Motors and Generators

Question 1

Describe the electrostatic forces present within the boundaries of the atoms and discuss the role these forces play in simple electrical/electronic applications.

Answer 1

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.

Question 2

Define voltage ((EMF).

Answer 2

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.

Question 3

Define current.

Answer 3

It is the movement of the flow of electrons

Question 4

Define resistance.

Answer 4

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.

Question 5

Define power.

Answer 5

It is the rate of doing work.

Question 6

Define frequency.

Answer 6

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.

Question 7

Define capacitance.

Answer 7

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.

Question 8

Define capacitive reactance.

Answer 8

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.

Question 9

Define inductance.

Answer 9

It opposes the change of current flow and is the result of the expanding and collapsing magnetic field caused by the changing current.

Question 10

Define inductive reactance.

Answer 10

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.

Question 11

Define impedance.

Answer 11

It the opposition to alternating current flow.

Question 12

State OHM’s Law.

Answer 12

“The voltage drop across any circuit or
component is proportional to both its resistance
and its current flow.” E = I x R

Question 13

State Kirchoff’s Law

Answer 13

“The sum of the voltages around any closed

loop is equal to zero.”

Question 14

State the Power Equation.

Answer 14

P = I x E or P = I^2 x R or P = E^2/R

Question 15

State the requirements for generator action.

Answer 15

(1) a magnetic field, (2) a conductor,
and (3) relative motion between the magnetic
field and conductor.

Question 16

State the requirements for motor action.

Answer 16

If a current-carrying conductor is placed in a magnetic field, a force will be exerted on the conductor.

Question 17

Explain the operation of a DC motor.

Answer 17

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.

Question 18

Explain Slip in an AC motor.

Answer 18

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

Question 19

Explain an AC induction motor.

Answer 19

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.

Question 20

Explain a Synchronous AC motor.

Answer 20

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.

Question 21

Explain why AC and DC motors draw large starting currents.

Answer 21

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.

Question 22

Explain how to limit high starting currents in AC and DC motors.

Answer 22

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.

Question 23

Identify the reason for limiting the number of motor starts in a given period.

Answer 23

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.

Question 24

Identify the effects of overheating insulation and bearings in motors and generators.

Answer 24

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.

Question 25

List the causes of excessive motor and generator currents.

Answer 25

Can arise from undervoltage and overloading the motor.

Question 26

List the indications of a locked pump rotor.

Answer 26

Instantaneous increase in pump current attempting to supply demanded torque. Instantaneous decrease in pump discharge pressure. Instantaneous decrease in system flow rate, Instantaneous increase in motor winding temperatures. Possible breaker trip.

Question 27

Differentiate between a locked rotor and a sheared rotor.

Answer 27

While you get a decrease in pump discharge pressure and system flow rate with both, running amps go to no load on a sheared rotor.

Question 28

What is the relationship between centrifugal pump motor speed and fluid flow rate?

Answer 28

Fluid flow rate is directly proportional to speed.

Question 29

What is the relationship between centrifugal pump motor speed and pump head?

Answer 29

Pump head is directly proportional to the square of pump speed.

Question 30

What is the relationship between centrifugal pump motor speed and pump power?

Answer 30

Pump power is directly proportional to the cube of pump speed.

Question 31

What is the relationship between centrifugal pump motor speed and pump current?

Answer 31

Pump current is directly proportional to the cube of pump speed.

Question 32

What is the relationship between centrifugal pump motor current and stator temperature?

Answer 32

As stator temperature rises, the resistance of the windings increases. Current flow through the increased resistance is partially given off as heat. Therefore, some portion of the current drawn by the motor is lost as heat energy, and additional current is required to maintain the
equivalent power and flow rate from the pump.

Question 33

Describe how alternating current flow is generated.

Answer 33

The alternate cutting and paralleling of magnetic flux lines by a conductor.

Question 34

Describe how direct current flow is generated.

Answer 34

A commutator reverses the polarity of the alternate cutting each time it goes to zero and switches. In this way pulsed DC is generated (180 positive to 0 to 180 positive)

Question 35

Define Apparent Power relative to the power triangle.

Answer 35

Apparent power is the product of the voltage and

current delivered to an electrical circuit.

Question 36

Define True Power relative to the power triangle.

Answer 36

The NET power that is actually transferred from the generator to the load (averaged over time). It is the power consumed by the resistive loads in an electrical circuit.

Question 37

Define Reactive Power relative to the power triangle.

Answer 37

The part that is returned to the load (averaged
over time) from the collapsing of magnetic fields. It is the power consumed in an AC circuit because of the expansion and contraction of magnetic (inductive) and
electrostatic (capacitive) fields.

Question 38

Define the Power Factor relative to the power triangle.

Answer 38

It is the ratio between true power and apparent power.

Question 39

List the three conditions required for paralleling AC sources.

Answer 39

1. The generator voltage must be the same or slightly higher than the grid voltage. 2. The frequency of the incoming generator must be slightly greater than the operating grid frequency. 3. The generator must be in phase with the grid voltage.