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Electric Motor Driven Systems

There are many types of alternating current (AC) motors available. Direct current (DC) motors are also used but to a limited degree.

AC Motors
The important characteristics of an AC power supply include voltage, number of phases, frequency, voltage regulation and continuity of power.

Various voltage systems, such as 115v, 230v and 460v are used along with a standard voltage rating for 60 Hz electric power systems which has been established by the American National Standards Institute. Motors usually operate satisfactorily and deliver their full power at the rated frequency and at a voltage 10% above or below rating, or at the rated voltage and frequency 5% above or below rating.

The phase voltages of three-phase motors should be balanced. If not, a small voltage imbalance can produce a greater current imbalance and much greater temperature rise, which can result in nuisance overload trips or motor failures.

Codes and Standards
Codes and standards for electrical motors and equipment are found in the National Electrical Code (NEC) NFPA 70.

Motor Efficiency
Factors which effect motor efficiency include: sizing of the motor to the load, type of motor specified, motor design speed and type of bearing specified. Oversizing the motor results in inefficiency. The efficiency of a motor falls off at loads lighter than rated full loads.

The type of motor specified is important because a permanent split capacitor motor is more efficient than a shaded-pole motor. A capacitor-start/capacitor-run motor is more efficient than a capacitor-start shaded-pole motor.

In polyphase motors, the lower the locked rotor torque specified, the higher the efficiency obtained in the design.