[b]Motor efficiency and a dynomometer[/b]

[hmic21]

Hello all,
I have been given the task to find the motor that works best as a generator. By best I mean as far as efficiency and output power go at the lowest rpm possible. I have set up a small experiment to test a pm dc motor,a stepper and a dc brushless. I know my experiment has quite amount of flaws to it and will not be extremely accurate.
I have mounted 2 motors on an alluminum plate and connected them via a belt and pulley system. I'm using a 1hp dc motor to drive the others and monitoring output and input current and voltage to check output and input power. It is hard at calulate the input power since the input power is not the power supplied to the driving motor, but to the motor being driven. I have to account for loss in the first motor and the belt and pulley system. What I did is drive an Identical 1hp dc motor with no load and monitored the input power to the driving one. Obviously all power is lost so I said that since they are identical we can estimate that half was lost in each one. I did this at different rpm to check the power lost at each of my test rpm.
At the end I subtracted the loss of the dc motor from the power supplied to the driving dc motor and said that was the input power to my test motor. From here I made speed vs. efficiency curves and speed vs. output power curves. My question is, how feasible of a test is this and is power lost by a motor the same when it is being a motor and being a generator?
I was told that using a dynomometer would be better to test efficiency and power but i could not find one out there that fit my budget.
Thanks....

 

[eemotor]

Often when testing for motor efficiency we use a torque tranducer between the motor and the generator to measure power output (torque and speed) at the motor's shaft. However, the transducer's can get a bit pricy.

 

[Skogsgurra]

Yes, the torque transducer is the best solution.

But, you are lucky to use DC motors. Their torque/ampere is quite constant, regardless of speed. So, you can calibrate at standstill using a beam that you attach to the shaft and lock so that it cannot rotate - just move a few degrees.

Then apply current to the motor and find out what torque it produces by means of a wheight that you slide along the beam until there is a balance between wheight and torque.

Remember to balance the beam first. You shall also work quickly so that you do not spot heat the commutator.

Do the calibration for four or five current levels to get an idea of how constant your Nm/A ratio is.

Gunnar Englund

http://www.gke.org

 

[hmic21]

So your saying that in DC motors there torque-amp curve should be pretty linear? What about for stepper motors and dc brushless? And also a side question, when using it as a generator whats the best way to choose your load? Thanks alot, I appreciate your help.

 

[Skogsgurra]

Yes, there is a linear relationship between armature current an torque - at least as long as there is no armature reaction (armature current makes net flux go down). If you make a curve like said in previous post, you will easily find if there is any non-linearity.

If you have a chance to find an old thyristor rectifier I think that is the best load. It can feed the energy back to the mains (no heat) and it usually has an easily settable current limit that makes running against constant torque an easy matter. In this configuration, you need just one motor (or brake, as it is).

Regarding steppers and BLDC, I wouldn't use them as brakes. A stepper is not at all suitable and a BLDC cannot brake (nowhere to get rid of absorbed power).

Gunnar Englund

http://www.gke.org