Calculating torque of dc motor - Page 1
Forum section: electronics / beginners

author: embedded.world@aol.com
author_rank: Contributor
timestamp: April 25, 2014, 06:22:00 am
content: How to calculate torque of a dc motor.I have got a dc motor with imprint:maxon motor314888swiss made1130329<>I can vary its speed, direction & control barke with a drive along with it, but don't know exactly how to calculate torque. It will be different with speed??

author: IanB
author_rank: Super Contributor
timestamp: April 25, 2014, 06:32:22 am
content: There will be a maximum torque available at any given speed and you would have to find this out from the data sheet. Probably there will be a curve.You can't calculate it from nothing, it is a property of the physical design and construction of the motor. You either have to measure it on the bench or make reference to manufacturer data.

author: Flenser
author_rank: Regular Contributor
timestamp: April 25, 2014, 08:51:39 am
content: This link describes how to calculate DC Motor characteristics and once you have these you can use them to calculate the torque.http://www.rci.rutgers.edu/~dbirnie/solarclass/MotorPrimerGaTech.pdfYou'll need to be able to measure voltage, current and RPM.

author: max_torque
author_rank: Super Contributor
timestamp: April 25, 2014, 07:28:25 pm
content: Within reason, the torque of an electrical machine is simply linearly proportional to the electrical current. You have to measure the motors current draw and use a "factor" to calculate the approximate motor torque. I'm sure Maxon quote this factor for their motors.

author: Witention
author_rank: Contributor
timestamp: April 28, 2014, 07:26:38 am
content: In normal day to day use the usual requirement is torque, knowing the mechanical power and rpm:torque_N.m = (60000 x Power_KW) / (2pi x RPM). This formula can be rearranged for different answers. Torque is turning force in newton meters here.An estimate of the torque can be made from the input electrical power, efficiency and the rpm. For motors of a few horsepower you might assume an efficiency of 80% or so.An estimate of the maximum allowed torque may be made using nameplate figures for power and rpm usually. Note that maximum continuous torque corresponds with maximum continuous current. Torque can exceed this for brief times during acceleration of the motor or changes in load, for example during starting. The peak torque may be as much as 3-10 times the maximum rating.The electrical power appears as a lower mechanical power (reduced by the efficiency). Then knowing the rpm and mechanical power, calculate the torque as above.The torque is proportional to the armature current. The field current (where present) is considered separately. As the motor current is specified at a maximum allowable continuous current (under certain conditions) this means that the current needs to be controlled, and there is a maximum continuous torque rating too..If you want to calculate a torque constant (e.g. newton meters per amp) this comes back to electric motor design, involving number of turns, current and magnetic field, but the calculation is involved, because the magnetic material and shape of the magnetic circuit needs to be considered. Normally the manufacturer provides the torque information in the motor specification (data sheet), or if this is not available, it could be measured. The most basic information is on the motor nameplate.The design considerations are complex enough to need a book or two and some learning (electrical engineering). Generally only manufacturers of motors and researchers are involved in this.If it is an AC motor, the measured current has to be corrected for power factor, as specified on the motor name plate. If power factor is specified as 0.8, multiply the measured current by 0.8 to get the real current to calculate the real power that produces mechanical output. If it is a 3 phase motor, the real current in one phase is multiplied by 3 phases and the phase to neutral voltage is used to calculate the power (for delta connected motors). In India this would be 240V or similar, though the 3 phases have a line to line voltage of 415V.These points can all be researched easily on the internet using the key words mentioned.