Motors and Generators Flashcards
(39 cards)
Describe the electrostatic forces present within the boundaries of the atoms and discuss the role these forces play in simple electrical/electronic applications.
It is the valence electrons that we are most concerned with in electricity. These are the electrons that most easily break loose from their parent atom. Normally, conductors have three or fewer valence electrons; insulators have five or more valence electrons; and semiconductors usually have four valence electrons.
Define voltage ((EMF).
An electric charge can do the work of moving another charge by attraction or repulsion. The ability of a charge to do work is called its “potential”. When one charge is different from the other, there must be a difference in potential between them. The sum of the difference of
potential of all the charges in the electrostatic field is referred to as electromotive force.
Define current.
It is the movement of the flow of electrons
Define resistance.
It is the opposition to current flow. It is proportional to the
length of the wire, and inversely proportional to
the cross-sectional area of the wire.
Define power.
It is the rate of doing work.
Define frequency.
For a waveform it is the number of times per second an identical pattern repeats itself. Each time the waveform changes from zero to maximum and from maximum to zero, or from zero to minimum and from minimum to
zero, is called an “alternation”. Two alternations form one cycle.
Define capacitance.
It is the ability to store a charge, and in storing that charge a capacitor opposes a change in voltage. Capacitors block direct current. Once charged, no current flows in the circuit. Alternating current flows in a capacitive circuit with AC voltage applied. A smaller capacitance allows less current.
Define capacitive reactance.
It is the opposition that a capacitor offers to AC. It decreases with increasing frequency, or for a given frequency, the capacitive reactance decreases with increasing capacitance. Capacitive reactance opposes a change in voltage. It offers high resistance to DC and
very low resistance to AC. Voltage lags current in capacitive circuits.
Define inductance.
It opposes the change of current flow and is the result of the expanding and collapsing magnetic field caused by the changing current.
Define inductive reactance.
It is the opposing force that an inductor presents to the flow of alternating current. Inductive reactance opposes a change in current. It offers low resistance to DC and
very high resistance to AC. Voltage leads current in inductive circuits.
Define impedance.
It the opposition to alternating current flow.
State OHM’s Law.
“The voltage drop across any circuit or
component is proportional to both its resistance
and its current flow.” E = I x R
State Kirchoff’s Law
“The sum of the voltages around any closed
loop is equal to zero.”
State the Power Equation.
P = I x E or P = I^2 x R or P = E^2/R
State the requirements for generator action.
(1) a magnetic field, (2) a conductor,
and (3) relative motion between the magnetic
field and conductor.
State the requirements for motor action.
If a current-carrying conductor is placed in a magnetic field, a force will be exerted on the conductor.
Explain the operation of a DC motor.
The DC motor operates because of the principle of motor action when a current-carrying conductor is placed in a magnetic field. This tends to move the conductor at right angles to the direction of the magnetic field.
Explain Slip in an AC motor.
If the rotor were to reach synchronous speed exactly, there would be no relative motion between the rotor and rotating magnetic field. Therefore, no voltage or current would be induced in the rotor and, subsequently no torque would be produced. Instead, the rotor runs just
enough below synchronous speed (under no load
conditions) to establish sufficient rotor current and torque to offset rotor losses. This difference between stator field synchronous speed and rotor speed is termed “slip”. Slip can be expressed as: Slip = (Ns - Nr)/Ns
Explain an AC induction motor.
A magnetic field rotates past the initially stationary rotor bars, a voltage is induced in the bars since relative motion has been provided between the magnetic
field and a conductor. Because the rotor bars are short-circuited or connected by conducting rings, a complete path or circuit is provided and the induced voltage causes currents to flow in the rotor bars.
Explain a Synchronous AC motor.
When three-phase current is applied to the armature of a synchronous motor, it produces a revolving magnetic field. This produces a starting torque that rotates the rotor. Since the motor starts similarly to a squirrel cage motor, the speed will be slightly less than synchronous
speed. When excitation is applied to the magnetic field coils, it produces alternate north and south poles which “lock into” position with the revolving magnetic field of the armature.
Explain why AC and DC motors draw large starting currents.
Because CEMF is not developed until rotation of a motor begins, the resistance of the armature is the only current-limiting factor during motor starting. Since armature resistance in most motors is designed low to minimize I^2R losses, initial armature current (or starting current) is
very high.
Explain how to limit high starting currents in AC and DC motors.
Starting resistors in series with the armature are used for limiting starting current in DC motors. The same principles operate in AC motors except that inductive reactance is involved, (discussed later in this chapter) This inductive reactance raises the total circuit resistance and limits the starting current to five to seven times its normal running current without the use of starting
resistors.
Identify the reason for limiting the number of motor starts in a given period.
To prevent excessive heat generation. Resistive losses in the copper coils and eddy currents within the rotor core produce a significant amount of heat, which breaks down insulation and reduces motor life expectancy.
Identify the effects of overheating insulation and bearings in motors and generators.
Insulation breakdown can result in short circuits and grounds, motor or generator trips, blown fuses and degraded resistance readings. Undervoltage can result in bearing problems since low voltage results in excessive torque loading on the machine. Overheating of bearings can result in early bearing failure.
