Get me up to Speed on DC Motors

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rbertalotto
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Re: Get me up to Speed on DC Motors

Post by rbertalotto »

All I know is what I know!

I'm using this SCR controller on two machines...A Jet 12 X 36 Lathe and a 14" band saw...Both using 90V DC - 3/4HP Permanent Magnet motors

http://www.surpluscenter.com/_MoreSpecs/as11-2269.pdf

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The lathe has been up and running for years with zero issues and I couldn't ask for better performance, power, speed, vibration etc. It has been a HUGE improvement over the 220v AC motor.

I recently upgraded the metal cutting converted bandsaw from a treadmill motor to a brand new 90V DC 3/4HP motor using this controller and it is fantastic. The TM motor would stall on any metal over 1/8"...This new motor with the SCR controller will slice through 1" steel with no issues (Might be able to do thicker but I've not had the need to try it. But I did cut up some 2" aluminum with no issues)

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And the cost for both units is $260....Hard to beat!
Carm
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Re: Get me up to Speed on DC Motors

Post by Carm »

wlw-19958 wrote:Hi There,
Carm wrote:
SteveHGraham wrote: I don't know what SCR is, but it sounds like it's less ripply.
Silicon Controlled Rectifier
I think it's more ripply because it is sine wave dependent, less so with three phase.
Maybe one of the knowledgeable posters will clarify.
Could one add some capacitors to smooth out the ripple?

Good Luck!
-Blue Chips-
Webb
Beyond my pay grade to answer your question, being a dinosaur. I recall when SCR's were high tech and replacing MG sets which as you know was bee's knees for the Monarch ten doublee.
What I grasp is that VFD's have replaced any sort of DC drive apart from the scrounge factor using treadmill motors or remote battery operated hydraulic drives.
Torch, Atunguyd or Dickeybird, want to expound? Type slowly please!
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BadDog
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Re: Get me up to Speed on DC Motors

Post by BadDog »

Carm wrote:Beyond my pay grade to answer your question, being a dinosaur. I recall when SCR's were high tech and replacing MG sets which as you know was bee's knees for the Monarch ten doublee.
So what you're saying is, if they were using vacuum tubes, you would be all over that... ;) (yes, I know that does not apply)

VFDs are not quite direct replacements for DC drives, though for many practical applications they have. Depending on the driver/controller, DC drives have relatively flat torque curves approaching 0 rpm. VFDs have relatively flat torque curves below rated RPM down to a point where they begin to drop off rapidly. That point varies by motor/controller and technology (sensorless vector, etc). But generally the point of fall off is somewhere between about 10% and 40% of rated rpm. For VFDs, under speed is (relatively) flat torque (to a point) and over speed is (relatively) flat HP. At least that's what I think I recall.
Russ
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Carm
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Re: Get me up to Speed on DC Motors

Post by Carm »

Russ
No, I couldn't answer Webb's question about capacitance to smooth rectified ripple.
Torch
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Re: Get me up to Speed on DC Motors

Post by Torch »

I think that VFDs are far more popular for large HP motors because of cost. AC motors are comparatively plentiful and therefore cheaper because AC power transmission is readily available. Large HP DC motors seem to be limited to a small market like elevators. Large HP PWM controllers are bloody expensive (but that may all change as electric vehicles are developed). Add that price to a niche market large DC motor and the combined price is even worse. VFDs themselves may also seem a bit expensive, but are a relatively simple way to add a usable variable speed range to an existing motor technology.

But for someone with an existing 1 to 2 HP DC motor, adding a PWM controller is cheaper and easier than converting everything over to AC with a VFD, IMHO.
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SteveHGraham
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Re: Get me up to Speed on DC Motors

Post by SteveHGraham »

Neglecting religion and politics, there are only two reasons why an approach to a tool problem would be avoided: technical issues and money.

Since capacitors are pretty simple, I'll guess the reason is money.

