Introduction to the UNIPOWER Family | Industrial Power Monitoring Guide

Introduction to the UNIPOWER family

A technical overview of power consumption measurement, supervision and control using the Unipower family of Intelligent Power-Control Units.

1 Introduction

Within the industrial sector it has become increasingly important to measure physical variables and use their values for control and regulation purposes. Examples include temperature, pressure, flow, current, voltage and revolutions. In addition, pH, conductivity, oxygen and other gases are measured without difficulty. However, torque and power consumption have often been ignored as variables for industrial supervision and control.

Most industrial applications use some type of electro-motor. The purpose of the motor is to supply the necessary torque transformed into linear translation, flow of current, pressure and similar outputs. Through measurement of the supplied torque, it is possible to monitor whether a process runs as expected. Continuous measurement of motor torque makes it possible to control or stop the process if it becomes unstable or moves outside predefined limits.

Direct torque measurement with strain gauges mounted on a rotating shaft is usually expensive and mechanically challenging. Rotational torque measurement systems exist, but they are costly, not always robust, and typically reserved for very expensive machinery. Torque can, however, be obtained indirectly from a fast and accurate measurement of motor power consumption.

The proportionality between power (P2) and torque is well known. This page argues for power consumption measurement as invaluable feedback in many industrial applications, with reference to the Unipower family of HPL and APM Intelligent Power-Control Units developed and manufactured by Hydria Elektronik ApS.

2 Power measurement

To use power consumption as an indirect torque measurement, the following points must be considered:

1. Power must be measured using the formula: P = √3 x U x I x Cosφ.
2. The measurement must be accurate, and repetition accuracy must be high.
3. The reaction time must be short. The shortest possible reaction time equals one half voltage period, which is 10 ms at 50 Hz.
4. The measurement must remain valid for non sine-shaped currents, such as measurement before frequency inverters that generate very high and short current peaks with crest factors up to 10.
5. The measuring unit should include support functions such as start timer, automatic zeroing, peak detector for maximum and minimum load, possible voltage compensation and Po correction.

All of these items are realised in the Unipower family. The HPL400-series also has an analogue output, 4-20 mA, and includes phase-order supervision.

The Unipower family is built around powerful microprocessors, enabling these functions to be implemented. Fast and accurate measurement was achieved through a specially developed 4-quadrant multiplier. This measurement principle makes kW measurement accuracy independent of the shape of the voltage and current signals. It also accepts high crest factors.

Besides accurate measurement, the Unipower family includes a number of control functions useful to industry.

2.1 Power versus cosφ and current measurement

Many existing power consumption measurement devices supplied to industry have been used to register total power consumption in kWh. These typically have a long reaction time and are therefore not very useful for machine supervision or control.

A number of load monitors based on cosφ or pure current measurement have been made. These systems are inadequate for torque control because only true power consumption is proportional to torque.

Current measurement shows a large non-linearity with respect to torque because current practically does not change from idle to about 50% load.

The angle φ and cosφ appear useful for control and supervision, but only when mains voltage is constant. When mains voltage increases, the power factor decreases if torque is constant. When mains voltage decreases, the opposite happens and this may lead to false alarms.

Mains voltage fluctuations also influence the proportionality between power and torque, though to a lesser extent. The power meter measures total motor power consumption: P1 = Po + P2. P2 is not related to mains voltage fluctuations, but Po changes quadratically with respect to ΔU. This can become critical, so different Po compensations are possible in the Unipower family.

With cyclic machines, such as tooling machines, it is advantageous to zero out idle power consumption Po for each cycle. This compensates for slow mains voltage fluctuations, friction in bearings, gear oil temperature and similar factors.

The Unipower family is designed for supervision and control of motors that appear as symmetric loads to the mains. Current is therefore measured in one phase. HPL units have an internal current converter up to 8A and four programmable current ranges: In = 1, 3, 5 or 8A. For currents greater than 8A, an external N/1 or N/5 current converter is required.

3 Standard Unipower family features

Besides power measurement and trip points, the Unipower family integrates several control functions. These functions are programmable from the keyboard and are needed to implement stand-alone supervision and control schemes.

3.1 Programming

Unipower products are generally programmed using only three keys on the front panel. The mode key switches the display from true kW or kW [%] to one of the programmable parameters. Red mode LEDs show which parameter may be changed. Once selected, the parameter value is shown on the display and may be altered with the two arrow keys.

