Using Current Monitoring for Load Analysis
Why Monitor Current?
Monitoring electrical current flow is an excellent method of gauging the performance of motor-driven devices (such as fans, pumps, and chillers) controlled by a building automation system (BAS). Solid-state current switches can be used to provide a contact closure to show that a motor is running, but many times it is helpful to be able to monitor the performance of motor loads. Current monitors are used to provide inputs to building automation systems or alarm circuits.
A lower than normal current flow could indicate a problem such as 1) a broken coupling on a pump, 2) a broken or loose belt on a fan, 3) a dirty strainer on a pump application, or 4) a dirty filter, a closed damper or the need to defrost on a fan application. A higher than normal current flow could indicate bad or worn bearings, or belts that are misaligned on motor loads. A monitored current can also be used to determine the percent loading of a chiller.
How To Monitor Current
Kele offers two current transducers-the 4CMA and the 4CTV. The 4CMA provides a 4-20 mA loop-powered output based on 0-20 amp line current (See Fig. 1). The 4CTV offers a 0-5 VDC output based on 0-20 amp line current, and no power supply is required (See Fig. 1). These current transducers (insulated for 600 volts) may be installed directly over the conductor.
If there is a possibility that the line current may exceed 20 amps, a current transformer (CT) will be needed with the current transducer (See Fig. 2). The secondary of the CT will be either 1 or 5 amps depending on the model used, but the 5 amp model is used most often.
The CT is installed on the conductor. One secondary lead of the CT is run through the window of the current transducer and then connected to the other lead. Since the CT has a 0-5 amp output and the current transducer accepts a 0-20 amp input for full output (4-20 mA or 0-5 VDC), it is advisable to loop the CT secondary through the transducer four times (twenty times for a 1 amp CT). This utilizes the full range of the transducer.
(Example: A 200:5 CT is installed with a 4CMA. The secondary leads from the transformer are looped through the 4CMA four times. The input to the BAS system will be 4-20 mA linear to 0-200 amps flowing through the line conductor.)
Remember, never leave the secondary of a CT open-circuited with line current flowing through the CT. Dangerous voltages will develop that could be hazardous to personnel and/or equipment.
Kele stocks a complete line of current, voltage, and power monitoring equipment. Call our experienced sales engineering staff for application assistance.
4CTV and 4CMA Applications
- Detect overcurrent or undercurrent conditions
Monitor:
- loads from 1-5000 amps
- chiller loading
- pump or fan performance
- load status with variable frequency drive applications
- multiple stage equipment
Definitions:
- Io = mA DC out of 4CMA
- Vo = VDC out of 4CTV
- Turns = Number of times conductor passes through 4CMA or 4CTV
