Mapping Drive Boundaries and Load Characteristics

The Torque-Speed Relationship

In AC induction and permanent magnet machines, output torque is directly proportional to stator current ($T \propto I$). Voltage controls magnetic flux, while current produces mechanical force. When pairing a motor and drive to an industrial mechanism, the load's torque curve over its operating speed range governs how the drive must deliver that current.

Industrial loads fall predominantly into two primary classifications, alongside a third specialized boundary condition:

• Variable Torque (Quadratic Load): Torque demand increases with the square of operating speed ($T \propto n^2$), meaning power scales cubically ($P \propto n^3$). Characteristic of unthrottled centrifugal fans, radial blowers, and centrifugal pumps.
• Constant Torque (Linear Power): Torque demand remains identical across the entire operating speed range ($T = k$), meaning required power scales linearly with speed ($P \propto n$). Characteristic of positive displacement pumps, screw extruders, industrial conveyors, and reciprocating compressors.
• Constant Power: Torque requirement drops inversely as speed rises ($T \propto 1/n$). Common in machine tool spindles, unwinders, and coil winders operating above base frequency in the field-weakening regime.

Understanding Dual-Rating Drive Specifications

Manufacturers typically publish two distinct power and current ratings for the exact same frame size:

• Normal Duty (ND) / Light Duty (LD): Sized for variable torque loads. Typically rated for 110% overload for 60 seconds (or 120% for 3 seconds). Ambient baseline is often rated at 40°C.
• Heavy Duty (HD) / Constant Duty (CD): Sized for constant torque loads. Typically rated for 150% overload for 60 seconds (or 180–200% for 2–3 seconds). Baseline ambient is often specified at 50°C.

Under the hood, the internal silicon (IGBT module) and DC-bus capacitors are identical. The 'Normal Duty' designation simply trades away transient overload headroom and elevated ambient thermal margins to offer an inflated continuous output current rating for benign loads.

Motor Cooling Boundaries at Reduced Speeds

When operating a constant torque load at reduced shaft speeds, the motor's internal cooling must be scrutinized. Standard totally enclosed fan-cooled (TEFC / IC411) motors use an internal fan mounted directly to the rotor shaft. The cooling airflow generated by this shaft-driven fan drops with the square of the rotational speed.

If you run a TEFC motor driving a constant torque load (full current draw) at 25 Hz (50% speed), the motor draws near-nominal current while receiving less than 30% of its designed cooling airflow. Without a separate force-ventilation blower (IC416 cooling) or an inverter-duty motor certified for a 10:1 or 20:1 constant-torque turndown, the motor stator windings will rapidly exceed their thermal class limit, even while the drive reports normal operating status.