AC Motor Speed. n s = synchronous speed (in RPM), f = frequency (in Hz), and p = the number of poles

(1)

AC Induction Motors

Simplest and most rugged electric motor _{Simplest and most rugged electric motor}

Consists of _{Consists of}wound statorwound stator and rotor assemblyand rotor assembly

AC in the primary member (stator)_{AC in the primary member (stator)} inducesinduces current in the secondary member (rotor) current in the secondary member (rotor)

Combined electromagnetic effects of the _{Combined electromagnetic effects of the} stator and rotor currents produce the force stator and rotor currents produce the force

(torque) to create rotation. (torque) to create rotation.

(2)

AC Induction Motors

Rotors typically consist of a laminated,

_{Rotors typically consist of a laminated,}

cylindrical iron core with slots for

receiving the conductors.

Common type of rotor has cast

_{Common type of rotor has cast}

-

-aluminum conductors and short

aluminum conductors and short

-

-circuiting end rings.

circuiting end rings.

This "squirrel cage" rotates when the

_{This "squirrel cage" rotates when the}

moving magnetic field induces a current

in the shorted conductors.

(3)

AC Motor Speed

The magnetic field rotates at the

_{The magnetic field rotates at the}

synchronous speed

of the motor

Determined by the number of poles in

_{Determined by the number of poles in}

the stator and the frequency of the AC

power

n

n_s_s = synchronous speed (in RPM), = synchronous speed (in RPM),

f

f = frequency (in Hz), and = frequency (in Hz), and

p

p = the number of poles= the number of poles

p

f

n

_s

=

120

(4)

AC Motor Speed

Synchronous speed is the absolute upper limit _{Synchronous speed is the absolute upper limit} of motor speed.

of motor speed.

When running, the rotor always rotates slower _{When running, the rotor always rotates slower} than the magnetic field (or no torque!)

than the magnetic field (or no torque!)

The speed difference, or _{The speed difference, or}slipslip, is normally , is normally referred to as a % of synchronous speed: referred to as a % of synchronous speed:

s

s = slip (in %), = slip (in %),

n

n_s_s = synchronous speed= synchronous speed

n

n_a_a = actual speed= actual speed

s a s

n

s

= 100

−

(5)

AC Motor

-

Speed vs. Torque

0 25 50 75 100 % Rated Torque % Synchronous Speed 100

Locked rotor/starting torque Minimum

acceleration torque

Service factor load / torque

Dashed line applicable to initial acceleration only

(6)

Single

-

phase AC Motors

Single phase AC motors require a "trick"

_{Single phase AC motors require a "trick"}

to generate a 2nd "phase" to develop

starting torque

Three common methods:

_{Three common methods:}

–

– splitsplit--phase (auxiliary winding is rotated phase (auxiliary winding is rotated 90

90°°)) –

– capacitorcapacitor –

(7)

Split

-

Phase AC Motor

0 100 200 300 0 25 50 75 100 % Synchronous Speed % Rated Torque switch opens 0 25 50 75 100 0 100 200 300 % Rated Torque % Synchronous Speed switch opens

Motor starts with both main and auxiliary winding A centrifugal switch opens and removes the aux winding

(8)

Disadvantages Disadvantages

Repeated start/stop _{Repeated start/stop} cycles heat the

cycles heat the

windings (high start windings (high start

resistance) resistance)

_{Less useful for large}_{Less useful for large}

inertial loads inertial loads

_{Requires large wiring}_{Requires large wiring}

to handle starting to handle starting currents currents Advantages Advantages

Split

-

Phase AC Motor

Operate at ~ constant _{Operate at ~ constant} speed, 4 pole, 60 Hz: speed, 4 pole, 60 Hz:

–

– 1780 RPM (no load)1780 RPM (no load) –

– 1700/1725 RPM at full 1700/1725 RPM at full load

load

Reversible at low _{Reversible at low} speed

speed

Rapid acceleration_{Rapid acceleration}

(9)

Single

-

Phase Capacitor Motors

AC Main_winding _Rotor

Auxiliary winding Capacitor

Permanent split capacitor (PSC)

_{Permanent split capacitor (PSC)}

Capacitor

_Capacitor

-

start (later switched out)

(10)

Permanent Split Capacitor (PSC)

0 100 200 300 0 25 50 75 100 % Synchronous Speed % Rated Torque 0 25 50 75 100 0 100 200 300 % Rated Torque % Synchronous Speed

(11)

More expensive for _{More expensive for} same HP

same HP

Lower performance _{Lower performance} when starting

when starting

Need to always use _{Need to always use} manufacturer's manufacturer's desired capacitor desired capacitor value value Advantages Advantages

Permanent Split Capacitor (PSC)

Quieter, smoother _{Quieter, smoother} than split phase

than split phase

Reduced starting _{Reduced starting} current

current

–

– Longer lifeLonger life –

– Higher reliabilityHigher reliability

Capable of frequent _{Capable of frequent} start/stop cycles

(12)

Shaded Pole AC Motor

0 100 200 300 0 25 50 75 100 % Synchronous Speed % Rated Torque 0 25 50 75 100 0 100 200 300 % Rated Torque % Synchronous Speed

(13)

Low starting and _{Low starting and} running torque running torque

Low efficiency_{Low efficiency}

Available in sub_{Available in sub}-

-fractional to ~ 1/4 hp fractional to ~ 1/4 hp sizes sizes Advantages Advantages

Shaded Pole AC Motor

Simple in design and _{Simple in design and} construction

construction

Suitable for low cost, _{Suitable for low cost,} high volume app's

high volume app's

Relatively quiet and _{Relatively quiet and} free from vibration free from vibration

"Fail safe" design _{"Fail safe" design}- -starts in only 1

starts in only 1 direction

(14)

NEMA

-

National Electrical

Manufacturers Association

NEMA is responsible for several electric

motor industry "standards"

Motor ratings (1/4 hp, 1/2 hp, 1 hp, etc.)

_{Motor ratings (1/4 hp, 1/2 hp, 1 hp, etc.)}

Frame size

_{Frame size}

–

– diameter, length, shaft size, etc.diameter, length, shaft size, etc.

Service factors

_{Service factors}

(15)

Service Factors

A multiplier applied to the rated

_{A multiplier applied to the rated}

horsepower

Indicate how much the motor can be

_{Indicate how much the motor can be}

overloaded without overheating

Generally used for

_{Generally used for}

–

– handling a known, occasional overloadhandling a known, occasional overload –

– provide a factor of safety where provide a factor of safety where

environment or service condition is not well environment or service condition is not well known

(16)

Motor Enclosures

DP

_DP

-

dripproof

DPFG

_DPFG

-

dripproof

, fully guarded

SP

_SP

-

splashproof

FV

_FV

-

forced ventilation

(separate/attached fan)

TENV

_TENV

-

totally enclosed, non

-

ventilated

TEFC

_TEFC

-

totally enclosed, fan cooled

TEUC

_TEUC

-

totally enclosed, unit cooled

(heat

(17)

AC Motor Efficiency

Efficiency,

_Efficiency,

η

= Power Output / Power

Input

Small universal motors have

_{Small universal motors have}

η

~ 30%

Large 3

_{Large 3}

-

phase motors have

η

~ 95%

Depends on actual motor load vs. rated

_{Depends on actual motor load vs. rated}

load

–

– efficiency best near rated loadefficiency best near rated load –

– efficiency drops rapidly for both underefficiency drops rapidly for both under-- and and over