“Made with pride in the USA”
Training
Guide
Effective Date:
August 1, 2011
CONTROL TRANSFORMER:
DEFINITION:
Control Transformers are virtually always single phase. They are also called "CPTs", Control Power
Transformers, High Inrush Transformers Step-Up and Power Transformers. They are designed to provide
a stable secondary voltage at high initial energizing or inrush currents typically 5 to 15 times normal operating
load current.
TERMS:
AC: Alternating current is the most common transmitted type of electricity. The flow begins by building up a
voltage which is positive on top and negative on the bottom, and therefore pushes electrons through the circuit in
the direction indicated by the solid arrows. However, then the source voltage starts falling off, and eventually reverses
polarity. Now current will still flow through the circuit, but this time in the direction shown by the dotted arrows. The cycle
repeats itself endlessly and as a result the current through the circuit reverses direction repeatedly. The number of alterations
per second is called frequency.
Hertz (Hz): The frequency (cycles) that AC reverses direction per second. 60Hz is common in NA, 50Hz in EU.
AMPS: The volume of electrical charge passing a point per unit of time. This "'flow rate" called current, is measured in amps.
Volts: The velocity of the electrical charge passing through a conductor.
Ambient: Refers to the ambient temperature of the air surrounding the transformer. Expressed in degrees Celsius (Centigrade).
The established industrial ambient is 40°C. Typically environments above this require component derating.
Regulation: The variation of transformer secondary voltage from no load to full load. In smaller sizes like 50VA, the difference
between no load and full load voltage may be as high as 12%-13%.
Inrush: The initial peak current (amp draw) measured as the transformer or the load becomes energized. This peak lasts only a
few cycles but can equal as much as 40 times the normal operating current. As the current spikes the voltage dips. Think of why
your house lights dim when the air conditioner turns on. Control transformers are designed to minimize the voltage drop at inrush.
Inrush is also a product of magnetizing the laminations and coil of the transformer itself. This causes a similar amperage peak on
the line feeding the transformer and may cause premature or nuisance tripping of fuses and breakers. The worst case inrush is always
at the peak of a cycle.
Types of Loads: The reversing action of alternating current makes no difference to some kinds of loads. For example, the light bulbs
in your home don't care which way the current flows through them. When the circuit is closed by turning on the switch, the light turns
on without regard for the direction of the current flow.
Inductive: Typically caused by energizing a magnetic field such as solenoids, relays, motors or magnetic starters. Initial energizing
(inrush) current is high which causes voltage to initially sag. After energizing occurs the current draw drops causing voltage to again
rise. This point defines "steady state" or "sealed" VA.
Resistive: Also described as linear. Loads that cause little or no initial energizing (inrush) current. Examples include incandescent
lighting or heating resistors. Since there is no inrush, current and voltage rise and fall in sync with each other.
Capacitive: The load presents a very high initial current draw similar to charging a capacitor. The initial high current draw causes a
momentary voltage sag. At high switching frequencies all loads contain some capacitive value. Examples are capacitors, power supplies
and variable frequency devices.
2
SELECTION:
You need to find out some of the information in ITALICS in order to properly
select a transformer.
1 Try to get a competitive P/N Quantity and if possible competitive price
If you get this information you will likely not need step two.
Primary competitors:
Sq-D
Acme Transformer
Sola/Hevi-Duty
Hammond Power Solutions
Dongan
GE
2 If competitive P/N not available you will need to get the following information:
b. Primary Voltage (input)
Single Phase primaries are typically expressed as: 120V, 208V, 230V, 277V, 480V
>>>>>The voltage potential measured between the two wires you intend to connect
to the transformer determines the primary voltage connections chosen.
c. Secondary Voltage (output)
Single Phase secondaries are typically expressed as: 24V, 120V, 240V
d. Load Rating
Volt-Amps (VA) & AC Watts
VA (Volts X Amps) EXAMPLE: 50VA, 500VA, 1500VA
Kva (Volts X Amps)X1000 EXAMPLE: .050Kva, .500Kva, 1.5Kva
EXAMPLE: If amp load and voltage is known then Volts X Amps = VA
EXAMPLE: If VA and Volts are known then VA / Volts = Amps
AC Watts = VA X Power Factor
e. Sizing the transformer
There are a number of places to quickly look this up.
