Electric Motor Selection and Sizing

Electric Motor Selection

_Two basic decisions to make:

_What type of motor is needed?

_ DC motor?

_ AC motor?

_Once type of motor is selected, what size

Motor is required?

Type Selection - DC Motor

_DC motors are typically used when

_ low-cost, _________________ is advantageous

_butt precise speed regulation not required

_ Starting torque required ___________________

Than running torque

_brief overloads OK, since motor has time tot cool

_ frequent start/stop cycles, reversing, or closed loop

Positioning required

Type Selection - AC Motor

_AC motors are typically used when

_ low-cost, _________________ is advantageous

_gearing required to deliver speeds that are

Significantly less than 1200 RPM

_ Starting torque ______________ running torque

_brief periods of high running torque frequently

Handled by flywheels

_ Available access to AC power

Brainstorming Scenario

_Your group has been hired to select motors for

Various applications and products

_Possibilities include:

_ DC motor (w/brushes)

_ Brushless DC motor

_ Split phase AC motor

_ Permanent Split Capacitor (PSC) AC motor

_ Shaded pole AC motor

_ Universal motor

In-Class Exercise

_Match one of the 6 different types of motors to

These applications & justify your selection:

_ Electric knife for carving turkey

_ Constant speed conveyor belt with frequent

Start/stops

_ Power window drive in auto

_ Fan in indoor HVAC unit

_ Industrial robot used in painting applications

Motor Sizing #1

_The major constraint on all motor operation

Is thermal in nature.

_The heat a DC motor must dissipate can be

Calculated:

Electrical Power = Power Dissipated + Mechanical Power

(Input) (As heat) (Useful output)

Electric Motor Selection and Sizing 33.2

Motor Sizing #2

• Each specific electric motor will have its own

Characteristic torque-speed “curve”

Speed, w

Torque, t

AC Motor

PMDC Motor

Universal Motor

Motor Sizing #3

_Each type of load will have its own torque--

Speed requirements

Speed, w

Torque, t Constant

Pumps and Fans Viscous Friction

(T µ speed2)

Motor Sizing #4

_Operating point occurs at intersection off motor

And load speed/torque “curves”

Speed, w

Torque, t

Pump

AC Motor

Speed, w

Torque, t

DC Motor

Constant

DC Motor Ratings

_DC motors are “rated” at a single speed and

Torque

_In most cases, the motor can operate at this

Point _______________________

_ Temperature rating will not be exceeded

_ DC motors rated with _____________________

_DC motors are “typically” used at

_ about ___________ of rated speed

_ about ___________ of rated torque

DC Motor Selection Example

_A 12 volt battery-powered DC motor is used on a

Child’s toy “Jeep”

_What power (in watts and hip) is required for this

Application? Hint – ignore acceleration

_Which motor should be selected from the spec

Sheet provided for the application?

V=3 ft. /sec

Total mass

m=40 kg o 5

AC Motor Ratings

_AC motors are also “ratted” at a single speed

And horsepower

_ 3450, 1725, 1140, 850 RPM are common

_AC motor can operate here continuously

_ Temperature rating will not be exceeded

_AC motors are “typically” used at

_ about ____________ of rated torque/power

_ much lower efficiency if motor is ____________

(“Over-rated”)