AC Motors 2

Introduction to motors cont.:
An object in motion travels a distance in a given time. Speed is the ratio of the distance traveled and the time it takes to travel the distance.

The linear speed of an object is a measure of how long it takes the object to get from point A to point B. Linear speed is usually given in a form such as meters per second (m/s).

The angular speed of a rotating object is a measurement of how long it takes a given point on the object to make one complete revolution from its starting point. Angular speed is generally given in revolutions per minute (RPM).

An object can change speed. An increase in speed is called acceleration. Acceleration occurs only when there is a change in the force acting upon the object. An object can also change from a higher to a lower speed. This is known as deceleration (negative acceleration).

Mechanical systems are subject to the law of inertia. The law of inertia states that an object will tend to remain in its current state of rest or motion unless acted upon by an external force. This property of resistance to acceleration/deceleration is referred to as the moment of inertia. The English system of measurement is pound-feet squared (lb-ft2).



If we look at a continuous roll of paper, for example, we know that when the roll is stopped it would take a certain amount of force to overcome the inertia of the roll to get it rolling. The force required to overcome this inertia can come from a source of energy such as a motor. Once rolling, the paper will continue unwinding until another force acts on it to bring it to a stop.

AC Motors 1

AC motors are used worldwide in many residential, commercial, industrial, and utility applications. Motors transform electrical energy into mechanical energy. An AC motor may be part of a pump or fan, or connected to some other form of mechanical equipment such as a winder, conveyor, or mixer. AC motors are found on a variety of applications from those that require a single motor to applications requiring several motors.

Before discussing AC motors it is necessary to understand some of the basic terminology associated with motor operation. Many of these terms are familiar to us in some other context. Later in the course we will see how these terms apply to AC motors.

In simple terms, a force is a push or a pull. Force may be caused by electromagnetism, gravity, or a combination of physical means.

Net force is the vector sum of all forces that act on an object,  including friction and gravity. When forces are applied in the same direction they are added. For example, if two 10 pound forces were applied in the same direction the net force would be 20 pounds.

If 10 pounds of force were applied in one direction and 20 pounds of force applied in the opposite direction, the net force would be 10 pounds and the object would move in the direction of the greater force.
If 10 pounds of force were applied equally in both directions, the net force would be zero and the object would not move.

Torque is a twisting or turning force that causes an object to rotate. For example, a force applied to the end of a lever causes a turning effect or torque at the pivot point. Torque (τ) is the product of force and radius (lever distance).
τ = Force x Radius
In the English system torque is measured in pound-feet (lb-ft) or pound-inches (lb-in).If 10 lbs of force were applied to a lever 1 foot long, for example, there would be 10 lb-ft of torque.

An increase in force or radius would result in a corresponding increase in torque. Increasing the radius to two feet, for example, results in 20 lb-ft of torque.

Circuit Breakers Types

Instantaneous magnetic-trip-only circuit breakers do not provide overload protection and are used on motor circuits where overload protection is provided by a motor starter. The current level at which an instantaneous trip circuit breaker trips is adjustable. The name comes from the electromagnet used to sense short circuit current. The purpose of overload protection is to prevent the motor from operating beyond its full-load capability. In the schematic illustrated below, a motor is supplied through a 3-pole circuit breaker, motor starter contacts and separately supplied overload contacts. Heat generated from excessive current will cause the overload contacts to open, removing power from the motor.

Thermal-magnetic circuit breakers have both overload and instantaneous trip features. When an overload condition exists, the excess current will generate heat, which is detected in the circuit breaker. After a short period of time, dependent on the rating of the breaker and amount of overload, the breaker will trip, disconnecting the load from the voltage source. If a short circuit occurs, the breaker responds instantaneously to the fault current and disconnects the circuit.

The user does not have access to the trip unit on some circuit breakers. This means the trip unit cannot be changed with another. Interchangeable trip is actually a design feature that is available on some thermal-magnetic and some solid state breakers. The advantage of a breaker with an interchangeable trip unit is the user can change the continuous current rating of the breaker without replacing the breaker. This is done by replacing the trip unit with one of a different rating.

molded case circuit breakers are available as a molded case switch. Molded case switches employ the same operating mechanism as the thermal magnetic and magnetic only units.  A preset instantaneous function is factory installed to allow the switch to trip and protect itself at a high fault current, but the switch provides no thermal overload protection.

Many electrical distribution systems can deliver large short circuit currents to electrical equipment. This high current can cause extensive damage. Current limiting circuit breakers will reduce the current flowing in the faulted circuit to substantially less magnitude. This helps protect expensive equipment. One way to accomplish current limiting is with an additional set of contacts that feature two moveable arms. These are referred to as dual-pivot contacts, which separate even more quickly than the single-pivot contacts. The dual-pivot contacts are connected in series with the single-pivot contacts. As with the single-pivot design, current flows in opposite directions through the contact arms, creating a magnetic repulsion. As current increases, the magnetic repulsion force increases.

In an overload condition where current may only be one to six times normal current, the contacts remain closed until the breaker trips. In a short circuit condition fault current is extremely high, both sets of contact arms may open simultaneously, generating high impedance arcs. The contact gap of the dual-pivot contacts increases more rapidly, therefore generating arc impedance more rapidly. Once the arcs are extinguished, the dual-pivot contacts close on their own due to spring tension. The single-pivot contacts are held open by the breaker mechanism, which will have tripped during the fault and must be manually reset.

Solid state circuit breakers function similarly to thermal-magnetic breakers. The basic breaker mechanism is still mechanical. The tripping unit is solid state.  As with the thermal-magnetic tripping unit, the Sensitrip circuit breaker tripping unit performs the following three functions:
 Senses magnitude of current flow,Determines when current becomes excessive,Determines when to send a trip signal to the breaker.


 Mechanism of these circuit breakers use a microprocessor to execute numerous functions programmed in the unit. These units have a greater degree of accuracy and repeatability. Adjustments on the trip unit allow the user to select numerical values the microprocessor will use in performing protective functions. Current sensors mounted in the trip unit monitor the value of load current. The value of current is reduced to a low level and converted to a digital voltage, which is used by the microprocessor. The microprocessor continuously compares the line current with the value set by the user. When current exceeds a preset value for the selected time, the trip unit sends a signal to a magnetic latch. The magnetic latch opens the breaker’s contacts, disconnecting the protected circuit from the power source.