immense ct is developed.
What’s the difference between dynamic or dc induction braking and plugging?
Plugging reverses the stator field. Dynamic makes it static.
Why use a larger vfd?
Runs cooler.
A single phase vfd, vs a 3 phase, gives how much Hp.
50% of the hp due to source.
Describe a vfd constant torque and hp design.
Torque will be the same at all times as long as voltage to hertz ratio is the same. The vfd keeps it the same. Until max voltage. Then, if freq is increased, torque decreases but hp stays constant as the speed goes up but torque goes down in proportion, meaning hp stays constant as per the formula.
Constant torque is good for…
Hoisting, gears, pulley
Constant horsepower is good for…
Windings, lathes, rotary cutting machines
What is carying both thehp and the torque good for
Fans and pumps.
What is hp vs torque
Horsepower is speed of work.
Torque is force of the twist.
What can a soft start not provide?
Speed control
Variable voltage inverter. Describe.
Works with all motors. It’s a vfd. In dc link, a capacitor then helps smooth out the voltage output from being jagged. Then, dc output after the igbt’s is sinusoidal.
(Igbt give speed control via firing angle btw; that’s why soft start doesn’t have speed control)
A csi. Describe.
Current source inverter.
No capacitor. A large inductor in dc link opposes excess current. (CHOKE).
Only works with large size motors.
In pulse width modulation, greater interval distance means what about frequency
Larger interval means less frequency.
Cogging is..
Jagged movement of motor. Happens at low rpm and low frequency.
How is a pwm wave varied?
Igbt on means current up. Off means down. The current wave is then made.
Where is the fan on a motor?
On the rotor. An auxiliary is attached if the rotor fan spills too slow to cool channels.
