How (non)linear is motor LRA? - Page 1
Forum section: electronics / general

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 28, 2013, 12:45:06 am
content: The textbook way of measuring LRA (Locked Rotor Amps) is to jam the motor and then measure the supply current when connected to its rated voltage. But that is generally a bad idea for larger motors since it would likely trip a circuit breaker and be hard on the motor.But what if I connected a 120V motor to a 12V transformer, then multiplied the measured current by 10? How close would that be to the actual full voltage LRA? (Assume the supply impedance is negligible, which really is the worst case situation as far as LRA is concerned.)

author: Kremmen
author_rank: Super Contributor
timestamp: June 28, 2013, 09:00:29 am
content: We are talking about brush DC motors, right? In that case the supply voltage to the motor armature (Us) is balanced by the back EMF generated by a turning motor (E) plus the voltage loss over the armature winding resistance (Ra*Ia) plus the inductive reactance of the armature winding L*di/dt so thatUs = E + Ra*Ia + L di/dtIf the rotor is stationary there is no E term and the equation devolves intoUs = RaIa + L di/dtFurther, if you use true DC for measuring, then the armature inductance term also vanishes and you get simply:Us = RaIafrom where it is easy to calculate Ra when you know Us and Ia. That term should be linear enough as long as you account for the possible resistance changes due to armature heating.There is no specific requirement for the supply impedance if you measure the true values of Us and Ia, as you should of course.This way of measuring does not account for the possible forward voltage losses (Vf) in the commutator so if you want to be real careful, do the measuring with several voltages and fit the result to a curve of the form y = ax + b where y = armature current, a=Us, b=-Vf and x=1/R. That might give you an idea about the magnitude of the commutator voltage loss as well. But that should be it.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 28, 2013, 12:55:10 pm
content: I'm dealing with AC induction motors.

author: Kremmen
author_rank: Super Contributor
timestamp: June 28, 2013, 03:25:42 pm
content: Quote from: NiHaoMike on June 28, 2013, 12:55:10 pmAll the easier then as you don't even have a commutator. Since the stator coils are copper the resistance (impedance) will be constant as far as voltage is concerned. What i said about heating still applies of course.

author: SeanB
author_rank: Super Contributor
timestamp: June 28, 2013, 06:42:04 pm
content: You will get a reading that is a little optimistic, as you will not be driving the core into saturation which drops the inductance a lot. Pretty much the calculation will be about 20% or so low, calculate the current that will flow if there were no inductance using the DC resistance and that will be the upper bound, the actual current will fall between those 2 points.If you have a clamp meter with peak capture this will give a pretty close estimate on startup, as the rotor will be effectively stopped for the first cycle. Inrush current will be pretty close to LRA current.