transformer

Drive isolation transformers protect the primary source from common-mode energies that originate with the motor drive system and can prevent ground current from transferring back through the primary ground system.

A(n) autotransformer is any transformer in which the primary and secondary circuits have a portion of their two windings in common.

The polarity of the voltage that causes current to flow from H1 to H2 in the primary results in current flow from X1 to X2 in the secondary.

The load power for a buck-boost transformer that boosts the source by 30 volts at a current of 40 amperes is 1.2 .kva

When current in a buck-boost transformer is more than nine amperes and the calculated fuse rating does not correspond to a standard fuse rating, the next higher fuse rating is permitted.

The overcurrent protection device in a buck-boost transformer should be wired in series with the shared winding. False

A drive isolation transformer is designed with additional reactance to reduce power quality problems associated with nonlinear loads.

The secondary of a drive isolation transformer represents a separately derived system.

Buck-boost transformers are typically available in sizes up to 10kva .

Buck-boost transformers can be used as isolating transformers.

Drive isolation transformers and line reactors both provide reactive control of distorted line current harmonics and line notching.

Drive isolation transformers can provide protection from common-mode transients that originate with the primary source.

When used during motor starting, an autotransformer must be allowed time to cool down before it is used again. A typical duty cycle is 10 seconds ON and 10 minutes OFF

For very large autotransformers, the cost of insulation increases faster than the reduction in copper reduces costs.

A reactor is a coil added in series with a load to reduce inrush current, voltage notching effects, and voltage spikes.

Reactors are used in the starting circuit.

The impedance of an autotransformer is greater than the impedance of a two-winding transformer. False

Since an autotransformer can be designed with only one winding, less copper can be used and costs may be lower than for an equivalent two-winding transformer.

Output reactors help eliminate voltage spikes by slowing down the rate of change in the drive output voltage.

To reduce notching, a reactor can be used to add impedance. The added impedance decreases notch depth and widens notch width

Transformers connected in a wye configuration require that the taps be changed on 3 coil(s).

Transformers configured in the open delta configuration require that the taps be changed on 2 coil(s)

A buck-boost transformer is typically used to change voltage by between 5% and 25%.

When a buck-boost transformer has current less than nine amperes, an overcurrent protection device is allowed to be rated at not more than 167% of the input current.

Electric motors typically draw many times their full-load current during startup. e

A(n) common-mode choke is a reactor used to reduce common-mode noise current generated by the rapid switching of a motor drive or a signaling device.

A saturable-core reactor is an inductor whose inductance is controlled through the use of a magnetic field created by a second winding wound around the same iron core as the primary winding.

Common-mode noise is a type of electromagnetic interference induced on power or communication lines.

Autotransformers have low impedance compared to a conventional transformer.

Autotransformers have a lack of isolation between the primary and the secondary.

Which of the following is NOT an application for autotransformers? Used in the switching circuitry for generator transfer switches

Because the impedance of an autotransformer is lower than that of an equivalent two-winding transformer, higher current flows when a short circuit fault occurs.

A reactor should have about 3% impedance to reduce notch depth by about 50%.

Zero-crossovers are often used for timing signals and to activate equipment.

When a buck-boost transformer has current of nine amperes or more, an overcurrent protection device is allowed to be rated at 125% of the full-load current.

Zigzag connected transformers are not permitted to be installed on the load side of any system grounding connections.

Which of the following are common methods of sizing a buck-boost transformer?

I. Determine the load current and the buck or boost voltage and calculate the required size of the buck-boost transformer. II. Determine the load power and calculate the required size of the buck-boost transformer.

Reduced voltage during motor starting results in reduced current and torque.

In an autotransformer, the open delta configuration provides better voltage balance than the wye configuration. False

For voltage regulation, an autotransformer can be installed in combination with the tap changer to split the percentage of change.

A(n) buck-boost transformer is a small transformer designed to lower or raise line voltage.

The opposition to current flow that is only present in AC is reactance

All two-winding transformers are isolation transformers.

An overcurrent protection device must be provided to cause the main switch or common overcurrent protection device to open if the load reaches 125% of its rating.

Buck-boost transformers have dual-voltage primaries and secondaries.

