# 115 shunt wound motor pwm circuit. Where to start?

#### Hamlet

Joined Jun 10, 2015
472
I have a 115v pump motor that I want to drive from about 140v dc.
I anticipate I might want to replace it in the future with a permanent magnet motor of 90v.

Both motors are rated for about 250w.

Should I use Mosfet or BJT to source the current to the motor?

Joined Jul 18, 2013
23,933
I have a 115v pump motor that I want to drive from about 140v dc.
What is the technology of the existing motor?
If wishing to replace with a DC motor do you need rpm control?
If not a simple bridge off of 120vac should suffice.
Max.

#### Hamlet

Joined Jun 10, 2015
472
Dang, I forgot to post the types of motors!
Both are d.c.,

#1 is a 1/3hp shunt wound motor rated at 115v for field &
armature, 1725 rpm, service factor unknown.
#2 is also 1/3hp, permanent magnet, 90v, 1725rpm. Service factor is 1.15.

In the past, I have had them pumping water from 50 to 100 psi,
using rotatory vane pump heads. In those applications,
I was using a 251/125v auto transformer with a bridge.
This was all on 110v mains. I was getting about 55v d.c,

With that said, I would like to run at rated voltage (and speed).
I could, and have, run these motors up on household mains
using nothing but a bridge rectifier...

However, this was all before I learned about "Form Factor" ratings of
d.c. motors:

"Form Factor (Power Supply Code): NEMA classification of quality of D.C. drive power quality based on the ratio of ripple D.C. current to average RMS D.C. output. 15 Typical codes are A, C, D, E and K."

From my understanding, some motors prefer less ripple than others.
So that got me to thinking that I should get the ripple as low as possible... for longevity of the motors.
So, sprinkling in a few smoothing capacitors...
Of course, now the voltage is too high!

Sigh, maybe I'm overthinking this Wouldn't be the first time.

Joined Jul 18, 2013
23,933
Have you tried the 90v P.M. motor on 120v with just a bridge? I would expect it to run with not problems.
Max.

#### Hamlet

Joined Jun 10, 2015
472
Yes, I had run it with just a bridge at 120v ac, with pump, driving a series of filters.
The pressures in the filtration units went too high, and thus I halved the voltage.
This was before I knew about F.F.

Only one filter in my new application, so back pressure shouldn't be a problem.

I probably will not bother with PWM with pumping this time around. However,
I sure would like to see a PWM circuit that can handle more than 12v, running
a 12 volt motor. I don't know if you need to isolate the control circuit from gate
on a Mosfet when controlling high voltages, or not. I can see now that there's a
large range of experiments waiting for me in Pulse Width Modulation...

Joined Jul 18, 2013
23,933
We seem to be jumping around from 90vdc motors to 12vdc?
There are many DC controllers out there, both SCR bridge and PWM made by KB who specialize in these motors, for 12v PWM they can be had on Ebay for ~$5.00. Max. #### tcmtech Joined Nov 4, 2013 2,867 Why bother is yo have a working system to power the motors? What do you expect to gain? Thread Starter #### Hamlet Joined Jun 10, 2015 472 Most PWM circuit discussion on the web are low voltage. We seem to be jumping around from 90vdc motors to 12vdc? There are many DC controllers out there, both SCR bridge and PWM made by KB who specialize in these motors, for 12v PWM they can be had on Ebay for ~$5.00.
Max.
(Most pwm circuit discussion on the web is 12v.)

I'll look into KB. Thank you.

#### Hamlet

Joined Jun 10, 2015
472
Why bother is yo have a working system to power the motors?

What do you expect to gain?
The ability to chose appropriate voltage, with low ripple.

#### tcmtech

Joined Nov 4, 2013
2,867
Is the lack of ripple all that significant to the motors operation or is it just a personal fixation?

If it was my project I would be looking at a common variable speed DC motor drive power supply or if you want to go DIY for the sake of learning just take an old power drill speed control trigger unit apart and rework it to use an external pot and if needed use a larger external switching device on its own heat sink if necessary.

Joined Jul 18, 2013
23,933
Most PWM circuit discussion on the web are low voltage.

(Most pwm circuit discussion on the web is 12v.)

.
The low level logic is 12v but you can use any H.V. Mosfet for the motor driver/control.
Max.

#### Hamlet

Joined Jun 10, 2015
472
The low level logic is 12v but you can use any H.V. Mosfet for the motor driver/control.
Max.
Great!

#### Hamlet

Joined Jun 10, 2015
472
Is the lack of ripple all that significant to the motors operation or is it just a personal fixation?
Some motors don't like ripple. Name Plate specifications/ratings for motors list tolerance for ripple (Form Factor or F.F). Some motors are more
tolerant than others. I'd like to eliminate ripple as a concern. I think brush & commutator life may be adversely affected by too much ripple.

#### tcmtech

Joined Nov 4, 2013
2,867
What's the power rating and anticipated service life these will be used in?

Reference to the motors themselves would be helpful too.

#### Hamlet

Joined Jun 10, 2015
472
What's the power rating and anticipated service life these will be used in?

Reference to the motors themselves would be helpful too.
I have two of these shunt wound types. 115v, 3A, 1750rpm, Gould.
The 90v pm motor is a 1/4hp, 2.5amp, 1750rpm Leeson (sorry, no picture).

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Joined Jul 18, 2013
23,933
Odd if they are shunt wound motors and the field voltage is not given for the rated 1750rpm?
And if they are shunt wound it is easy to control the rpm by the field voltage although torque will suffer unfortunately.
Max.

#### Hamlet

Joined Jun 10, 2015
472
Yes, annoying that the field voltage is not stamped.
The field & arm. are wired in parallel from the factory.

#### tcmtech

Joined Nov 4, 2013
2,867
I have two of these shunt wound types. 115v, 3A, 1750rpm, Gould.
The 90v pm motor is a 1/4hp, 2.5amp, 1750rpm Leeson (sorry, no picture).
I doubt that those motors could care less about ripple. I've seen similar general purpose type DC motors like those used on some pretty crude DC power supplies that ran for years in near continuous use without issues.

Given their low amp ratings I would just use a simple light dimmer unit ahead of a bridge rectifier and a large capacitor for the power supply. I've got several DC motors I run like that in variable speed applications that work just fine.

Joined Jul 18, 2013
23,933
Yes, annoying that the field voltage is not stamped.
The field & arm. are wired in parallel from the factory.
That would seem to imply that they require the same voltage.
On the question of ripple, both servo's and spindle motors have run off of full wave SCR bridge control for decades, mostly 3ph however.
Max.

#### Hamlet

Joined Jun 10, 2015
472
I doubt that those motors could care less about ripple. I've seen similar general purpose type DC motors like those used on some pretty crude DC power supplies that ran for years in near continuous use without issues.

Given their low amp ratings I would just use a simple light dimmer unit ahead of a bridge rectifier and a large capacitor for the power supply. I've got several DC motors I run like that in variable speed applications that work just fine.
Experience counts. Thanks.

I used a light 600w triac dimmer ahead of bridge rectifier for testing, but the dimmer eventually died. The uF value of cap I was using at the time
was small. Nothing got hot, I think it was EMF that killed it.

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