Are braking resistors or EMC filters necessary?

[Wade Lippman]

The VFD on my lathe has burnt out. I have found two good replacements; they are essentially identical except that one has a EMC filter and the other will accept a braking resistor. Are either of those necessary?

 

[Richard Coers]

Here is some Google help; https://www.manufacturingtomorrow.c...-how-should-i-select-a-braking-resistor/15756

EMC Filtering For industrial Applications - Roxburgh EMC

EMC (Electromagnetic Compatibility) filters are used to ensure that electrical and electronic equipment does not generate, or is not affected by, electromagnetic disturbance.

www.dem-uk.com

https://www.automationdirect.com/adc/overview/catalog/drives_-a-_soft_starters/ac_variable_frequency_drives_(vfd)/line_reactors_-a-_output_filters

You will like the braking resistor if you turn large bowls. My Oneway has a switch to turn the resistor circuit on and off. The only reason I've heard of EMC filters in a hobby shop is hearing static in the shop radio. I don't think they are expensive accessories.

 

[Bill Boehme]

In general, a braking resistor is needed in situations where there is an overhauling load for a significant portion of the drive's duty cycle. An overhauling load is basically an aiding load as opposed to the normal situation in which the load is opposing the drive. An example of an overhauling load would be a heavily loaded conveyor going downhill on a steep slope. In that kind of situation, the load is driving the motor which means the motor becomes a generator that is dumping energy back into the VFD. A braking resistor would be needed to dissipate that energy, converting it into heat. This isn't a situation that would be encountered in woodturning other than a few seconds when ramping down a very heavy inertial load (very large diameter and very heavy, more than 100 pounds). If you are concerned, the easiest solution would be to increase the deceleration ramp time.

 

[Don Wattenhofer]

To add to what Bill said: To fast a decel. ramp time also could cause the chuck to unscrew with a heavy load.

 

[Wade Lippman]

In general, a braking resistor is needed in situations where there is an overhauling load for a significant portion of the drive's duty cycle. An overhauling load is basically an aiding load as opposed to the normal situation in which the load is opposing the drive. An example of an overhauling load would be a heavily loaded conveyor going downhill on a steep slope. In that kind of situation, the load is driving the motor which means the motor becomes a generator that is dumping energy back into the VFD. A braking resistor would be needed to dissipate that energy, converting it into heat. This isn't a situation that would be encountered in woodturning other than a few seconds when ramping down a very heavy inertial load (very large diameter and very heavy, more than 100 pounds). If you are concerned, the easiest solution would be to increase the deceleration ramp time.

I see that as a setting. What does ramping down actually do? The VFD recommends a setting of 5 seconds.

They also say that if you want a braking resistor to use theirs for $125, so that the VFD can monitor the temperature and shut it off if it is overheating.
I have the 200ohm breaking resistor from my old VFD. Would it be safe to use on a lathe, as opposed to a conveyer belt?

 

[Wade Lippman]

Here is some Google help; https://www.manufacturingtomorrow.c...-how-should-i-select-a-braking-resistor/15756

EMC Filtering For industrial Applications - Roxburgh EMC

EMC (Electromagnetic Compatibility) filters are used to ensure that electrical and electronic equipment does not generate, or is not affected by, electromagnetic disturbance.

www.dem-uk.com

https://www.automationdirect.com/adc/overview/catalog/drives_-a-_soft_starters/ac_variable_frequency_drives_(vfd)/line_reactors_-a-_output_filters

You will like the braking resistor if you turn large bowls. My Oneway has a switch to turn the resistor circuit on and off. The only reason I've heard of EMC filters in a hobby shop is hearing static in the shop radio. I don't think they are expensive accessories.

What difference do you see in stopping with a bowl, with and without the resistor? Is it 15 second compared to 5 seconds, or 120 seconds compared to 5 seconds?

The EMC filter is probably unnecessary; apparently they are required by the EU.

 

[Richard Coers]

What difference do you see in stopping with a bowl, with and without the resistor? Is it 15 second compared to 5 seconds, or 120 seconds compared to 5 seconds?

The EMC filter is probably unnecessary; apparently they are required by the EU.

If I am turning a huge blank like 18"-22" diameter, and forget to set the braking to slow, it will trip the VFD when I hit the stop button. That is when I'm checking progress of the outside roughing and the interior has not been touched. I have to press a reset button for the lathe to run again. If I was to guess on the time difference, I'd say 10 seconds vs 30 seconds.

 

[Don Wattenhofer]

I see that as a setting. What does ramping down actually do? The VFD recommends a setting of 5 seconds.

