Unit 4-6 AC/DC MOTORS Test

The turns ratio of the transformer can be changed by looping the power wire through the opening in the transformer to produce a primary winding of more than one turn.

True

Transformer formulas can be used to determine the number of secondary turns needed to produce desired ranges.

True

When the primary of a transformer is connected to power but there is no load connected to the secondary, current is limited by the inductive reactance of the primary.

True

Autotransformers, like isolation transformers, can be used as step-up or step-down transformers.

Ture

The polarity of transformer windings can be determined by connecting one lead of the primary to one lead of the secondary and testing for an increase or decrease in voltage. This is often referred to as a ____ connection.

buck or boost

Magnetic materials contain tiny magnetic structures in their molecular material known as magnetic ____.

domains

The volts-per-turn for any transformer is determined by the ____.

primary winding

The autotransformer has one disadvantage: Since the load is connected to one side of the power line, there is ____ between the incoming power and the load.

no line isolation

The ____ transformer is basically an isolation transformer with a high-voltage rating on the primary winding, and that is intended for use in metering circuits.

potential

The amount of core material needed for a particular transformer is determined by the ____ rating of the transformer.

power

CTs are designed to operate with ____ winding shorted.

the secondary

A problem with many clamp-on ammeters is that ____.

their lowest scale value is too high to accurately measure low-current values.

A(n) ____ transformer is a transformer constructed with a hollow core, similar to a donut in that it has a hole in the middle.

toroid

Current transformers are used to change the full-scale range of ____.

AC ammeters

Autotransformers rarely have multiple taps to provide different voltages

False

To calculate the current flow supplied to the primary, use the formula IP = ____.

Is/ Ratio

The number of turns needed for a full-scale range of amperes can be calculated using the formula ____.

Np/Ns=Is/Ip

A basic law concerning transformers is that all values of a transformer are proportional to its turns ratio.

True

Almost all transformer nameplates will list the primary voltage, secondary voltage, and kVA rating.

True

An autotransformer is literally a self-transformer.

True

Current transformers differ from voltage transformers in that the primary winding is generally part of the power line.

True

Many electricians use the clamp-on type of AC ammeter.

True

The autotransformer's one disadvantage can cause problems with certain types of equipment and must be a consideration when designing a power system.

True

Assume that an ammeter provides range values of 5 A, 2.5 A, 1 A, and 0.5 A. Also, assume that the meter movement requires a current flow of 100 mA (0.100) to deflect the meter full scale and that the primary of the current transformer contains 5 turns of wire. The number of turns needed for a full-scale range of 5 amperes is ____ turns.

250

The primary winding of a transformer is between points B and N, and has 120 volts applied to it. There are 120 turns of wire between points B and N. The selector switch is set to point D, and the load is now connected between points D and N. The secondary of this transformer contains 40 turns of wire. Assume that the load connected to the secondary has an impedance of 10 ohm. The amount of current flow in the secondary circuit is ____ amps.

4

Autotransformers have very high efficiencies, most in the range of ____.

95%-98%

An ammeter with a full-scale range of 0-6 amperes would not be able to measure accurately a current flow of 0.2 amperes. The answer to this problem is to ____.

wrap multiple turns of wire around the jaw of the ammeter to change the secondary-scale value

The primary current can be computed by using the formula ____.

Ep/Es= Is/Ip

If the amount of voltage applied to the load is to be computed, the formula ____ can be used.

Ep/Es=Np/Ns

A good rule for any type of transformer is that power in is always slightly greater than power out.

False

Transformers can be divided into several classifications, which include ____.

isolation, auto, and current.

A very common type of isolation transformer is the distribution transformer, which is used to supply power to

most homes and many businesses

To use the clamp-on type of AC ammeter, clamp the jaw of the meter around ____ of the conductors supplying power to the load.

only one

____ of a current transformer must be connected in series with the load.

the primary winding

A transformer nameplate does not list the current rating of the windings. The current rating for a winding can be determined by dividing the kVA rating by the ____.

winding voltage

In-line ammeters (ammeters that must be connected directly into the line) that have multiple-range values cannot use a current transformer to provide the different ranges.

