What is the torque of a motor?
Torque: A measure of how much a force acting on an object causes that object to rotate. The tendency of a force to rotate an object about an axis, fulcrum, or pivot.
When energized, the rotor of a motor produces a twisting force. That force is a function of the design of the motor, both the stator and the rotor itself. In the U.S. the design of electric motors is done to the specifications of the National Electrical Manufacturer's Association (NEMA) specification MG 1.
The torque a three-phase AC motor can produce is a function of its design, the applied voltage and frequency, and the speed of the motor shaft at the moment of measurement.
A NEMA Design B motor has the following speed/torque curve when operating at designed voltage and frequency:
Other designs have different curves. The full-load torque is the torque the motor is designed to produce continuously without overheating. In English units, the equation for the torque of a motor is:
T = HP x 5252 / RPM
Where T is torque in ft.*lbs, HP is self-explanatory, RPM is the actual operating speed of the motor per the nameplate, and 5252 is the constant that yields the proper units for torque. For the metric equivalent, it's:
T = kW x 9549 / RPM where torque is measured in N*m
So, a 10Hp 460V 60Hz motor operating at its nameplate speed of 1750RPM produces 30 ft.*lbs of torque. If the load does not require that much torque, the motor will run a little faster. If there is no load attached to the motor at all, it will spin at very nearly, but not quite 1800RPM. Per the graph above (which is generic, and meets minimum NEMA standards) that same motor will produce approximately 45 ft.*lbs. at zero speed, approximately 40 ft.*lbs at 20% speed, and approximately 60 ft.*lbs. at 75% speed. At the lower speeds, it will draw significantly higher than its full-load rated current.
When energized, the rotor of a motor produces a twisting force. That force is a function of the design of the motor, both the stator and the rotor itself. In the U.S. the design of electric motors is done to the specifications of the National Electrical Manufacturer's Association (NEMA) specification MG 1.
The torque a three-phase AC motor can produce is a function of its design, the applied voltage and frequency, and the speed of the motor shaft at the moment of measurement.
A NEMA Design B motor has the following speed/torque curve when operating at designed voltage and frequency:
Other designs have different curves. The full-load torque is the torque the motor is designed to produce continuously without overheating. In English units, the equation for the torque of a motor is:
T = HP x 5252 / RPM
Where T is torque in ft.*lbs, HP is self-explanatory, RPM is the actual operating speed of the motor per the nameplate, and 5252 is the constant that yields the proper units for torque. For the metric equivalent, it's:
T = kW x 9549 / RPM where torque is measured in N*m
So, a 10Hp 460V 60Hz motor operating at its nameplate speed of 1750RPM produces 30 ft.*lbs of torque. If the load does not require that much torque, the motor will run a little faster. If there is no load attached to the motor at all, it will spin at very nearly, but not quite 1800RPM. Per the graph above (which is generic, and meets minimum NEMA standards) that same motor will produce approximately 45 ft.*lbs. at zero speed, approximately 40 ft.*lbs at 20% speed, and approximately 60 ft.*lbs. at 75% speed. At the lower speeds, it will draw significantly higher than its full-load rated current.
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