THREE-PHASE ALTERNATING CURRENT MOTORS - Free Online Tutorial Part 3
MOTOR NAMEPLATE DATA
Motors are designed and developed for specific applications. Identifying their proper usage may be difficult. To ensure the correct component for the correct application, all government regulatory societies require a minimum of specific information to be printed on the motor's nameplate. Additional information may be obtained in IEEE Standard 45, Section 24, and (NEC) Article 430. This data includes:
- Manufacturer's name.
- Motor frequency. This may be represented as Hz for hertz or as CPS for cycles per second. This is always an indication of AC application.
- Phases (either three phase or single phase). This is also an indication of AC application.
- Voltage. The motor is designed to operate at this voltage or within a specified voltage range. Two voltages separated by a slash, such as 450/225, indicate a two-voltage system. Either voltage may be used by connecting the electrical stator leads as directed in the manufacturer's manual or on the data plate.
- Full-load current (FLC). This is the current required to operate the motor at its rated load and speed. This is not the current draw when the motor is started. If two current values are given, this indicates the current when supplied with one of the two possible voltage connections. When the higher voltage is used, less current is necessary to operate the motor.
- Full-load speed. This is the speed in revolutions per minute the rotor will turn under full load.
- C rise. This Celsius value plus the motor's rated ambient temperature add together to determine the maximum temperature range the motor can obtain under full-rated load (40C equals 104F).
- Time rating. This is the time the motor can operate continuously without stopping. Usually 5, 15, 30, or 60 minutes or continuous ratings are specified.
- Rated horsepower.
- Code letter. This indicates the highest current the motor will draw when the rotor is physically prevented from moving initially. The current is rated in kVA per horsepower. This is a measurement of locked rotor amperage. Table 16-3 lists code letters from the National Electrical Code.
- Design. This provides starting kVA, running kVA, and running KW characteristics. This is a product of the internal resistance of the rotor. Generally, designs B, C, and D are used:
-- Design A is of limited usage. This motor has extremely high starting kVA, as much as 50 percent higher than the B, C, or D design motors.
-- Design B is a standard rotor design. This type of rotor has a low internal resistance. It has normal starting torque, low starting current, and low slip at full load.
-- Design C has a higher internal rotor resistance. This improves the rotor power factor at the start, providing more starting torque. Fully loaded, the extra resistance creates a greater slip.
-- Design D has more resistance. The starting torque is maximum.
-- Serial number. The serial number or identification number is extremely useful when dealing with the manufacturer. The serial number and appropriate information is maintained on file with the company.
-- Type. This is the manufacturer's specific application information. This will also identify the housing characteristics (waterproof, drip-proof, and so forth).
--Service factor. This is an allowable overload above the full-load current. It is expressed as a decimal. Multiplying the full-load current by the service factor establishes the maximum allowable current acceptable above full-load current for a short period of time.
-- Frame. Many of the dimensions found on a blueprint are incorporated in the frame identification. Some of these specifications may include the rotor shaft length, diameter, and machining the motor housing and bolting placements; and so forth.
When a motor is ordered, all the data plate information must accompany the supply document. There is no substitute for the correct electrical component. Universal equipment does not exist in a marine distribution system unless the specifications can be matched exactly.
The table below provides a sample of some three-phase mot or starting characteristics for design B, C, or D. Design A motors may have starting kVA values that are as much as 50 percent higher. Many 3,600 RPM motors are design A.
MOTOR EFFICIENCY
Efficiency is the ratio of output to input. Only part of the power going into a motor is actually delivered to the load in the form of mechanical power. Some power is lost in the resistance in the stator windings and in the stator core. Other losses are transmitted across the air gap to the rotor. Resistance in the rotor uses up power. Finally, the power needed to overcome windage and friction losses reduces the mechanical output even further.
The copper losses are proportional to the current squared (P = I2R). This is the only variable loss. Rotational and core losses do not change as the motor becomes loaded.