In a saturable-core reactor, large amounts of DC current can be controlled by a small amount of AC current. False

Electrostatic shields can prevent common-mode noise from transferring from the transformer's primary to the secondary only if the secondary is grounded. False

For a step-down autotransformer application, the source is applied across the entire coil , which acts as the primary.

For a step-up autotransformer application, the source is applied across only part of the coil .

Buck-boost transformers should not be used to create a 120/240-volt single-phase service fed by a 208Y/120-volt 3-phase, 4-wire source.

For buck-boost transformers, the NEC® makes a distinction between those rated at less than 9A and those rated at that amount of current or more.

Different loads require different starting torque. For a load with a very high starting torque, the autotransformer can be tapped to deliver 80% voltage.

For loads with a lower starting torque, the autotransformer can be tapped to deliver 50% voltage. The lower voltage start draws a smaller line current than the higher voltage.

Which of the following are benefits associated with grounding the secondary of a drive isolation transformer? I. Grounding provides superior transient and impulse immunity to the load side of the transformer.

II. Grounding reduces common-mode noise coupling from primary to secondary. III. Grounding prevents induced ground current from transferring upstream into the primary system.

In autotransformer starting, the motor terminal voltage depends on the load current. False

In a variable transformer, it is possible to move the contact under load without interrupting the circuit.

Buck-boost transformers can be wired with subtractive polarity to buck the source voltage.

Length of a power line does not significantly impact voltage drop on the line. False

A buck-boost transformer needs to have a rating equal to the maximum possible unbalanced load current on the neutral of a 4-wire system.

Which of the following is not a common buck-boost transformer application? Creating a 3-phase, 4-wire wye circuit from a 3-phase, 3-wire delta circuit

A(n) fault-sensing system can consist of current transformers wired to sense when an unbalance of 50% or more is present in the line current.

A buck-boost transformer is not an optimal choice for use with a fluctuating load because the voltage drop on a line varies with load.

Which of the following is NOT a basic function of a drive isolation transformer? Increase of notch depth

A choke is a reactor used to restrict the current to AC or DC drives in the case of short circuits in the drive itself.

A 50% reduction in notch depth is enough to eliminate extra zero crossovers that cause problems.

A saturable-core reactor consists of one winding. False

In autotransformers used for motor starting, the wye configuration is used for small motors and the open delta configuration is used for large machines. False

A(n) variable transformer is a continuously adjustable autotransformer consisting of a single layer of wire wound on a toroidal core, and a carbon brush that traverses this winding.

In an autotransformer, Tap H2 connects to Tap X2 .

An autotransformer has a higher power rating than an equivalent two-winding transformer.

In autotransformer starting, transformer motor current and line current are not equal.

Autotransformer reduced-voltage motor starting reduces the applied motor voltage to 50%, 65%, or 80% of the line voltage when starting.

Autotransformers are not able to block the flow of transients and harmonics throughout a 3-phase power system.

Autotransformers are designed to be overloaded during motor starting. This allows a less expensive autotransformer design to be used.

A notch is a distortion in a voltage waveform where the voltage quickly drops toward zero and then returns to the correct value.

In general, transformer secondary conductors are not considered to be protected by the primary overcurrent device.

Which of the following is a use for line input reactors? A motor drive causes notching, Low line impedance allows high inrush current, Power factor correction capacitors are used

Nonlinear rectified power supplies are a common cause of power quality problems.

The National Electrical Code® requirements found in Section 450.3 are for the protection of . the windings of the transformer only

Where primary-only protection for transformers is provided with rated currents of nine amperes or more, the basic rule requires that each transformer rated 1000 volts, nominal, or less be protected by an individual overcurrent protective device on the primary side, rated or set at not more than 125% of the rated transformer primary current.

With a variable-frequency drive, unwanted pulses of current are caused by fast-changing voltages from the drive.

With 3-phase motor drives using SCR 6-pulse (6-diode) rectifiers, there are short intervals of time when more than one SCR is ON. This causes a transient short circuit to flow six times per line power cycle. This short circuit current flow causes nonlinear voltage to drop across the system impedance and results in distortion in the voltage waveform called a notch.