When the VFD receives the stop command it decelerates or ramps the motor down to zero speed and then shuts itself down. If you have an emergency stop that disconnects power or you just loose power, then it would coast to a stop and probably take more then 5 seconds to come to a stop.

 

[Wade Lippman]

When the VFD receives the stop command it decelerates or ramps the motor down to zero speed and then shuts itself down. If you have an emergency stop that disconnects power or you just loose power, then it would coast to a stop and probably take more then 5 seconds to come to a stop.

So it stops faster if you ramp down the power than if you simply stop the power? That is counter intuitive, but I'll take your word for it.
Do that eliminate the need for a brake resistor?

 

[Don Wattenhofer]

So it stops faster if you ramp down the power than if you simply stop the power? That is counter intuitive, but I'll take your word for it.
Do that eliminate the need for a brake resistor?

Refer back to Bill B's reply! The brake resistor is needed only if you have an overhauling load or try to stop in too short a time and on a lathe as long as you go with the 5 second deceleration recommended you should not need a brake resistor. I have been using VFD's on wood lathes for 30 years and I have never had a need for or had a brake resistor.

 

[Bill Boehme]

What difference do you see in stopping with a bowl, with and without the resistor? Is it 15 second compared to 5 seconds, or 120 seconds compared to 5 seconds?

The EMC filter is probably unnecessary; apparently they are required by the EU.

The casual observer wouldn't see any difference in stopping with or without the resistor. The difference is that the stored mechanical energy has to go somewhere when stopping the lathe. If the VFD uses a braking resistor then that's where the energy gets dissipated as heat. If there isn't a braking resistor then the energy is dissipated in the motor windings and VFD electronics as heat. For a woodturning lathe, unless the VFD manufacturer says you must have a braking resistor, then in all likelihood, you don't need it. The exception would be if you are turning massive tree trunks and stopping frequently. Even then it would be better to just cut power and let the lathe free-wheel to a stop. Otherwise, you would need to make sure that the chuck/faceplate doesn't unscrew during braking. Many years ago, I was quick enough to catch a heavy spinning chunk of wood before it hit the floor. This primal instinct to catch falling things isn't always a good idea, especially if they are heavy and spinning. Now, I just hit the emergency stop button if there is any possibility of the chuck unscrewing when stopping.

In the US, the FCC is the regulatory agency that sets what is known as Part 15 requirements for conducted and radiated interference. Theoretically, this would apply to lathes with variable frequency drives. There are some exceptions for industrial equipment and it is bending the rules a bit to apply it to consumers, but the ultimate responsibility for insuring that the lathe isn't causing harmful interference lies with the end user.

So it stops faster if you ramp down the power than if you simply stop the power? That is counter intuitive, but I'll take your word for it.
Do that eliminate the need for a brake resistor?

Actually, this is perfectly intuitive ... that is, if you know how an AC variable frequency drive works. Think about your bench grinder. When you shut off power, it slowly coasts to a stop. My bench grinder takes 1 minute 45 seconds to wind down. The speed of the VFD-controlled three-phase motor on your lathe is controlled by the frequency, not by power. This means that the motor speed is in lockstep with the frequency that is applied to it. So, if the VFD ramp-down time is 5 seconds from full speed to complete stop, this means that the frequency of the current to the motor is ramping down from 60 Hz at the rate of 12 Hz/sec.

 

[Wade Lippman]

The casual observer wouldn't see any difference in stopping with or without the resistor. The difference is that the stored mechanical energy has to go somewhere when stopping the lathe. If the VFD uses a braking resistor then that's where the energy gets dissipated as heat. If there isn't a braking resistor then the energy is dissipated in the motor windings and VFD electronics as heat. For a woodturning lathe, unless the VFD manufacturer says you must have a braking resistor, then in all likelihood, you don't need it. The exception would be if you are turning massive tree trunks and stopping frequently. Even then it would be better to just cut power and let the lathe free-wheel to a stop. Otherwise, you would need to make sure that the chuck/faceplate doesn't unscrew during braking. Many years ago, I was quick enough to catch a heavy spinning chunk of wood before it hit the floor. This primal instinct to catch falling things isn't always a good idea, especially if they are heavy and spinning. Now, I just hit the emergency stop button if there is any possibility of the chuck unscrewing when stopping.

In the US, the FCC is the regulatory agency that sets what is known as Part 15 requirements for conducted and radiated interference. Theoretically, this would apply to lathes with variable frequency drives. There are some exceptions for industrial equipment and it is bending the rules a bit to apply it to consumers, but the ultimate responsibility for insuring that the lathe isn't causing harmful interference lies with the end user.


