What is IR compensation?

IR compensation is a voltage compensation function used particularly in AC motor variable frequency drives operating with open-loop V/F control. It helps reduce the effect of the I × R voltage drop across stator winding resistance on low-speed performance. The purpose is to support motor flux and usable torque at low frequency.

It is not directly a speed regulation setting. Correcting load-related shaft-speed deviation is more directly associated with slip compensation or suitable feedback-based speed control. IR compensation addresses the voltage and flux side; slip compensation addresses the speed deviation associated with motor slip.

Where does the I × R voltage drop come from?

Stator windings have electrical resistance. Current through a winding produces a resistive drop of U_R = I_phase × R_phase. Current and resistance must refer to the same per-phase equivalent circuit. Line current, phase current and resistance measured between two motor terminals cannot simply be interchanged. The stator resistance, star/delta connection and drive motor model therefore matter.

On a linear V/F curve, the basic voltage command falls with frequency. A resistive drop that appears small at rated frequency can become a substantial share of the applied voltage at 3–5 Hz. Less voltage remains to establish magnetic flux, which can weaken loaded starting, low-speed traction and torque production.

The actual relationship between terminal voltage, resistive drop and flux is vectorial. The simple addition and subtraction below is a teaching equivalent; it does not represent every motor voltage component or the complete drive algorithm.

What does IR compensation add?

The basic approach is to add an appropriate voltage correction to the low-frequency voltage command. Depending on the manufacturer, this may be a manually set boost, a frequency-dependent curve or an automatic calculation based on current and motor parameters. Not every function labelled IR compensation calculates instantaneous I × R.

Conceptual relationship: Voltage command ≈ basic V/F voltage + suitable compensation voltage. Manufacturers may use terms such as IR compensation, voltage boost or automatic torque boost. Similar terminology does not guarantee identical algorithms or parameter scaling across drives.

A numerical low-frequency example

For illustration only, take a linear V/F equivalent based on 230 V per phase at 50 Hz. The basic voltage at 5 Hz is approximately 23 V. If phase current is assumed to be 5 A and phase resistance 1 Ω, the resistive drop is 5 V. In this simple model, 18 V remains without compensation. Adding 5 V raises the applied command to 28 V, leaving approximately 23 V after the resistive drop.

The same 5 V drop represents approximately 2.2% of 230 V but approximately 21.7% of 23 V. This illustrates why the effect becomes more significant at low speed. The 5 V figure is not a recommended drive setting, and actual current and resistance are not assumed to remain unchanged at all frequencies.

IR compensation versus slip compensation

IR compensation supports low-speed flux and torque production by compensating for the effect of stator resistance on voltage. Slip compensation applies a suitable frequency correction to reduce the load-related speed drop of an induction motor. Neither setting is a substitute for the other.

Schneider Altivar documentation may identify these as separate parameters such as UFr and SLP. Check the actual series and firmware for parameter codes, access conditions and ranges. Increasing voltage compensation simply because speed falls under load can overlook the real cause.

Is it the same as torque boost?

Torque boost is a broader term. Additional low-frequency voltage may be set manually or calculated automatically using a motor model. IR compensation can be treated as a particular implementation of low-speed voltage compensation, but the manufacturer's terminology and parameter description govern its use.

The frequency range also matters. Some drives progressively reduce the extra voltage until it reaches zero at a defined frequency. That cutoff may be separate from base frequency. There is no universal rule that compensation ends at 10 Hz or remains active up to 30 Hz on every drive.

Is manual IR adjustment available in every control mode?

No. A drive using vector control can account for stator resistance and flux within its motor model. Selecting sensorless vector control may disable a separate manual IR/boost setting, change its function or manage it automatically. The selected control mode determines which parameters apply.

For example, the ABB ACS355 manual describes 2603 IR COMP VOLT and 2604 IR COMP FREQ and identifies the relevant function's use in scalar frequency control. These are ACS355 examples, not parameter codes or values to transfer directly to a different drive.

What happens with excessive compensation?

Excessive low-frequency voltage can cause over-fluxing, high magnetising current, motor heating, noise or instability. The drive may enter current limiting, and a higher setting may fail to produce the expected torque improvement. ABB likewise cautions against unnecessarily high IR compensation voltage to avoid overheating.

The commissioning objective is not the highest IR setting. It is stable starting and load response within permitted current and temperature limits. Motor overload protection and temperature feedback are not obstacles to disable in pursuit of more torque.

Why can behaviour change as the motor warms up?

Copper winding resistance increases with temperature. A cold motor and a motor that has been running under load therefore do not have the same resistance. A setting that appears suitable when cold may produce different current and stability behaviour when warm. Assess both cold starting and warm operation.

Within the supported identification method, auto-tuning helps determine stator resistance and other electrical motor-model parameters. After changing the motor, winding connection or cable arrangement, check the manufacturer's requirements for repeating identification. Raising compensation is not a sound remedy for incorrect motor nameplate data.

The Inovance MD310 manufacturer manual lists stator resistance, rotor resistance, inductances and no-load current among its motor parameters. However, stationary and rotating identification do not necessarily identify the same parameters. Consider whether the load can move, which procedure is supported and the drive version. The manufacturer PDF is linked below.

Fans, pumps and constant torque loads

Constant torque applications continue to need meaningful torque at low frequency. Suitable voltage compensation can therefore be particularly useful on conveyors, feeders and some extruders or mixers. A loaded machine's breakaway torque may also exceed its continuous running torque.

Centrifugal fan and pump torque generally falls with speed, so a strong boost is not necessary in every application. Positive displacement pumps cannot automatically be assigned the same load characteristic. Assess differential pressure, friction and process conditions. The word “pump” alone is not a basis for the setting.

Is IR compensation enough at very low speed?

Not always. It reduces one limitation of low-frequency V/F control; it does not turn the system into encoder-based speed control. For high continuous torque near zero speed, precise regulation or rapid load response, compare open-loop and closed-loop control options against the actual motor/drive performance data.

A standard self-cooled motor can lose cooling capacity as speed falls. Improved torque after voltage compensation does not establish that the motor can operate for hours within its thermal limits. Danfoss's low-speed guidance also considers load, operating duration and cooling together. An independent cooling fan or a motor suitable for low-speed duty may be required.

A practical commissioning sequence

Start with motor data and the mechanical load. Check nameplate entries, star/delta connection, free mechanical movement and loaded starting conditions. Establish whether the symptom is a genuine low torque fault, mechanical restriction or a current limit.

Then check the control mode and ramp. Review the V/F curve, motor identification requirements and acceleration/deceleration time. A short ramp requires additional acceleration torque; IR adjustment does not remove the torque required by inertia. Assess carrier frequency within manufacturer limits for noise and capacity considerations; it is not a substitute for torque compensation.

Finally, adjust compensation carefully and monitor the result. Follow the manufacturer's method, making small changes while observing current, temperature, starting and warm-motor stability. Interpret VFD fault codes together with load and speed at the time of the event. If speed deviation remains, assess slip adjustment or the control method separately.

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