Electric Motors

EngineeringMVRT10-21-2006

ENGINEERING

A motor generates rotational motion.

WHAT IS A MOTOR?

Conventional motors use chemical or electromagnetic and electric PE.

ie: Car engines (motors) burn gasoline to turn wheels.

THE ELECTRIC MOTORAn electric motor uses electric

energy.

A simple DC motor. Current flow generates an EMF around the armature.

The armature continues to rotate.Once it has reached the zero position, the current is reversed and the process repeats.

Magnet

Magnet

Armature

Electromagnets

ROTATIONAL MOTIONRotational motion has two

components.

TORQUE SPEEDTorque is the force of the rotation.Speed is the rate of rotation.

Power describes energy over time in Watts (W).

POWER

There are two power equations that deal with electric motors:

ELECTRIC ROTATIONAL

P=IV P=TV

Power = Current times Voltage

Power = Torque times Speed

Power = Current times Voltage

ELECTRIC POWER

Voltage is difference in charge, or potential.

Current is the quantity of electrons moving over time.

V

I

Volts

Amps

V

A

P=IV

Power = Torque times Speed

ROTATIONAL POWER

Torque is the force of the rotation.

Speed is the rotations per unit of time.

T

V

Newton-meters

Rotations per Minute

Nm

P=TV

RPM

Electric motors behave with certain characteristics.

MOTOR CHARACTERISTICS

With no load and constant voltage, motors output a certain rpm.

T

V

This is called the free load rpm.

This is the maximum speed of the motor at this voltage.

With maximum load and constant voltage, the motor outputs a certain torque.This is called the stall torque.This is the maximum torque at this voltage.

The graph of the relationship between torque and rpm looks like this.

Ideally, the graph should look like this.At any point, the product of T and V is the same.However, in reality, the power is not constant.

As the armature of the motor turns, it generates backward current.

Thus, the faster the motor turns, the greater its resistance to current flow.

This is the graph of speed versus power.

Even as you put in a huge amount of power, the graph begins to flatten out.

V

PAt these high power inputs, most of the energy is converted to heat.

Here is the graph of power vs. speed.At low speed, you either have low power or high power.At those points, you have either low torque or high torque.

To find the best power output of a motor...We take our real torque vs. speed graph...And find when their product is greatest.This is always halfway between the two extremes.At this point, the motor is most efficient.The torque at this point is called the max power torque.

The speed at this point is called the max power rpm.

P

Power Output:

THE CIM

Free RPM:Stall Torque:Ideal for long running times.

343 Watts5310343.4 oz-in

Power Output:

THE FISHER PRICE

Free RPM:Stall Torque:Hi power, but suited for medium loads due to plastic gears.

387.5 Watts12651.79 Nm

Power Output:

THE WINDOW

Free RPM:Stall Torque:Useful for preventing backdrive.

22 Watts8410.6 Nm

Power Output:

THE GLOBE

Free RPM:Stall Torque:Low power motor for low-medium load applications.

55 Watts81150 in-lb

Power Output:

THE MABUCHI

Free RPM:Stall Torque:Counterclockwise rotation with additional spur gear.

16.8 Watts470060.8 mNm

Sadly, the Mabuchi was not around when motor photos were being taken.

Nobody missed it.

Electric Motors

By Humphrey HuPhotographs from Wikipedia, USFIRSTYou may not modify this Powerpoint without permission

ENGINEERING