Actually, this is perfectly intuitive ... that is, if you know how an AC variable frequency drive works. Think about your bench grinder. When you shut off power, it slowly coasts to a stop. My bench grinder takes 1 minute 45 seconds to wind down. The speed of the VFD-controlled three-phase motor on your lathe is controlled by the frequency, not by power. This means that the motor speed is in lockstep with the frequency that is applied to it. So, if the VFD ramp-down time is 5 seconds from full speed to complete stop, this means that the frequency of the current to the motor is ramping down from 60 Hz at the rate of 12 Hz/sec.

Thank you. My old VFD came with a resistor, and since it was a cheap one, I assumed they were necessary. Someone on line said that the cheap ones might come with a resistor but the terminals aren't connected to anything. I checked mine, and sure enough, the terminals aren't connected to anything.
The resistors must cost something. Why on earth would they wire them in if they are just for show?

 

[Bill Boehme]

Why on earth would they wire them in if they are just for show?

Apparently, "just for show" must be worth something.

 

[Ric Williams]

Many years ago, I was quick enough to catch a heavy spinning chunk of wood before it hit the floor. This primal instinct to catch falling things isn't always a good idea, especially if they are heavy and spinning.

Very, very true. I have a deep 6" scar across my thigh to remind me. I've never been curious enough about physiology to want to see the inside of my leg like I did that day.

 

[Larry Copas]

I have four VFDs on different machines in my shop beside the wood lathe. They can be programmed internally for different acceleration and deceleration speeds depending on the application. All four can also accept an exterior braking resistor. I've never needed an exterior resistor. The internal options were always enough resistance to slow the machine.

On my General lathe when I'm turning something more than 50 pounds it will trip the VFD overload when I hit the off switch. Always thought I would re-program the VFD but never have. I just slow the speed a little before hitting the off switch. Don't turn that much over 50 pounds anyhow.

 

[Dean Center]

This is a very interesting discussion. The Powermatic 3520s have a braking resistor, at least on the A and, I believe, on the B. Powermatic password protected the controls on their VFDs, to keep us non-electrical engineers from screwing things up. With your new VFD, you'll have the freedom to set the parameters as you like.

Personally, I would just reuse the old one, since your lathe manufacturer thought one would be of benefit, and not change the parameters. Braking resistors are simple devices that shouldn't
'wear out', AFAIK. If necessary, they can be had online for $30 or so.

 

[Don Wattenhofer]

On my General lathe when I'm turning something more than 50 pounds it will trip the VFD overload when I hit the off switch. Always thought I would re-program the VFD but never have. I just slow the speed a little before hitting the off switch. Don't turn that much over 50 pounds anyhow.

That is a good thing best to leave it alone unless you want to increase the deceleration time. The VFD is protecting itself and likely preventing the possibility of the chuck unscrewing.

 

[Bill Boehme]

On my General lathe when I'm turning something more than 50 pounds it will trip the VFD overload when I hit the off switch. Always thought I would re-program the VFD but never have. I just slow the speed a little before hitting the off switch. Don't turn that much over 50 pounds anyhow.

That sounds like the deceleration time is too short which will cause the DC bus voltage in the inverter to exceed the automatic trip threshold. There are two simple no-cost solutions:

1. Do what you have been doing ... just use the speed dial to slow the lathe a bit before hitting the stop switch OR
2. Use Don's solution, which is to increase the deceleration time by a few seconds.

Personally, I would just reuse the old one, since your lathe manufacturer thought one would be of benefit, and not change the parameters. Braking resistors are simple devices that shouldn't wear out, AFAIK. If necessary, they can be had online for $30 or so.

Before doing that, It would be wise to check the documentation for the min and max resistance values as well as the wattage rating. Since the resistor would probably need to be installed outside of the new VFD, it might need a cooling fan and a safety enclosure.

 

[Wade Lippman]

FWIW I got a VFD and reused my old resistor.
The VFD runs at about 335v and sends current to the resistor when the voltage goes over 360v. The 360v is changeable.
With a chuck and a modest bowl blank, at 5 seconds stop time, no current went to the resistor when stopping from full motor speed and 1015 rpm.
At 2 seconds stop time, it got 0.3a and 39v for less than a second. Since it is a 100 ohm resistor, that actually makes sense.
I could change the pulley to get 3,000rpm and use a bigger blank, but I am happy. I will just set it 7 second stop time and all will be fine.