1. Single Phase selection Guide in the LVGP catalog
2. Transformer Selection Process in the CPT catalog
3. Amp ratings in the CPT catalog
Just remember:
Take all the components that may be energized simultaneously and add their listed
(they are always listed by the mfr) energizing currents (Inrush VA) and then add
to that the running current of any device that may be already operating. Take that
value and find a comparable value on the Regulation Data Chart under the 90%
column. Alternately, the √((inrush VA)² + (sealed VA)²) formula can be used.
Make sure when selecting that all the items sealed VAs together also agrees with the
minimum VA of the selected transformer.
The values in the table are finite. If your calculated value exceeds a listed
valued selected the next largest.
3
COMMON QUESTIONS
My output voltage
seems high.
1. Typically transformers are designed to provide nameplate voltage at full rated load. When little or no load
is attached the output may be as much as 13% high. The no load to full load voltage differential decreases
in proportion to the load.
2. Obtain the correct voltage measurement for the primary voltage. Just because the breaker is labeled
480 volts, is the actual voltage 480 volts? The secondary will be affected by the same percentage as the
actual primary differs from the label primary. 492 volts applied to a 460 volt primary would cause a 7%
increase in the measured secondary.
EXAMPLE: "I wired up a B100BTZ13JK and see 140 volts on the secondary. Something is wrong with
the transformer."
ANSWER:
1. Ask what the measured primary voltage is. (492V)
2. Ask what the actual transformer load is. (20VA Essentially no-load)
3. Calculate what the actual secondary would be based on actual primary voltage and actual load.
Measured Sec.volts/estimated Sec. volts based on actual VA load. (in this case near enough to use no-load
value). (140/132.3) = 5.8% high. If the no-load 132.3 value was increased by another 7% it would equal
141.6. In this case with a 20VA load the actual design voltage would be approximately 130V (just slightly
below no-load).
4. If your calculations do not meet that of the customer it may be worthy of further investigation. In the example,
the measured primary was 492 volts. Without that 7% overage, the secondary voltage would have been
130V which is within the specifications.
Help me size a
Remember that you need both the inrush and sealed VA for all of the primary devices used. In this case the 3
starters as the lamps resistive loads that draw only milliamps.The customer states that each starter draws 170VA
inrush and 40VA sealed and starts imultaneously. If you add the 3 inrush VAs (170 X 3) = 420VA the choice
would point to a 100VA using the 90% table. 100VA was chosen because the 420VA exceeds the maximum
inrush load for a 75VA (410VA). If the 3 starters energized at different times but eventually all ran together, then
the calculation would include the sealed VA values as well ((170 X 3) + (40 X 3)) = 540VA, still pointing to
100VA. However, remember that the 3 starters have a sealed VA of 40 (40 X 3) = 120VA. This means the
minimum available selection becomes 150VA.
Alternately, the calculation can be made by applying the universal formula for inrushVA selection found on Page 4
of the catalog. The formula is: √((inrush VA)²+(sealed VA)² )
ANSWER: √((3 x 170)² + (3 x 40)²) = 523.92VA. Again pointing to 100VA. However, the total
sealed VA=120 which means that the minimum transformer available is 150VA.
transformer.
I have 3 Sq-D
Size One starters
plus 2 lamps.
Can you help me?
Can you help me
wire this up?
