Budget VFD with external braking resistor support

[jwmelvin]

Safe Torque Off is the right way to do this.

...

SVC and FOC require knowledge of more motor nameplate data than V/Hz for most drives.

Thank you. I’ll use STO for my e-stop. Any thoughts on circumstances that would cause one to use the EF/EF1 functions?

It seems that the G20 does auto tune for the FOC setup. I’m hopeful this works out but will be back here if it’s not as smooth as I’d like. For now I’m leaving my low/high switch in place (and obviously won’t change it while the spindle is on). I’ve seen your advise that the low speed (4-pole) is likely to work better. The drum switch seems to have terminals that I could use to switch motor parameters in the drive, so I’m considering trying that, depending on how some initial testing goes.

 

[macardoso]

Thank you. I’ll use STO for my e-stop. Any thoughts on circumstances that would cause one to use the EF/EF1 functions?

It seems that the G20 does auto tune for the FOC setup. I’m hopeful this works out but will be back here if it’s not as smooth as I’d like. For now I’m leaving my low/high switch in place (and obviously won’t change it while the spindle is on). I’ve seen your advise that the low speed (4-pole) is likely to work better. The drum switch seems to have terminals that I could use to switch motor parameters in the drive, so I’m considering trying that, depending on how some initial testing goes.

The GS20 manual indicates the drive is capable of "Multi-Motor" control. I *believe* this means you can configure 4 different sets of parameters and switch between them at runtime if you were to change the motor connected to the drive. This would be an excellent feature to leverage to select different settings for the low windings and high windings. You would need an extra contact on your motor switch to go to a drive input to indicate which parameter set to be using. I am not an expert on this, but a call to AD support would help you out!

There are a plethora of safety functions with motors than go beyond just a basic Safe Torque Off. The next up is a Time Delay STO where the drive gets 3 seconds to stop the motor under power before the STO kicks in. This is usually accomplished with an external Safety-Rated time delay relay. There is also Safe-Limited Speed, Safe-Limited Torque, Safe-Limited Position, and many others, although the more complicated ones require high end drives and safety rated encoders on the motor. They are used in industrial settings to make it safe for workers to work on equipment without stopping it.

The time delay STO is useful because it allows you to use the braking resistor even in the case of an ESTOP press. Normally STO just kills the drive and the motor coasts to a stop. I can talk about this more if you are interested.

 

[jwmelvin]

The next up is a Time Delay STO where the drive gets 3 seconds to stop the motor under power before the STO kicks in. This is usually accomplished with an external Safety-Rated time delay relay.

I like this idea, for the reason you suggest (to actively brake before relinquishing control if necessary). I imagine that one could use an external PLC to receive the ESTOP command, activate the drive's EF command, and then activate STO if EF is not successful (based on feedback from the drive). If using the time-delay relay, then I suppose the ESTOP would always activate STO after EF. That seems pretty straightforward though.

 

[macardoso]

I like this idea, for the reason you suggest (to actively brake before relinquishing control if necessary). I imagine that one could use an external PLC to receive the ESTOP command, activate the drive's EF command, and then activate STO if EF is not successful (based on feedback from the drive). If using the time-delay relay, then I suppose the ESTOP would always activate STO after EF. That seems pretty straightforward though.

The *right* way to do this is to use a dedicated safety-rated time-delay relay. The reason for this is that the relay as well as the drive have specially designed safety hardware which have been certified to have an extremely low probability of dangerous failure.

The relay wires right into the ESTOP. When the ESTOP is pressed, there is an immediate output which goes to the drive digital input to command a max deceleration stop. There is then the time delayed safety outputs which trigger the STO after the timer expires. The timer is set at the worst case deceleration (as short as possible), but not to exceed 3 seconds.

The STO should always activate in this method otherwise there is nothing preventing a start command from starting the drive while the ESTOP is pressed. This method only ensures a brief moment to allow the drive to stop the motor under power. If it fails to do that, it will still stop due to the ESTOP, hence why this is an acceptable safety function.

Here is an example of a time delay safety relay product, although there are tons of options:

https://www.automationdirect.com/adc/shopping/catalog/safety/safety_relay_modules/e-stop_-z-_safety_gate_time_delay_relays/dold_e-stop_-z-_safety_gate_time_delay_relays/lg5928-41-61-3

I am certainly guilty of not following best engineering practices in my machinery safety systems at home (I used Mach 4's software ESTOP - on Windows! - for years). But, I do feel like I need to recommend the *right* way to do things here. I don't want anyone to think there is an acceptable alternative to doing safety systems the right way.

 

[mksj]

There are a number of different options with the way that an E-Stop can function, and also depends on the type of machine and if it is an industrial operation or home. The safety E-Stop systems in industrial settings use double switches in a serial configuration, or two separate loops along with a monitoring loop. If any switch fails or power loss it goes open, the purpose to prevent any potential of a system restart with dual redundancy. In addition often an emergency stop button's purpose is to shut down the machine as fast as possible to prevent injury. This often can be a destructive mechanical braking system. I always use a dual circuit E-Stop in my designs, which does three separate actions: deactivates power to the system run relays which are used to activate the VFD run inputs, cuts power to the latched power relay which also stops the the possibility of the VFD receiving any run command and also activates a emergency fast stop to the VFD. In some cases a VFD reset may be required depending on the programming. There are also various other interlocks which always require the machine to be in the stop mode when there is a fault and a reset is performed. This is similar to what you see with factory lathes and mills, that have relays/contactors and/or VFD's. Additionally there are other fault triggers such as the belt cover switch, chip guard or foot brake that once activated require a reset to prevent a restart.

The GS20 looks like a nice reasonable costing replacement for the older GS1/2 VFD's with a lot more features, and nice to see that it has the circuitry for an external braking resistor. The Teco L510 is an inexpensive VFD, which works ok, but does not support an external braking resistor. It is also quite limited as to the number of inputs and programming. The Teco E510 is more comparative to the GS20, the price of the E510 has gone up significantly this last year, so the GS20 is a good choice and Automation Direct has great support.

 

[jwmelvin]

Thanks for the input. I feel better equipped to set up my system. Probably I’ll go with STO and not worry about active breaking with Estop for the mill.

 

[mksj]

Ont thing to note on STO is that you need to review the other parameters and how it is wired into your system. When resetting the STO, you need to make sure that the run command does not restart the machine upon reset of the STO should there be an active run command (i.e. the E-Stop was pressed while the machine was running). Typically there are default parameters which require the run command to be cycled through the stop position, but you need to check how the STO is implemented/programmed specific to the VFD you are using (see video below). The other point is that STO will not quickly stop the motor drive with electronic braking.