General Terminology on motor nameplate
(1) Service Factor:
A service factor is a multiplier that indicates how much overload a motor can take. For example, a motor with a 1.15 Service Factor can safely handle intermittent loads up to 15% above its nominal horsepower.
Slip can be used in two ways. The slip RPM is the difference in the entire load and synchronous speeds.
This slip RPM can be expressed as a percentage of the synchronous speed. It is also known as percent slip. Standard motors have a load slip of between 2% and 5 %.
(3) Synchronous Speed:
This is the speed at which the magnetic field inside the motor rotates. This is also the speed at which the engine can run without load. A 4-pole motor that runs on 60 cycles would have a magnetic speed of 1800 RPM.
That motor shaft’s no-load speed would be close to 1800, most likely 1798 or 1799 RPM. The motor’s total load speed could be 1745 RPM. The slip RPM of a Motor is the difference between the full load speed and the synchronous speed.
(1) Pull Up Torque:
as the motor accelerates, the torque decreases until it drops to a point called the pull-up torque. The Pull-up Torque refers to the minimum torque the electric motor develops when running at full load speed.
Pull-up torque refers to the minimum torque produced during acceleration from the locked rotating shaft to the speed at breakdown torque. Motor designs may not have a pull-up value because the lowest point could be at the locked point. In this instance, pull-up torque is equal to locked rotor torque.
Motors without a definite breakdown torque (NEMA design D) have pull-up torque. This is the minimum torque that can be developed at full load speed. It is often expressed as a percentage of full-load torque.
(2) Starting Torque (Locked Rotor Torque):
Start torque is the torque the motor produces when it’s energized at full power, and the shaft is locked in place. The Starting Torque, or Locked Rotor Torque, is the torque of an electric motor when it starts at zero speed or at rest.
This is the torque available when power is applied to accelerate the load. A high Starting Torque is essential for applications and machines that are difficult to start, such as cranes, such as cranes, positive displacement pumps, or cranes.
Applications such as centrifugal fans and pumps with a low start load can accept a lower Starting Torque.
(3) Full Load Torque:
Full load torque is the maximum torque the motor can sustain without overheating.
The Full-load Torque in imperial units can be expressed as T full load torque (lb ft) = (Rated horsepower Motor X 5252). / Rated rotational velocity (rpm)
The rated torque can also be expressed in metric units as Full load torque (Nm =)
(Rated KW Motor X 9550) / Rated rotating speed
Example: The torque produced by a 60 HP motor at 1725 rpm is T full load torque = 60 X 5,252/1725 (rpm).
T full load torque = 182.7 lb/ft
(4) Peak Torque:
Many loads, such as reciprocating compressors, have cycling torques. The amount of torque required depends on the machine’s position.
The Peak torque requirement is the actual maximum torque needed at any point. Peak torques are necessary for punch presses and other loads requiring oscillating torque.
(5)Pull out Torque (Breakdown Torque),
Breakdown torque refers to the maximum torque a motor can produce with rated voltage and frequency applied at the rated frequency. It is not affected by sudden drops in speed.
The breakdown torque is often expressed in percentages of full-load torque. This load is then increased until it reaches its maximum.
(1) Full Load Amps:
Total load amps refer to the amount of current the motor can draw under full load (torque). This is also called nameplate amps.
(2) Locked Rotor Amps:
Also called starting inrush current, the motor draws under full voltage.
Lock Rotor Current (IL) Three Phase Motor: 1000x HP x (KVA/HP) / 1.732 x Volt Lock Rotor Current (IL) Single Phase Motor: 1000x HP x (KVA/HP) / Volt