Motor Load Monitors (Shaft Power Monitors)

Motor load monitors, often called shaft power monitors, are protection relays that measure the true power (kW) drawn by a motor and use it as an indirect but highly accurate indication of shaft load and torque. Unipower digital load monitors continuously supervise motor load to detect overload, underload, jamming and dry running conditions, providing fast and reliable protection for pumps, conveyors and other driven machines without the need for extra mechanical sensors.

🔧 How Unipower Motor Load Monitors Work

• True kW measurement: The monitor calculates motor input power using the three phase formula P = sqrt(3) x U x I x cos(phi). Because power is directly related to torque, changes in mechanical load show up immediately as changes in measured kW.
• Digital motor load display: Units such as the Unipower HPL series display motor load as kW and as percent of a scaled range, making it easy to see normal operating load and set precise trip limits.
• Overload and underload trips: Programmable high and low kW limits allow the relay to trip or alarm on both overload (jam, blockage, mechanical fault) and underload (dry running, empty feed, broken belt) conditions.
• Fast response and surge masking: Reaction times in the order of tens of milliseconds detect genuine faults quickly, while adjustable start up and trip delay timers prevent nuisance tripping during motor start or brief transients.
• Min and max load memory: Peak detector functions store minimum and maximum load values to simplify commissioning and make it easier to diagnose process problems or mechanical changes over time.
• Flexible current measurement: Smaller motors can be monitored directly through the unit (feed through up to a defined current), while standard current transformers (n:5 or n:1) are used for higher motor currents.

✅ Benefits of Unipower Motor Load Monitors

• Detects overload, underload, jamming and dry running early, helping prevent damage to pumps, gearboxes, couplings and driven equipment.
• More accurate and stable than current-only or power factor based devices, especially where supply voltage and load conditions vary.
• Reduces or eliminates the need for extra field sensors such as pressure, flow or level switches in many applications.
• Analogue outputs and configurable relay contacts allow easy integration into PLC and SCADA systems for alarms, trips and process control.
• Helps implement predictive maintenance by revealing gradual changes in load caused by wear, fouling or product build up.
• Assists energy optimisation by showing when motors are running underloaded or inefficiently.

🚀 Typical Applications for Motor Load Monitors

• Pumps: Protection against dry running, closed valves, blocked filters and cavitation by monitoring motor kW instead of relying only on pressure or level switches.
• Conveyors and feeders: Detect jammed screw conveyors, overloaded belt conveyors, blocked screens and transfer points before serious damage occurs.
• Crushers, grinders and mixers: Monitor changing torque due to product build up, varying density or foreign objects entering the machine.
• Fans and blowers: Protect against broken belts, closed dampers and abnormal airflow conditions by monitoring motor load.
• General machine protection: Use motor load as a process variable for end point detection, batch control and protection of gearboxes, couplings and mechanical drives.

Unipower motor load monitors give engineers a simple way to add fast, accurate shaft power protection to new and existing motor installations. By supervising real motor kW instead of only current, they provide a much clearer picture of what is happening at the machine, helping you reduce downtime and protect critical assets.

Choosing the Right Motor Load Monitor

• Voltage and current range: Select a model that covers the mains voltage and motor current (direct measurement or via CT) for the installation.
• Number of relays: Decide whether you need one or two relay outputs for separate alarm and trip, or for independent over and underload limits.
• Analogue output and communications: For PLC and SCADA integration, choose a monitor with 0-20 mA or 4-20 mA output, and optional serial communication where required.
• Mounting and access: DIN rail or panel mount versions are available; consider where the display and buttons need to be for commissioning and monitoring.

Motor Load Monitor FAQs

• Q: What is a motor load monitor?
  A: A motor load monitor is a protection relay that measures the true power drawn by a motor in kW and uses this as an indication of shaft load. It trips or alarms when the load goes outside preset limits, protecting the machine from overload, underload, jamming or dry running.
• Q: How is a motor load monitor different from a shaft power monitor?
  A: In most industrial applications the terms are used interchangeably. Both monitor motor power to infer shaft load. Some devices can be configured to show either input power or calculated shaft output power, but the protection principle is the same.
• Q: Why use a motor load monitor instead of a simple current relay?
  A: Current relays are affected by supply voltage and power factor, so they do not always follow mechanical load. A motor load monitor calculates true power (kW), which is directly related to torque, giving a much more accurate and stable indication of real load on the machine.