PanarchyMills
Member
- Location
- NE California
- Occupation
- Engineer, Design & Build
I'm sizing some 3 phase motors into a system and need to solve for torque but I'm having trouble knowing which values to use. Questions are numbered
I can calculate motor power (kW) = V*I*cos(theta)*sqrt(3) <- this is actual power (electrical)
Often this exceeds the nameplate power by, what seems, the value of the efficiency.
(1) So I should be using the nameplate power (because this is the mechanical power at the shaft) for torque calculations? Or my calculated actual power?
Moving to calculated torque: T = (60/2pi)*(power/rpm)
Sometimes I can find full load torque in data sheets, other times its just locked rotor/breakdown/pullup torque, often express as % of FL torque. But often the torque value in the data sheet is way off from my calculated value
(2) what is the best way to calculate or find the motor torque, delivered at the shaft at full load amps?
Torque Example:
1.5kW (dataplate)
860RPM
Torque calculated from power: (60/2pi)*power/RPM = 16.65 Nm
From datasheet:
breakdown Torque: 17.4Nm, 424% FL Torque
FL torque (calculated): 17.4/4.24 = 4.1 Nm
(3) what's going on here? Is there a typo on the datasheet or am I doing something wrong? and if the data sheet doesn't provide torque values, what's the best way to calculate them? I suppose part of my question relates to the calculated vs actual motor power. Is torque the same and the datasheet torque the best value to use rather than the calculated value?
Thanks for the help
I can calculate motor power (kW) = V*I*cos(theta)*sqrt(3) <- this is actual power (electrical)
Often this exceeds the nameplate power by, what seems, the value of the efficiency.
(1) So I should be using the nameplate power (because this is the mechanical power at the shaft) for torque calculations? Or my calculated actual power?
Moving to calculated torque: T = (60/2pi)*(power/rpm)
Sometimes I can find full load torque in data sheets, other times its just locked rotor/breakdown/pullup torque, often express as % of FL torque. But often the torque value in the data sheet is way off from my calculated value
(2) what is the best way to calculate or find the motor torque, delivered at the shaft at full load amps?
Torque Example:
1.5kW (dataplate)
860RPM
Torque calculated from power: (60/2pi)*power/RPM = 16.65 Nm
From datasheet:
breakdown Torque: 17.4Nm, 424% FL Torque
FL torque (calculated): 17.4/4.24 = 4.1 Nm
(3) what's going on here? Is there a typo on the datasheet or am I doing something wrong? and if the data sheet doesn't provide torque values, what's the best way to calculate them? I suppose part of my question relates to the calculated vs actual motor power. Is torque the same and the datasheet torque the best value to use rather than the calculated value?
Thanks for the help