TechArchive

Original article date: March 1998

Monitoring the load on a motor that is driving a machine or process can give valuable information. On a mixer or agitator for example as the viscosity increases it will take more power to stir the mixture. When cutting metal as a tool gets dull it takes more power to make the cut. And if a machine cycle is completed without any increase in load it means that the tool is broken or the part is not present.

A power sensor can measure these load changes and send a signal to meters computers programmable controllers recorders or data collection systems. A load control senses the load and has built-in relays to sound alarms change feed rates and in extremis stop the machine.

The power cell an unique power sensor uses three balanced Hall Effect devices. Hall Effect semiconductors sense a magnetic field. When a current-carrying conductor passes through a magnetic flux concentrator (doughnut) and the Hall Effect sensor is placed in a gap in the concentrator the signal is proportional to the current.

The Hall Effect semiconductor can also multiply two signals. The Hall Effect sensor is excited with a signal that comes from the voltage sample for that phase. The Hall device multiplies these two signals.

The resulting output current times voltage is then proportional to power. This is an instantaneous vector multiplication which also calculates the lag or lead of the current (power factor).

The signals for each of the three phases are summed and the analogue output is proportional to the three-phase power (kW).

Using Hall Effect devices instead of traditional current transformers and voltage transformers greatly simplifies installation. Accuracy is also improved by eliminating the phase shift errors from CTs and PTs which can be large at low power factors. The Hall devices will also work on the output of variable frequency drives.

The analogue output can be hooked up to meters programmable controllers chart recorders and data loggers. It can also be used with a V series load control for setting up trip points and relay outputs.

In many load controls the single element wattmeter is used for sensing power. A 120V signal is taken from two phases with a transformer. The assumption is that the 120V signal changes in the same ratio as the primary voltage. This 120V signal typically comes from a control transformer that is also used for other purposes.

The current signal is taken from the REMAINING phase with a current sensing toroid that is either built into the control or excited externally. For large motors (over 10-20kW) a current transformer is used together with the toroid.

The control calculates the power factor by sensing the “zero crossing” of the AC voltage and the “zero crossing” of the AC current. This lag is the power factor.

The technique assumes that the load is balanced. Since a load control is normally used on a single motor the imbalance between phases is small. If more than one motor is being sensed from a single location the power cell should be used which measures current and voltage in all three phases. The single element wattmeter also does not work well with variable frequency drives. Here again the power cell should be used.

Why traditional techniques of power sensing don’t work well with variable frequency drives

• Current transformers and clamp-on ammeters don’t work at low frequency
• Voltage transformers don’t work at low frequency
• The wave shape as it leaves the drive is too distorted to use zero crossing techniques
• Many lower cost instruments are designed for sine wave calculations of current and voltage

Traditional schemes for measuring the power on the input to the drive are also not reliable

• The drive doesn’t take its power in sine waves. It takes power only during part of the cycle as it is charging capacitors
• The drive often takes power at a high power factor regardless of the motor load. This won’t give a true value for lightly loaded motors

The solution is to use the power cell since the Hall Effect sensors are not affected by the odd wave shapes and frequency. Also no current transformers and voltage transformers are used.

• Vydas Marketing
• 01428 751822

March 1998