There are two good reasons why your concrete mixer slump meter should use a watt transducer and not just measure Amps:
An Ammeter (or electrical current meter) measures the current drawn by your mixer motor during mixing of a full batch of concrete. As the aggregates and cement flow into the mixer, the current, and the power drawn by the motor(s) increases. When water mixes into the other ingredients, the current and power increase even more. It peaks when the mix reaches a doughy consistency, or “zero slump”. As the concrete starts to flow, it now has a “slump” value and the current or power starts to decrease in proportion to the flow characteristics of the mix. This drop in current and power is dramatic, with a small amount of extra water giving a large drop in resistance to the motion of the paddles. The “Slump Meter” in your Batch Controller measures current or power to detect this drop and, as a result, control the quantity of water to give the desired slump.
Why not use an ammeter?
If you use an ammeter to measure the slump, the current measurement is inversely proportional to the voltage and if the voltage drops, the current will rise. This is a simple fact of the way electric motors work. Not so obvious but equally critical, as the load on the motor increases, the phase angle decreases and the power factor increases. These terms and their explanation are more complex than I can describe in a short article. The result, however, is to reduce the range of the ammeter from no load to full load on the motor. In effect, it limits the sensitivity of the meter.
By multiplying volts and amps we can make the result, VA, independent of the applied voltage and remove one problem. But when true motor power, Watts, is used, we now get a larger variation from no load to full load. As a result, we get more sensitivity from the slump meter.
A Watt Transducer is better
The measurement of true motor power is more expensive, but the result is worth the extra cost. Instead of a simple ammeter or current transducer, we now need a more complex electronic box. The watt transducer measures true rms volts, true rms amps and phase angle. In addition, we need a voltage step-down transformer and one or two current transformers. The cables carrying the motor power are threaded through these. The transducer sends its output to a panel meter or, more often, the PLC or control system for the batch plant. The control system then compares the motor power, as we add water to the mixer, to the setting for the desired slump value. It either controls the water directly or, more usually, monitors the slump after the addition of the calculated volume of water.
Scale-Tron’s BatchTron control system operates in this way. It has a watt transducer as its slump meter and a bar-graph displayed on the touch screen, next to the mixer symbol. The operator can check the motor power from this bar-graph and optionally he can set upper and lower limits to warn if the slump is too high or low. Because it uses a watt transducer, it is sensitive and independent of the supply voltage.