* Lagging DC Amps

R

Randy Gross

Guest
This question concerns the amperage supplied to a dc motor (in this case 10
amps). Would it be harmful to the motor if the amperage dropped to say 7
amps? Would the rpms drop as a result of reduced amps?

Curious,
Randy

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Randy Gross wrote:
This question concerns the amperage supplied to a dc motor (in this case 10
amps). Would it be harmful to the motor if the amperage dropped to say 7
amps? Would the rpms drop as a result of reduced amps?
Click to expand...
A DC motor with a fixed field magnet strength, (permanent magnet field
or shunt wound field) the torque (at any given speed) is approximately
proportional to the current, while the speed (at any given torque
load) is approximately proportional to applied voltage. Reducing the
current from 10 to 7 amps will reduce the torque the motor can produce
at that speed by 30%. If the torque load is constant the motor will
slow till the current increases high enough to supply the torque, or
the motor will stall. As long as the motor does not depend on its
rotation to drive a fan to carry the heat away, a stalled motor
drawing 7 amps will not get as hot as a running motor drawing 10
amps. The current heats the armature windings in proportion to the
current squared.

--
John Popelish
 
Torque, in this scenario, is important initially because, there is a
flywheel in the device I'm powering with the DC motor: 1/10hp, 2800rpm.

What I have is an errant 33 rpms' (900 rpm target) and, trying to avoid the
expense of a DC motor control, I was thinking I could "tweak" such a small
amount.

Am I thinking wrong?
Randy

























John Popelish <jpopelish@rica.net> wrote in article
<405CF162.948C63EF@rica.net>...
: Randy Gross wrote:
: >
: > This question concerns the amperage supplied to a dc motor (in this
case 10
: > amps). Would it be harmful to the motor if the amperage dropped to say
7
: > amps? Would the rpms drop as a result of reduced amps?
:
: A DC motor with a fixed field magnet strength, (permanent magnet field
: or shunt wound field) the torque (at any given speed) is approximately
: proportional to the current, while the speed (at any given torque
: load) is approximately proportional to applied voltage. Reducing the
: current from 10 to 7 amps will reduce the torque the motor can produce
: at that speed by 30%. If the torque load is constant the motor will
: slow till the current increases high enough to supply the torque, or
: the motor will stall. As long as the motor does not depend on its
: rotation to drive a fan to carry the heat away, a stalled motor
: drawing 7 amps will not get as hot as a running motor drawing 10
: amps. The current heats the armature windings in proportion to the
: current squared.
:
: --
: John Popelish
:
 
<Snip>

"That depends on both the stability of the load and the tightness of your
speed requirement."

--
John Popelish

I see your point: What's important! Speed, torque and accuracy. I think
I'll buy the controller.

Thanks John,

Randy
 
Randy Gross wrote:
Torque, in this scenario, is important initially because, there is a
flywheel in the device I'm powering with the DC motor: 1/10hp, 2800rpm.

What I have is an errant 33 rpms' (900 rpm target) and, trying to avoid the
expense of a DC motor control, I was thinking I could "tweak" such a small
amount.

Am I thinking wrong?
Randy
Click to expand...
To lower the speed, you need to lower the applied voltage. This may
or may not lower the current. That depends on how the load acts as
the speed changes. Most loads require less torque at lower speed.
The flywheel loads the motor with torque only when you try to raise
the speed. Limiting the available current to a lower value will slow
the acceleration of that flywheel. I can't say if you can avoid a
feedback control for the motor. That depends on both the stability of
the load and the tightness of your speed requirement.

--
John Popelish
 
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