UNIT 13
PHASE CONVERTERS
OBJECTIVES
After studying this unit, the student will be able to
• describe the purpose of a phase converter.
• give at least one major limitation of a phase converter.
• discuss several conditions where it is advisable to install a phase converter.
• name the two major types of phase converters.
• select a phase converter and motor to power a load.
• wire a phase converter circuit.
Phase converters are used on farms to operate 3phase electric motors from a single-phase electrical suppi y, Figure 13-1. Three-phase electric motors, particularly in larger sizes, are generally less expensive and
more maintenance free than single-phase motors of the
same horsepower rating. Besides, large-horsepower, single-phase electric motors are not readily available. A
farmer installing a grain dryer requiring a 20-hp motor
probably will have to install a 3-phase motor. Ideally,
this farm should be· supplied with 3-phase power, but the
electric utility primary lines at the farm may only be able
to supply single-phase power. Three-phase can usually
be provided, if the farmer is willing to share the cost of
extending the additional primary wire. If this cost is too
great, a phase converter may be an acceptable alternative.
262
Unit 13
Phase Converters
PHASE CONVERTER LIMITATIONS
Phase converters are not the solution to all farm electrical power problems. Some types of loads are difficult
to power with a 3-phase motor operating from a phase
converter. These are loads which require high starting
torque, such as silo unloaders, and gutter cleaners.
The phase converter limits the starting current draw
of the motor. When starting current is limited, motor
starting torque is greatly reduced. However, many phase
converter applications can be used for easy-starting fann
loads, such as fans or pumps. A grain dryer is a typical
example. Some equipment can be started empty, with
the load applied after the motor has obtained full operating speed.
Another limitation to the use of phase converters is
t
• A single-phase motor is available, but it has such a
high starting current that the electric utility will not
permit its operation because it causes objectionable
primary line voltage drop. This voltage drop causes the
lights to flicker at neighboring farms and residences
every time the motor starts. A 3-phase motor operated
from a phase converter draws less current, so that line
flicker may not be a problem.
• A single-phase electric motor for the job is not available, and a 3-phase motor is the only alternative.
• Temporary 3-phase service is required until permanent, utility-supplied 3-phase service is available.
• New equipment is provided with a 3-phase motor as an
integral part of the unit, and replacement with a singlephase motor is either too difficult or more costly than a
phase converter installation.
• Several motors are required, and the cost of 3-phase
motors plus a phase converter is less than it is for single-phase motors.
Single-phase
power
2 wires
Phase converter
disconnect
I= =:1
=
3-phase
motor starter
Figure 13-1 A phase converter provides a means of powering a 3-phase motor from a single-phase electrical supply.
that voltages between phase wires to the 3-phase electric
motor are generally not exactly balanced. Voltage unbalance generally results in slightly unbalanced currents
within the different motor windings. Slight unbalances
need not be of concern if properly sized motor running
overcurrent is provided.
TYPES OF PHASE CONVERTERS
Two types of phase converters are available: static,
and rotary. Each type offers advantages for specific appiications.
Static Phase Converter
WHEN TO CHOOSE A PHASE
CONVERTER
The static phase converter, Figure 13-2, has no moving parts. It consists of a cabinet containing capacitors
and, sometimes, an autotransformer. Static phase converters are selected for a specific size of 3-phase motor.
It is possible to operate more than one motor from a
single static phase converter, but extra capacitors are
required, as well as careful balancing of the line currents. More than one motor for a static phase converter
should not be installed unless recommended by the phase
converter manufacturer.
A simple static phase converter uses a bank of capac-
There are various reasons for choosing to install a
phase converter.
• The cost of primary line extension to provide 3-phase
power to the farm is much higher than it is for a phase
converter installation.
• The 3-phase electrical energy utility rate, plus the cost
of rebuilding the farm electrical service, is enough
higher than it is for single phase that the phase converter is the less costly solution.
Single-phase
/disconnect
Ll
I
I
L2
L3
l Capacitors
3-phase
motor
Figure 13-2 A simple static phase converter uses a bank of capacitors to develop the third phase.
Unit 13
Phase Converters
263
itors to supply one of the 3-phase motor lead wires (a
bank of capacitors consists of several capacitors connected in parallel). The other two motor lead wires are
supplied directly from the single-phase line.
Excessive line current to the motor can occur with
this type of phase converter. To minimize the risk of
damage to the motor due to excessive current in one or
more motor windings, two precautions are recommended.
