TYPES OF LOSSES IN ELECTRIC MOTOR OPERATION

What are the typical losses that occurs in electric motor operations?

Energy losses in electric motors fall into four categories:

• Power losses (Stator and Rotor Losses)
• Magnetic core losses
• Friction and windage losses, and
• Stray load losses.

Figure shown. A typical NEMA Design B motor showing components that can be modified to increase the motor's efficiency: (a) Stator windings; (b) Rotor length; (c) conductor bars and end rings; (d) air gap; (e) laminations; (f) bearings; (g) fan.

Power losses and stray load losses appear only when the motor is operating under load. They are therefore more important — in terms of energy efficiency — than magnetic core losses and friction and windage losses, which are present, even under no-load conditions (when the motor is running, of course).

Power losses, also called I²R losses, are the most important of the four categories and can account for more than one-half of a motor's total losses. Power losses appear as heat generated by resistance to current flowing in the stator windings and rotor conductor bars and end rings.

Stator losses make up about 66% of power losses, and it is here that motor manufacturers have achieved significant gains in efficiency. Since increasing the mass of stator windings lowers their electrical resistance (and therefore reduces I²R losses), highly efficient motors typically contain about 20% more copper than standard efficiency models of equivalent size and rating.

Rotor losses are reduced by decreasing the degree of slip. This is accomplished by increasing the mass of the rotor conductors (conductor bars and end-plates) and/or increasing their conductivity (see below), and to a lesser extent by increasing the total flux across the air gap between rotor and stator.

Magnetic core losses arise from hysteresis effects, eddy currents and magnetic saturation, all of which take effect in the steel laminations. Magnetic losses can account for up to 20% of total losses. With proper design, use of better materials and stringent quality control, these losses can be reduced considerably.

The most effective means to reduce hysteresis and saturation losses is to utilize steels containing up to 4% silicon for the laminations in place of lower-cost plain carbon steels. The better magnetic properties offered by silicon steels can reduce core losses by 10 to 25%. Reducing the laminations' thickness also helps: substituting 26-ga or 29-ga steel for the 24-ga steel found in standard-efficiency motors lowers core losses by between 15 and 25%.

Lengthening the lamination stack, which reduces the flux density within the stack, also reduces core losses. Eddy current losses can be reduced by ensuring adequate insulation between laminations, thus minimizing the flow of current (and I²R losses) through the stack.