Induction Motor Restart: Kinetic Buffering and Flying Restart After Brief Power Dips

One of the most frustrating situations in a plant running asynchronous motors is the entire process stopping after a brief voltage dip or momentary grid outage. Even an interruption of a few hundred milliseconds can halt a whole production line if the drive faults and stops the motor. Yet modern variable frequency drives have powerful functions designed to keep the process alive in such cases: flying restart (catch a spinning motor / speed search), kinetic buffering and automatic restart. In this article we cover how restarting an asynchronous motor after a short outage or voltage dip works, which parameters matter, the out-of-phase reclosing risk and the right drive-control selection. At HEM Motor our goal is to make your process resilient to interruptions by supplying drive-friendly motors with the correct insulation class quickly from stock.

The Problem: Why Does a Short Outage Stop the Process?

While feeding the motor, a variable frequency drive also monitors the DC bus voltage. When there is a short outage or a deep voltage dip on the grid, this DC bus voltage drops rapidly. When it falls below a certain lower limit, the drive faults (undervoltage) to protect itself and cuts its output. Meanwhile the motor is still spinning due to inertia; especially with high-inertia fan, pump and centrifuge loads the motor keeps free-spinning for seconds. When the grid returns, if the drive does not restart the spinning motor correctly, the motor's current speed and the drive's generated frequency do not match; this means high current, a fault and mechanical shock.

• Voltage dip/sag: The grid voltage drops briefly; it may be due to another large load starting or a line fault.
• Short outage: The grid goes fully out for a few hundred milliseconds and returns; typical during an automatic recloser operation.
• Result: If no measure is taken, the drive stops, the motor starts free-spinning and the process is interrupted.

Catching a Spinning Motor (Flying Restart / Speed Search)

Flying restart is often called catching a spinning motor or speed search. In this function, before re-applying output, the drive detects the motor's current speed and direction. To do this the drive sweeps the output frequency from high to low (or with a defined scanning logic) to catch the motor's real speed; it then synchronizes the current to this speed and smoothly takes the motor back over. This way the motor goes from free-spinning to controlled drive without stopping and the process continues uninterrupted.

Without flying restart, starting a spinning motor from zero frequency draws very high current, because there is a large difference between the rotating field the drive produces and the rotor's physical speed. This causes both an overcurrent fault in the drive and mechanical shock on the shaft and coupling. Speed search eliminates this difference.

Kinetic Buffering

Kinetic buffering is another clever way to keep the process alive during a short outage. In this function, when the grid goes away the drive uses the kinetic energy in the motor's rotating mass to maintain the DC bus voltage. The drive lowers the motor speed in a controlled way; during this deceleration the motor briefly behaves like a generator and the energy it produces feeds the DC bus. Thus the DC bus voltage is kept above the undervoltage fault threshold and the drive keeps running without faulting. When the grid returns the drive accelerates the motor again.

This method is very effective especially with high-inertia loads (large fans, centrifuges, thick rolls), because the energy stored in the rotating mass is large and enough to bridge a short outage. With low-inertia loads the stored energy is small, so the kinetic buffering time shortens. Kinetic buffering and flying restart are usually used together: kinetic buffering tries to bridge the outage, and if it cannot, flying restart engages when the grid returns and takes the motor back over.

• High inertia = long kinetic buffering time = full bridging of short outages.
• The DC bus is kept above the fault threshold; the process does not stop.
• When the grid returns, acceleration is controlled; the current surge is limited.

Automatic Restart (Auto Restart)

Automatic restart lets the drive start running again by itself, without operator intervention, after an outage or fault is cleared. This function is valuable in unmanned or remote plants (water lift stations, irrigation, ventilation) so the process recovers on its own. However, auto restart must be configured carefully; for safety, uncontrolled restart must not be allowed on machines whose rotation is dangerous or which require operator intervention.

• Retry count: How many automatic starts will be attempted.
• Retry interval/wait: The wait time between each attempt.
• Counter reset: Resetting the attempt counter after a period of trouble-free running.
• Safety interlock: Auto restart must be disabled or interlocked on dangerous machines.

Auto restart and flying restart work together: the drive decides to restart automatically, and flying restart safely takes over the spinning motor.

Out-of-Phase Reclosing Risk

In asynchronous motors running directly from the grid (DOL) without flying restart there is a serious risk: out-of-phase reclosing. During a short outage, while the motor keeps free-spinning, a residual voltage remains in the stator for a while and this voltage slowly drifts in phase. If at that exact moment the grid returns and the contactor recloses the motor, the grid voltage and the motor's residual voltage may be out of phase. If the two voltages are in anti-phase, the resulting sudden voltage difference causes very high current and a severe mechanical torque shock; this shock can break the coupling, shaft or gearbox.

To manage this risk, critical motors running directly from the grid use a wait time before reclosing (for the residual voltage to decay) or use synchronous controlled closing. In drive systems, flying restart already removes this problem, because the drive detects and synchronizes the motor's speed and phase state before closing. Therefore, in plants that experience frequent outages and have high-inertia loads, the drive solution both protects the process and improves mechanical safety.

