Assignment 2
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 type of single-phase motor does not use a squirrel-cage rotor assembly?
Universal uses an armature type of assembly
List two factors that determine the amount of torque developed by a motor?
1. Strength of the magnetic fields
2. Amount of current flow
Why are pole pieces laminated?
Pole pieces are laminated to reduce iron losses (eddie currents)
What type of single-phase motor is typically not reversible?
Shaded-pole induction motor
In an AC universal motor, are torque and speed directly or inversely proportional to each
other?
Inversely proportional (speed decreases torque will increase)
If the number of poles in a shaded-pole motor is doubled, does the speed of the motor
increase or decrease?
Double the number of poles will decrease the motor speed by half.
Syn speed = (120 X F) / (# of poles)
What is the typical phase displacement angle in a shaded-pole motor?
10° phase angle displacement created by the shading coil
What happens to the torque of an AC universal motor as the load is increased?
Torque is increased
The amount of CEMF produced in a rotor assembly is proportional to what three factors?
1. The amount of conductor
2. The Magnetic field strength
3. The speed of rotation
Is the shaded-pole motor an induction motor?
Yes
Why are pole pieces constructed with silicon-steel?
Reduce Hysteresis Losses
In universal motors, are the armature and field coils connected in series or parallel?
Series
Which single-phase motor is considered to be the most simple and least expensive?
Shaded Pole induction motor
What is the formula for synchronous speed?
Syn speed = (120 x f) / (# of poles)
What is the formula for rotor speed?
Rotor speed = Syn speed – Slip Speed
How should the direction of rotation of an AC universal motor be reversed?
Change the armature leads with respect to the field leads.
Does a shaded-pole induction motor rotate in the direction towards the shading coil
potion of each pole piece or away from the shading coil portion of each pole piece?
Towards the shading coil
Does the counter-emf generated increase or decrease as load is added to an AC series-
universal motor?
Counter-emf increases
List three applications of shaded-pole motors.
1. Fans, blowers, timers
2. Small auxiliary pumps, small appliances
3. Toys, computers, recorders
List three applications of AC universal motors.
1. Vacuum cleaners, power tools, mixers, drills
2. Sewing machines, razors
3. Appliances
List three types of power losses that occur in AC machines.
1. Mechanical - windage and friction (heat) losses
2. Electrical - Conductor resistance I2R losses
3. Magnetic - Core losses eddy currents and hysteresis
Calculate the synchronous speed of a four-pole AC machine operating at 60 Hz
frequency.
Syn speed = 120 X hz / # of poles
Syn speed = 120 X 60 / 4
Syn speed = 1800 rpm
Calculate the rotor speed of a six-pole AC machine operating at 60 Hz with a 5% slip.
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 .05 = 60 rpm
Rotor speed = 1200 - 60 rpm = 1140 rpms
A motor operating at 60 Hz has a nameplate rotational speed of 3450 rpm. Calculate the
number of poles the motor has and the percent slip at full-load condition.
Syn speed = 120 x F / # of poles
# of poles = 120 X 60 Hz / 3600 RPM
# of poles = 7200 / 3600 RPM
# of poles = 2
Slip Speed = Syn Speed – Rotor Speed
Slip Speed = 3600 RPM – 3450 RPM
Slip Speed = 150 RPM
% Slip = (Slip Speed / Syn Speed) X 100
% Slip = (150 RPM / 3600 RPM) X 100
% Slip = 4.167 %
A four-pole, single-phase motor has a nameplate rating of 1720 RPM on a 60 Hz system.
The motor has a running speed of 1755 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 = ((1755 RPM – 1720 RPM) / 1720 RPM) x 100
% regulation = 2.035 %
Shaded-pole motor components
1. Squirrel-Cage rotor assembly
2. Rotor Bars
3. Iron Core
4. Shaft
5. Shading Coil
6. Pole Piece
7. Main field windings
Single-phase universal motor components
1. Bell Housing (End Plate)
2. Terminal Box
3. Commutator
4. Armature Winding
5. Iron Core
6. Main Pole Piece
7. Field Winding
8. Bearings
9. Shaft
10. Cooling Fan
11. Enclosure
12. Base
13. Brushes