GARYLAND M/C QUIZ #5

Generally, less than ____ percent error per angle of rotation exists.

5.

Starting methods for single-phase motors involve disconnecting the start winding of a split-phase motor when the motor reaches about ____ percent of its rated speed.

75

____ starting is the simplest of all starting methods.

Across the line.

The term ____________________ means that there are two windings wound together.

Bifilar.

Give an overview of four-step switching.

Each time one of the switches changes position, the rotor will advance one-fourth of a tooth. After four steps, the rotor has turned the angular rotation of one "full" tooth. If the rotor and stator have 50 teeth, it will require 200 steps for the motor to rotate one full revolution. This corresponds to an angular rotation of 1.8 degree per step. (360 degrees/200 steps = 1.8 degree per step.)

Resistor starting and reactor starting differ in the means used to reduce the amount of torque.

False.

Some resistor and reactor starters use multiple steps of starting. This is accomplished by tapping the resistor or reactor to provide different values of resistance or ____ reactance.

Inductive.

What is the advantage of eight-step switching over four-step switching?

The advantage of this arrangement is that each step causes the motor to rotate one-eighth of a tooth instead of one-fourth of a tooth. The motor now requires 400 steps to produce one revolution, which produces an angular rotation of 0.9 degree per step. This results in better stepping resolution, and greater speed capability.

What is a thermistor?

The solid-state relay uses a solid-state component called a thermistor and, therefore, has no moving parts or contacts to wear or burn. A thermistor exhibits a rapid change of resistance when the temperature reaches a certain point. This particular thermistor has a positive coefficient of resistance, which means that it increases its resistance with an increase of temperature.

Current ____ are used to sense the amount of motor current.

Transformers.

Some circuits may use several steps of starting, depending on the circuit requirements.

True.

Split-phase motors intended to operate in the open accomplish this by the use of a(n) ____________________ switch connected to the shaft of the rotor.

centrifugal.

Current rises at a(n) ____________________ rate in an inductive circuit.

exponential.

Inrush current is referred to as ____________________ current.

locked rotor.

Resistor starting is accomplished by connecting resistors in ____________________ with the motor during the starting period.

series.

What is reactor starting?

A reactor starter is the same basic control except that reactors or choke coils are used to limit in-rush current instead of resistors. Reactors limit in-rush current with inductive reactance instead of resistance. Reactors have an advantage in current-limiting circuits because of the rise time of current in an inductive circuit. In a resistive circuit, the current will reach its full Ohm's Law value instantly. In an inductive circuit, the current must rise at an exponential rate. This exponential rise time of current further reduces the in-rush current.

To allow for better stepping resolution, most stepping motors have ____ stator poles.

Eight.

Stepping motors are similar to other types of DC or AC motors in that their output shaft moves through a specific angular rotation each time the motor receives a pulse.

False.

A resistive-capacitive network can be used to provide the ____________________ -degree phase shift needed to change single-phase AC into two-phase AC.

Ninety.

What is a current relay?

The current relay operates by sensing the amount of current flow in the circuit. This type of relay operates on the principle of a magnetic field instead of expanding metal. The current relay contains a coil with a few turns of large wire and a set of normally open contacts. The coil of the relay is connected in series with the run winding of the motor, and the contacts are connected in series with the start winding.

When is it impossible to start a motor across the line?

There are conditions where it is not possible to start a motor across the line due to excessive starting current or excessive torque. Some power systems are not capable of providing the high initial currents produced by starting large horsepower motors. When this is the case, some means must be employed to reduce the amount of starting current. Two of the most common methods are resistor starting and reactor starting. Although these two methods are very similar, they differ in the means used to reduce the amount of starting current.

Stepping motors can be operated on AC voltage.

True.

The potential starting relay is used with a different type of split-phase motor called the capacitor start-capacitor run motor.

True.

Three-phase squirrel cage motors are the most popular motors used in industry.

True.