Mass is the amount of matter contained within an object. The mass of an object does not change unless matter is added or removed. For this reason, the mass of a  person on earth would be the same quantity if that same person were on the moon.

The weight of an object is the result of the force that gravity exerts on an object.

Weight and mass are often confused. To help explain the difference, imagine you weighed 180 pounds on earth. If you were now on the moon, you would only weigh 30 pounds! This is due to the fact that the moon’s gravity is one-sixth that of earth’s. However, your mass did not change. You are still the same size and shape on the moon as you were on earth. Your weight changed because the force exerted on you by the moon’s gravity is less than the force exerted on you by the earth’s gravity.

Speed is the rate of motion (how far an object moves within a specified amount of time).

You are probably most familiar with the speed of an automobile, such as 60 miles per hour or 60 mph. If you break this down, it means that the automobile has driven 60 miles (how far) in one hour (in how much time). This is its speed.

Velocity denotes not only the rate of motion but also the direction of the motion.

Velocity is often confused with speed, and sometimes these two terms can be used interchangeably, especially when the direction is not known or not important. As before, we can say that an automobile is traveling at 60 miles per hour. This is the speed of the automobile. If we said that the automobile was traveling 60 mile per hour north, then we would be speaking of the automobile’s velocity. Notice the addition of a direction. This is what distinguishes velocity from speed.

Work is done when a force, applied to an object, results in movement of that object in the direction of the applied force.

For example, assume your car is stuck in some mud. You get behind the car and push with all the force you can muster and the car moves! You have just done some work. You applied a force to an object, which resulted in the movement of that object.

Electrical current is the movement of electrons. Electrons move from atom to atom as electrical current flows in a conductor. The rate at which the electrons move past a point is called an ampere (or amp). One ampere is equal to 6.25 × 1018 electrons traveling past a given point in one second of time.

Mathematically, electrical current is defined as:

$$I = \frac{Q}{t}$$

Where:

• I = current (amperes)
• Q = electric charge (coulombs)
• t = time (seconds)

Electrical voltage is the electrical pressure or potential difference that causes electrons to move through a conductor and produce electric current. It represents the amount of energy available to move electric charge between two points in an electrical circuit. Voltage is measured in volts (V).

Mathematically, voltage is defined as the work done per unit charge:

$$V = \frac{W}{Q}$$

Where:

• V = voltage (volts)
• W = work or energy (joules)
• Q = electric charge (coulombs)

Resistance is the property of an electrical circuit or component that opposes the flow of electrical current.

Reactance is the property of an electrical circuit or component that opposes the flow of an alternating current (AC). There are two types of reactance—inductive and capacitive. A determining factor in the amount of reactance is the frequency of the applied AC. This can be seen from the formulas for inductive and capacitive reactance:

For inductive reactance:

$$X_{L}=2\pi fL$$

Where:

• X_L = inductive reactance (measured in ohms);
• π = 3.14; 
• f = frequency (in hertz); 
• L = inductance (measured in henry).

For capacitive reactance:

$$X_{C}=\frac{1}{2\pi fC}$$

Where:

• X_C = capacitive reactance (measured in ohms);
• π = 3.14; 
• f = frequency (in hertz); 
• C = capacitance (measured in farads).

Notice that f appears in both formulas. If the applied current is AC, there will be a value for f. As a result, there will be some value of reactance. However, if the applied current is direct current (DC), the value of f will be zero (0). As a result, there will be no reactance.

Real or true power is resistive power. That is, the amount of power consumed by the resistive elements of the circuit. Resistive power is measured in watts (W).

Reactive  power  is  the  power  consumed  by  the  reactive  components  in  an alternating current circuit. Reactive power is measured in VAR (volt-amps-reactive).

Apparent power is the combination of the resistive and reactive powers and is measured in volt-amperes (VA).

The torque that a motor develops when the rotor is at a standstill and full power is applied is called locked rotor torque.

A motor’s rotor is considered locked at two different times: at start-up, when the rotor has yet to rotate and full power is applied to the motor, and when the load on a running motor is so great that the motor stalls to a standstill even though full power is applied. The torque that a motor develops at these times is called locked rotor torque.

Breakdown or pull-out torque.

This is the amount of torque produced by the motor, operating at rated voltage and frequency, at the instant the load causes the motor to stall.

In dynamic braking, the motor, simultaneously acting as a generator, produces a counter-torque that opposes the direction of rotation. This energy is applied to high-wattage, low-resistance resistors known as DB (dynamic braking) resistors. This causes a braking action to occur.

Regenerative braking is similar to dynamic braking in that a counter-torque is produced, which provides a braking action to the rotating motor. The difference is that in regenerative braking, the energy is fed back into the energy source.

Drives that are able to provide regenerative braking usually are more expensive; however, this form of braking is the most preferred method because there is less wasted energy in the form of heat and it produces a usable energy that offsets the costs.