• Provide properly sized fuses or thermal overload relays
in each of the three wires to prevent motor damage due
to excessive winding current.
• Load the motor to only 75% of its nameplate horsepower rating.
The proper-horsepower 3-phase motor, operated from a
capacitor-type static phase converter, can be determined
using Equation 13.1. First, the horsepower rating of the
farm equipment must be determined.
Static phase converter
Eq. 13.1
motor horsepower
Equipment horsepower required
0.75
Problem 13=1
A large poultry house ventilation fan requires a
7 1J2 hp electric motor. A 3-phase motor will be operated
from single-phase power, using a capacitor-type phase
converter. Determine the minimum recommended hors
power rating for the 3-phase motor for this applicatio
Solution
A motor operated from a capacitor-type static pha
converter should be loaded to not more than 75% of i
horsepower rating. Use Equation 13.1 to size the mote
Static phase converter
motor horsepower
7.5 hp
--= 10hp
0.75
A 10-hp, 3-phase electrical motor should be selected
power this load.
The second type of static phase converter uses :
autotransformer in addition to capacitors. The main a
vantage of this type is that the amount of capacitance m
the autotransformer voltage can be adjusted to balan
motor line currents so the motor can develop its ft
horsepower rating. As an example, a 10-hp submersib
pump may be operated at full nameplate rating, providt
proper balancing procedures are followed. Significa
variation in motor load will require rebalancing. Tl
manufacturer should provide adjustment instructions.
clamp-on type ammeter is required to make this adjm
ment. An autotransformer-type static phase converter
shown in Figure 13-3.
Three-phase motors operated from static phase co1
verters generally have reduced starting torque. The stat
ing torque can be improved by adding extra capacito
while the motor is starting. Once the motor reaches ft
Disconnect
3-phase
motor
Phase converter
Figure 13-3
264
Unit 73
An autotransformer-type static phase converter with starting capacitors
Phase Converters
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operating speed, these capacitors are disconnected. Manufacturers provide some starting capacitors and the control circuit as an integral part of the phase converter.
However, with extra starting capacitors, a 3-phase motor
can develop about as much starting torque as it would if
operating from a utility-supplied 3-phase supply.
Two types of capacitors are used in phase converters:
oil filled, and electrolytic. The oil-filled capacitors remain in the circuit continuously. They are large and can
dissipate the heat produced by the alternating current
flowing in the circuit. Electrolytic capacitors can only be
energized for a short time, or they will become overheated and fail. This type provides a large amount of
capacitance in a small size and, therefore, it is used as a
starting capacitor. The electrolytic type must not be energized continuously. Static phase converters should be
limited to applications where the load power requirement
is fairly constant.
Rotary Phase Converter
The rotary phase converter consists of a rotating unit
and a bank of oil-filled capacitors, Figure 13-4. The rotating unit is usually a 3-phase motor without an external
shaft. The single-phase lines are connected to two leads
of the rotating unit, as shown in Figure 13-5. The capacitors are connected between one of the single-phase leads
and the third lead. The rotating unit acts as a generator
and, with the aid of the capacitors, produces a third
phase in the third lead. The 3-phase converter leads can
then be connected by a 3-phase motor through a motor
starter.
Figure 13-4
Rotary phase converter (Courtesy of Ronk
Electrical Industries)
An advantage of the rotary phase converter is that
more than one 3-phase motors can be operated from a
single phase converter. The motors can also drive loads
which have a fluctuating power requirement. This is the
major advantage of rotary converters over the static type.
The size of the rotary phase converter is determined
by the horsepower rating of the largest motor to be
started and the total of all motors to be operated. For
example, assume that a farm materials handling system
requires a single 10-hp motor and two 5-hp motors. This
load can be supplied with a single rotary phase converter·
with a single motor rating of 10 hp. This phase converter
must also have a total connected load rating of at least
20 hp. When starting the motors, the largest is started
first, and then each of the others individually. Manufacturers may rate their phase converters in kilovolt-amperes (kVA) rather than in horsepower.
Single-phase
line
11
- Disconnect
Fuses
3-phase
wires
Windings
Capacitor bank
Rotating unit
Figure 13-5
A rotary phase converter consists of a rotating unit and a bank of capacitors.
Unit 13
Phase Converters
265
Phase converter
capacitors
3-phase motors
Figure 13-6 Extra oil-filled capacitors may be required at each motor when a single rotary phase converter supplies severa
3-phase motors.
The phase converter must first be turned on with the
rotating unit reaching full speed before any motor is
turned on. Then, each motor is started individually.
