AC Induction Motor Design (1)

1. Introduction

Electric motor systems consume 20% of all energy generated in the United States, 57% of all electrical energy, and 70% of electrical energy consumed by industry. Over 1.1 billion motors, of all types, are presently in use in the United States at this time.

Induction motors were invented by Nikola Tesla in 1888 while he was a college student. In the present day, induction motors consume between 90 to 95 percent of the motor energy used in industry.
We are going to discuss:
– The purpose of induction motors
– Induction motor construction
– Operating principles
– NEMA Designs
– Design E motor discussion
– Motor insulation
– Inverter duty motor construction

2. The Purpose of Induction Motors

Contrary to popular belief, induction motors consume very little electrical energy. Instead, they convert electrical energy to mechanical torque (energy). Interestingly enough, the only component more efficient than the motor, in a motor system, is the transformer. The mechanical torque that is developed by the electric motor is transferred, via coupling system, to the load.
The electrical energy that is consumed by electric motors is accounted for in losses. There are two basic types of losses, Constant and Variable, both of which develop heat (Figure 1):

• Core Losses: A combination of eddy-current and hysterisis losses within the stator core. Accounts for 15 to 25 percent of the overall losses.
• Friction and Windage Losses: Mechanical losses which occur due to air movement and bearings. Accounts for 5 to 15 percent of the overall losses.
• Stator Losses: The I2R (resistance) losses within the stator windings. Accounts for 25 to 40 percent of the overall losses.
• Rotor Losses: The I2R losses within the rotor windings. Accounts for 15 to 25 percent of the overall losses.
• Stray Load Losses: All other losses not accounted for, such as leakage. Accounts for 10 to 20 percent of the overall losses.

FIGURE 1