# Budget VFD with external braking resistor support



## FliesLikeABrick (Apr 22, 2021)

Hi team,

I purchased a cheap $90 KCLY "2HP" VFD off eBay last year for my new drill press.  It is amazing what $100 gets you in terms of configuration (and documentation) for industrial lego pieces on a budget!

The manual indicated that the unit had terminals and configuration for external power dissipation, as well as some internal capacity when external resistors are not available.

In the months I have owned it, the internal braking does largely meet my needs -- but in certain situations I can trip it, unless I adjust many settings to be very conservative.

So, I removed the plastic knockouts covering the PB+ and PB- terminals, and wired in the correct resistors mounted on an aluminum heatsink.  However, no combination of settings results in the external resistors being used.  I have reached the conclusion that this $90 product, unsurpririsingly, doesn't have the extra bits to support external power braking.

--
That all said, I need a VFD for another project, and can move this one to it.  This leaves me looking for a VFD that supports external power braking resistors, so I can put it on the larger drill press.  I'd love if it could use the existing resistors I have mounted, 100 ohm (two 200ohm in parallel), good for up to 100W of dissipation, which I can of course reconfigure to 50ohm or 200ohm ohr 400ohm if needed to suit a particular VFD

Any suggestions for budget VFDs that actually work with external power resistors? I do not mind cheaper import devices, after being impressed with what is available - of course I will take the ratings at face value and over-size the unit...

Thanks all - I just don't trust my eBay browsing to turn up results that actually will use the external resistors, after being burned last time.  I am of course not complaining - you get what you pay for...


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## macardoso (Apr 22, 2021)

I can help you out here. Will post some ideas.

As an industrial controls guy, your braking resistor setup scares me a bit. Assuming your VFD is running off of 240VAC, those braking wires will be carrying ~325VDC which is dangerous from a shock hazard. Also if they were to short to the metal strip the resistors are mounted to, the VFD would probably be fried from the overcurrent. Let's get you a properly made VFD braking resistor with high temperature insulated leads.


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## FliesLikeABrick (Apr 22, 2021)

macardoso said:


> I can help you out here. Will post some ideas.
> 
> As an industrial controls guy, your braking resistor setup scares me a bit. Assuming your VFD is running off of 240VAC, those braking wires will be carrying ~325VDC which is dangerous from a shock hazard. Also if they were to short to the metal strip the resistors are mounted to, the VFD would probably be fried from the overcurrent. Let's get you a properly made VFD braking resistor with high temperature insulated leads.


Thank you for your input - I appreciate your concern for these failure modes.  These resistors are so far a proof-of-concept, I to intend to add extra insulation and human protection to it, but was trying to prove that the VFD had external braking capacity before going down that path.  Hopefully I can put your mind at ease a bit while we discuss VFD options, with a few notes:

The metal mounting plate has its own dedicated ground run to the upstream contactor's junction box and tied into the incoming ground from the power supply
I will be pop-riveting insulating fiber board behind the exposed electrical connections, to reduce the chances of an impact pushing the exposed contacts against the backplate
I have a cover that will mount over this entire setup, similar to the perforated cages around commercial products
This whole setup can be unscrewed and re-mounted to the backside of the plate, away from the operator
The conductors are 90C-rated, and can additionally be sleeved with fiberglass (including between the resistors, with a bit of un-and-re-soldering) to prevent chafing or reduce the probability of contact in the case they were otherwise damaged.

That all said, I am not above replacing this with a commercial solution, once I know I have a VFD that can properly use the additional investment.


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## macardoso (Apr 22, 2021)

AutomationDirect is my source for low cost *good* automation components. No guesswork with Amazon or Ebay, real manuals, real support.

For a basic VFD, I'd recommend the GS20 series VFDs. The 2HP model (230V 1P) is GS21-22P0 ($188) and has an option for an external 200W braking resistor GS-BR-200W091 ($30). The drive has Sensorless Space Vector Control (SVC) and Sensorless Field Oriented Control (FOC) which gives much better low speed torque in the motor compared to a basic V/Hz drive from eBay. It also has Safe Torque Off (STO) inputs which are a true safety rated method to ensure the motor has no power when an ESTOP is hit.



			https://www.automationdirect.com/adc/shopping/catalog/drives_-a-_soft_starters/ac_variable_frequency_drives_(vfd)/general_purpose/gs21-22p0
		




			https://www.automationdirect.com/adc/shopping/catalog/drives_-a-_soft_starters/ac_variable_frequency_drives_(vfd)/vfd_accessories/braking_units_-a-_resistors/gs-br-200w091
		


This particular VFD does not appear to have an internal resistor for dynamic braking. The external resistor is rated for braking at 125% of motor max rated torque, which will stop the motor FAST.