Parameters are stored in EEPROM and therefore remain present when the unit is turned off. The function of the keys repeats if they are held down continuously.

3.2 Measurement ranges

All units have an internal current transformer that measures current up to 8A. The units have four current ranges: 1, 3, 5 and 8A. When measuring currents above 8A, an external current transformer N/1 or N/5 must be used. If an N/1 current transformer is used, the 1A current range should be selected; similarly, an N/5 current transformer uses the 5A range.

The display of power is in percent of the measurement range. The power in kW corresponding to 100% may be calculated as:

P = 1.73 x U x I
U: Nominal voltage
I: Selected current range or primary current of the external current transformer

Example: With current range = 1A and nominal voltage 400VAC, P = 1.73 x 400 x 1 = 0.692kW. This means that 100% on the display corresponds to 0.692kW. A display of 40% corresponds to 0.4 x 0.692kW = 0.277kW.

3.3 Trip points

Trip points in the Unipower family are always programmed in percent of the measurement range. Choosing the trip point may be done theoretically or practically.

Md = P2 x 60 / 2πn
Md: Torque where an alarm is wanted
P2: Corresponding shaft power
n: Revolutions in rev/min
P1 = P2 + P0, or from the motor efficiency curve
Trip point [%] = 100 x P1 / P, where P is the unit's measurement range

3.4 Peak detectors

Leave the machinery running with normal load for a suitable time duration. Read the maximum and minimum values by activating the arrow keys in kW mode: arrow-up for maximum peak and arrow-down for minimum peak. Program the trip points with appropriate tolerances into the Unipower module.

3.5 Start-timer, Ts

To avoid alarms generated by start current, supervision is not activated until the motor is running. This is done with a start timer, Ts, typically programmable from 0.1-25 seconds. When consumption exceeds 5%, Ts is activated. After Ts expires, limits, hysteresis and reaction timers become active. If consumption drops below 5%, supervision is disabled.

3.6 Reaction timer, Tr

Reaction timers avoid alarms generated by peaks in the power signal. If a Tr of 1 second is used, for example, an alarm is not generated unless the measurement has exceeded the trip point for 1 second. Tr may typically be programmed from 0.01-25 seconds.

3.7 Resetting alarms

Alarms are reset either by the Reset key on the front plate or via the digital input S1.

3.8 Reset input, S1

Through digital input S1, an alarm may be reset manually or automatically. Auto reset is enabled when S1 is connected to Gnd, which activates the hysteresis function.

3.9 Hysteresis

A maximum or minimum hysteresis band is placed relative to the trip points. The hysteresis band is always placed below a maximum limit and above a minimum limit. The band size is programmed in percent of the measurement range.

With a maximum limit of 80% and a hysteresis band of 10%, consumption must drop below 70% before the relay is on again. The hysteresis function is activated when a trip point is exceeded and the external reset is active. Hysteresis can establish 2-point or 4-point control.

3.10 Alarm blocking, S2

If a short predictable overload or underload occurs during the supervision cycle, an alarm may be avoided by informing the unit about the incident. This is done through digital input S2, which must be activated as long as the alarm condition is present or should be ignored.

This also applies if a motor is intentionally stopped while using a minimum limit. If nothing is done, the unit will generate a minimum alarm as the minimum limit is exceeded.

3.11 Relay polarity

If inverted relays are needed, the units allow relay polarity to be programmed as inverted. Please note that inverted relays are off during normal operation, which means the ability of self-supervision is lost. For this reason, non-inverted relays are generally advised where possible.

3.12 dP/dt supervision (HPL430)

dP/dt supervision monitors changes in consumption rather than absolute values. This is useful where absolute values are inadequate for efficient supervision, such as conveyor belts or bucket conveyors with variable static load. The variable load may make static trip points impractical, while dP/dt supervision may solve the task.

The dP/dt limit is programmed in percent of the measurement range. A high measurement range therefore results in less sensitive supervision, and vice versa. If power is at 10% and the dP/dt limit is 10%, the measurement signal must rise to double the normal load within 20 ms before the relay trips, because the dP/dt limit is relative to the measurement range rather than the current measurement.

In addition to the dP/dt limit, the HPL430 also contains a maximum limit so that a slowly increasing load can be detected.

dU/dt supervision

If voltage alterations are not considered during dP/dt supervision, false alarms may occur because idle power in the motor changes quadratically in proportion to voltage alterations. An increase in voltage can therefore cause a power increase and eventually an alarm even if shaft power has not changed. HPL430 includes dU/dt supervision, which ignores a dP/dt alarm if voltage change has been greater than a user-programmed level.

