Motor controller warning.

Many forum members use tread mill motors and similar commutated DC motors for driving various tools.
I have just been browsing the net looking at motor controllers and noticed many inaccurate descriptions. Many adverts claim PWM motor control and SCR control. In fact these are just SCR controls. Real PWM controllers tend to be more expensive. I have also seen claims of PWM only for what is plainly an SCR controller.

So what is the problem? Well it depends on what you use the motor for. An SCR chops away part of the normal rectified sinewave shaped input to reduce the average voltage and hence reduce motor speed. A PWM system uses a much higher frequency than the 50/60 Hz mains and varies the density of a square wave pulse train to vary average voltage and speed.

The SCR gives an output with sharp spikes when not at full voltage. This does not matter too much for applications such as a saw or even a milling machine or lathe spindle but the motor speed will be pulsing to some degree due to both the low drive frequency (double the mains value) and aliasing with the commutator segments. Mechanical inertia and motor inductance will tend to smooth these effects.

Due to its much higher frequency, 1-10 kHz being typical, PWM control is inherently much smoother and so makes it infinitely preferable for applications such as tool post and other grinders. You would probably get better finish on small lathes also. Use PWM when you want the smoothest drive, motors will last longer too.

Anyway this warning is not to say don't get an SCR controller but to alert forum members to misleading for sale notices on the net. Whatever type you go for make sure that you get one with sufficiently high voltage and current rating for your motor.

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Originally Posted by tonyfoale

Many forum members use tread mill motors and similar commutated DC motors for driving various tools.
I have just been browsing the net looking at motor controllers and noticed many inaccurate descriptions. Many adverts claim PWM motor control and SCR control. In fact these are just SCR controls. Real PWM controllers tend to be more expensive. I have also seen claims of PWM only for what is plainly an SCR controller.

So what is the problem? Well it depends on what you use the motor for. An SCR chops away part of the normal rectified sinewave shaped input to reduce the average voltage and hence reduce motor speed. A PWM system uses a much higher frequency than the 50/60 Hz mains and varies the density of a square wave pulse train to vary average voltage and speed.

The SCR gives an output with sharp spikes when not at full voltage. This does not matter too much for applications such as a saw or even a milling machine or lathe spindle but the motor speed will be pulsing to some degree due to both the low drive frequency (double the mains value) and aliasing with the commutator segments. Mechanical inertia and motor inductance will tend to smooth these effects.

Due to its much higher frequency, 1-10 kHz being typical, PWM control is inherently much smoother and so makes it infinitely preferable for applications such as tool post and other grinders. You would probably get better finish on small lathes also. Use PWM when you want the smoothest drive, motors will last longer too.

Anyway this warning is not to say don't get an SCR controller but to alert forum members to misleading for sale notices on the net. Whatever type you go for make sure that you get one with sufficiently high voltage and current rating for your motor.

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Whilst your comments are valid for the operation of the device, unless tread mill motors are different in America (quite possible) in Australia they are DC motors, so after you've rectified the output from either of those systems the results are the same so it doesn't matter how you arrived at the most appropriate voltage/speed, so long as it doesn't exceed the max for the motor

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Originally Posted by old kodger

Whilst your comments are valid for the operation of the device, unless tread mill motors are different in America (quite possible) in Australia they are DC motors, so after you've rectified the output from either of those systems the results are the same so it doesn't matter how you arrived at the most appropriate voltage/speed, so long as it doesn't exceed the max for the motor

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The results are not the same. The output in each case is not steady DC, it is pulsing. The pwm pulses will be of higher frequency and easier to smooth. Motors run smoother and last longer when fed with PWM supply. The comment about rectifying the output of these controllers is irrelevant because their output is DC.

In any case, as I pointed out, the post was not about triacs or scr vs pwm, it was about misleading adverts.

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Originally Posted by tonyfoale

In any case, as I pointed out, the post was not about triacs or scr vs pwm, it was about misleading adverts.

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This is a real problem that has been magnified significantly of late by places like E-Bay & Amazon where there are far too many people selling stuff that they have no idea what it is. I'm sure there is intentional false advertising but the little bit of on line dealings I've had with these fly by night operations indicates to me that most of these sellers have no idea what an SCR is or what PWM means.

So it comes down to people taking warnings like this seriously and researching what they are buying. Some suggestions:

1. Know the manufactures name and model number of the item being sold.
2. If you can't find on line documentation do not buy! This might seem obvious but your difficulties can be magnified significantly by the lack of documentation.
3. Get the sellers contact information, phone and business address. Also assure your self that they have a real return policy even if it is no returns.

would adding smoothing capacitor after rectifier help smooth signal and increase motor life? also would the varying/pulsating speed you mentioned possible cut down on harmonics/chatter when used on a lathe.
thanks

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Originally Posted by vortexwizard

would adding smoothing capacitor after rectifier help smooth signal and increase motor life? also would the varying/pulsating speed you mentioned possible cut down on harmonics/chatter when used on a lathe.
thanks

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In general adding a capacitor helps with smoothing.
Your second point is very interesting but there is no simple answer. Sometimes adding a source of extra vibration can help kill the effects of another, like chatter. However, this is unlikely to be a general answer and in some cases it might make thing worse.
My approach would be to feed the motor as smoothly as practicable and look for chatter cures elsewhere, machine rigidity, cutting tool angles, depth of cut, speeds and feeds, work stick out or overhang etc.

