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Power Rating
The power rating of control transformers generally ranges from 0.75 kilovolt-amperes, or 75 volt-amperes, to 1 kilovolt-ampere, or 1000 volt-ampere. The rating is indicated in volt-amperes, not watts, because transformers generally supply power to operate inductive devices such as the coils of relays and motor starters (Fgr. 8). The volt-ampere rating indicates the amount of current the transformer can supply to operate control devices. To determine the maximum output current of a transformer, divide the volt-ampere rating by the secondary voltage. The transformer shown in Fgr. 8 has a power rating of 250 volt-amperes. If the secondary voltage is 120 volts, the maximum secondary current would be 2.08 amperes.
A control transformer intended to operate a single motor starter may have a rating of 75 to 100 volt amperes. Transformers intended to supply power to an entire relay cabinet will have much higher ratings, depending on the number of devices and their current requirements.
Fgr. 8 The power rating of a transformer is listed in volt-amperes.
Fgr. 9 One side of the transformer has been grounded.
Grounded and Floating Control Systems
One side of the secondary winding of a control transformer is often grounded (Fgr. 9). When this is done, the control system, is referred to as a grounded system. Many industries prefer to ground the control system, and it's a very common practice. Some technicians believe that it's an aid when troubleshooting a problem. Grounding one side of the control transformer permits one lead of a voltmeter to be connected to any grounded point and the other voltmeter lead to be used to test voltage at various locations throughout the circuit (Fgr. 10).
However, it's also a common practice to not ground one side of the control transformer. This is generally referred to as a floating system. If one voltmeter probe were to be connected to a grounded point, the meter reading would be erroneous or meaningless since there isn't a complete circuit (Fgr. 11). High impedance voltmeters would probably indicate some amount of voltage caused by the capacitance of the ground and induced voltage produced by surrounding magnetic fields. These are generally referred to as ghost voltages. A low impedance meter such as a plunger type voltage tester would indicate no voltage.
Accurate voltage measurement can be made in a float control system, however, by connecting one voltmeter probe directly to one side of the control transformer (Fgr. 12). Since both grounded and floating control systems are common, both will be illustrated throughout this text.
Fgr. 10 Voltage can be measured by connecting one meter probe to any grounded
point.
Fgr. 11 Floating control systems don't ground one side of the control
transformer. Connecting a voltmeter probe to a grounded point would provide
meaningless readings because a complete circuit does not exist.
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EXAMPLE:
What is the maximum fuse size permitted to protect the primary winding of a control transformer rated at 300 volt-amperes and connected to 240 volts?
I = 1.25 A
Fuse size = 1.25 x 5
Fuse size = 6.25 A
NEC Section 240.6 indicates that a standard fuse size is 6 amperes. A 6 ampere fuse would be used.
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Transformer Fusing
Control transformers are generally protected by fuses or circuit breakers. Protection can be placed on the primary or secondary side of the transformer, and some industries prefer protection on both sides. NEC Section 430.72(C) lists requirements for the protection of transformers employed in motor control circuits.
This section basically states that control transformers that have a primary current of less than 2 amperes shall be protected by an overcurrent device set at not more than 500% of the rated primary current. This large percentage is necessary because of the high in-rush current associated with transformers. To determine the rated current of the transformer, divide the volt-ampere rating of the transformer by the primary voltage.
NEC Section 430.72(C)(2) states that fuse protection in accordance with 450.3 is permitted also.
This section states that primary protection for transformers rated 600 volts or less is determined in NEC Table 430.3(B). The table indicates a rating of 300% of the rated current.
The secondary fuse size can also be determined from NEC Table 450.3(B). The table indicates a rating of 167% of the rated secondary current for fuses protecting a transformer secondary with a current of less than 9 amperes. Assuming a control voltage of 120 volts, the rated secondary current of the transformer in the previous example would be 2.5 amperes (300/120). The fuse size would be:
2.5 x 1.67 = 4.175A
The nearest standard fuse size listed in 240.6 without going over this value is 3 amperes. The secondary fuse size can be set at a lower percentage of the rated current because the secondary does not experience the high in-rush current of the primary. Since primary and secondary fuse protection is common throughout industry, control circuits presented in this text will illustrate both.
Fgr. 12 Connecting one meter probe directly to one side of the transformer
will provide accurate readings on a floating control system.
QUIZ:
1. What is the operating voltage of most magnetic control systems?
2. How many primary windings do control transformers have?
3. How are the primary windings connected when the transformer is to be operated on a 240 volt system?
4. How are the primary windings connected when the transformer is to be operated on a 480 volt system?
5. Why are two of the primary leads crossed on a control transformer?
6. You are an electrician working in an industrial plant. You are building a motor control cabinet that contains six motor starters and six pilot lamps. All control components operate on 120 volts AC. Two of the motor starters have coil currents of 0.1 amperes each and four have coil currents of 0.18 amperes each. The six pilot lamps are rated at 5 watts each. The supply room has control transformers with the following rating (in volt-amperes): 75, 100, 150, 250, 300, and 500. Which of the avail able control transformers should you choose to supply the power for all the control components in the cabinet? (Choose the smallest size that will supply the power needed.)