Correct Motor and Drive Selection

To get full benefit from the restart functions, the motor and drive must be compatible. It is important that the motor has inverter-duty winding insulation, withstands du/dt voltage spikes and is connected with a properly grounded shielded cable. Also, since frequent start-stop and restarting increase heating, the motor's duty type and thermal protection should be selected to suit this operating mode. With high-inertia loads, the inertia of the motor and load is also considered to extend the kinetic buffering time.

• Inverter-duty winding insulation and suitable insulation class (F/H).
• Winding temperature monitoring with a PTC thermistor or PT100.
• Proper grounding and shielded cable (for EMC and bearing current).
• Sizing to the duty type; thermal margin for frequent starting.

For the basics of the drive-fed asynchronous motor see our VFD with asynchronous motor, and for starting methods our star-delta versus soft starter articles. For single-phasing and protection see single-phasing (phase loss), for heating from frequent starting our starts per hour and for grounding our grounding and EMC in VFD systems guides.

Commissioning and Parameter Setting Steps

The trouble-free field operation of the restart functions depends on correct commissioning and parameter setting. When the drive arrives with default settings these functions are often passive or at conservative values; they need to be enabled and fine-tuned per the application. The first step is to introduce the motor and load character to the drive correctly; for this, running an autotune (automatic motor identification) is recommended. Autotune measures parameters such as the motor stator resistance, leakage inductance and magnetizing current; without these values being correct, speed search will not work correctly either.

The second step is to enable the flying restart and kinetic buffering functions and set the scan current, scan direction and ramp times per the application. In the third step, the auto restart parameters (retry count, wait time, counter reset) are defined in line with safety requirements. The final step is a real test: by simulating a short outage under controlled conditions, it is verified that the drive catches the motor smoothly, the current surge stays limited and the process continues uninterrupted. Without these tests, one should not assume the parameters will behave as expected in the field.

• Introduce the motor parameters correctly with autotune.
• Enable flying restart and kinetic buffering and configure the scan settings.
• Set the auto restart count and wait time according to safety rules.
• Verify the behavior by simulating an outage under controlled conditions.

In Which Applications Does It Make the Most Difference?

The restart functions do not provide the same value in every plant; you see the greatest benefit in applications where the cost of an outage is high and the load has high inertia. In water and wastewater lift stations a pump stopping can cause flooding or a process interruption; here auto restart and flying restart let the process recover on its own. In large ventilation and stack exhausters, high inertia lets kinetic buffering fully bridge short outages. In continuously producing textile, paper and plastics lines, even a short stop means product loss, quality deviation and restart time; on these lines restart functions directly protect production continuity.

By contrast, on machines requiring position precision, whose rotation is dangerous or which mandate operator approval, automatic restart should be approached cautiously. In such applications flying restart can still be used, but the restart decision must be tied to safety logic and uncontrolled starting must not be allowed. The right approach is to assess each application's risk profile and configure the functions accordingly.

The Effect of Restarting on Motor Life

While the restart functions protect the process, they also positively affect the motor's mechanical and thermal life. A correct flying restart greatly reduces the starting current surge and the related heating because it does not force the motor to start from zero. By eliminating the out-of-phase reclosing risk, mechanical components such as the shaft, coupling, key and gearbox are protected from sudden torque shocks. These shocks can, over time, cause fatigue cracks and premature failure in systems running directly from the grid; drive-based restarting removes this load and lowers maintenance cost. Still, very frequent restarting creates heat buildup in the motor, so the starts per hour and duty type must not be overlooked, and continuous temperature monitoring with a PTC or PT100 keeps the motor safe.

Frequently Asked Questions

Can flying restart be used on every motor and load?

Flying restart is designed for loads with a rotating mass that keep free-spinning after an outage; it is very effective in fans, pumps, centrifuges and similar applications. On machines requiring position control or whose rotation is dangerous, it must be configured carefully and used with a safety interlock if needed. The drive must support this function and be parameterized correctly.

Does kinetic buffering bridge an unlimited outage time?

No. The time kinetic buffering can bridge is limited by the energy stored in the rotating mass. With high-inertia loads this time is longer and short outages can be fully bridged; with low-inertia loads the time is short. If the outage is long the drive may still stop, but when the grid returns flying restart engages and safely takes the motor back over.

Is automatic restart safe?

It is safe when configured correctly and on suitable machines; it lets the process recover on its own in applications such as unmanned lift stations, irrigation and ventilation. But on machines where rotation creates a hazard or which require operator approval, auto restart must be disabled. Safety standards and risk assessment always take priority.

Strengthen Your Process Against Interruptions

If short voltage dips and grid outages stop your process, you can gain resilience against these interruptions with the right motor and drive selection. With HEM Motor's broad manufacturer stock and fast delivery, we supply drive-friendly asynchronous motors with the correct insulation class quickly. Share your application, load type and the interruption problem you experience; contact us to request a quote for the most suitable motor.