It's a B150-3460-1
Although the product has a descriptive label which outlines the correct terminals to connect, neither may be handy
to the caller or to the person receiving the call. In those cases it is best to contact Micron customer service as they
can open the drawing file, view the label directly and e-mail you the file. Once you have the drawing you can:
1. Ask if the customer has the transformer in front of them or where they can easily see it "head-on". (the normal
mounting has the primary on top)
2. Ask what primary and (if applicable) secondary they want.
3. On the B150-3460-1, if the customer wants a 460 primary the unit is jumpered across the H₃ - H₂ (center
terminals) and the input wires connected to H₁ - H₄ or the two outer terminals. Since this unit only offers a 115V
4
I need a step-up
transformer.
Read reverse connecting below. If necessary for loads under 3Kva it
may be best to design a transformer to do the job or allow for the
voltage drop when run backward.
Can the transformer be
reverse connected?
Typically, above 3Kva, a transformer can be wired either as a step-down or
step-up device. Windings on transformers under 5Kva are typically
compensated" toprovide nameplate voltage at full rated load. The compensation
value can be as much as 13% on a 50Va and lessens as the VA increases.
EXAMPLE: putting 120V into the secondary of a B050BTZ13JK might yield
418V from the 480V primary at no load and 355V at full load.
EXAMPLE: putting 120V into the secondary of a B3K0BTZ13JXH would yield
475V from the 480V primary at no load and 470V at full load.
Remember to remind the customer when asked about reverse connecting.
I get a strange voltage
when measuring voltage
from X₁ or X₂ to ground.
The only meaningful reference is a voltage measurement between X₁ and X₂.
A measurement between a terminal and ground is that of a stray voltage and
meaningless as there is no connected ground reference between the windings.
I need an isolation
transformer.
1. The definition of an isolation transformer is that the primary and secondary
windings are connected magnetically but not physically. Also called
"Double Wound".
2. An isolation transformer can also be defined as one having equal voltage
primaries and secondaries. The B050LP1519JJ fits both categories of isolation.
I need a shielded
transformer.
Shielding is typically a conductor wound between major windings which
is then bonded to the core. The shield acts to mitigate line-to-line or
line-to-ground noise. Typically, US built control transformers do not have
shielding. The type of noise may define the type or placement of shielding
required.
I need to get 24Vdc out,
which transformer?
Transformers are AC to AC devices. You cannot transform DC. What you
are describing for is a power supply.
I have a high operating
ambient. Do I need a high
temperature class
transformer?
Will a higher temperature
class transformer solve a
high ambient problem?
The standard operating environment is based at 40°C. As a rule-of-thumb,
for each 10° increase over 40°C, derate the maximum load by a
minimum of 10%.
The standard operating environment is based at 40°C. The temperature class
of the transformer is based on internal temperature rise above 40°C
Higher temperature classes run hotter and further add to the ambient.
This may necessitate additional derating.
I have 480 3-phase how
The voltage potential as measured between the two chosen legs should match
do I connect to a
B050MBT13XK?
Isn't a B050MBT13XK
a 3-phase transformer?
to the primary terminals selected. Open the appropriate drawing to assist.
The typical control transformer is a single-phase device that allows the connection
of two legs from a three phase source to power the primary.
5
The transformer seems
to run "hot"
Define hot? A 105°C system/55°C rise transformer is defined as its maximum operating
temperature measured at the hottest spot on the coil and at the max ambient of 40°C.
Defined as: 40°C ambient + 55°C allowable rise + 10°C max allowable hot spot.
105°C = 220°F. Measure with a thermocouple and not a hand. If at less than full load
and less than 40°C, a measured temperature above the transformer rating is worthy of
further investigation.
What fuse do I use?
1. Primary: 250% of the rated primary current. This is valid with secondary protection
per NEC 450.3(B).
2a. Secondary: 167% of the maximum rating if secondary current < 9A.
2b.
125% of the maximum rating if secondary current ≥ 9A.
Divide the given VA by the given voltage to get primary amps or multiply primary amps by
given voltage to get VA).
Current fusing chart resides in the catalog or you can call Micron for the information.
Note 1: Micron recommends slow-blow fuses for the primary since the transformer will see
an inrush load as it itself energizes and as the load energizes. Under certain situations the first
few cycles might be as high as 40 times running current.
