1.4: Motors
Student Learning Outcomes
After reading this chapter, you should be able to:
- List the various types of electric motors
- Describe the construction of electric motors
- Explain motor control equipment
- Identify the types of motor maintenance
Motors
In order to pump water throughout a distribution system, a motor is needed to run the pump. Motors and engines are an integral part of moving water from the source to the customer. Almost all electrical motors used by water utilities to operate pumps are powered by alternating current (AC). AC flows in one direction and then the other. The current strength rises from zero to a maximum, returns to zero, and then falls and rises in the opposite direction.
This sequence is called a cycle. The frequency of AC is the number of cycles that are completed in one second. For example, 60 hertz (Hz) is equivalent to 60 cycles per second. The number of peaks per cycle equals the phase power. Larger motors are usually powered by three-phase power.
Electric motors are available in a wide range of types, speeds, and power capabilities. Smooth power output and high starting torque is suited for direct connection to centrifugal pumps. While most motors used in the water industry, internal combustion engines are used and have their place of function. Internal combustion engines are primarily used for standby service during emergencies or when there are power outages.
Motor Definitions
As with most things, there are certain terms, phrases, and words associated with motors. Therefore, this section will handle several definitions, which will help in the overall understanding as we progress through this chapter.
- Horsepower (hp) is the unit of measure of power for electric motors. A Scottish engineer adopted the term by the name of James Watt. Watt compared the output of steam engines with the power of horses.
-
Watt is the standard unit of measure of power. One watt is equivalent to one joule per second. It is used to quantify the rate of energy transfer. The conversion between watts and horsepower is shown below:
- 0.746 watt = 1 hp
- Volt is a measurement of electrical pressure. It is similar to pounds per square inch of pressure in water. Common voltages are 110/120 and 220/240 volts for lighting and operation of small motors. Large motors require voltages of 440/480 or higher.
- Ampere is the unit used to measure the flow electrical current
- Ohm is a measure of electrical resistance or impedance. Electrical pressure drops due to resistance.
- Stator is the stationary part of a motor. It usually consists of a steel core with slots in which insulated coils of copper or aluminum winding are placed.
- Rotor is the rotating part of the motor. It consists of a steel core with copper or aluminum windings. The rotor is located on the motor shaft within the stator and is separated from it by only a small air gap.
There are other terms associated with motors, which will be discussed later in this chapter.
How a Motor Works
As previously mentioned, motors are found throughout the water utility industry. However, there are motors everywhere in our day to day lives. There are motors in computers, hairdryers use a motor, fans, appliances, toys and so many other things use motors. When an electrical current starts to move along a wire, it creates a magnetic field around it. This magnetic field can cause movement, which can propel a motor. The link between electricity, magnetism, and movement is the basic science behind an electric motor.
The attracting and repelling forces of a magnet within a motor are what create the rotational motion. One pole of the magnet is designated as north and the other south. A current passing through a wire wrapped around a wire rod is called an electromagnet. A simple motor is formed when a rotating magnet is placed near an electromagnet. When AC power is connected, the current is passed through the electromagnet (stator) creates a magnetic pole that attracts the unlike pole of the other magnet (rotor). Reversing the current changes the pole and the rotor spins and rotates the shaft. The magnetic field in the stator induces a current in the rotor, which then rotates, turning the motor and pump shafts.
The speed at which the magnetic field rotates is called the motor’s synchronous speed. This speed is expressed as revolutions per minute (rpm). A frequency of 60 Hz has a maximum synchronous speed of 3,600 rpm or 60 revolutions per second. Remember, in 60 Hz, there are 60 cycles per second. The 60 cycles multiplied by the 60 revolutions equals 3,600 rpm. Motors can run at fractions of 3,600 rpm by increasing the number of poles in the stator. For example, a four-pole motor has a synchronous speed of 1,800 rpm and a six-pole has a synchronous speed of 1,200 rpm. Motors may also run at their synchronous speed or slightly lower.
More electrical current is needed to start motors than there is needed to keep the motor running. The motor starting current (locked-rotor current) is the current drawn by the motor the instant the motor is connected to the power supply system. The locked-rotor current is often five to ten times the normal full load current. The current starts out at its maximum value and then decreases to the motor’s ordinary current draw as the motor reaches full speed.