Another motor is not turned on until the previous motor
has reached full operating speed. Typically, a rotary
phase converter can supply two times its individual
motor rating. Extra oil-filled capacitors are usually required when several motors are operated from a single
rotary phase converter, Figure 13-6. The manufacturer
provides instructions for multi-motor installations.
Three-phase motors operated from a rotary phase
converter develop only about half as much starting
torque as when operated directly from a utility-supplied
3-phase supply. However, starting torque can be improved by adding a starting capacitor unit to any motor
requiring additional torque. These units generally consist
of electrolytic capacitors which are disconnected automatically when the motor achieves full operating speed.
Starting torque can also be improved by installing an
oversized phase converter. A 5-hp motor will develop
higher starting torque when operated from a 1 0-hp phase
converter than from a 5-hp phase converter.
The rotary phase converter has a minimum horsepower rating for a single motor installation. Generally,
the smallest motor which can be operated alone is onefourth the horsepower rating of the rotary phase converter. The manufacturer should be consulted if smaller
motors are to be operated alone from a rotary phase converter.
266
Unit 73
Phase Converters
Problem 13-2
A grain dryer has a fan requiring a 25-hp, 3-phasl
electric motor. The grain dryer has three additional mo
tors rated at 5 hp, 3 hp, and I hp. Determine the mini
mum size rotary phase converter required to supply al
four motors.
Solution
The minimum size rotary phase converter is deter
mined by the size of the largest motor which, in thi
case, is 25 hp. The individual motor horsepowers mm
be added together to make sure that they do not excee
two times the phase converter rating, which is 50 hr
25
+ 5 + 3 + 1 = 34 hp
The manufacturer must be consulted to determine if extr
capacitors are required for the additional motors.
480-volt Phase Converters
The phase converters discussed thus far connect to
240-V, single-phase system and produce power suitabl
for operation of 240-V, 3-phase motors. Phase conver
ers are available which produce power suitable to opera1
480-V, 3-phase motors from a 240-V, single-phase SUI
ply. Some require a transformer to step up the volta
from 240 V to 480 V.
PHASE CONVERTER WIRING
Information pertaining directly to the wiring of phase
converters is not contained in the National Electrical
Code. All of the previous diagrams in this unit show the
minimum necessary components of a phase converter
installation:
• A single-phase disconnect must be installed which will
disconnect power from the entire installation.
• The phase converter.
• If more than one motor is supplied by the phase converter, a separate disconnect and overcurrent protection is required for each motor.
• A controller for each motor.
The wiring of a phase converter installation requires
good judgment. NEC Article 430 provides most of the
information necessary for a safe and efficient installation. The requirements of NEC article 460 must be met
because capacitors are used in the phase converters. NEC
Article 450 requirements must be met if a transformer is
included as a part of the phase converter.
tl
Sizing the Wires
e
Two sets of wires must be sized in a 1-motor phase
converter installation: 1) the 3-phase wires to the motor,
and 2) the single-phase wires supplying the phase converter. The first step in the procedure involves the determination of the motor full-load current. This is found in
NEC Table 430-150. For example, the full-load current
of a 10-hp, 230-V, 3-phase motor is 28 A. The full-load
current of the motor must be multiplied by 1.25 (125%)
to determine the minimum ampere rating of the motor
circuit wire, NEC Section 430-22(a). See Equation 13.2
ll
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d
,,
the single-phase wires? To get a clue to the answer, look
up the full-load current of a 10-hp, 230-V, single-phase
motor in NEC Table 430-148. The single-phase, 10-hp
motor would draw 50 A; nearly twice as much as the
3-phase, 10-hp motor. If the wire size were based upon
the single-phase motor full-load current, then the minimum· single-phase wire rating would be 62 A.
50
1.25 = 62 A
X
The same result would be obtained by multiplying the
10-hp, 3-phase, 230-V motor full-load current by 2.2
(220%). The single-phase input wires to the phase converter supplying one motor are determined by Equation
13.3.
Minimum ampere rating of phase
Eq. 13.3
converter input wires, one motor
= 2.2 X motor full-load current
Problem 13-3
A phase converter supplies a 25-hp, 230-V, 3-phase
motor. The circuit wires are copper with THWN insulation. Determine the minimum size of 1) the 3-phase wire
from the phase converter to the motor, and 2) the singlephase input wire to the phase converter.
