Electrical Induction Motors  Torque vs. Speed
Full load operating torque vs. break down, pull up and locked rotor torque.
Torque is the turning force through a radius  with the units Nm in the SIsystem and the units lb ft in the imperial system.
The torque developed by an asynchronous induction motor varies when the motor accelerates from zero to maximum operating speed.
Locked Rotor or Starting Torque
The Locked Rotor Torque or Starting Torque is the torque an electrical motor develops when starting at zero speed.
A high Starting Torque is more important for application or machines hard to start  like positive displacement pumps, cranes etc. A lower Starting Torque can be accepted for centrifugal fans or pumps where the start load is low or close to zero.
Pullup Torque
The Pullup Torque is the minimum torque developed by an electrical motor when it runs from zero to fullload speed (before it reaches the breakdown torque point).
When a motor starts and begins to accelerate the torque in general will decrease until it reach a low point at a certain speed  the pullup torque  before the torque increases until it reach the highest torque at a higher speed  the breakdown torque  point.
The pullup torque may be critical for applications that needs power to go through some temporary barriers achieving working conditions.
Breakdown Torque
The Breakdown Torque is the highest torque available before the torque decreases when the machine continues to accelerate to working conditions.
Fullload (Rated) Torque or Braking Torque
The Fullload Torque is the torque required to produce the rated power of an electrical motor at fullload speed.
In imperial units the Fullload Torque can be expressed as
T = 5252 P_{hp} / n_{r} (1)
where
T = full load torque (lb ft)
P_{hp} = rated horsepower
n_{r} = rated rotational speed (rev/min, rpm)
In metric units the rated torque can be expressed as
T = 9550 P_{kW} / n_{r} (2)
where
T = rated torque (Nm)
P_{kW} = rated power (kW)
n_{r} = rated rotational speed (rpm)
Example  Electrical Motor and Braking Torque
The torque of a 60 hp motor rotating at 1725 rpm can be calculated as:
T_{fl} = 5252 (60 hp) / (1725 rpm)
= 182.7 lb ft
NEMA Design
NEMA (National Electrical Manufacturers Association) have classified electrical motors in four different designs where torques and startingload inertia are important criteria.
Accelerating Torque
Accelerating Torque = Available Motor Torque  Load Torque
Reduced Voltage Soft Starters
Reduced Voltage Soft Starters are used to limit the start current reducing the Locked Rotor Torque or Starting Torque and are common in applications which is hard to start or must be handled with care  like positive displacement pumps, cranes, elevators and similar.
Related Topics

Electrical
Electrical units, amps and electrical wiring, wire gauge and AWG, electrical formulas and motors.
Related Documents

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