What is the difference between a Motor Starter and Contactor? What ...

What is the difference between a
Motor Starter and Contactor?
Is Arching worse in DC or AC
systems? Why?
If an Optoisolator has to send a
signal into an AC system what
type would be used?
What is the difference between a
NEMA and IEC Motor Starter?
What device is typically used to send
information from one voltage system
to another using only light?
(Say from a computer to a welder)
If an Optoisolator has to send a
signal into a DC system what
type would be used?
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What are some advantages and
disadvantages of SSRs over
Magnetic Control Relays?
Begin with the End in Mind
• Review Chapter 6 questions
• Chapter 5 and Chapter 6 will be combined
for one homework assignment.
What is a Transformer?
• A magnetic machine used to raise and
lower alternating voltage or current
– Only works on AC
CHAPTER 6
The Control Transformers
Transformer Fundamentals
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The Turns Ratio (1)
• The main relationship between
the voltage and current going to
and from a transformer is the
“Turns Ratio” (N)
N
N
p
s
E
E
P
S
I
I
S
P
Power
Step-Down Connection
13
17
Turns Ratio (2)
Normally the Turns Ratio is expressed
as a ratio If the Primary Voltage is 120
V, and the secondary Voltage is 24V,
the turns ratio is “5 to 1” (5:1)
N
E
E
P
S
120V
24V
5
" 5:
1"
The First Law of
Thermodynamics applies
• “Energy is neither lost nor destroyed”
• Means that Energy In = Energy Out or
“Power In = Power Out”
VA
IN
VA
OUT
Step-Up Connection
16
18
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Multiple Tap Transformers
Allow user to adjust for changes in primary voltage
Control Transformer Label
With 120V on the secondary, how much
current will this transformer supply?
Answer:
• First you need to step the Voltage down
from 480V to 120V (4:1)
• Second, the secondary has to supply:
• 500mA x 4 = 2A
• 250mA x 1 = 250 mA
• 100mA x 6 = 600mA
• Total = 2.85A
19
Multiple Tap Control Transformer
With Label
Question:
• You need to power a control cabinet that
will operate 4 Motor Starters, 6 Pilot lights,
and an E-Stop Relay for a 480V motor
circuit. If each MS Coil draws 500mA,
each light 100mA and the E-Stop 250mA,
what size transformer do you need?
Answer Continued:
• Calculate the KVA (Power)
• 2.85A X 120V = 342VA
• You should up-size it to the next available
• (500 VA or .5KVA)
You need a 480V to 120V, .5KVA
Transformer
20
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The Control Transformer (1 of 6)
• Control transformers are used to step
down load voltages of 240V and 480V to a
control circuit voltage of 120V.
The Control Transformer (3 of 6)
• Primary connected for 480V
The Control Transformer (5 of 6)
• Metal links used to make a 240-volt
connection
The Control Transformer (2 of 6)
• Primary connected for 240V
The Control Transformer (4 of 6)
• Primary leads are crossed.
The Control Transformer (6 of 6)
• Metal link used to make a 480-volt
connection
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Transformer Protection
• Fuses and circuit breakers can be used to
protect the transformer.
• Requirements are listed in the NEC
Section 430.72(c), 240.6.
• Usually Transformers are fused on the
Primary and Secondary sides, though
Primary Fusing alone is allowable.
Toyota Fusing (When a blown fuse can cost
$20,000 a minute)
Usually 2 fuses are
used on the primary
because 2 lines are
used.
One side of the
secondary is either
grounded or fused
Grounding and Floating
Control Systems (2 of 4)
• Voltage can be read by connecting one meter lead to
any grounded point of system.
Transformer Fusing
• If transformers are fused on the primary
alone, the fuse will have to be smaller than
if it is fused on both the primary and
secondary.
• The primary fuse may be left out if the
system protection is sized for than the
primary fuse rating would be.
• Fusing may be left out for small
transformers in “Supervised Locations.”
Grounding and Floating
Control Systems (1 of 4)
• When one side of the secondary winding is
grounded, the control circuit is a grounded system.
Grounding and Floating
Control Systems (3 of 4)
• When secondary system is not grounded, it is a
floating system. No voltage would be read to ground.
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Grounding and Floating
Control Systems (4 of 4)
• Voltage will be read:
Connect the transformer below
Connect the transformer below Connect the transformer below
Summary
• In this chapter, we discussed:
– The use of control transformers in a control
circuit
– How to connect a control transformer for
the operation on a 240- or 480-volt system
– Grounded and floating control systems
– Control transformer protection
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