Newbie here with a question. If you are using, say a 10 hp 460v/3ph/60hz VFD to power a motor driving a high inertia load. This motor is on an industrial machine and is only one piece of equipment on the machine. The motor needs to be braked upon stopping it - say within 5 seconds. The VFD parameters are set for a decel rate of 5 seconds and the VFD has an optional braking resistor attached. All is good so far, the system works properly. Now you need to incorporate a safety relay to an emergency stop button for this unit - running wiring to the VFD's onboard safety input in order to kill power to the drive if the emergency stop is activated - Is this going to affect the decel time of the drive, disable braking or will the DC regenerative portion of the VFD that does the braking be unaffected ? Thanks ahead of time for your answers!
question for VFD gurus
- Thread starter om21braz
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petersonra
Senior Member
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- Northern illinois
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- engineer
The safety input on the VFD removes power from the motor. There is no braking going to be happening once it is tripped, no decel, nothing. just coast to stop.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
Sounds like if the 5 second stop requirement is safety related (personnel injury or machine damage), you would either have to not use the VFD safety input for this purpose or else provide some form of backup (mechanical or other) braking.
In my experience, it depends on the configuration of the VFD.
I like to setup my VFDs such that if the Safety input on the drive were to go low, the VFD would not only disconnect power to the motor (as per the norm), but would also immediately disconnect power to the brake release coil. Brakes are typically spring-return, so removing power would set the brakes. With a high-inertia load, the motor may still continue to spin through the brakes as the drive is letting it freewheel.
If you're not sure of this, add a second relay or contactor on the brake lines. Parallel the safety input on the drive to hold this contactor closed; when the safety input goes low, the contactor opens and the brakes set.
I like to setup my VFDs such that if the Safety input on the drive were to go low, the VFD would not only disconnect power to the motor (as per the norm), but would also immediately disconnect power to the brake release coil. Brakes are typically spring-return, so removing power would set the brakes. With a high-inertia load, the motor may still continue to spin through the brakes as the drive is letting it freewheel.
If you're not sure of this, add a second relay or contactor on the brake lines. Parallel the safety input on the drive to hold this contactor closed; when the safety input goes low, the contactor opens and the brakes set.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Wow, did not even think about the possibility of the motor itself energizing the brake coil as it spins down!
Are electrically-actuated mechanical brakes usually capable of stopping the motor without burning up, or are they just there to hold the shaft in position after the motor has stopped?
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
That's going to depend on how the brake was selected, you can do it either way. Usually (but not always) if a VFD is involved the designer will opt for sizing the brake as a holding brake since the VFD braking capability will bear the brunt of the work. But if it needs to stop the load unassisted for safety, then he will size it for that instead.
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