What is free (coast) stop ?
When a motor is running and the start signal is cut, the motor will lose power and the mechanism will coast to a stop. The coast-to-stop feature of the drive, which is set in free-run mode, allows the motor to coast to a stop without external braking or resistance.
In an AC motor speed control system, the coast-to-stop feature can be used to help the motor slow down by reducing the frequency of the power supplied to the motor. This helps reduce mechanical stress on the motor and improve performance and reliability of the drive system.
It should be noted that the coast-to-stop feature should not be used in applications where sudden stopping is required, as this may result in a longer stopping distance and damage to the system. In these cases, external braking or resistance should be used to bring the motor to a stop.
On the other hand, the coast-to-stop feature is necessary in high-inertia machines, such as those with large-diameter flywheels. In these types of machines, the motor's braking capacity may not be sufficient to stop the mechanism, so mechanical braking units (such as hydraulic or brake pads) are used. If the coast-to-stop feature is not activated in this situation, the drive will go into "Ov: DC bus voltage high" error and become inactive.
Some users may misunderstand the coast-to-stop concept and think “no braking means unsafe.” In fact, this feature should be selected based on the needs of the application. Especially in low-inertia systems like fans or pumps, letting the motor stop by itself can save energy and extend equipment life.
From my experience, the coast-to-stop option is often ignored in small motors, but using the wrong stop method can cause unnecessary activation of brake resistors or braking units. In the right place, coast-to-stop is actually an economical solution.
However, since the stopping distance will be longer, operators and maintenance staff must be aware of this. My advice is to always test the coast-to-stop mode before applying it to production, to confirm that the stopping time is acceptable for the process. Otherwise, system performance may be negatively affected.
Finally, if a machine uses both coast-to-stop and mechanical brake units, the drive’s stop algorithm must be tuned to make them work together. Otherwise, the drive may trip with a DC Bus overvoltage error and unexpectedly stop the production line. Correct parameter settings are critical in such cases.
+ What is brake resistor?
+ What is Acc/Dec time ?
+ What is Safe torque off (STO) ?
In short, it is the same as stopping the motor with a mechanical contactor or stop button. The motor stops uncontrollably due to its own inertia.
In an AC motor speed control system, the coast-to-stop feature can be used to help the motor slow down by reducing the frequency of the power supplied to the motor. This helps reduce mechanical stress on the motor and improve performance and reliability of the drive system.
It should be noted that the coast-to-stop feature should not be used in applications where sudden stopping is required, as this may result in a longer stopping distance and damage to the system. In these cases, external braking or resistance should be used to bring the motor to a stop.
On the other hand, the coast-to-stop feature is necessary in high-inertia machines, such as those with large-diameter flywheels. In these types of machines, the motor's braking capacity may not be sufficient to stop the mechanism, so mechanical braking units (such as hydraulic or brake pads) are used. If the coast-to-stop feature is not activated in this situation, the drive will go into "Ov: DC bus voltage high" error and become inactive.
Coast Stop vs Ramp Stop
This simulation shows a VFD’s response in two stopping strategies. It plots frequency, current, and DC Bus voltage live.
Change inertia, ramp time, and brake resistor to observe different behaviors.
Change inertia, ramp time, and brake resistor to observe different behaviors.
Parameters
Start Frequency (Hz): 50
Inertia (J): 1.0
Ramp Time (s): 5.0
DC Bus Class:
Educational Summary
Coast: Drive output is disabled, current drops quickly; the motor coasts on inertia. High inertia + low friction → longer stop; regenerative energy can raise the DC Bus.
Ramp: Output frequency follows a deceleration ramp, producing a controlled slowdown. Too short ramp + no BR → OV (overvoltage) risk.
BR (Brake Resistor): Dissipates regenerative energy as heat to keep DC Bus within limits.
Ramp: Output frequency follows a deceleration ramp, producing a controlled slowdown. Too short ramp + no BR → OV (overvoltage) risk.
BR (Brake Resistor): Dissipates regenerative energy as heat to keep DC Bus within limits.
DERELL Note: Choose ramp time balancing mechanical safety and line efficiency. If OV appears, first extend ramp; if not enough, enable BR.
Coast Stop
Frequency (Hz), Current (A), DC Bus (V)
Ramp Stop
Frequency (Hz), Current (A), DC Bus (V)
Learning Tip: Keep start frequency and inertia the same. Shorten ramp time to 2 s with BR disabled – watch the DC Bus peak and trigger OV in Ramp Stop. Then enable BR and see OV vanish.
From my experience, the coast-to-stop option is often ignored in small motors, but using the wrong stop method can cause unnecessary activation of brake resistors or braking units. In the right place, coast-to-stop is actually an economical solution.
However, since the stopping distance will be longer, operators and maintenance staff must be aware of this. My advice is to always test the coast-to-stop mode before applying it to production, to confirm that the stopping time is acceptable for the process. Otherwise, system performance may be negatively affected.
Finally, if a machine uses both coast-to-stop and mechanical brake units, the drive’s stop algorithm must be tuned to make them work together. Otherwise, the drive may trip with a DC Bus overvoltage error and unexpectedly stop the production line. Correct parameter settings are critical in such cases.
+ What is brake resistor?
+ What is Acc/Dec time ?
+ What is Safe torque off (STO) ?
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