Single- Phase AC Motors

8/9/2019 Single- Phase AC Motors

1/40

Single- Phase AC Motors


2/40

Single Phase Motors are the most common of

all motors.


3/40

A single phase induction motor is composed

of a Squirrel-Cage rotor and a Stator.


4/40


5/40

The following Slides show the progressive

steps in winding a 4- pole, 36 slot stator.


6/40

Starting with the laminated iron stator, paper

insulators- called Slot Liners are first inserted

in the slots.

The Squirrel-

Cage Rotor is

Identical to

that of a 3-

phase motor.


7/40


8/40

Next, the Auxiliary winding is embedded so that its

poles straddle those of the main winding.


9/40

Each pole of the main winding consists of a group of

four concentric coils, connected in series.

Adjacent poles are connected so as to produce alternate

North/ South Polarities.


10/40

The empty slot in the center of each pole- shown as

a vertical line- and the partially filed slots on eitherside of it are used to lodge the auxiliary winding.

The latter has only 2 concentric coils per pole.


11/40

The Synchronous speed of all single

phase induction motors Equation: ns=

Where: NS= Synchronous Speed [rpm] = Frequency of the Source [Hz]

P = # of Poles

120

P


12/40

Synchronous Speed: the speed at which analternating-current machine must operate to

generate electromotive force at a given

frequency.


13/40

The rotor turns at slightly less thansynchronous speed. The full-load slip is

typically 3 percent to 5 percent in fractional

HP motors.


14/40

Slip is given by:

S= *100%

Where: S= Slip

Ns= Synchronous Speed

n= Motor Speed

(ns-n)

ns


15/40

T

hus, motor speed may be found by:

n=Ns(1-s/100%)

Where: n= motor speed

Ns= Synchronous Speed

s= slip


16/40

Problem: Calculate the speed of the 4 pole

single phase motor if the slip at full load is3.4%, and the frequency is 60 Hz.


17/40

Torque Speed Characteristics

Under locked rotor conditions, when a voltage

is applied to the stator, a large current

develops in the rotor which acts like the

shorted secondary of a transformer.

Consequently, the motor has no tendency to

start by itself.


18/40


19/40

The motor develops a torque as soon as itstarts to spin. The torque increases as the

motor nears synchronous speed, as shown in

the Torque-Speed Curve.


20/40

Principle of Operation

Due to the inductance of the rotor, the flux inthe rotor lags the flux in the stator by 90.


21/40

Principle of Operation

The rotor flux increases with the motor speed

and is nearly equal to the stator flux when the

speed is nearly synchronous speed. The

combination of the two fluxes produces a

revolving field that rotates at synchronous

speed.


22/40

Locked-Rotor Torque

To produce a starting torque in a single-phasemotor, a revolving field must be created.


23/40

Locked-Rotor Torque

This is done by adding an auxiliary winding

that is out of phase with the main winding. Animpedance is placed in series with the

auxiliary winding.

The Impedance may be resistive, capacitive or

inductive and determines the type of split-phase motor.


24/40

Locked-Rotor Torque

A speed-sensitive centrifugal switch is also

connected in series with the auxiliary winding.

When the motor reaches 75% of its

synchronous speed, the switch opens and the

auxiliary winding is no longer energized.


25/40


26/40

The main winding of a single phase motor is

made of large wire and a large number of

turns. This causes the main winding (stator)

current to lag the source voltage.


27/40

The auxiliary winding of a resistance split

phase motor- most commonly split phase

motor- is a small wire and a small number of

turns. This causes the auxiliary windingcurrent to be in phase with the source voltage.


28/40

Thus the main winding current lags the

auxiliary winding current by 25 and the phase

shift between the two generates the motors

starting torque.


29/40

The starting period should not last longer thanfive seconds- to prevent the auxiliary winding

from melting under the starting current- six to

seven times the full load current.


30/40

The low cost makes resistance split-phaseinduction motors the most popular single-

phase motors. They provide moderate torque

for infrequent starts. They are 1/3 hp to hp

motors that drive fans, pumps, small tools and

other residential machines.


31/40

Capacitor Start Motor

The design of the capacitor start motor is the

same as that of the split phase motor, except

that the auxiliary winding has about as many

turns of wire as the main winding and a

capacitor and a centrifugal switch are in series

with the auxiliary winding.


32/40


33/40


The capacitor is chosen so that the current in

the auxiliary winding leads the current in the

main winding by 80. The larger phase shift

between the two reduces the starting current

in the auxiliary winding to half that in a split

phase motor with the same starting torque.


34/40


The design allows for a high starting torque

with a lower starting current- four to five

times the full load current. There is less

heating in the auxiliary winding allowing it to

be energized more often- frequent starting.


35/40


Once the centrifugal switch of the capacitorstart motor opens it behaves the same as the

running split phase motor, because the main

windings are identical.


36/40


Capacitor-start motors are used when high

starting torque is required. They are built in

sizes ranging from 1/6 hp to 10 hp. Typical

applications are compressors, large fans,

pumps and other high inertia loads.


37/40


Electrolytic Capacitors are used in capacitorstar motor.


38/40

Efficiency and Power Factor of Single

Phase Induction Motors

The efficiency of fraction horsepower singlephase AC motors is typically low due to losses

1 hp or larger during starting. It improves to

80% for motor.


39/40

Vibration

There is greater vibration in the single phase

AC motor than there is in the three phase AC

motor due to the pulsating power supplied to

the AC motor while it produces constant

power.


40/40

Vibrations

Rubber rings at the motors end bells are used

to reduce vibrations.

Single- Phase AC Motors

Documents

Transcript of Single- Phase AC Motors