False

The amount of current flow in a secondary circuit can be computed using the formula I = Z / E.

False

The clamp-on ammeter does not have different range settings.

False

The secondary winding of a CT should never be opened when there is power applied to the primary.

True

If a rotary switch were to be removed and replaced with a sliding tap that made contact directly with the transformer winding, the turns ratio could be adjusted continuously. This type of transformer is commonly referred to as a ____ depending on the manufacturer

Variac or Powerstat

An autotransformer has a 2:1 turns ratio. A voltage of 480 volts is connected across the entire winding. Since the transformer has a turns ratio of 2:1, a voltage of 240 volts will be supplied to the load. Ammeters connected in series with each winding indicate the current flow in the circuit. It is assumed that the load produces a current flow of 4 amperes on the secondary. A current flow of ____ amperes is supplied to the

2

One of the formulas for determining current in a single-phase circuit is ____.

I= VA/E

According to ANSI rules, the high-voltage leads of a transformer are to be marked H1 - H2, and so forth, and the low-voltage leads are marked ____, and so forth.

X1-X2

Assume that an isolation transformer has 240 turns of wire on the primary and 60 turns of wire on the secondary. This is a ratio of 4:1 (240/60 = 4). Now assume that 120 V is connected to the primary winding. The voltage of the secondary winding is ____V.

30

A current transformer has a ratio of 600:5. If the primary power wire is inserted through the opening, it will require a current of 600 amps to deflect the meter full scale. If the primary power conductor is looped around and inserted through the window a second time, the primary now contains two turns of wire instead of one. It requires ____ amps of current flow in the primary to deflect the meter full scale.

300

The primary winding of a transformer is between points B and N, and has 120 volts applied to it. There are 120 turns of wire between points B and N. The selector switch is set to point D, and the load is now connected between points D and N. The secondary of this transformer contains 40 turns of wire. The amount of voltage applied to the load is ____ V.

40V

A common type of isolation transformer found throughout industry is the control transformer, which is used to increase the line voltage to the value needed to operate control circuits.

False

The clamp-on type of AC ammeter uses a current transformer to operate.

True

An autotransformer uses the same winding as the primary transformer, but does not use the same winding as the secondary transformer.

False

If the number of primary turns is the sum of the turns between taps (such as 80 + 100 + 60 = 240 turns), and 120 volts are connected across the 240 turns, this transformer will have a volts-per-turn ratio of 2.0.

False

If the primary section of a transformer is connected across part of a winding, and the secondary is connected across the entire winding, the secondary section contains more windings than the primary. This type of transformer is a step-down transformer.

False

The reason that the isolation transformer can greatly increase any voltage spikes before they reach the secondary is because of the rise time of current through an inductor.

False

There will always be some amount of current flow in the primary of any voltage transformer even if there is no load connected to the secondary. This is called the relaxation current of the transformer.

False

When current transformers are used, the main power line is inserted through the opening in the transformer. The power line acts as the primary of the transformer and is considered to be two turns.

False

When the secondary winding of a CT is opened when there is power applied to the primary, this will cause the transformer to produce a step-down in voltage that could still be high enough to kill anyone who comes in contact with it.

False

Factors such as the amount and type of core material, wire size used to construct the windings, number of turns, and the degree of magnetic coupling between the windings greatly affect the transformer's impedance.

True

The amount of inrush current in the primary of a transformer is limited by amount of applied voltage, resistance of the wire in the primary winding, and flux change of the magnetic field in the core.

True

The amount of power input and output for an autotransformer must be the same.

True

The primary current can be computed by using the same formula that is used to compute primary current for an isolation-type transformer.

True

The primary winding of a transformer is the side of the transformer that is connected to the driven load.

False