Solution
The full-load current of the motor from NEC Table
430-150 is 68 A. Determine the minimum ampere rating
of the motor circuit wire from Equation 13.2.
1.25
J
Minimum ampere rating
of motor circuit wire
= 1.25
X
motor full-load current
For the 10-hp motor in the example, the minimum rating
of the wire is 35 A.
Minimum ampere rating
of motor circuit wire = 1.25
X
28
= 35
A
t-
te
)-
e
68
= 85
A
The minimum size copper THWN wire from NEC Table
310-16 is No. 4 AWG, which is rated at 85 A. Determine the minimum ampere rating of the phase converter
input wires from Equation 13.3
2.2
a
le
X
Eq. 13.2
Using copper wire with TW insulation, look up the minimum wire size inNEC Table 310-16. The minimum wire
size is AWG No. 8, which is rated at 40 A.
The question now is, how much current will flow on
X
68 = 150 A
The minimum size copper THWN wire from NEC Table
310-16 is No. 0 AWG.
A phase converter that supplies more than one motor
is only slightly more involved than the case of a single
motor. Consider an installation similar to that shown in
Figure 13-6. The wires are shown in schematic form in
Unit 73
Phase Converters
267
10 hp, 230 V, 28 A
5 hp, 230 V, 15.2 A
3 hp, 230 V, 9.6 A
(4) Single-phase
input wires
(2) 3-phase feeder
wires to group of
motors
The minimum feeder current rating is determined by
using Equation 13.4
Minimum motor feeder ampere rating
= (1.25 X 28) + 15.2 + 9.6
= 35 + 15.2 + 9.6 = 59.8 or 60 A
(3) Wires to
rotating unit
10 hp
10 hp
5 hp
3 hp
Figure 13-7 Several different sizes of wire may be required for a phase converter installation involving several
motors.
Figure 13-7. The wires to the individual motors, as
shown by (1) in Figure 13-7, are determined by using
Equation 13.2, as discussed previously.
The minimum ampere rating of a feeder wire, as
shown by (2) in Figure 13-7, supplying two or more
motors is required to be the full-load currents of the motors added together, plus one-fourth of the largest motor
full-load current, NEC Section 430-24. Equation 13.4 is
stated in a slightly different form.
Minimum motor feeder ampere rating
Eq. 13.4
= 1.25 X Full-load current of largest motor
+ Full-load current of all other motors
If there are two or more motors of the largest horsepower, then multiply the current of only one of them by
1.25.
Problem 13-4
The 230-V, 3-phase motors of Figure 13-7 are rated
10 hp, 5 hp, and 3 hp. Determine the minimum size
copper THWN three-phase feeder conductor.
Solution
. The full-load current of each motor from NEC Table
430-150 is:
268
Unit 13
Phase Converters
The minimum size motor THWN copper feeder wire
from NEC Table 310-16 is No. 6 AWG.
The wire from the phase converter capacitor panel to
the rotating unit, as shown by (3) in Figure 13-7, will not
carry more current than the motor feeder. Therefore, the
minimum size for this wire is the same as it is for the
motor feeder wire.
The single-phase input wire to a phase converter supplying a group of motors carries almost twice the current
as the 3-phase feeder supplying the motors. The singlephase input wire is sized at 2.0 (200%) times the sum of
the full-load currents of all the motors, Equation 13.5 ·
Phase converter input wire ampere
Eq. 13.5
rating for group of motors
= 2 X Full-load current of all motors
I
Problem 13-5
Determine the minimum size copper THWN singlephase input wire size for the installation described in
Problem 13-4.
Solution
Add the full-load currents of all motors and multiply
by two, as in equation 13.5
Phase converter input wire ampere rating for
group of motors
= 2 X (28 + 15.2 + 9.6)
= 2 X 52.8 = 105.6 or 106 A
The minimum size phase converter THWN copper inpu
wire from NEC Table 310-16 is No. 2 AWG, which i
rated at 115 A .
Most phase converter installations involve 230-\
3-phase motors. Thus, the previous calculations .apply t<
the sizing of the circuit wires. The procedure varies only
slightly when 460-V, 3-phase motors are supplied from a
230-V, single-phase electrical supply.
• For 460- V motors, find the motor full-load current in
the 460-V column of NEC Table 430-150.
• Use Equations 13.6 through 13.9 for determining the
following.
