Motor speed vs voltage?
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Posts: 46,084
Hello Everyone,
Does anyone know the general relationship between DC
motor voltage and RPM? I'm trying to come up with a <very rough>
measurement of the speed of a DC motor by reading the input voltage. This
is a tiny motor (~3.7V @ 70Ma) with virtually no load, but I am varying the
speed by changing the voltage.
All input appreciated, Duncan
Does anyone know the general relationship between DC
motor voltage and RPM? I'm trying to come up with a <very rough>
measurement of the speed of a DC motor by reading the input voltage. This
is a tiny motor (~3.7V @ 70Ma) with virtually no load, but I am varying the
speed by changing the voltage.
All input appreciated, Duncan
Comments
An ideal DC motor's RPM is directly proportional to applied voltage.
RPM=K1*V where K1 is a constant, and V is applied volts.
Real-life motors have armature resistance and losses, which causes variation
from this equation. Think of the motor as a generator which turns as fast
as needed to generate a back emf = applied emf. However, armature
resistance drops some of the applied voltage, so that:
RPM=K2*(V-I*R)=K2*V-K2*I*R where K2 is a constant not=K1 above.
I is armature current, and R is armature resistance.
If you can measure armature current, you can empirically determine the K2
times R and add this term for more accurately determining RPM. (This is
often done in practice for speed control).
Summary:
* The first equation is approximate, depending on motor quality,
(i.e.,armature resistance and losses).
* The second equation can be used for more accuracy if you are willing to do
some calculations.
Regards,
Ray McArthur
Original Message
From: <orthner@s...>
To: <basicstamps@egroups.com>
Sent: Thursday, August 31, 2000 11:29 PM
Subject: [noparse][[/noparse]basicstamps] Motor speed vs voltage?
>
> Hello Everyone,
> Does anyone know the general relationship between DC
> motor voltage and RPM? I'm trying to come up with a <very rough>
> measurement of the speed of a DC motor by reading the input voltage. This
> is a tiny motor (~3.7V @ 70Ma) with virtually no load, but I am varying
the
> speed by changing the voltage.
I think you will need to record data by counting the rpm of the shaft at
different motor voltages. Then it is a simple matter to do a linear curve
fit and come up with a multiplier for the voltage to obtain rpm. This would
hold true unless your motor is subject to a changing load. If this be the
case, wou will need to do a little feedback to arrangement, possibly with
optical encoder (not entirely difficult to build) which would measure rpm
and adjust the motor voltage accordingly.
Good Luck
Gary
g.shearer@v...
Free Electron Laser Research Center
Vanderbilt University
Nashville, Tennessee
Original Message
From: <orthner@s...>
To: <basicstamps@egroups.com>
Sent: Thursday, August 31, 2000 10:29 PM
Subject: [noparse][[/noparse]basicstamps] Motor speed vs voltage?
>
> Hello Everyone,
> Does anyone know the general relationship between DC
> motor voltage and RPM? I'm trying to come up with a <very rough>
> measurement of the speed of a DC motor by reading the input voltage. This
> is a tiny motor (~3.7V @ 70Ma) with virtually no load, but I am varying
the
> speed by changing the voltage.
>
> All input appreciated, Duncan
>
>
>
> Hello Everyone,
> Does anyone know the general relationship between DC
>motor voltage and RPM? I'm trying to come up with a <very rough>
>measurement of the speed of a DC motor by reading the input voltage. This
>is a tiny motor (~3.7V @ 70Ma) with virtually no load, but I am varying the
>speed by changing the voltage.
With the resistance constant (windings), the voltage and curent can change.
It seems to me if you're varying the voltage, you need to check the current
and work backwards from there. Just my 2 cents.
Regards,
Bruce
>All input appreciated, Duncan
>An ideal DC motor's RPM is directly proportional to applied voltage.
>
>RPM=K1*V where K1 is a constant, and V is applied volts.
>
Thanks Ray, so it is a linear relationship. I was going to have the stamp
read (ADC) the motor voltage and vary the 'time on' of the motor to get
some rough positioning. On closer examination of the machine I'm working
on, I realize that it has a limit switch on the mechanism. Ha! So it'll
likely be better to use that to stop and start the motor. 8^]
Thanks again, Duncan
Yep, that was interesting info from Ray. (Thanks Ray!)
Thought I would mention a chip that I'm in the process of hooking up, its a
DRV102 PWM driver, 2.7A 8 to 60v, from Burr Brown.
It looks really cool, but don't know how to get one of these T0-220 packages
hooked up into a bread board yet.
Must be a socket or clever work around out there somewhere.
Mark
Original Message
From: orthner@s... <orthner@s...>
To: basicstamps@egroups.com <basicstamps@egroups.com>
Date: Friday, September 01, 2000 7:26 AM
Subject: Re: [noparse][[/noparse]basicstamps] Motor speed vs voltage?
>At 12:06 AM 9/1/00 -0400, Ray wrote:
>
>>An ideal DC motor's RPM is directly proportional to applied voltage.
>>
>>RPM=K1*V where K1 is a constant, and V is applied volts.
>>
>
>Thanks Ray, so it is a linear relationship. I was going to have the stamp
>read (ADC) the motor voltage and vary the 'time on' of the motor to get
>some rough positioning. On closer examination of the machine I'm working
>on, I realize that it has a limit switch on the mechanism. Ha! So it'll
>likely be better to use that to stop and start the motor. 8^]
>
>Thanks again, Duncan
>
>
>
>
>
>
>
>
>
t-49) wire wrap pins, put them in the bread board and connection is easy.
alternativly you can use the machined pin ic sockets, most to220 pins will
go into these.
norm
>From: "techno_masai" <plunkettm@e...>
>Reply-To: basicstamps@egroups.com
>To: <basicstamps@egroups.com>
>Subject: Re: [noparse][[/noparse]basicstamps] Motor speed vs voltage?
>Date: Fri, 1 Sep 2000 11:45:59 -0700
>
>Hi Duncan,
>
>Yep, that was interesting info from Ray. (Thanks Ray!)
>
>Thought I would mention a chip that I'm in the process of hooking up, its a
>DRV102 PWM driver, 2.7A 8 to 60v, from Burr Brown.
>
>It looks really cool, but don't know how to get one of these T0-220
>packages
>hooked up into a bread board yet.
>
>Must be a socket or clever work around out there somewhere.
>
>Mark
>
>
>
Original Message
>From: orthner@s... <orthner@s...>
>To: basicstamps@egroups.com <basicstamps@egroups.com>
>Date: Friday, September 01, 2000 7:26 AM
>Subject: Re: [noparse][[/noparse]basicstamps] Motor speed vs voltage?
>
>
> >At 12:06 AM 9/1/00 -0400, Ray wrote:
> >
> >>An ideal DC motor's RPM is directly proportional to applied voltage.
> >>
> >>RPM=K1*V where K1 is a constant, and V is applied volts.
> >>
> >
> >Thanks Ray, so it is a linear relationship. I was going to have the stamp
> >read (ADC) the motor voltage and vary the 'time on' of the motor to get
> >some rough positioning. On closer examination of the machine I'm working
> >on, I realize that it has a limit switch on the mechanism. Ha! So it'll
> >likely be better to use that to stop and start the motor. 8^]
> >
> >Thanks again, Duncan
> >
> >
> >
> >
> >
> >
> >
> >
> >
>
>
>
>
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