Single Phase Motors
Single-phase motors operate in the same fashion as 3-Phase motors, except they are only run off of one phase. The instantaneous power of single-phase motors is not constant. This is because the system reaches a peak value twice in each cycle. Typically they are only used in fractional horsepower sizes, but they can be furnished up to 10 hp at 120 or 240 V. No power is required to bring these motors up to speed and they must be started by some outside device. A starting winding is usually built into the motor to provide initial high torque. As the motor comes to high speed a centrifugal switch changes connections to running winding.
There are three basic types of single-phase motors:
- Split-phase motors use a rotor with no windings. They have a comparatively low starting torque so a low starting current is needed.
- Repulsion-induction motors are more complex and expensive than split-phase motors and require a higher starting current
- Capacitor-phase motors have a high starting torque and high starting current. They are used in applications where the load can be brought up to speed very quickly and infrequent starting is required.
Three-Phase Motors
Three-phase motors are used when more than ½ horsepower is needed. A three-phase motor has two main parts. A rotor is the turning component and the stator is the part that turns the rotor. The rotor is also referred to as a squirrel cage. The squirrel cage consists of a circular network of bars and rings, which look similar to a cage connected to an axle. The stator within a three-phase motor consists of a ring with three pairs of coils. The coils are evenly spaced around the rotor. Each pair of coils is attached to one phase power. Since each is out of phase with each other, a rotating magnetic field is created and spins around the stator at a continuous rate. Three phase motors are operated at 230, 460, 2,300, or 4,000 V. There are three main classes of three-phase motors.
- Squirrel-Cage Induction motors
- Synchronous motors
- Wound-Rotor Induction motors
Various motors are used to operate pumps in the water industry. The table below identifies the motor type, phase, application, and provides some additional comments.
|
Motor Type |
Phase |
Application |
Comments |
|---|---|---|---|
|
Induction (Squirrel cage rotor) |
Single |
Jet pumps, small centrifugal pumps |
<1 hp, requires switch |
|
Induction Squirrel Cage (split-phase) |
Three |
General centrifugal |
Low maintenance, single speed |
|
Phase-Wound |
Three |
Variable speed |
Speed adjustable |
|
Synchronous |
Three |
Used where power efficiency is critical |
No slip, efficient |
|
Vertical Hollow-Shaft |
Three |
Vertical Turbine |
Mounted on pump discharge head |
|
Submersible |
Three |
Submersible |
Submersible |
Principal Motor Components
There are several components, which make up a motor. This section discusses five (5) principal components.
-
Frame—The frame of a motor provides protection. A frame is usually made of cast iron or steel. There are four (4) common frame types.
- Open drip-proof—This frame type has openings that allow air to pass through and cool the motor. The openings are constructed so drops of liquid or solid particles will not normally interfere with motor operations. These frames are suitable for most indoor installation, but should not be used if water or chemicals will splash on the frame.
- Totally Enclosed Fan-Cooled—In a totally enclosed frame, it is constructed so that outside air cannot enter the motor. Cooling is provided by a built-in fan. These frames are suitable for outside use and in moisture-laden atmospheres.
- Totally Enclosed Explosion-Proof—This type of frame is constructed to withstand an explosion of gas or vapor within the motor. It also prevents the ignition of gas or vapor surrounding the motor by sparks within the motor. This type of frame should be used whenever the motor is located near an explosive atmosphere such as chemical feeding areas.
- Submersible—A submersible frame is designed to be totally submerged in water. It is equipped with special seals to keep the water out and retain the oil surrounding the motor.
- Stator—As previously explained, the stator is the stationary part of the motor. It usually consists of a steel core with slots that are insulated coils of copper or aluminum winding.
- Rotor—The rotating part of a motor is called the rotor. It is located on the motor shaft within the stator.
- Bearings—In order for the motor shaft to be held in position with minimal frictional resistance, bearings are used. The rotor is in turn supported by bearings, which allows the rotor to turn. The motor housing supports the bearings. Bearings are either lubricated with oil or grease to prevent metal surfaces from wearing.
- Shaft—The shaft is a rod that extends through the bearings and rotor. The rotor turns the shaft to deliver mechanical power.
- Windings—The windings are wires laid in coils wrapped around a magnetic core. This forms magnetic poles when energized with electrical current.
- End Bells—Motor end bells or shields are the main support for the bearings
Motors are designed for a wide range of loads, environmental conditions, and mounting configurations. Many motor configurations are standardized is sizes up to 200 hp. Larger motors are not usually standardized. As motors convert electrical energy into mechanical energy, heat is generated. Therefore, motors must be designed with some type of ventilation. In external temperatures greater than 104 ° F, the life of a motor can be shortened.