Single 460 V motor supplied by phase converter from a
230 V supply:
Eq. 13.6
Minimum ampere rating of motor
circuit wires
= 1.25 x Motor full-load current
gle-phase fusible safety switch or a suitable circuit
breaker. This disconnect switch must be rated in horsepower, NEC Section 430-109. A circuit breaker rated in
amperes is acceptable, but is less desirable as a disconnect. The disconnect switch must have a single-phase
horsepower rating not smaller than the sum of the horsepower ratings of the motors to be supplied, NEC Section
430-112.
Problem 13-6
Determine the horsepower rating required for the
phase converter disconnect supplying motors with ratings of 25, 5, 3, and 1 horsepower.
Solution
Minimum ampere rating of phase
Eq. 13.7
converter input wires, one motor
= 4.4 X Motor full-load current
Two or more 460 V motors supplied by one phase converter from a 230 V supply:
Individual motor circuit wires; use Equation 13.6
Minimum motor feeder ampere rating Eq. 13.8
= 1.25 X Full-load current of largest motor
+ Full-load current of all other motors
Phase converter input wire ampere .
Eq. 13.9
rating for group of motors
= 4 X Full-load current of all motors
PHASE CONVERTER DISCONNECT
l
1
A means must be provided to disconnect all ungrounded single-phase input wires for the phase converter, NEC.Section 460-B(c). This disconnect can serve
as the motor circuit disconnect provided only one motor
is supplied by the phase converter. This disconnect must
also be within 50 ft (15.24 m) and within sight from the
motor controller, NEC Section 430-102. Individual
motor circuit disconnects are required when one phase
converter supplies several motors, NEC Section 430112. These individual motor circuit disconnects are in
addition to the phase converter main single-phase disconnect (as shown in Figure 13-6).
The main phase converter disconnect should be a sin-
Disconnect switch rating
= 25 + 5 + 3 + 1 = 34 hp
Choose a disconnect switch which has a single-phase
horsepower rating not smaller than 34 hp. Single-phase
horsepower is used because the switch is installed in the
single-phase input wire.
Single-phase and 3-phase horsepower ratings are not
the same. A 3-phase disconnect switch with a 10-hp rating cannot be used as the disconnect for a 10-hp, singlephase motor. The single-phase motor draws 1.73 times
as much current as the 3-phase motor. Therefore, a single-phase, 10-hp disconnect switch must have a higher
current rating than a 10-hp, 3-phase disconnect switch.
Table 13-1 gives approximate equivalent single- and 3Table 13-1 Approximate equivalent 3-phase and singlephase horsepower ratings for 240-V safety switches
Horsepower
Three phase
Single phase
3
7 1/2
10
15
20
30
40
50
60
75
100
125
150
200
1 112
3
5
7 1/2
10
15
20
30
35
40
60
70
85
115
Unit 73
Phase Converters
269
phase horsepower ratings. For example, a disconnect
switch rated at 100 hp, three phase can usually be used
as the disconnect for a 60-hp, single-phase motor load.
OVERCURRENT PROTECTION
Overcurrent protection must be provided for each
ungrounded single-phase input wire to the phase converter. Time-delay fuses are usually installed in the
phase converter disconnect switch. The time-delay fuse
size is determined by the same procedure used to size the
single-phase input wires, Equation 13.3, 13.5, 13.7, or
13.9. The next standard fuse size larger than the value
determined by one of the input wire ampere rating equations may be chosen to protect the input wires. In Problem 13-5, the proper size time-delay fuse for the No. 2
AWG copper single-phase input wires is 110 A.
Overcurrent protection for individual motor circuit
wires is determined by the procedures required in NEC
Article 430, as discussed in Unit 17.
MOTOR CONTROLLERS
The controller for a motor supplied by a phase converter is permitted to be any appropriate type described
in NEC Article 430. Some types of motor controllers do
not provide adequate overload protection for the motor.
It is extremely important in a phase converter installation
that some type of overload protection sensing motor current be provided. Three types of motor controllers that
provide this protection are:
• Magnetic motor starter with properly sized thermal
overload relays
• Manual motor starter with properly sized thermal overload relays
• Safety switch with horsepower rating equal to or
greater than motor horsepower, and time-delay fuses
sized not greater than 1 .15 (115%) times the motor
full-load current
A rotary phase converter must always be turned on
and reach full operating speed before any motor supplied
is turned on. Should there be a temporary power interruption, it is important that the motors be turned off, or
the phase converter may become damaged when power
is restored. A manually operated controller will not turn
the motors off during a temporary power outage. When
power is restored, the motors will try to start. Magnetic
motor starters must always be used to control motors
supplied by a rotary phase converter. Also, do not use
270
Unit 73
Phase Converters
2-wire control, because this will cause immediate restarting of the motor. Time-delay relays may be used with
2-wire control to prevent immediate restarting of the
motor. When power is lost, the magnetic motor starters
will shut off the motors so they cannot restart when
power is restored. The motors must be restarted manually; or an automatic restart control can be added to start
each motor, in sequence, once the rotary phase converter
has been restarted.
