Garyland Super Long Exam #3

Why is it necessary to reduce the voltage to a motor when the frequency is reduced?

A reduction in frequency causes a corresponding reduction of inductive reactance. The voltage must be decreased to prevent the motor from being damaged by excessive current.

What is a synchronous motor called when it is operated without load and used for power factor correction?

A synchronous condenser.

What is a thyristor?

A thyristor is a solid state device that operates in only two states. It is either completely turned on or completely turned off.

What are the four primary parts of programmable logic controllers?

A. The power supply B. The central processing unit (CPU) C. The programming terminal or program loader D. The I/O (pronounced eye-oh) rack

After an SCR has been turned on, what must be done to permit it to turn off again?

After an SCR has been turned on, the current flow through the anode-cathode section must drop below the holding current level before it will turn off.

What is an easy way to remember how an OR gate functions?

An easy way to remember how an OR gate functions is to say that any 1 input a 1 output.

The amount of counter torque developed by a direct - current generator is proportional to what?

Armature current.

Why are mechanical brakes often used on cranes?

Because they can hold a suspended load.

Why are universal motors so named?

Because they can operate on AC or DC current.

How is dynamic braking for direct - current motor accomplished?

By connecting direct current to the stator winding.

How is the speed of an eddy current clutch controlled?

By controlling the amount of direct current excitation current applied to the clutch.

How is plugging stop accomplished?

By momentarily reversing the direction of rotation of a motor.

How can a synchronous motor be made to produce a leading power factor?

By over exciting the rotor.

In variable frequency drives that employ IGBTs, how is the output to the motor controlled?

By pulse width modulation.

How is the dynamic braking force of an alternating - current motor controlled?

By the amount of direct current connected to the stator winding.

Explain why controlling the rotor current controls the stator current also.

Controlling the rotor current also controls the stator current because an induction motor is basically a transformer. When Nikola Tesla patented the induction motor, he patented it as a rotating transformer.

Explain why the wound rotor motor produces the greatest of starting torque per ampere of starting current of any three - phase motor.

Controlling the rotor current also controls the stator current because an induction motor is basically a transformer. When Nikola Tesla patented the induction motor, he patented it as a rotating transformer.

Name two factors that determine the synchronous speed of a motor?

Frequency of the applied voltage Number of stator poles per phase

What is the primary difference between a GTO and an SCR?

GTOs can be turned off by applying a negative pulse to the gate with respect to the cathode.

What will happen if the motor is overloaded in a basic stop-start, push-button circuit with overload protection?

If the motor is overloaded, it will cause too much current to flow through the circuit. When a current greater than normal flows through the overload heater, the heater produces more heat than it does under normal conditions. If the current becomes high enough, it will cause the normally closed overload contact to open. Notice that the overload contact is electrically isolated from the heater. The contact, therefore, can be connected to a different voltage source than the motor. If the overload contact opens, the control circuit is broken and the relay de-energizes as if the STOP button had been pushed. After the overload contact has been reset to its normally closed position, the coil will remain de-energized until the START button is again pressed.

Why are the resistors used in the rotor circuit smaller for a starter than for a controller?

In a starter circuit the resistors are in the circuit for only a very short period of time. In a controller circuit, the resistors must have a high enough wattage rating to operate continuously.

____________________ contacts are contacts that exist in the logic of the program only.

Internal

What type of motor is generally used with IGBT drives?

Inverter rated motors.

What is the purpose of the DC coil on a polarized field frequency relay?

It is used to polarize the magnetic core of the relay.

How are junction transistors driven into saturation, and what is the advantage of driving a transistor into saturation?

Junction transistors can be driven into saturation by supplying them with an excessive amount of base-emitter current. The advantage is that they can produce a very low voltage drop in the range of 0.04 to 0.03 volts.

Name three factors that determines the amount of induced voltage.

Length of conductor or number of turns of wire, and Speed of the cutting action.

What is the advantage of consequent pole motors over some other types of variable speed motors?

Maintain high torque at low speeds

Name three methods of braking a motor.

Mechanical brakes, Dynamic braking, Plugging.

Should the excitation current be applied to the rotor of a synchronous motor before it is started?

No. Excitation current should be applied only after the rotor has accelerated to a speed close to synchronous speed.

Does varying the voltage to an AC induction motor cause a change in synchronous speed?

No. Frequency and number of stator poles determines synchronous speed.

What device is generally used to accurately stop a motor when a plugging stop is used?

Plugging switch or zero speed switch.

What is meant by "ramping" and why is it used?

Ramping means to increase or decrease motor speed over some period of time. It is used to bring a load up to speed slowly instead of connecting it directly to the power line.

Name three factors that determines the amount of torque developed by a wound rotor induction motor.

Strength of the magnetic field of the stator, Strength of the magnetic field of the rotor, and Phase angle difference between stator and rotor flux.

What is the advantage of an IGBT over a junction transistor?

The IGBT is a voltage controlled device, not a current controlled device. This gives it the ability to turn on and off very fast.

What controls the output voltage of the alternator when a brushless exciter is used to supply the excitation current of the rotor?

The amount of direct current supplied to the pole pieces of the brushless exciter.

What is rotor slip?

The difference in speed between the synchronous speed of the rotating magnetic field and the speed of the rotor.

What is the function of a field discharge resistor?

The field discharge resistor prevents high voltage from being induced into the rotor winding of a synchronous motor.

What is the disadvantage of a motor that a high - impedance stator winding?

The high impedance prevents the motor from producing a large amount of torque.

Name two requirements that must be met before a motor can be used for wye-delta starting?

The motor must be designated for the stator windings to be connected in delta during the run period All stator windings leads must be accessible

What is the purpose of an out-of-step relay?

The out-of-step relay disconnects the motor from the circuit in the event that excitation current is not applied within a certain length of time.

If an alternator is used to provide variable frequency, how is the output voltage of the alternator controlled?

The output voltage of an alternator is controlled by the amount of DC excitation current.

What is the main disadvantage of using SCR's in a variable frequency drive?

The switching of the SCRs can cause harmonics on the line.

Why is it possible for a synchronous motor to operate at the speed of the rotating magnetic field?

The synchronous motor can operate at synchronous speed because it is not an induction motor and does not depend on induction to produce a magnetic field in the rotor.

What is the disadvantage of driving a junction transistor into saturation?

The transistor cannot turn off as quickly which greatly limits the frequency response.

What are the three main electrical test instruments used in troubleshooting?

The voltmeter, ammeter, and ohmmeter.

What is the advantage of dynamic brakes over mechanical brakes?

There are no brake shoes to wear out.

What is the purpose of the slip rings located on the rotor shaft of a wound rotor motor?

They permit external resistance to be connected in the rotor circuit.

Why are SCR's used to construct a bridge rectifier in many solid-state variable frequency drives?

They permit the output voltage to be controlled.

What solid-state device is generally used to produce variable frequency in drives designed to control up to 500 horsepower?

Transistors.

What happens when a normal snap-action switch is used with an electronic control system?

When this type of switch is used with an electronic control system, contact bounce can cause a great deal of trouble. Most digital logic circuits are very fast-acting and can count each pulse when a contact bounces. Depending on the specific circuit, each of these pulses may be interpreted as a command. Contact bounce can cause the control circuit to "lose its mind."