A 1 HP 90V DC motor that operates at an impossible 100% efficiency has to suck at least 8.28 amps. Isn't that right? So you will need a capacitor (bunch of caps in parallel, whatever) that can put out a lot of current without fast dips in voltage. I think. Wikipedia says DC motors are around 75% efficient, so 11 amps at top speed, assuming I am correct in guessing that voltage determines speed. It sounds like ripple would get worse with increased speed. Maybe.

I am too lazy to figure out the answer, but caps have a time constant (determines speed of voltage drop) related to their size, and if you want a big time constant, you have to have a big cap.

Someone please figure this out for us.
Every hard-fried egg began life sunny-side up.
atunguyd
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Re: Get me up to Speed on DC Motors

Post by atunguyd »

Carm wrote:


Beyond my pay grade to answer your question, being a dinosaur. I recall when SCR's were high tech and replacing MG sets which as you know was bee's knees for the Monarch ten doublee.
What I grasp is that VFD's have replaced any sort of DC drive apart from the scrounge factor using treadmill motors or remote battery operated hydraulic drives.
Torch, Atunguyd or Dickeybird, want to expound? Type slowly please!

Yes advances in power electronics has created small cheap VFD's that can supply some impressive power. Since AC Induction motors are cheaper to make than DC motors this results in a trend away from DV motors and on to AC motors with VFD's.
atunguyd
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Re: Get me up to Speed on DC Motors

Post by atunguyd »

wlw-19958 wrote:
Could one add some capacitors to smooth out the ripple?
Not advisable, I would need to sit and think why, but I suspect that this may not be good for the SCR', and you could very well remove the speed control aspect of your circuit. If you think logically about it, the SCR speed controller only applies a portion of the waveform to the motor and this has the effect of controlling the speed. If you now add a capacitor, when the SCR is off this capacitor will provide power to the motor (depends on value of capacitor) so you will effectively have a DC power supply at full power all the time.

Anthony
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wlw-19958
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Re: Get me up to Speed on DC Motors

Post by wlw-19958 »

Hi There,
atunguyd wrote:Not advisable, I would need to sit and think why, but I suspect that this may not be good for the SCR', and you could very well remove the speed control aspect of your circuit. If you think logically about it, the SCR speed controller only applies a portion of the waveform to the motor and this has the effect of controlling the speed. If you now add a capacitor, when the SCR is off this capacitor will provide power to the motor (depends on value of capacitor) so you will effectively have a DC power supply at full power all the time.
That make sense to me. I guess that is why some use a variac
with a full wave rectifier and caps to smooth out the ripple (and
maybe a choke or too).

Good Luck!
-Blue Chips-
Webb
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SteveHGraham
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Re: Get me up to Speed on DC Motors

Post by SteveHGraham »

I found an equation for finding capacitance for the desired ripple on a DC motor supply.

I don't see how this can be right, but it comes out to 16.5 F for one volt of ripple. That's huge. For 5 volts of ripple, it would be 3.3 F, which is still gigantic at 90V. Capacitance is charge/volt, so for a given capacitance, when you increase the voltage, you have to crank up the size of the capacitor.

The capacitance is a ratio. The top part is the current times the voltage, so that maxes out at 990AV for 11 A and 90 V. The frequency is on the bottom, and that's 60 Hz. The desired ripple voltage is on the bottom, so if you plug in one volt of ripple, you get 990/60 = 16.5 Farads.

On the Mouser site, the price for an 80 V 12 F capacitor is around a thousand dollars.

Someone who actually knows should come along and clear it up.
Every hard-fried egg began life sunny-side up.
Carm
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Re: Get me up to Speed on DC Motors

Post by Carm »

Steve
Your logic has already cleared it up.
"All things are possible, but not all are desirable". The paraphrase here, the cost of the application.

Atunguyd
Thanks for the explanation. Makes sense to me too.
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SteveHGraham
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Re: Get me up to Speed on DC Motors

Post by SteveHGraham »

Here is the page I referred to, in case anyone--preferably an actual engineer--wants to look.

http://electronics.stackexchange.com/qu ... alculation
Every hard-fried egg began life sunny-side up.
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