Whilst I cannot comment about treadmill motors in the states, it would surprise me if they were any different to similar motors in oz.
All of the treadmills I have dismantled for their motors have had capacitive/inductive smoothing built into the internal circuitry.
With respect to mini lathes, as I do not own one, I cannot comment, however, I do own a mid sized lathe (6"x39" or 155 x 1000mm) carrying an 8" three jaw chuck, I can tell you from empirical experience, that a 2.5hp treadmill motor who's power source is from one of the devices in question, without the motor's smoothing components, driving a 2" 'a' section pulley by belt into a 4.5" diametre driven pulley, then through the gearbox, with the amount of inertia generated by the gears and the weight of the chuck, produces no vibration,or chatter what so ever. If I make any deleterious comment at all, it is that despite it's claim to be 2.5hp, it does not have the grunt that the original squirrel cage motor had at 2hp.
The question of longevity of the motor under these conditions is still open to debate, but since I can buy a complete treadmill from the local dump for $10, it's hardly an issue.
regards,
Rob.

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Originally Posted by old kodger

Whilst I cannot comment about treadmill motors in the states, it would surprise me if they were any different to similar motors in oz.
All of the treadmills I have dismantled for their motors have had capacitive/inductive smoothing built into the internal circuitry.
With respect to mini lathes, as I do not own one, I cannot comment, however, I do own a mid sized lathe (6"x39" or 155 x 1000mm) carrying an 8" three jaw chuck, I can tell you from empirical experience, that a 2.5hp treadmill motor who's power source is from one of the devices in question, without the motor's smoothing components, driving a 2" 'a' section pulley by belt into a 4.5" diametre driven pulley, then through the gearbox, with the amount of inertia generated by the gears and the weight of the chuck, produces no vibration,or chatter what so ever. If I make any deleterious comment at all, it is that despite it's claim to be 2.5hp, it does not have the grunt that the original squirrel cage motor had at 2hp.
The question of longevity of the motor under these conditions is still open to debate, but since I can buy a complete treadmill from the local dump for $10, it's hardly an issue.
regards,
Rob.

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Rob,

Nice summary, nothing to argue with there. Empirical experience beats theory every time.

Tony, I have used one of the 2 treadmill motors I have and in that project I used the treadmill speed controller that was with the motor. I attached a trim pot to the board like a lot of people do to adjust the speed. But in addition to the speed controller board I also used the large choke coil too. I noticed when I put the choke in series of the output to the motor, the motor ran a little smoother under a load, but could tell no difference when not loaded. I think this would be reducing any non pwm ripple, would it not? When I built it, I didn’t have a oscilloscope to test my theory, maybe I should now. BTW the project is a 12” softball pitching machine. The motor is attached to a plate to hold the motor and a small trailer wheel and rim is mounted directly to the motor. If you are not familiar with a pitching machine, look up Jugs pitching machines. It is basically a cloned idea of how they make theirs.

Output reactor chokes can be effective at smoothing the ripple, capacitors too but they often need to be big in capacity (and expense). A microwave transformer core can be rewound to produce a reactor choke which works for a motor application but for DC welder conversions not so much. Been there, done that, bought the tee shirt.

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Originally Posted by Big Sexy

..... I didn’t have a oscilloscope to test my theory, maybe I should now.

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For anyone contemplating getting an oscilloscope, or lusting after one I would suggest having a look at those boxes which use a laptop or other PC for the display and processing options. Basically these are a box of silicon bits connected to a laptop through USB and connected to the work with normal instrument leads. Picoscope seem to be the leaders in this field, check out https://www.picotech.com/products
The 2000 series start from around $100 up to the 9000 series at over $11000.
You have to spend a lot more for an equal digital oscilloscope without the extra power that a PC gives. There are 2 and 4 channel models, ..... heck, I am not going to list all the features here, read the specs if you are interested. A few days back I planned to mill some PCBs but as I was doing a test cut in air I noticed that the Z axis was ratcheting down with each drilling up and down. I spent too long testing using the CNC MDI features with gauges on the quill. Then I connected a two channel Picscope up to the Z scale A & B channels and the problem was obvious within a few seconds and I was easily able to confirm the fix without having to run the mill.
Just in case anyone gets the wrong idea, I have no association with Picoscope, I am only a satisfied paying customer. My only regret is that I did not get a four channel model.