Acceleration Methods 385

accelerating torque

Another name for pull up torque. It is called accelerating torque because this is the moment at which speed transitions from starting speed to full speed.

acceleration

The rate at which speed increases.

base speed

The speed (in rpm) at which the motor runs with full-line voltage applied to the armature and the field.

breakdown torque

The maximum torque a motor can produce without abruptly losing motor speed.

constant horsepower/variable torque

A load that requires high torque at low speeds and low torque at high speeds. Horsepower remains constant as speed and torque are inversely proportional.

constant torque/variable horsepower

A load that requires a constant torque throughout acceleration. If the load needs to move faster, the horsepower is increased.

copper loss

A power loss due to current flowing through wire. The lost power is converted into heat.

counter emf

The voltage induced in the armature of a DC motor that opposes the applied voltage and limits armature current.

frequency

A measurement of the number of complete AC cycles that occurs in one second. Frequency is measured in Hertz (Hz).

friction

The resistance to motion between the contact surfaces of two objects. Friction generates heat and increases the wear between components.

full load torque

The torque needed to produce the rated power at the full speed of the motor.

horsepower

A unit of power used to describe machine strength. One horsepower equals 33,000 ft-lbs of work per minute, or 746 watts.

inertia

The tendency of an object to stay in its state of rest or motion until acted on by an external force. Torque and braking must overcome inertia to accelerate or decelerate a motor.

inrush current

The initial surge of current into the windings. Inrush current can be up to ten times higher than the continuously needed current because there is low initial resistance.

locked rotor torque

The torque that a motor produces when full power is supplied to the motor and the rotor is not yet moving.

NEMA

The National Electrical Manufacturers Association. Motor nameplates carry NEMA letter designations that indicate the design characteristics of the motor.

percent slip

The difference between a motor's synchronous speed and its speed at full load. Percent slip is a way to measure the speed performance of an induction motor.

pull up torque

The torque needed to cause a load to reach its full rated speed.

rheostat

A continuously variable electrical resistor used to regulate current.

speed control

The external means of varying the speed of a motor under any type of load.

speed regulation

The ability of a motor to maintain its speed when a load is applied. A motor's speed regulation is fixed based on its design.

starting torque

Another name for locked rotor torque. It can be called starting torque because it is the torque applied by the motor at startup.

synchronous speed

The speed of the rotating magnetic field of an AC induction motor.

torque

A force that produces rotation. Torque is measured in pounds-feet in the English system and Newton meters in the metric system.

variable frequency drive

A device that converts incoming 60Hz AC power into other desired frequencies to allow for AC motor speed control.

variable torque/variable horsepower

A load in which both torque and horsepower requirements can change depending on the needed speed.

watt

A unit used to measure power. 746 watts equals one horsepower.