140303A - Three Phase Motors Flashcards
What are the three most important terms related to 3P induction motors?
- torque
- mechanical power
- efficiency
What is torque?
Also called moment force, torque is the force that moves a rotating body
Rotational motion is a result of the interaction between which two forces?
driving force and restraining force
What is a type of restraining force?
friction or some other conversion of mechanical energy
Give the formula for torque
Torque = Force (F) x Radius (R)
T = F x R
what are the units for torque?
Newton meters or N*m
What are the units of force?
Newtons or N
What are the units of Radius?
Meters or m
What is a result of imposing torque on a load?
counter torque
In what direction will the counter torque of a load be applied?
in the opposite of the direction of the motor torque
When are motor torque and counter torque balanced?
When the motor is run driving the load at a constant speed
What happens if the load on the motor increases? 5 points
- Counter torque increases
- motor slows down
- motor draws more current
- motor increases torque to match new counter torque
- motor speed is now lower than it was before
What happens when load decreases? 3 points
- less torque
- less current
- higher speed
What is the mechanical power of a rotating machine the product of?
Rotational speed
torque
a numerical constant
Formula for mech power in SI units
P = [2(pi)nT] / 60
What are the units of mechanical power?
Watts or W
what is 2(pi)?
the conversion between rotational and linear motion
what is n?
the rotational speed/ frequency in rpm
what is T?
Torque in N*m
How many Watts in 1 horsepower?
1 HP = 746 Watts
What is efficiency?
The ratio of output energy/ power over input energy / power
What is efficiency in relation to an electric motor?
Ratio of the mechanical power output to the electrical power input for any given load
Formula for elec motor efficiency?
P (in electrical) = P (out mechanical) + P(loss)
What are some electrical losses in an induction motor?
- iron losses in the stator and rotor
- copper losses in the stator and rotor
What are some induction motor mechanical losses that ultimately convert to heat?
- friction in the bearings and between brushes and slip rings
- windage, the friction of air movement produced by the motor
Give formula for % motor efficiency
% Eff = (Pout / Pin) x 100
Give the 3 types of mechanical loads
- variable torque
- constant torque
- constant horsepower
Describe a variable torque load’s characteristics. 2 points
- low torque at low speed
- high torque at high speed
Name some types of variable torque loads
- centrifugal pumps
- centrifugal compressors
- centrifugal fans and blowers