Motor Control Equipment
Smaller motors are usually started by directly connecting line voltage to the motor. However, in larger motors (greater than fractional horsepower) a motor started is needed. A typical motor starter includes a main disconnect switch, fuses or circuit breakers, temperature monitors, and a means for operating the motor remotely.
The functions of motor control fall into two main categories. Much of the functions of motor control are for the protection of the motor and associated feeder cables. The other function determines when and how a motor operates.
There are full voltage and reduce voltage motor controllers. Full voltage controllers use the full line voltage from the electrical source to start the motor. The starting current is drawn directly from the power line. In a reduced voltage controller the starting current of the motor is too high and may damage the electrical system. The controller uses a reduced voltage and current to start the pump motor.
Motor control systems are either automatic or manual. Manual systems are usually less expensive and require employee labor to operate. These types of control systems are generally located in a central control room. Automatic controllers are commonly operated remotely and reduce the need for manual operation. Either type of control system should be included with high and low-level alarms as an early warning system.
In order to prevent or reduce the likelihood of motor failure, motor protection equipment is often used. Thermal overload relays on starters prevent a motor from burning out if abnormal operating conditions increase the load beyond the design capacity. Fuses and circuit breakers are placed in the main power wiring of a motor to protect against short circuits. The fuses or circuits fail and shut down the motor. Overcurrent or overload relays are used to sense current surges in the power supply. In the event of a power surge, these relays disconnect the motor from the power supply. In areas where lightning may occur, lightning arresters are used to prevent damage from high voltage surges. Voltage relays are frequently used to detect a loss of power and to initiate a switchover to an alternate power source. There are a variety of other relays to protect against things such as reverse currents, phase reversals, and frequency changes. Sensors are also used to protect against overheating, increases in speed, and other operational variables, which are not considered normal.
Motor Maintenance
As with all mechanical equipment, a regularly scheduled maintenance program is prudent. General inspection and maintenance items include good housekeeping in order to keep the area around the motor clean and free of things, which can contribute to premature failure. An inspection checklist to routinely examine things such as alignment and balance of the motor, proper lubrication, adequate insulation, phase imbalance, and connections of switches and circuitry should be followed.
Bearings need to be properly maintained as well. The bearing housing should be filled with oil. With new pumps, the oil should be completely replaced after the first month of operation. Then routine oil changes should occur every 6 to 12 months. In grease lubed bearings the temperature should be monitored closely, especially during the first month. Regreasing should be completed per manufacturer specifications.
Proper records should also be maintained. The make, model, capacity, type, serial number, and warranty information should be kept in order to replace or repair the same or compatible motor. The installation date and name of the company that installed the motor should also be kept with all the other records of the motor. Manufacturers provide suggested inspection and maintenance schedules. It is also important to maintain records of the names and addresses of the manufacturer and local repair representatives.
Results from any testing should also be kept on file. Depending on the size and type of motor, testing can vary. Some common testing parameters include, but are not limited to motor vibration and operating temperatures. Routine checking of cables and grounding is also recommended.
Sample Questions
-
Volt is a measurement of electrical ___________.
- Resistance
- Pressure
- Power
- Current
-
Ampere is a measurement of electrical ___________.
- Resistance
- Pressure
- Power
- Current
-
Ohm is a measurement of electrical ___________.
- Resistance
- Pressure
- Power
- Current
-
Which of the following is not a three-phase motor?
- Repulsion induction
- Squirrel-cage induction
- Synchronous
- Wound-rotor induction
-
A four-pole motor has a synchronous speed of ___________.
- 1,200 rpm
- 1,800 rpm
- 2,400 rpm
- 3,600 rpm
-
The speed at which the magnetic field rotates is called the motor’s ___________.
- Run speed
- Rotation speed
- Synchronous speed
- Dynamic speed
-
Which of the following is a basic single-phase motor?
- Repulsion-induction
- Squirrel-cage induction
- Synchronous
- Wound-rotor induction
-
The stator is the ___________ part of the motor.
- Rotating
- Stationary
- Insulating
- Not part of a motor
-
Motors are designed for an external temp ___________.
- less than 150F
- less than 125F
- less than 110F
- less than 104F
-
PLC stands for ___________.
- Programmable Logic Center
- Pump Local Control
- Programmable Logic Controller
- Pump Local Center