Designing drive systems without an understanding of electric motor losses.

Not understanding motor losses can create major building construction problems.  The losses for the motor can be broken down into two basic characteristics; fixed and variable.

            Fixed Losses

a)      Core losses represent the energy required to overcome Hysteresis.  This includes the loss created by the eddy current that flows within the core.  Core losses can be reduced by various design methods concentrating on the design of the laminations and the material utilized for the core.  Today silicon steel and the use of thinner steel laminations with better inter-laminar insulation are utilized to keep these losses to a minimum.

b)      Next are the losses associated with bearing friction and the resistance presented by the rotation of the armature through the surrounding air.  This loss is known as windage.

c)      These fixed losses are all independent of motor load and are dependent on the motor design.

Variable Losses

a)      The stator losses also known as I²R are due to heating which occurs as current flows through the stator winding.  Manufacturers reduce this loss by modifying the stator slot design or by decreasing insulation thickness and increasing the volume of wire in the stator.

b)      There are also rotor losses, which are also I²R losses and occur due to the heating in the rotor windings.  Manufacturers reduce this loss by increasing the size of the conductive bars and end rings, to produce a lower resistance.

c)      Fluxes that are induced by the motor's load currents create what is commonly called stray load losses.  All the variable losses are affected by the load imposed on the motor and therefore increase with motor load for any specific motor.

Table 1-5

Fixed Motor Losses

Fixed Losses

Typical Losses %

Factors Affecting Losses

Core Losses	15-25		Type and quantity of Magnetic Material
Friction &  Windage	5-15		Selection and design of fans, bearings and seals

Table 1-6

Variable Motor Losses

Variable Losses

Typical Losses %

Factors Affecting Losses

Stator Losses	25-40		Stator conductor size.
Rotor Losses	15-25		Rotor conductor size and material
Stray Load Losses	10-20		Manufacturing and design methods

          

                      Table 1-7

Typical Distribution Of Motor Losses - %

        (1800 Rpm Open Drip-Proof Enclosure)

Type Of Loss	Motor Horsepower
	25hp	50hp	100hp

Stator	42%	38%	28%
Rotor	21%	22%	18%
Core Losses	15%	20%	13%
Windage & Friction	7%	8%	14%
Stray Load	15%	12%	27%