Assignment 5
What happens to the speed of an AC universal motor as the load is decreased?
Speed Increases
What is the primary difference between a start-type and run-type capacitor?
Start-type is Electrolytic and the Run-type is oiled filled
When creating a magnetic field, do the lines of flux flow from south to north across the
air gap?
No flux lines do not flow they are directional leaves the north enters the south
What is the rotational force produced by a motor called?
Torque
What type of AC machine converts electrical input to mechanical output?
Motors
In an AC universal motor, are torque and speed directly or inversely proportional to each
other?
Inversely Proportional
What approximate percentage of rated speed does a centrifugal switch open at?
75% – 80%
If the centrifugal switch fails to open as a capacitor-start induction-run motor accelerates to its rated speed, what could happen to the starting winding?
Over heat and burn out
In reference to the left-hand rule for conductors, what do the fingers and thumb represent?
Thumb - Direction of current flow
Fingers - Direction of the magnetic field around the conductor
In reference to the left-hand rule for electromagnets, what do the fingers and thumb represent?
Thumb - Points to the direction of the north pole
Fingers - Direction of current flow
What is a measure of a material’s ability to become magnetized called?
Permeability
What component of an AC machine supports the shaft and allows it to rotate freely?
Bearings
Does the stator of an AC machine produce a stationary magnetic field or an alternating
magnetic field?
Alternating Magnetic Field
What are the terminal markings for start winding leads in a 6-lead split-phase induction
motor?
T5 – T8
In a capacitance-start induction-run split-phase induction motor, start field windings are wound with ________ turns of
_______ gauge wire relative to the run field windings
Few, small
To reverse a 4-lead permanent-split capacitor motor, which wiring connections are
interchanged?
T5 – T8
List three types of power losses that occur in split-phase induction motors.
1. Mechanical - windage and friction (heat) losses
2. Electrical - Conductor resistance I2R losses
3. Magnetic - Core losses eddy currents and hysteresis
Does the counter-emf generated increase or decrease as load is added to an AC series-
universal motor?
CEMF Increases
What type of 1∅ AC motor has the highest starting torque characteristics?
AC Universal Motor
A rotating magnetic field is produced in an AC motor by “splitting” the phase through
two separate field windings. What type of motor uses this principle?
Resistive Start Induction Run
Capacitor Start Induction Run
Permanent Split Capacitor
Capacitor Start Capacitor Run
Why are pole pieces constructed of laminated sheets?
Reduce the amount of Eddy Currents
What are the brushes in an AC universal motor made of?
Carbon Graphite Brushes
What is the minimum number of terminal leads required for connection of a dual-voltage
115/230V resistance-start induction-run motor?
6 Lead
In an AC universal motor, when proper commutation is taking place, what color is the
commutator?
Dark Chocolate Brown Color
In a permanent magnet motor, where are magnetic flux lines most densely concentrated?
At the poles
What type of capacitor does a permanent-split capacitor motor use?
Oil Filled Capacitor Type
What type of capacitor does a capacitor-start induction run motor use?
Electrolytic Type Capacitor
What are two factors that determine the amount of torque developed by a motor load?
1. Strength of the magnetic fields
2. Amount of current flow
What is the portion of the stator pole piece is closest to the rotor assembly called?
Pole Shoe
How is the capacitor in a capacitor-start induction-run split-phase motor connected?
Connected in Series with the Start winding and the Centrifugal Switch
What solid-state device can be used to disconnect the start winding in a resistance-start
induction-run split-phase motor?
Thermistor
Does a current relay use normally open or normally closed contacts connected in series
with the start winding?
Normally open contacts
What type of motor has a short-circuited turn of heavy copper wire wound around part of
the stator pole?
Shaded Pole Motor
When an AC motor is first started, why is the in-rush current so high?
Rotor current is only limited by the very low rotor circuit resistance
What are two advantages of a resistance-start induction-run motor?
1. Cheaper, No Capacitors, Low running Torque, Low starting Torque
2. Can be started frequently, Constant Fixed Speed
What are two disadvantages of a resistance-start induction-run motor?
1. Limited to 1/3 Hp, Low Starting Torque
2. Noisy, High starting surge current, Needs Starting Circuit
What are two advantages of a capacitor-start induction-run motor?
1. Less inrush current at start up, Higher Starting torque
2. Constant Fixed Speed, Can be sized up to 3 Hp, Good for heavy starting loads
What are two disadvantages of a capacitor-start induction-run motor?
1. Can’t be frequently started,
2. Low running Torque, More Expensive then resistive start induction run
What are two advantages of a permanent-split capacitor motor?
1. No Starting Switch, Good Running Torque
2. Less running Current, Lower starting current, good for high cycle loads
What are two disadvantages of a permanent-split capacitor motor?
1. Lower starting torque, Cost due to oil filled capacitor
2. Higher Hp ratings, Lower break down torque
What are two advantages of a capacitor-start capacitor-run motor?
1. Good starting and run torque, lower starting and running current
2. Has higher Hp ratings, better efficiency
What are two disadvantages of a capacitor-start capacitor-run motor?
1. Requires Two Capacitors, larger size, Higher cost
2. Requires starting switch
A 4 HP, two-pole, single-phase, split-phase induction motor has a nameplate rating of
3420 RPM on a 60 Hz system. The motor has a running speed of 3528 RPM under a no-load condition. Calculate the speed regulation of this motor.
% regulation = ((No-load speed - full-load speed) / full-load speed) x 100
% regulation = ((3528 RPM – 3420 RPM) / 3420 RPM) x 100
% regulation = 3.158 %
Calculate the rotor speed of a 4 HP, 230V, 1∅, six pole, continuous duty split-phase induction motor operating at 3% slip with 80% power factor and 80% efficiency connected to a 60 hertz source.
Rotor speed = syn speed - %slip
Syn speed = 120 x hz / # of poles
Syn speed = 120 X 60 / 6
Syn speed = 1200 rpm
Slip = 1200 rpm X .03 = 36 rpm
Rotor speed = 1200 - 36 rpm = 1164 rpms