# motor speed control

#### mahmoud shendy

Joined Dec 23, 2007
21
Dear All..

this is the fourth time I post, sorry but actually I am confused..

my problem is to control the speed of a 4hp DC motor, its rating is

180VDC
20A

I measured the Armature resistance and Field Resistance, both are (0.6 Ohm)

I want to know how to measure the lasting parameters:

Ke and Kt conatants
J (monent of inertia)
B (friction losses)
La and Lf (armature and field inductance)

after determining this parameters, I will be able to model the motor to control
it..

by the way, I need to brake the motor after the rotation, since it still rotates
for about 15 minutes after power of.. AND SO, what is the recommended, series wound
or shunt wound or separately excited?

what is the maximum current can field coil passes without damage, and also the starting
current can the armature passes?

I use external power resistor for Armature and Field, but it runs VERY VERY VERY hot !!!!!

I need your help, thanx a lot, and GOOD BYE
... Shendy

#### scubasteve_911

Joined Dec 27, 2007
1,202
Finding this information requires more sophisticated testing. If you know what these are, then you should be able to devise a test procedure.

For example, for Ke, apply known voltages and measure the speed of the motor. Plot a few points, then use a best fit line, the slope of which is your Ke constant.

Kt, you need to provide known torque loads on the motor, then measure the change in current. Plot the points, best fit, etc. You may be able to use a characterized motor with a constant current source to provide the variable torque load.

For friction losses, I would run the motor without a load connected, then measure the currents under steady state. These currents represent the torque needed to overcome the frictional resistance (actually, all losses..).

Inductances, use an inductance meter. Eventhough, I believe, when you rotate the motor you may get a range of inductances, but you should be able to get an average.

For inertia, that's another story. Basically, you need to accelerate the motor and see it's response to a step input. This will include your inductance, or electrical response, but it is usually much slower than your inertia.

As far as shunt, series, seperately excited, I am not certain.

Steve

#### Bernard

Joined Aug 7, 2008
5,227
As you say there are hot resistors in armature and field ckts. ; at 4A field current ,resistor about 40 Ω and 640W , might run a little warm. How you connect the motor depends greatly on the load. For series connection it needs a fulltime load or commutator segments might start flying from over speed. I willnot ask any more questions because you do not give answers per your past threads.

#### scubasteve_911

Joined Dec 27, 2007
1,202
Bernard,

You are absolutely correct, he has abandoned far too many threads, many with responses from myself. If or when I leave this forum, this will certainly be why.

Steve

and this will be the last second I waste on this poster