AC Induction Motor Design (5)

7. Design E Motor Discussion

The design E motor was specified to meet and international standard promulgated by the International Electrotechnical Commission (IEC). IEC has a standard which is slightly less restrictive on torque and starting current than the Design B motor. The standard allows designs to be optimized for higher efficiency. It was decided to create a new Design E motor which meets both the IEC standard and also an efficiency criterion greater than the standard Design B energy efficient motors.

For most moderate to high utilization application normally calling for a Design A or B motor, the Design E motor should be a better choice. One should be aware of slight performance differences.
Although the NEMA standard allows the same slip (up to 5%) for Designs A, B, and E motors, the range of actual slip of Design E motors is likely to be lower for Designs A and B.
There are a number of considerations which must be observed with Design E motors:

• Good efficiency - as much as 2 points above Design B energy efficient.
• Less Slip - Design E motors operate closer to synchronous speed.
• Lower Starting Torque - May not start "stiff" loads.
• High Inrush - As much as 10 times nameplate full load amps.
• Availability - Presently low as the standard has just passed.
• Starter Availability - Control manufacturers do not have an approved starter developed at this time.
• National Electric Code - Has no allowance for higher starting amps. Design E motors will require changes to NEC allowances for wire size and feed transformers.
• Limited Applications - Low starting torque limits applications to pumps, blowers, and loads not requiring torque to accelerate load up to speed.
• Heavier Power Source Required - High amperage and low accelerating torque mean longer starting time and related voltage drops. May cause nuisance tripping of starter of collapse of SCR field with soft starters.