3.13 Reversing (HPL440)

The HPL440 is the most advanced stand-alone control unit in the Unipower family. Without connection to a PLC, it is able to reverse a conveyor and potentially remove a blocking. The reversing time and number of reversing attempts are programmable.

3.14 Frequency adapted trip point (HPL450)

In connection with mechanical variable gear, power measurement on the motor may not directly serve as shaft power measurement. Adjustment for revolutions, or frequency, is required. The HPL450-series was developed for this purpose.

In addition to power measurement, a frequency signal such as 4-20mA, 0-10V, 5-1000Hz square or Namur is connected to the unit. The unit adjusts the trip point from user-programmed coordinates: minimum, break point and maximum, depending on frequency.

It would go too far to describe every unit of the Unipower family in detail, but the HPL403 is a pure measurement transducer for kW and kWh and is usually used as a PLC-interface unit. It measures and displays kW in percent or absolute value. The HPL410-430 and HPL110 implement different trip points for supervision and control. TMS units are used for supervision of cutting tool machinery for tool-break and tool-blunt conditions.

4 Applications

The Unipower family has many applications, which may be divided into supervision, control and tool monitoring tasks. Tool monitoring is not treated here; interested readers are referred to the company's homepage: www.unipower.dk.

4.1 Supervision

It should be stated that it is usually the machinery, rather than the motor, that requires supervision. At the same time, supervision of the motor may be a valuable benefit.

1. The Unipower modules replace friction clutches, breaking bolts, tacho controllers and similar devices in connection with conveyor belts, screws, elevators, ventilators, pumps and more. Depending on the application, a unit with one, two or more trip points is used. Maximum and minimum trip points are often useful with ventilators and pumps. Two-speed motors use two trip points, one for each speed. Transport elevators in the food processing industry often use a unit with a maximum trip point and a dP/dt maximum limit. The HPL440 is used with screw transporters and can change the revolution direction of the screw for a set time and number of attempts.
2. Examples of applications include coal transporters at power plants in Austria and Denmark, conveyor belts in industrial washing machinery in Sweden, elevators and transporters in automatic computer-controlled stock systems in Switzerland and Belgium, cleaning plants and pumps in Denmark, internal transport and gate supervision in Germany, and motor/gear supervision in reactor control at a nuclear power plant. Because of their very fast reaction time, Unipower modules have also been used as end stops.

4.2 Control

Some relatively simple control tasks are implemented by the Unipower alone. More complex control schemes must be established by a PLC with assistance from one or more Unipower modules. As stand-alone controllers, Unipower modules are used as two-point or four-point controllers with a suitable hysteresis band.

a) Control of flow of material

The power consumption of a grinding mill working with stone, coal or similar material is measured with the Unipower HPL410. Material is supplied from a conveyor belt that would otherwise continue filling and possibly overfill the mill.

The maximum trip point may be programmed to 80% with a hysteresis band of 10%. As the mill fills, power consumption rises. When it reaches 80%, the HPL410 maximum trip point relay changes state. The relay stops the conveyor belt feeder. As power consumption falls to 80% - 10%, the relay activates the feeder again.

If motor consumption is below 50% of nominal load and the hysteresis band is narrow, such as 5-10%, Po compensation may be necessary, particularly where large mains voltage fluctuations exist.

b) Supervision of pumps

Depending on pump construction, breakdown may occur if the pump runs dry. To prevent this, an HPL110 with a minimum limit is widely used.

c) Controlling mixer

The power consumption of a mixer is a measure of the mixture's viscosity. If consumption reaches a certain maximum trip point, the relay changes and should be used to start the flow of low-viscosity material to the mixer. This system is currently used in the ice-cream industry in Denmark.

d) Controlling number of revolutions

The inverted analogue output signal, 4-20 mA, may be used to control a frequency inverter so the machine's power consumption can be kept constant by changing revolution speed. Within certain limits, the same can be achieved with mechanical variable gears using maximum and minimum trip points plus hysteresis.

These examples are intended as general information about the possibilities of measuring torque indirectly through power measurement.

5 Panel instruments

If the unit needs to be panel mounted, the Unipower family also includes units with housing according to DIN43700 with standard dimensions 72 x 72 mm. HPL110A and HPL130A are stand-alone units containing both measurement and control ability, while d10 and d382 require APM110 and APM382 respectively.

