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2008 Ag Tech Conference
Electric Motors
Problem Solving Examples
Fonnulas:
Low Voltage
E=IR
Slip Percent.= SmRfM·- ActRPM x 100
Synchronous Speed
wattsoc
=
v x
A
746 Wattslhp
R(t)-
·,
=_
_,_...:=:.1_ _
1
+ l
R(l)
R(Z)
High Voltage R(t) ::::i.R(l) +R(2)
\
II
II
I
Watt.SAC
=
V x AxPF
746 Wattslhp
Torque (lbft) =
H.P.
x ~ 5252 and liLP. = 550 ft/sec.
RPM
Size X RPM = Size X RPM
__ The following dia~ show motor belt a¢ gear drives. netennine the sheave and gear.
speeds from. the supplied infonnation
What is the RPM. of sheave C?_ _ _ __
7~
T
W)lat is the RPM of gear D?_ _ _ __
.,. ':.J
.,.
.··'".
t. Ollowmg questions.
Below is a dual volhlfte iudu,...;· ·
~
fl"-.vu. tun ~r ~for low volta~ (115 V) at A and high voltage
(23() V) at B.
.
..
·
A
B
·€>·
~
Run2
· Assume thit· each set of numiDg winctiDgs bas. a resiStance of 16 olmis.
l.What is the
voltage?
toUat resistance of~ lUDDing winding$ on tho motor~ for low
·
·
2. What is the total resistance of the nmniDg ~ on the motor wired for high
vo~~----------3. How Jilany amps of cuueat draw does the low voltage motor have? ---~4. How many amps of cun:eut draw does the bigb voltage motOr have? _...;.__ __
S. Which1110t0f (lOw YOJtage) or (high vobge) will use~ ddlliLS~.&~b!b~'J~?P_.....__ __
.
·
·
_ o.wrrs
You purchased and elec:tPc motor at an auction but it has no nameplate. The size of the shaft ·
is 7/16 jDCh BDd will fit your appJication. 1be motor is wired for 120 VC)Its and the ampere
reading is 3.2 under load. The motor has a power factor of .S. What is the horsepower of the
(
mototl
A.
"3/4hp
c.
l/3.hp
B.
lhp
D.
1/4 bp
EXTRA CREDIT
37.
The recommended .fuse size for a small motor iS _ _ _ times the nameplate
current.
a. 2.0-b. 1.25
38.
Which of the following types ofmotors would you expect to find 011 a portable power
drill?
a. repulsion start-induction run.
b. universal
39.
c. 0.8
d. 0.6
c. split phase
d. shaded ~Ie
When making mnning winding connections to provide for motor operation on 120
volts instead of240 vol~ ____
a. nmning windings should be in parallel
b.- mnning windings should be in series
c. "R" windings j.n series and "R" winding in series with starting winding
_d. running windings and starting windings must be connec,Ud opposite ways
40.
A capacitor start, induction run motor:
a. has a capacitor to help start the motor but not when running
b. has a capacitor to help start and run the motor
c. bas a capacitor to -run the motor
d. none of the above
41.
The motor..,...--- consists of a laminated steel co~ that holds coils of copper wire
in place.
a. annature
b. tUrbine
42.
c. stator
d. terminal board
Squirrel cage is a term used to describe _ __
a. theshaft
b. theframe
c. the windings
d. therotor
43.
The term impedance when refetring to electric motors is defined as:
a." total ppposition to current flow in ~ circtiit
b.
olltJ>!it ofa motor · .
power
of
c. the property an electrical circuit that opposes any change in current flow
d. cur.rent in a coil
44.
Universal motors can operate on _ _ _ current
a. AC
·c. ACorDC
b. DC
45.
d. neither AC or DC
motors are used to power clocks and timms due to their precise speed.
~
Universal
b. Synchronous
46.
b.. repulsOr
47.
·. c. capacitor start
d. capacitor start·induction run
type of motor enclosures have no ventilation openings but are not airtight or
c. Explosion proof
d. Drip proof ·
Motors are best protected from burnout by-----'
a. circuit breakers
b. fuse
SO.
c. sensor
d. voltage reducer
waterproof.
a. Splash proof
b~ Totally enclosed
49.
uP to speed is a _ __
This style ofmotor does not haVe a starting ·winding•
. ·a. shaded pole
b. split phase
48.
d. Capacitor·start
The_ device that disconnects a cap~itor after a motor is
. a. · cent:rifugal switch
C_
c. Split phase
c. overload protection
d. fusetron
Unless discharged after power is turned off, a serious electric shock can occur from a
a. transistor
b. rheostat
c.. capacitor
d. limit switc~