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## macardoso (Apr 22, 2021)

Also, with a trusted supplier, I see no reason to arbitrarily oversize a VFD. See the duty rating tables in the manual to understand the overload ratings. These are usually under the CT/VT (Continuous/Variable Torque) section in the manual.


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## FliesLikeABrick (Apr 22, 2021)

macardoso said:


> Also, with a trusted supplier, I see no reason to arbitrarily oversize a VFD. See the duty rating tables in the manual to understand the overload ratings. These are usually under the CT/VT (Continuous/Variable Torque) section in the manual.


100% agreed, the oversizing was arbitrary and based on being skeptical of the cheap eBay options.  I will look at your recommendation and comment back here if I have any questions or end up placing an order.  This pricing is much more reasonable than I expected as well, thank you!

As for E-Stop - thanks, I was trying to figure out the best way to implement emergency stop, and this sounds like a good balance (via the VFD, but not just a normal "stop" command; and not just cutting power to the VFD from upstream)


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## macardoso (Apr 22, 2021)

Yup! AutomationDirect is a great middle ground of cheap enough to be reasonable + good free phone support, manuals, and quality products. They are my go-to for all home projects.

Safe Torque Off is an industry standard and each product is qualified by a standards agency. 









						What is Safe Torque Off (STO)? | Library.AutomationDirect
					

Safe Torque Off, (STO) is a safety feature on newer VFDs that turns off power on the VFD output to prevent the motor from producing torque.



					library.automationdirect.com
				




This would 100% be my method to implement ESTOP control on the machine.


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## FliesLikeABrick (Apr 22, 2021)

Order placed - thank you for the quick reply and great information.


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## jwmelvin (Apr 22, 2021)

I recently bought a G20 to get my mill running. I have a couple questions, for which I’m happy to start a new thread but are prompted by the discussion above. 

1. For an e-stop, there appear to be a couple options. One is the STO inputs, which kills motor drive power and coasts to a stop. The other is setting a digital input to a value of 10 (external fault, EF) or 28 (emergency stop, EF1). EF may be configured to decelerate; EF1 is coast to stop. Using EF to stop ASAP seems beneficial to me in many circumstances but I’d like to better understand the trade offs. Seems like STO is the highest-reliability approach to killing power. 

2. When would one choose Sensorless Vector (SVC) versus Field-Oriented Control (FOC)? It sounds like FOC is generally better, but with an older motor as I have (2-speed 2-hp from 1980), does that mean I should not be using FOC, or does it just limit my frequency range?


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## macardoso (Apr 22, 2021)

jwmelvin said:


> I recently bought a G20 to get my mill running. I have a couple questions, for which I’m happy to start a new thread but are prompted by the discussion above.
> 
> 1. For an e-stop, there appear to be a couple options. One is the STO inputs, which kills motor drive power and coasts to a stop. The other is setting a digital input to a value of 10 (external fault, EF) or 28 (emergency stop, EF1). EF may be configured to decelerate; EF1 is coast to stop. Using EF to stop ASAP seems beneficial to me in many circumstances but I’d like to better understand the trade offs. Seems like STO is the highest-reliability approach to killing power.
> 
> 2. When would one choose Sensorless Vector (SVC) versus Field-Oriented Control (FOC)? It sounds like FOC is generally better, but with an older motor as I have (2-speed 2-hp from 1980), does that mean I should not be using FOC, or does it just limit my frequency range?


Safe Torque Off is the right way to do this. The reason is that the function is executed in specially designed hardware with an *extremely *low probably of dangerous failure (i.e. the motor does not stop when the ESTOP is hit). The function also allows you to wire dual channels into the function. This help limit risk of shorted or broken wiring leading to a dangerous condition where the ESTOP does not function as required. IF you use a digital input, you are trusting software to take care of the function for you, and you do not get the drive's safety rating.

SVC and FOC are both higher performance motor control algorithms which help reduce motor heating and provided a wider speed band (how slow you can go before the VFD can no longer provide the motor rated torque). V/Hz is typically between 5:1 - 10:1, SVC can be 100:1 (GS20 drive w/o encoder offers 50:1), and FOC is 1000:1 or better (GS20 drive w/o encoder offers 100:1). You typically need to add an encoder to the motor to get the maximum speed band. These control methods should be applicable to all motors provided you know the motor data. SVC and FOC require knowledge of more motor nameplate data than V/Hz for most drives.


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## jwmelvin (Apr 22, 2021)

macardoso said:


> 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.


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## macardoso (Apr 22, 2021)

jwmelvin said:


> 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.


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## jwmelvin (Apr 22, 2021)

macardoso said:


> 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.


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## macardoso (Apr 22, 2021)

jwmelvin said:


> 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.


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## mksj (Apr 22, 2021)

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.


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## jwmelvin (Apr 22, 2021)

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.


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## mksj (Apr 23, 2021)

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.


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