The laws of our land now demand more efficient electric motors. Over the years, the motor manufacturers responded with higher efficiency units, plus they commented that they could increase operating efficiency even more if industry would accept increased starting currents.
To meet this challenge, NEMA developed standards for what is now known as the NEMA Design E motor. This standard establishes efficiency values that exceed the required nominal efficiencies for energy- efficient Design B motors, Figure 1.
(Incidentally, the “E” designation is unrelated to the energy aspect. That letter was chosen because it was the next available letter in the NEMA design alphabet, because A, B, C, and D were already taken.)
However, a few less-than-obvious factors enter the picture when it comes to making the decision to specify or install a NEMA Design E motor:
• Some ratings by some manufacturers of high-efficiency Design B motors may exceed the required efficiency of a Design E motor. But this relationship may vary from one rating to another and from one manufacturer to another.
• Design E motors have significantly higher starting (locked rotor) currents, Figure 2 and Figure 3, than do typical Design B units. This may or may not require larger power wiring and motor starter than required for a Design B motor of equivalent rating, but it must be checked.
• Rating-for-rating, Design E motors produce less locked-rotor (accelerating) torque than do Design B motors, Figure 4. This seems contrary to the higher starting currents, but that is one of the tradeoffs. The difference in starting torques is especially important for motors rated less than 20 hp. Therefore, if your application is a constant-torque load and with limited acceleration times, select carefully.
Moreover, many Design B motors, exceed the minimum NEMA torque capabilities for a specific motor rating. Therefore, if a Design E motor replaces an above-the-curve Design B motor, the new E motor may not “cut the mustard” and may be incapable of starting the load.
On the other hand, depending where a specific Design B motor falls in the characteristic spectrum, the Design E may do just fine. To determine these factors without using the trial-and-error method, determine (don’t guess) the load requirements before specifying either motor.
Bottom line
If your application runs for long periods at full load and starting torque is not a limiting factor, then a NEMA Design E motor may be a wise investment. Careful calculations will be a wise investment.
One way to sort through all these variables is to request a certified specification sheet that includes locked rotor and fullload currents plus a torque-speed curve from the prospective manufacturer then work together as a selection team.
Mr. Crouse is chief engineer for Lincoln Electric’s Motor Division, Cleveland.