Specify the work cycle before choosing a drive

A drive can run a motor comfortably at steady speed and still struggle when the machine accelerates a loaded conveyor every minute. Current demand has a duration and a repetition rate. Those details belong in the specification alongside the motor nameplate. The variable frequency drive guide explains the basic equipment; duty selection asks what that equipment must deliver throughout the job.

IEC TR 61800-6:2003 is a technical report on determining load-duty types and corresponding current ratings for adjustable-speed electrical power drive systems, with particular attention to the basic drive module, or BDM. It presents guidance on rating methods. The general requirements for AC systems are addressed in IEC 61800-2; IEC 61800-1 covers the DC-system branch.

Turn the machine cycle into a load profile

Record starting, acceleration, running, deceleration and waiting. For each stage, identify the required current, its duration and the time until the next occurrence. “Eight hours per day” says little about repeated peaks. Start with the mechanical requirement: torque, the difference between torque and power, and the relationship between torque and speed explain why equal power labels can hide different operating demands.

A loaded conveyor may need constant torque across its operating range. A centrifugal pump or fan often has a speed-dependent load, but the word “pump” does not establish a universal duty. Use the pump controller guide, examine the actual torque curve, and consider torque when selecting the motor. The machine's load profile matters more than its application label.

Read all three current limits

Continuous current is available under the manufacturer's stated conditions. Overload current comes with an allowable duration and recovery or repetition conditions. Maximum or peak current may describe a much shorter capability. A catalogue entry saying “150% for 60 seconds” is incomplete for selection unless you also know the reference current, repeat interval and operating conditions.

One drive can have different continuous-current ratings for different duties. Compare the relevant current columns rather than relying on “normal duty” or “heavy duty” alone. Motor overload protection serves a separate purpose: protecting the motor's thermal condition. Choosing between speed control and torque control changes how the load is controlled, but does not remove the drive's electrical limits.

Why the average current can mislead

Consider this illustrative 60-second cycle. The values below represent RMS current during each stage; this is an engineering example, not a prescribed IEC duty profile.

The arithmetic average is about 19.2 A. A simple square-law equivalent gives I_eq = √[(30² × 10 + 20² × 40 + 5² × 10) / 60] ≈ 20.5 A. The high-current stage carries more weight when considering losses that rise with current squared.

This calculation does not qualify a drive or motor for the application. The equipment must also support 30 A for ten seconds every minute. Thermal time constants, cooling and semiconductor limits can make that repeated demand unacceptable even when an equivalent-current calculation looks reasonable. Check the manufacturer's duty data or sizing method.

Acceleration sets its own requirement

A shorter acceleration ramp can increase the torque needed to accelerate inertia. Calculate or measure that demand separately from steady operation. Sudden torque increases also load the mechanics. A torque boost setting cannot create additional hardware current capacity; excessive boost may increase heating.

Neither torque control nor direct torque control cancels continuous or short-time current limits. If a loaded motor cannot maintain speed, the low-torque troubleshooting guide can help distinguish an incorrect setting from a load or capacity problem. Control strategy and equipment sizing must work together.

Include the stop, not just the start

An inertial load can return energy during deceleration. Check dynamic braking hardware against braking duration and repetition, and relate the stop to the required motor braking torque. DC injection braking works differently and can add motor heating. Different stopping methods require different checks.

Apply the rating to the actual installation

Ambient temperature, altitude, switching frequency and cabinet ventilation can affect available current. Apply the derating instructions for the selected model. A self-ventilated motor running slowly may also need a separate cooling assessment. The balance between torque and speed therefore includes thermal behaviour as well as mechanical output.

Other drive requirements remain relevant: IEC 61800-5-1 drive safety, power-interface voltage and drive energy efficiency address different aspects of the system. A suitable current rating does not establish compliance with every other requirement.

Related reading

Variable frequency drives · Constant-torque loads · Motor overload protection · Ramp times · Dynamic braking · General requirements for AC drive systems

References

IEC TR 61800-6:2003 — official scope and publication details
ABB ACS580 — manufacturer catalogue example of duty and current ratings