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    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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    Quote Originally Posted by tonyfoale View Post
    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.
    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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    Quote Originally Posted by old kodger View Post
    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
    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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    Quote Originally Posted by tonyfoale View Post

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

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    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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    Quote Originally Posted by vortexwizard View Post
    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
    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.

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    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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    Quote Originally Posted by old kodger View Post
    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.
    Rob,

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


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