INSTALLING EXTRA CAPACITORS
Extra capacitors are often required when two or more
motors are operated from one rotary phase converter.
These capacitors are most easily installed at the magnetic
motor starter. A disconnect is required for the capacitor
bank unless the capacitors are on the load side of the
motor running overcurrent protective device, NEC Section 460-8(b )( 1 ), Exception.
The capacitors may change the amount of current
drawn by the motors. The amount of current from the
supply up to the capacitors will be affected, but not the
current flow from the capacitors to the motor. The capacitors are to be connected to the load side of the thermal
overload heaters (Figure 13-8). Therefore, the current
A B C
B
Gdc5ers
Rotary unit
contacts
Thermal sensing
heaters
capacitors
figure 13-8 Extra capacitors, if required, should be connected on the load side of the thermal sensing heaters in
the motor starter.
flow through the thermal overload heaters may be affected. The overload heaters' sensing current setting may
need to be adjusted if the addition of these capacitors
significantly changes one or more of the motor line current values, NEC Section 460-9, Exception.
GROUNDING
A grounding path must be provided for all exposed
metal parts of the phase converter installation, including
phase converter, capacitors, disconnects, motor controllers, and motors. Procedures for grounding are covered
in NEC Section 250, as discussed in detail in Unit 10.
REVIEW
Refer to the National Electrical Code when necessary to complete the following review
material. Write your answers on a separate sheet of paper.
1. Is the purpose of a phase converter to provide a means of operating a 3-phase electrical motor from a single-phase electrical supply?
2. Name the two types of phase converters.
3. Capacitors are used in phase converters. Which type of capacitor can carry alternating
current continuously?
4. Is it true that a 3-phase electric motor supplied by a phase converter generally does
not develop as much starting torque as it would if operated from utility-supplied
3-phase power?
5. Which of the following is the major reason for choosing a phase converter rather than
utility-supplied 3-phase power?
a. Its overall cost is lower.
b. The power supplier does not produce 3-phase power.
c. A single-phase motor of the required size is not available.
d. The farm is not wired for 3-phase power.
6. A grain dryer fan requires a 15-hp electric motor. Determine the minimum size
3-phase electric motor and capacitor-type static phase converter required to power
this load.
7. Identify the static phase converter which, through proper adjustment of the number of
capacitors, allows a motor to develop full nameplate horsepower.
8. If a farmer wants to operate several motors from one phase converter, then which
type of phase converter should be chosen?
9. A phase converter is used to supply, 7 IJ2-, 3-, and 2-horsepower, 3-phase, 230-V
electric motors. Determine the minimum size copper THW single-phase input wires.
10. A farmer can buy a used 30-hp rotary phase converter from a neighbor. If the phase
converter is in good operating condition, should this phase converter be used to
supply a 5-hp, 3-phase motor?
11. Would a I0-hp rotary phase converter be adequate to supply the motors in Problem
No. 9 of this review?
12. Which of the following types of controllers should be used for a motor supplied by a
rotary phase converter?
a. Circuit breaker
Unit 73
Phase Converters
27 7
b. Fusible switch
c. Manual motor starter
d. Magnetic motor starter
13. A 3-phase, 3-pole disconnect switch can be used on the input wires to a phase
converter if a single-phase, 2-pole disconnect is not available. Determine the minimum 3-phase horsepower rating required for a 3-phase disconnect used on the input
wires to a rotary phase converter supplying motors rated at 15, 10, 3, and 2 horsepower.
14. A phase converter supplies one 15-hp, 230-V, 3-phase electric motor. Determine the
recommended size time-delay fuses used to protect the single-phase converter input
wires.
15. Extra oil-filled capacitors are required for a phase converter supplying several motors. Refer to the diagram and indicate the most appropriate location (A, B, C, or D)
to connect the capacitors to the motor circuit.
From phase -------- --------- To other
converter
motors
Motor disconnect
Motor circuit
fuses
B
Motor starter
contacts
Moto
r current
thermal sensing
elements
3-phase
motor
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Unit 73
Phase Converters
L
C
D