Motor Controls Final Exam

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True

If the current flow through the shunt field is interrupted, a compound wound, DC motor will become a series motor and race to high speeds.

False

If the load is added to a motor, the motor speed increases.

True

Many autotransformer starters contains taps that can be set for 50, 65, or 80 percent of the line voltage.

False

Mechanical type brakes cannot hold a suspended load.

False

Open transition starting means that the motor is connected to the power line during the transition from wye to delta.

True

Synchronous motors are able to operate at the speed of the rotating magnetic field because they are not induction motors.

True

If the winding is such that the series delta gives the low speed and the parallel wye gives the high speed, the torque will be the same for both speeds.

False

In a resistive circuit, the current will take some time to reach its full Ohm's law value.

False

In a wye connection, the line current and phase current are not the same.

True

In an inductive circuit, the current must rise at an exponential rate.

True

In order to produce a rotating magnetic field, there must be more than one phase.

True

In part winding starting, neither of the two windings is individually capable of withstanding the starting current for more than a few seconds.

True

Inverter rated motors are specifically designed to be operated by variable frequency drives.

False

It is possible to troubleshoot a circuit without a working knowledge of schematics.

True

Most autotransformer starters use two transformers to reduce the voltage applied to the motor during the period of acceleration.

True

Most motor circuits are protected by dual-element time delay fuses or inverse time circuit breakers.

True

Motors that used a centrifugal switch to disconnect the start windings cannot be used with variable voltage control.

False

Multiple speeds are obtained by connecting inductance in parallel with the main run winding.

True

One method of providing plugging control is with the use of an automatic timed circuit.

True

One of the factors that must be taken into consideration when determining the conductor size is the temperature rating of the devices and terminals.

True

One way of determining when to shunt the resistors out of the circuit is by sensing the motor current.

True

Overload relays are intended to disconnect the motor from the power line in the event that the current draw becomes excessive.

True

Part winding starters provide closed transition starting.

True

Part winding starting is typically used for motors that supply the moving force for centrifugal pumps, fans, and blowers.

False

Plugging is used with direct current motors more than with three phase squirrel cage motors.

False

Plunger type voltage testers can be used to test control systems that operate on low voltage, such as 24 volt systems.

True

Ramping permits the motor to bring the load up to speed slowly as opposed to simply connecting the motor directly to the line.

True

Resistors connected in series with the motor limit the amount of in-rush current.

False

Series field windings are marked F1 and F2 in the terminal connection box.

True

Shaded pole motors are used in applications where it is necessary to reverse the direction of rotation of the motor.

True

Shunt field windings exhibit a much higher resistance than the series field windings.

True

Small wound rotor motors are often controlled manually by a three-pole make-before-break rotary switch.

True

Starting circuits generally employ resistors of a lower wattage value than circuits that are intended for speed control.

False

Table 430.250 is used to determine the full load current of two phase motors

False

The Insulated Gate Biolar Transistor (IGBT) is a current controlled device, not a voltage controlled device.

True

The NEC tables list the amount of the current that the motor is expected to draw under a full load condition.

False

The amortisseur winding provides good starting torque and a relatively high starting current.

False

The armature of a large DC motor has high resistance.

True

The excitation current should be connected to the rotor only after it has accelerated to a speed that is close to synchronous speed.

True

The hot-wire relay functions as both a starting relay and an overload relay.

True

The magnetic field travels through the same number of poles during each complete cycle.

False

The problem with eddy current clutch circuits is that they lack a speed sensing device that automatically increases or decreases the DC excitation current when the load is added or removed.

True

The shunt field power supply is generally designed to remain turned on even when the main (armature) power supply is turned off.

True

The type of bars used to construct the rotor has a great effect on the operating characteristics of the motor.

False

The values listed in Table 430.248 are for motors that operate under high speeds and torques.

True

The wound rotor motor can be started in multiple steps to provide smooth acceleration from 0 RPM to maximum RPM.

False

The wound rotor motor has a lower starting torque per amp of starting current than any other type of three phase motor.

True

Three-speed consequent pole motors can be wound to produce constant horsepower, constant torque, or variable torque.

True

When deenergizing or energizing a control cabinet of motor control center module, the electrician should be dressed in flame retardant clothing while wearing safety glasses, a face shield, and hard hat.

False

When dual voltage motors are used, the motor must be operated on the high voltage setting of the motor.

False

When using an ohmmeter, make certain that the power is connected to the circuit.

False

Wound rotor type motors and synchronous motors generally use autotransformer starters.

False

Capacitive reactance is proportional to the rotor frequency.

False

Centrifugal switches can be used on all types of split phase motors.

True

Changing frequency causes a corresponding change in the inductive reactance of the windings.

True

Closed transition starting does not disconnect the motor from the power line.

False

Clutches cannot provide a smooth start for loads that can be damaged by sudden starting, or for high inertia loads such as centrifuges or flywheels.

True

Compound motors contain both a series and shunt field.

True

Compound motors generally employ a field loss relay (FLR) that is intended to disconnect the motor from the power line in the event that shunt field current should be interrupted or drop below an acceptable level.

True

Contactors intended to connect direct current motors to the line often have arc suppression and magnetic blowout coils.

False

Delta connected dual voltage motors are acceptable for part winding starting.

False

Direct current motors depend on inductive reactance to limit current flow.

True

Double-face clutches have both the armature and field discs mounted on the same hub.

True

Dual voltage motors designed for wye-delta starting will supply all 12 T leads at the terminal connection box.

True

Dual voltage motors not intended for wye-delta connection will have certain terminals tied internally.

True

Dynamic braking for a DC motor is very effective, but the braking effect becomes weaker as the armature slows down.

False

For closed transition starting, four separate start contactors are used.

False

Frogleg wound armatures are most often used in small machines.

True

A DC motor is generally operated at or below normal speed.

False

A synchronous motor can not be made to produce a leading power factor.

True

A synchronous motor should never be started with excitation applied to the rotor.

False

A three phase, two-pole motor contains eight actual poles.

False

Across-the-line starting is only used for alternating current.

True

Across-the-line starting is the simplest of all starting methods.

False

Alternators are often used to control the speed of several induction motors that require different changes in speed.

True

Autotransformer starters provide greater starting torque per ampere drawn from the line than any other type of reduced voltage starter.


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