Note 2: The NEC fusing values are maximum values. While maximum values are
recommended for the primary, the customer may wish to size the secondary more tightly
to protect his load.
Can I get fusing installed
on a 3 or 5Kva?
Micron does not offer either secondary or primary fusing capability above 2Kva because
the current draw exceeds the capability of the midget or Class "CC" fuses used. The small
fuse body size cannot dissipate that amount of energy.
Can I get fusing installed
on a Globaltran?
Micron does not recommend as the fuse size and style used in the US is not generally
accepted in EU.
Should I ground the
transformer?
If the transformer has a ground lug installed, the lamination is typically attached via
a ground wire to that lug. Alternately the "0" voltage terminal on the secondary can be
grounded as long as no additional grounds are connected to that secondary.
Can I run a 60 Hz
transformerat 50Hz?
If I put in 60Hz can
I get out 50Hz?
Hertz is a measure of electrical frequency. The typical North American frequency is 60Hz.
The nameplate frequency can be run at higher, but not lower than nameplate frequency.
No. The frequency in is the frequency out.
CE just means
"finger-safe"?
Actually, CE has to do with isolating insulation and the electrical creepage distances between
windings. It differs from UL standards. Micron's GlobalTRAN line, meets the requirements of
EN 61558-2-2 which refers to Safety of Power Transformers. Finger-safe is defined by IP
numbers under another EN standard.
6
Is Impervitran a Class 2
transformer?
No. Class 2 refers to a design which is inherently current limiting and cannot provide power at
levels of 100VA and over and often does not need overcurrent protection. The basic
difference is that a control transformer cannot tell the difference between inrush and a dead
short. It will try to energize until the windings burn through. It cannot be used in a Class 2
circuit.
What class is a
control transformer?
Although not often referred-to, control transformers qualify as Class 1 transformers.
Can you design
(xxx) voltage?
(xxx) VA?
match(xxx) footprint?
The answer to these is, of course, but its volume related. Typically if its only going to be a
one-shot order for 10 or so pieces, it cannot justify the engineering time necessary to
support. If it sounds like a good opportunity, pass it along and we'll quote it.
Footprint is a problem if an existing bobbin does not match up.
Can you cross this
"Acme" P/N?
It is often quite easy to "search" the part on the internet. If unable call Micron customer service.
NOTE: When many of the competitors relocated to China, they redesigned their units with
multiple primary voltages to reduce inventories. Micron offers as many options, but not
on one transformer. Our philosophy is "'why add extra voltage taps if they are used less
than 20% of the time". We can often offer a more economical solution for the customer's
high volume needs.
My machine is down,
can you cross a
Sq-D "xxx"P/N?
If the customer has a machine down then they know what voltage they need for the primary
or they can tell you what screws are loose on the transformer or where the jumpers are
connected. Once you know the primary voltage, VA and secondary voltage, offer the
product you have in stock that matches what they actually need.
HINT 1: Don't hesitate to tell the caller you'll get right back to them. Micron has most of the
competitor's catalogs. If not, Google the competitor's P/N. Most sites will lead you
the wiring diagram.
HINT 2: You can always offer the next VA size up.
I got a B050MBT13XK
to replace a
B050BTZ13JK.Where do
the jumpers go?
Don't use any jumpers on the B050MBT13XK. The primary coil is wound with one wire
with voltage stops (taps) at 208, 230 & 460. On the B050BTZ13JK the primary coil is made
up of two windings which are connected by jumpers in parallel (high current, low voltage)
or in series (low current, high voltage). If you jumpered across the primary of a tapped
transformer, you will have shorted out a portion of the winding.
Do you have a rail mounted
transformer?
By "rail" they mean DIN-Rail, which is the way most contactors, terminal blocks and
power supplies are mounted. Micron does not offer this option because the
majority of our product is deemed too heavy to DIN-Mount.
7