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DC MOTOR.

C

CLT

Jan 1, 1970
0
If i have a 12V 10.5 Nominal Amps DC Motor and i want to regulate the
speed whit a reostat, what are the ohms and watts for that reostat?
Thanks in advance.
 
C

CLT

Jan 1, 1970
0
I have the 12VDC in the mashine connected to the motor but is too
fast, so i whant to put this reostat betwin the 12V and the motor to
slow it down .
 
D

Don Kelly

Jan 1, 1970
0
CLT said:
I have the 12VDC in the mashine connected to the motor but is too
fast, so i whant to put this reostat betwin the 12V and the motor to
slow it down .
--------
Your question is simply too vague to be answered. What kind of DC motor -
shunt, series or separately excited or permanent magnet field ?
Application - i.e. what is the motor driving (as this will affect the speed
reduction you will get)? What speed reduction is wanted at full load? (speed
at no load may not be much different from that with no rheostat. )
There is no cut and dried answer.
 
J

John Gilmer

Jan 1, 1970
0
in advance.
Oh, come on.

12 volts, 10 amps? DC motor (not ac/dc)

Just a WAG but a 1.2 ohm resistance would have that motor running at about
1/2 speed. That's ASSuming it's providing the same torque.

To a "zero order" approximattion, the torgue is proportional to the current
and the speed it proportional to the voltage (at the motor0.

He could borrow a trick used for multi-speel operation of small motors in
automotive application and use a 4 position switch to give 0,1,2,3 (where 3
is full speed) control. Low speed would have two resistors in series,
medium would have one, and full speed would have direct connection to the
motor.

To WAG things: use 1 ohm resistors. Experiment to find real values that
will give the speeds you want. But with fans, for example, the running
torque is ABOUT proportional to the square of the shaft speed. So, half
speed, you only need 3 amps to supply the torque and only 6 volt to supply
the speed. The resistor needs to drop 6 volts at 3 amps or 2 ohms.

If a rheostat is used, he might be able to use a 2 or 3 ohm model. The
power rating? Maybe 10 watts to be on the safe side. If he can get it
specified, he should ensure that the rheostat can also handle the the 10
amps within it's power rating.
 
D

Don Kelly

Jan 1, 1970
0
John Gilmer said:
in advance.
Oh, come on.

12 volts, 10 amps? DC motor (not ac/dc)

Just a WAG but a 1.2 ohm resistance would have that motor running at about
1/2 speed. That's ASSuming it's providing the same torque.
--------------
Recheck this. As you have done it correctly for your fan example, I ASSume
this is a slip.

I did start to give an example similar to what you have done but then I
realised that the type of DC motor and the speed- torque characteristics of
the load can throw such estimates out the window. In addition the range of
control wanted would be handy to know for choosing a rheostat.
Overcautious- maybe.

Given that -the assumptions that you have made are the most likely but
without further information one can't be sure.

Assuming that the field is constant (permanent magnet or connected on the
supply side of the resistor) at 1.2 ohms- the "no load" speed would be
nearly the same as it was without the resistor and at rated current, the
speed would be 0 as the rotor voltage would be 0. This is not half the
original speed at this current (and torque). At 5A the speed would be a bit
less than half the speed without the resistor (at the same torque). At 2.5A
the speed would be 75% of the original. This does not take into account the
load's speed torque characteristic as you have done below.

If it is a series motor the results would generally be the same -half speed
at 5A, 75% speed at 2.5A and lets avoid no load.

If the motor has a wound shunt field and is such that some expertise would
be needed to separate field and armature connections, then use of an
external rheostat would not be effective (i.e. at 5A the speed would be near
normal but the torque would be halved)- hence the desire for more knowledge
of the motor and application.

As for what you have said below- no problems -assuming the motor is as you
assume.
 
C

CLT

Jan 1, 1970
0
The motor has permanent magnets and the load is not constant, becouse
the motor shake a food dispenser for animals and some times it has
more food then others.
Thanks for your help.
 
D

Don Kelly

Jan 1, 1970
0
CLT said:
The motor has permanent magnets and the load is not constant, becouse
the motor shake a food dispenser for animals and some times it has
more food then others.
Thanks for your help.
OK then what Gilmer suggested is in line. If you want a minimum speed at
rated current then set the rheostat to give this. If the desired minimum
speed is 25% of nominal at 10A then there will be a voltage drop across the
resistor of 9V which requires 0.9 ohms and a power dissipation capability of
90 (say 100 + watts to be safe). The rheostat must handle 10A at all
settings. However at part load, a higher resistance will be needed so a
higher resistance rheostat would be needed. Don't expect constant speed as
load varies. Some playing around as suggested by Gilmer would be worth
while.
It might be worthwhile looking into electronic controllers which will be
more efficient and can control speed as desired.
 
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