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Unformatted text preview: un/Start (Permanent Split Capacitor, PSC)
• Shaded Pole Fractional hp (up to ½ hp)
Very small motors (up to 1/10 hp) ASIDE: Reminder (?)…
Power Consumption in Alternating Current (AC)
Electrical Systems
AC I “line” (“hot”) 120 V
60 hz
1) Single Phase AC Power: V load = * ∙ +#! ∙ #! “neutral” single
phase volts
watts amps “power factor”
(dimensionless) = cos / “rms” = rootmeansquared “Power Triangle”
Apparent Power
(VA)
Reactive
Power
(VAr) /
Active Power
(W) = * ∙ +#! ∙ cos /
#! = 1 for purely resistive load
< 1 when current is lagging
or leading voltage Example: Device connected to household power
(singlephase AC) “hot”
watt
meter power device “neutral” Readings: = * ∙ +#! ∙ #! 114 volts
7.4 amps
pf = 0.77 (114 V) ∙ (7.4 A) ∙ (0.77)
= 650 watts 650 watts
Note: “Apparent Power”
(114 V) ∙ (7.4 A) = 844 watts 3Phase AC Power A
C
B LA
3phase
motor LB
LC
“linetoline voltages” *010,#! = *34 , *35 , *45 Electrician’s
Measurements…
LA
LB
LC *010,#! = *34 = *35 = *45 “line currents”
+0,#! = +3 , +4 , +5 balanced?
balanced? 3Phase AC Power = 3 ∙ *010,#! ∙ +0,#! ∙ volts
watts amps = 3 ∙ * ∙ + ∙ (End of aside.) AC INDUCTION MOTORS
• An induction motor is an AC motor where power is
supplied to the rotating device via electromagnetic
induction.
• In general, the rotor lies within a magnetic field created
by the stator. An electric current is induced within the
rotor, and the resulting force causes the rotor to turn. • The speed at which the magnetic field rotates
depends on the number of magnetic poles in the
stator, and is referred to as the “synchronous speed”
Synchronous
Speed = At 60 Hz AC power: 120 × frequency
Number of Poles Poles
2
4
6
8 frequency of AC
power supply
(e.g. 60 Hz) Synchronous Speed (rpm)
3600
1800
1200
900 • The rotor of an induction motor usually does not rotate
at synchronous speed, but lags it slightly. This lag is
usually expressed as a percentage of the synchronous
speed, called the “slip”.
% Slip = Synchronous Speed − Rotor Speed × 100% Synchronous Speed
• The “rated speed” is the nominal speed of the rotor
when operating at full load.
e.g. A motor with synchronous speed of 1800 rpm may
operate with rated speed of 1750 rpm (i.e. magnetic field
spins at 1800 rpm, rotor spins at 1750 rpm.) • In general, the amount of slip is approximately
proportional to the load imposed upon the motor by
the driven equipment:
100% Percent of
FullLoad
Slip 50% 0%
0% 50% No Load 100%
Full Load Load Typical Range of FullLoad Efficiency Values
by Motor Type:
• Squirrel Cage ~ 75 – 95% • PSC ~ 40 – 70% • Shaded Pole ~ 20 – 35% For each category, the fullload efficiency generally will
increase with motor hp. Approximate Range of Full Load Efficiency Values for
Squirrel Cage Motors (3 phase, induction motors)
100%
95%
90% Full
85%
Load
Efficiency
80%
75%
70%
65%
1 1.5 2 3 5 7.5 10 15 Motor Rated hp 20 25 30 40 50 Efficiency at partload can be different than at fullload:
Approximate Variation in Motor PartLoad Efficiency
for Induction Motors % of
FullLoad
Efficiency Motor Load (% of Full Load) Usually, motors in HVAC systems do not run at 100% load (fullload)—that is, the brake power required by the driven device
(e.g. pump, fan) is less than the motor capacity.
For Example
Say a centrifugal pump requires ∼32 bhp from an 1800 rpm
motor to operate at the required flow and head. The motor
selected to drive the pump would likely be 40 hp (i.e. the
nearest size that can provide at least 32 hp).
boiler Motor
power
supply Pump flow
95 L/s
204 kPa heat in heat out heating
loads Centrifugal Pumps
• commonly used in
HVAC applications Pump Performance Curve (for a particular pump) 95 L/s,
204 kPa Note: 204 kPa ≈ 68 feet of H20 (head) Say available motors are as follows:
Rated hp
20
25
30
40
50 6789:;
92%
93%
93%
94%
95% Recall…
Estimating pressure drop
(head loss) through a
section of piping:
flow mean fluid velocity pipe “friction factor”
length of pipe section head
loss ∆ =
+ 2 “fitting loss coefficients” gravitational constant pipe diameter 1 ∆ = + 2 ∆ ∝ Head loss through the pipe section is approx
proportional to flow velocity squared Also, volumetric flowrate ()
is proportional to mean fluid
velocity (): =∙ crosssectional area
(for fluid flow) If we consider the bracketed
term as fixed/constant for a
particular piping section (i.e.
once it has been built), then So, head loss through a section
of pipe varies approx with
volumetric flow () squared: ∆ ∝ “system curve” ∆ Centrifugal Pump Performance Curve
For a particular impeller size and
rotational speed (rpm).
pump curve ∆ Operating Point: At intersection of the two curves operating point ∆ Building Energy Performance – Spring 2012  Topic 14  “Coefficient of Performance” Refrigeration System (or Heat Pump):
Used to move heat from a colder environment
to a warmer environment
colder
environment cooling warmer
environment Refrigeratio...
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 Spring '12
 DavidMather

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