Measuring single phase AC and DC motor currents - Page 1
Forum section: electronics / projects

author: Kalvin
author_rank: Super Contributor
timestamp: October 25, 2018, 06:12:00 am
content: I have been looking for a solution for measuring currents of two max. 10A electric AC and DC motors. The motors are already wired and installed in place, and I not allowed to insert any current transformer in series with one of the phase leads ie. I am not allowed to touch the existing wiring.So, it looks that my options are clamp-type current transformer and clamp-type hall sensor. The clamp-type current transformer can be used with the AC motor and the clamp-type hall-sensor can be used with both AC and DC motors.The motors and the measurement system will be placed outside buildings inside the motor cabinet, so the measurement system will be exposed to temperature variations from winter -30C ... summer +60C, but otherwise protected from rain and snow. The measurements will be performed once in an hour for 60 seconds 24/7. The consecutive measurements should be repeatable within 5% - 10% over few months, so the sensors should not drift too much in time. Temperature compensation might be possible in software. The measurement will be done by a MCU with 12-bit ADC at 200 Hz for each channel.The measurement system will be battery-operated (car battery) and it will collect the measurement data into a SD-card for later analysis. The sensors will be turned off when motors are not running, but the energy consumption of the sensors should be kept as low as possible in order to maximize the available measurement time.I have no real-world expertise on this area of using clamp-type current transformers nor clamp-type hall-sensors for building this kind of measurement system, so I would be very grateful for any information what I should take into consideration when selecting the actual sensors for best overall accuracy of 5% - 10% and less than 50 USD per sensor. If someone knows good and practical application notes for installation and analog interfacing, I would appreciate any pointers as well.Br,Kalvin

author: ivaylo
author_rank: Frequent Contributor
timestamp: October 25, 2018, 06:49:35 am
content: That’s a strange requirement, you are installing a permanent measurement system, but not allowed to unscrew a bolt? What are the voltages? If not too high and you convince the higher powers just a shunt may do. With transformers and shunts your ADC may be more sensitive to a 90C temperature swing than your sensors (not sure about hall effect devices). Also this motor or motors are either AC or DC, not same motor is sometimes ran AC, sometimes DC?

author: Kalvin
author_rank: Super Contributor
timestamp: October 25, 2018, 07:10:34 am
content: Quote from: ivaylo on October 25, 2018, 06:49:35 amThe system will not be permanent, although it will run as long as the car battery lasts - maybe two-three months at a time. I am not allowed to modify the existing wiring, but I am allowed to install a clamp-type sensor onto one of the phases for each motor, for example. There are two motors to be monitored, one AC and one DC motor, max. 10A each. The voltages are 240V AC/50Hz and/or 48V DC. The MCU will be using external reference for the ADC if necessary in order to keep the measurements sufficiently accurate over wide temperature range. The MCU will sample currents of two motors at 200 Hz and store the sampled data for later processing.

author: Kalvin
author_rank: Super Contributor
timestamp: October 25, 2018, 08:11:27 am
content: The system will be used to monitor the trend in the motor currents in order to determine whether the mechanical system driven by the motors is requiring maintenance, like lubrication etc.

author: _Wim_
author_rank: Super Contributor
timestamp: October 25, 2018, 08:04:28 pm
content: I think this doc should get you started selecting your sensor:https://www.lem.com/en/file/2666/downloadMore info can also be found here:https://www.lem.com/en/downloads#documentation

author: Kalvin
author_rank: Super Contributor
timestamp: October 25, 2018, 10:43:20 pm
content: Quote from: _Wim_ on October 25, 2018, 08:04:28 pmPerfect! This is exactly what I was looking for. Thank you very much!