Power in Motors

Power in Motors

AC motors are either single-phase or three-phase. For a single-phase motor, the electrical input power is given by Equation (A).

 (A) Pin = VIcosθ  V is the rms line voltage, I is the rms line current, and cosθ is the power factor.
 As we learned in the lesson on three-phase,

 (B) Pin = √3 VIcosθ  The output mechanical power is:

 (C) Pout = Toutωm  Pout is the output mechanical power, Tout is the output torque, and ωm is the angular frequency of the shaft.

 (D) Pin = Pout + Ploss 
Ploss refers to the copper, eddy current, hysteresis, friction, and windage losses.
Pout is usually given in horsepower (746 W/HP). The efficiency of a motor (η) is the ratio of the powers:

 (F) η = Pout/Pin
Typical values for industrial motors are between 85% and 95%. It should be understood that these efficiency are for full load. Efficiency decreases significantly for light loads.

 Since motor speeds are usually given in rpm rather than shaft angular frequency, conversion is frequently necessary:

 (G) ωm = nm(2π)/60  nm is the motor shaft rotation in rpm.

Motor Enclosures

Motor Enclosures 

•  ODP (Open Drip Proof) - Motors with open enclosures, typically used in clean, non hazardous areas. Vents are designed to prevent liquids and debris from entering the motor from an angle of 0 to 15°.

•  TEFC (Totally Enclosed Fan Cooled) = Motor with a small fan on the rear shaft of the motor. Typically used in very wet, dirty or dusty areas. 

•  TEAO (Totally Enclosed Air Over) - Motor with dust-tight enclosure and an aerodynamic design. This type of motor is desined to allow the air flow of the fan/blower keep the motor cool. 

•  TENV (Totally Enclosed, Not Ventilated) - Motor with a dust-tight, enclosure with no air vents. These small hp motors are cooled by convection to the frame.

•  XPRF (Explosion Proof) -Motors designed to withstand an internal explosion of gas or vapor. If such an explosion happens, the gas/vapor will not ignite.

Electric motor Formulas

Losses in AC Motors

Losses in AC Motors

 The electrical input power to a motor is always greater than the mechanical output power. There are five different kinds of power losses:

•  Copper losses - Since the windings in a motor are made of copper, there will be i2R losses due to the resistance of the copper.
•  Eddy current losses - The changing magnetic fields in an AC system cause currents to flow in the iron of the rotor and stator. These currents flow in small circles, like the eddies in a river. Most rotors and stators are made of laminated iron to reduce the loss due to eddy currents.
•  Hysteresis loss - In a 60 Hz system, the current, and thus the magnetic field, reverses 120 times per second. Some power loss occurs with each reversal.
•  Friction loss - Friction exists in all mechanical systems.
•  Windage loss - This is loss due to the generation of air movement.

 All of these losses manifest as heat generated in the motor.

Service Factor

The service factor - SF - is a measure of periodically overload capacity at which a motor can operate without overload or damage

The service factor - SF - is a measure of periodically overload capacity at which a motor can operate without overload or damage.

The NEMA (National Electrical Manufacturers Association) standard service factor for totally enclosed motors is 1.0.

 A motor operating continuously at a service factor greater than 1 will have a reduced life expectancy compared to operating at at its rated nameplate horsepower.

 NEMA Service Factor at Synchronous Speed (RPM) for drip proof motors: 

Example - Service Factor

 A 1 HP motor with a Service Factor - SF = 1.15 can operate at

 1 HP x 1.15 = 1.15 HP

 without overheating or otherwise damaging the motor if rated voltage and frequency are supplied to the motor.

 Insulation life and bearings life are reduced by the service factor load.

Effect of Voltage Unbalance on Motor Performance

Effect of Voltage Unbalance on Motor Performance

 When the line voltages applies to a polyphase induction motor are not equal, unbalanced currents in the stator windings will result. A small percentage voltage unbalance will result in a much larger percentage current unbalance. Consequently, the temperature rise of the motor operating at a particular load and percentage voltage unbalance will be greater than for the motor operating under the same conditions with balanced voltages. Should voltages be unbalanced, the rated horsepower of the motor should be multiplied by the factor shown in the graph below to reduce the possibility of damage to the motor. Operation of the motor at above a 5 percent voltage unbalance condition is not recommended.

 Alternating current, polyphase motors normally are designed to operate successfully under running conditions at rated load when the voltage unbalance at the motor terminals does not exceed 1 percent. Performance will not necessarily be the same as when the motor is operating with a balanced voltage at the motor terminals.

Reference: NEMA Standards MG 1 14.35.

Resistor color code

Resistor color-coding 
 To distinguish left from right there is a gap between the C and D bands.

band A is first significant figure of component value (left side) 

band B is the second significant figure (Some precision resistors have a third significant figure, and thus five bands.) 

band C is the decimal multiplier
band D if present, indicates tolerance of value in percent (no band means 20%)