CONTROL TRANSFORMER, the majority of industrial motors operate on voltages ranging from 240 to 480 volts. Magnetic control systems, on the other hand, are often powered by 120 volts. To run the control system, a control transformer steps the 240 or 480 volts down to 120 volts. A control transformer is nothing unique, save that most of them have two main windings and one secondary winding. Each main winding has a 240-volt rating, while the secondary winding has a 120-volt rating.
This signifies that the turns ratio between the primary and secondary windings is 2:1 (2 to 1).
Assume that each main winding has 200 turns of wire and each secondary winding has 100 turns. Each main winding has two twists of wire for every one turn of wire in the secondary winding.
The control transformer’s principal windings are designated H1 and H2. H3 and H4 are the labels for the other major windings. The secondary windings are denoted by the letters X1 and X2. If the transformer is to step 240 volts down to 120 volts, the two primary windings are connected in parallel, as illustrated in Figure 6-1.
Figure 6-1 Shows Parallel primaries for 240 volt operation.
Figure 6-1 shows the H1 and H3 leads linked together, as well as the H2 and H4 leads connected together. The impact is the same as having only one primary winding with 200 turns of wire in it since the voltage supplied to each primary winding is the same. This indicates that the turns ratio is 2: 1 when the transformer is connected in this manner. The secondary voltage is 120 volts when 240 volts are applied to the primary coil.
Figure 6-2 Shows Series connection of primaries for 480 volt operation.
Screw terminals are often attached to the primary and secondary leads of a control transformer. Figure 6-3 shows how the H2 and H3 leads are bridged to make connecting the primary winding easier. For example, if the transformer is to be wired for 240 volts, the two primary windings must be linked in parallel, as illustrated in Figure 6-1.
This connection may be formed on the transformer by connecting leads H1 and H3 with one metal link and leads H2 and H4 with another metal link (Figure 6–4).
Figure 6-3 The primary leads have been crossed. Figure 6-4 A 240 volt connection is made using metal links.
The primary windings must be linked in series if the transformer is to be utilized at 480 volts, as illustrated in Figure 6-2. This connection may be created on the control transformer by connecting H2 and H3 with a metal link, as illustrated in Figure 6-5.
Figure 6-5 A 480 volt connection is made using a metal link.
Figure 6-6 depicts a typical control transformer.
Instead of two independent windings, some control transformers include a multi-tapped primary (Figure 6–7).
In this example, the transformer is intended to step voltages of 480, 277, 240, or 208 down to 120.
Figure 6–6 Shows Control transformer.
Figure 6-5 A 480 volt connection is made using a metal link.
Power Rating
Control transformer power ratings typically vary from 0.75 kilovolt-amperes (75 volt-amperes) to 1 kilovolt-ampere (1000 volt-amperes). Because transformers typically deliver power to inductive equipment such as relay coils and motor starters, the rating is given in volt-amperes rather than watts (Figure 6–8).
6-8 Figure A transformer’s power rating is indicated in volt-amperes.
The volt-ampere rating of a transformer specifies how much current it can give to control devices. To find the maximum output current of a transformer, divide the volt-ampere rating by the secondary voltage. Figure 6-8 shows a transformer with a power rating of 250 volt-amperes. The maximum secondary current is 2.08 amperes if the secondary voltage is 120 volts.
I= VA/E
I= 250/120
I= 2.08 A
A control transformer with a rating of 75 to 100 voltamperes may be used to power a single motor starter.
Transformers designed to power a whole relay cabinet will have substantially higher ratings, depending on the number of devices and their current needs.
Control Systems (Grounded and Floating)
A control transformer’s secondary winding is frequently grounded on one side (Figure 6–9). When this is done, the control system is said to be grounded. Grounding the control system is a frequent procedure in many businesses. Some technicians feel it might help them solve an issue.
Figures 6–9. The transformer has been grounded on one side.
Grounding one side of the control transformer allows one voltmeter lead to be attached to any grounded point and the other voltmeter lead to be used to evaluate voltage at various points throughout the circuit (Figure 6–10).
Figure 6-10 By attaching one meter probe to any grounded spot, voltage may be monitored.
It is, nevertheless, normal practice not to ground one side of the control transformer. This is commonly known as a floating system. If only one voltmeter probe is attached to a grounded location, the meter reading will be incorrect or meaningless since there is no complete circuit (Figure 6–11). High impedance voltmeters would most likely show some voltage due by ground capacitance and induced voltage produced by nearby magnetic fields. These are commonly known as ghost voltages. No voltage would be indicated using a low impedance meter, such as a plunger type voltage tester.
Figure 6-11 One side of the control transformer is not grounded in floating control systems.
A voltmeter probe connected to a grounded location would provide false results since no full circuit exists.
A float control system may, however, monitor voltage accurately by attaching one voltmeter probe directly to one side of the control transformer (Figure 6–12). Because grounded and floating control systems are both widespread, both will be illustrated in this article.
Figure 6-12 On a floating control system, connecting one meter
probe directly to one side of the transformer will produce precise readings.
Transformer Fusing
Fuse or circuit breakers are commonly used to safeguard control transformers. Protection can be installed on either the main or secondary side of the transformer, and certain businesses want both sides to be protected. NEC
Section 430.72(C) specifies the criteria for transformer protection in motor control circuits.
This provision essentially specifies that control transformers with primary currents less than 2 amps must be protected by an overcurrent device set at no more than 500% of the rated primary current. This high percentage is required due to the high in-rush current associated with transformers. To calculate the rated current of a transformer, divide its volt-ampere rating by its main voltage.
EXAMPLE:
What is the maximum fuse size permissible to protect the primary winding of a 300 volt-ampere control transformer connected to 240 volts?
Solution:
I = VA/E
I = 300/240
I = 1.25 A
Fuse size = 1.25*5
Fuse size = 6.25 A
According to NEC Section 240.6, a standard fuse size is 6 amperes. A fuse of 6 amps would be used.
NEC Section 430.72(C)(2) says that fuse protection in line with 450.3 is permissible also.
This section specifies that NEC Table 430.3 is used to establish primary protection for transformers rated at 600 volts or less (B). According to the table, the rating is 300% of the rated current.
NEC Table 450.3 can also be used to determine the secondary fuse size (B). For fuses protecting a transformer secondary with a current less than 9 amps, the table shows a rating of 167% of the rated secondary current. In the above example, assuming a control voltage of 120 volts, the rated secondary current of the transformer would be 2.5 amperes (300/120). The fuse size would be:
2.5*1.67 = 4.175 A
The closest standard fuse size stated in 240.6 that does not exceed this amount is 3 amperes. Because the secondary does not encounter the high in-rush current of the primary, the secondary fuse size can be set at a lower percentage of the rated current. Because primary and secondary fuse protection is ubiquitous in the industry, the control circuits provided in this article will demonstrate both.