5.1 HPL110A

HPL110A is functionally identical to HPL110. Additionally, the unit is available with 0-20mA analogue input and 4-20mA analogue output. HPL110A may be supplied for single phase measurement and/or supply.

5.2 HPL130A

HPL130A includes dP/dt supervision in addition to the standard control functions. The unit is available with 0-20mA analogue input and 4-20mA analogue output. HPL130A may be supplied for single phase measurement and/or supply.

5.3 d10

The Unipower d10 is a display/control unit for the APM110. When connected, the APM110 functions only as measuring and relay unit. The d10 is connected via two wires. All control functions are taken over by the d10.

If correctly set up, the d10 displays power measurement as kW, HP or kW%. Limit setpoints may also be set in kW, HP or kW%. The d10 has an analogue output, either 4-20mA proportional to the power measurement or a separate alarm output.

5.4 d382

The APM382 may be connected to a display unit called d382 for readout of measured power. It connects to the APM382 through the same serial connection used to connect the APM382 to a PC. Therefore, an APM382 cannot be connected to both a PC and a d382 at the same time.

The d382 can display measured power or program the APM382. All control functions are performed by the APM382.

5.5 d382-dA

To accommodate data acquisition demands, the d382 display unit is available with extra memory as d382-dA. In addition to d382 functions, d382-dA contains settings for data acquisition: sampling interval, automatic or manual acquisition start, and trigger function. The sampling interval lies between 20 ms and 1 s. The d382-dA has capacity for 1 million measurements.

6 The APM-family

The APM family consists of compact units without a built-in display and includes measurement transducers and load monitors. The functions described in the previous chapters also apply to APM units.

6.1 APM100B

The APM100B is a 3-phase measurement transducer for symmetrical loads. The measurement range may be set up in steps of 0.1kW from 0.1kW to 80kW. Analogue outputs are 0(4)-20mA and 0(2)-10V. An S0-1 output for kWh pulses is also included.

6.2 APM110

The APM110 is a 3-phase load monitor for symmetrical loads. The measurement range may be set up in steps of 10W from 0.01-80kW. The unit has a maximum and a minimum limit with a common output relay. The maximum limit equals the measurement range, while the minimum limit is set in percent of the measurement range. The unit includes the necessary timers to function as a load monitor.

6.3 APM300B

The APM300B is a 3-phase asymmetrical measurement transducer to be connected to external current transformers. Analogue outputs are 0(4)-20mA and 0(2)-10V. An S0-1 output is also present for kWh pulses.

6.4 APM380

The APM380 replaces the previous PWM325. It measures power on 3-phase asymmetrical loads, including after frequency inverters. The unit has built-in current transformers for currents up to 80A. Supply voltage is 24V=, and the unit may be connected to mains voltages from 3 x 230V to 3 x 575V.

The voltage range is set on the unit front plate, where the current range is also selected from 1A to 80A. Analogue outputs are 0(4)-20mA and 0(2)-10V. One of four analogue filters with short time constants may be chosen, as well as one of four digital filters with long time constants.

APM380 is primarily used as a measurement transducer for Tool Monitors (TMS) in tooling machines, but may be used in any application where power measurement after frequency inverters is needed.

6.5 APM382

The APM382 is identical to APM380 with respect to measurement principles, but it also includes control machines for limits and related functions. Programming is done with the PC software 382Mon or with the display unit d382.

This concludes the examination of the Unipower products. Hydria Elektronik ApS is a company in constant development, meaning new units are developed frequently. If an application needs modifications to an existing unit, contact the company to discuss the new requirements. A variety of customer-specific units already exist, and new requirements emerge all the time.

Frequently asked questions

What is the UNIPOWER family used for?

The UNIPOWER family is used for industrial power consumption measurement, motor load monitoring, supervision and control. It supports applications such as conveyors, pumps, ventilators, mixers, tooling machines and material handling systems.

Why measure true power instead of current or cosφ alone?

True power consumption is proportional to motor torque. Current and cosφ measurements can be misleading when load conditions or mains voltage change, while true kW measurement gives a more reliable control signal.

Which UNIPOWER products are covered?

This page covers HPL-series units, panel instruments including HPL110A and HPL130A, and APM-family products including APM100B, APM110, APM300B, APM380 and APM382.

What control functions are included?

Functions include programmable trip points, start timers, hysteresis, dP/dt supervision, analogue outputs, peak detection, automatic zeroing, phase-order supervision and support for external current transformers.