Voltage Divider methods?

Hi all,

I have 2 24VDC geared motors, which supposed to be running at the same speed.

Both motors are supplied with same 24V source. 1 was measured to turn 1 round at 70secs, the other at 61secs, which came out a ratio of 70/61=1.1147

Each motor was measured to draw around 6amp.

I'm thinking if I could just step down the 24V source on 1 motor side to 24/1.1147=21.5V, so that both motors will run at the same speed?

Hence I'm thinking of using 1kohm and 10kohm resistors for the voltage divider.

How do you guys think? Thanks.

Rgds.

First lets find the resistance of your motor R=E/I R=24/6 gives you 4 ohms. If you use a 1K and 10K divider and put the 4 ohm motor across the 10K you will end up with a 1K and 4 ohm divider. The 10K will have almost no effect. Now lets calculate the current through a 1K at 24V. I=E/R I=24/1000 gives you 0.024 amps or 24 milliamps. Since current in a series circuit is the same throughout the circuit, you will only have 24 milliamps going through your motor and it will most likely not run at all. You could use series resistance to reduce the motor voltage but you will have to use a very small resistance value (and high wattage value) and deal with the loss of torque. As you load the motor it's current draw increases (effective electrical resistance decreases) and that causes a voltage decrease across the motor and voltage increase across the resistor.

What you will need to control the voltage to the motor is a voltage regulator, or regulated power supply. The supply will have to be capable of delivering the maximum full load amps of the motor.

Sorry to burst your bubble, but it's the law... Ohms law :)

Steve

080212-1048 EST USA

rainman:

No way. 1 kohm at 6 A = 6000 V.

How accurately do you need to synchronize the speed of the two motors? How much are you willing to spend is another factor.

I will let others tell you how.

.

Um, what I think Vger is saying that if the 1k is in series with the motor, he won't draw 6A anymore. 24 VDC/ 1004 ohms = less than 24mA.

Rainman has the correct idea from the theory perspective, but a voltage divider won't make the circuit he wants. It is not regulated and as soon as the motor is tapped into the divider point - WHACK there goes the divider due to the low (as compared) motor resistance.

Rainman better to find a suitable rheostat (yeah really old school) to put in series with the motor that is too fast. You could experiment with power resistors in series with the too fast motor. We are talking about low resistance, high wattage units though. Verrry lossy and a stoneage method of motor control. (Think bedspring resistors in ancient golf carts type of control).

Like gar asks, how critical is the speed match?

Thanks Vtxstar and rest who replied,

The maximum speed difference I can allow is around 2-3%, for instance 1000rpm for 1 motor and 1020rpm for the other.

The motor inductance is 19mH and 6.4ohm.

I do not have any experience in using rheostat, what should be the rheostat value range like, to effective control the motor speed in terms of +-10%?

Thanks.

Rgds.

You haven't mentioned how the motor's are being used, but by the sound of it you will need to make a bit of circuitry to achieve your goal.

My suggestion would be to research PWM control of a DC motor, very commonly used and reasonably simple to achieve quite good speed regulation. DC motor's respond well to this type of control.

Good Luck !

Russell.

080213-0719 EST USA

rainman:

A rheostat is a name for a variable resistor.

If your torque load on the motor is relatively constant, then an appropriate series resistor may be satisfactory. From your information 3.5/6 ohms is the approximate resistance required and power of 3.5 * 6 watts. If you can find a 1 ohm rheostat its power rating will need to be greater than 6 * 6 * 1 watts or this means 50 or 100 watts. You could use an adjustable resistor called a Dividohm made by Ohmite. This is a fixed adjustable resistor.

If your motors had wound fields, then field adjustment might be the better and easier means of balancing the motor speeds and less sensitive to torque variation.

.