Need help figuring out Resister on a 12v Power Supply

[pcw]

Hi

I recently purchased a used CNC router that has a Stepperworld FET-3 controller. It has two power supplies 5v 3amp and a 12v 7 amp. I am using Vexta PH268 motors rated at 5v 1.5 amp. At the motors I have 5.36v and would like to increase the voltage to gain some speed. Right now I can not go faster than 12 ipm.

The previous owner built the system using six 5 ohm resister to reduse the current from 11.8v to 5.36v. What would be a safe voltage and what resister would I need to accomplish this, Any help would be appreciated. Dan

[cbcnc]

Hi Dan,

The resistors are going to control the amperage going to your motors and not the voltage.
By my figuring 2 - 5Ω, 15 watt resistors, one for each phase of your motor, would deliver 1.44 amps @ 12v.
To get the full capacity of the motor at 1.5 amps you would need a resistance of 4.67Ω , 15 watts, each resistor. Probably, the only way you could get that would be to put a couple of resistors in series.

Chris

[pcw]

Hi Chris

I am new to all of this so please forgive me. What I am confused about is I have 12v form the controller to the resister but on the bottom of the resister I only shows 5.36v to the motors.

So if I get this right the voltage to the motors will always be 5.35v. If I remember the resister's where 5 ohm 40 watt.

I ran down the wires a couple of times and it appears to we wired correctly but it just seems that these 8 wire bipolar motor should be able to run faster that 12 ipm. I believe the PH268M motors are only 118oz. I am using Mach 3 software and have tuned them as best as I could.

Would you opt out to just replace the controller and motors? I have a nice 1425 K2 table but it seems like it has a weak link. Dan

[cbcnc]

A higher voltage supply will get the motors to spin faster.
Changing the resistors won't work. Sorry I had misread what size resistors you had so all I did was to verify that they were right for the motors.
If your controller will handle it you could power it with 24v or more. Of course, you would need to recalculate the resistors to limit the amperage. I don't know what the specs of the Stepperworld board are.
Did K2 supply the motors? What do they recommend for your machine?

Chris

[pcw]

Chris

Not really sure of the origin of the motors since I purchase the unit used. I would sure like to hear from someone who has had this combination to see if they can get there motors to run faster than 12 ipm.

The documentation from Steperworld state's that you can increase the voltage twice the motors rated volts with limiting resisters. My motors are rated at 5.4v 1.5 amp and they recommend 10
.8 with limiting resisters.

From Steeperwords manual.
Full Article
Selecting a current limiting resistor

It is important that neither the motor nor controller exceed their rated currents.

There is a precise relationship between the Voltage, Resistance, and Current ratings of both the motor and controller which must be understood before experimenting with motors. If you do not understand the following section, use only the motors provided with your project, which are matched to the control circuit.

The most straightforward way of reducing current to the motor is to reduce the voltage. With power supplied to the motor at its rated voltage, calculations are easy-- the motor will draw the rated current.

There may be times, however, when the exact motor supply voltage is not available, or you simply wish to run the motors below the rated current. (There is a benefit of running motors at much higher than rated voltages with proper current-limiting -- the motors can achieve higher RPMs.)

A current-limiting resistor in series with the motor can be used to effectively limit current to the motor, at the cost of wasted power. The resistor used is a special "power resistor" which must dissipate power and heat. Ideally, the resistor should be a non-inductive type so as not to interfere with the inductance of the motor and control circuit (especially when current-limiting to achieve higher RPMs).

The value of a current-limiting resistor in series with the motor can be determined from the following equation...

Vsupply = Vdrop + (i * Rmotor) + (i * Rlimit)

Vdrop = voltage drop of Transistors used (1 to 2 volts in a unipolar controller)
i = current in circuit
Rmotor = resistance of single motor coil
Rlimit = current limiting power resistor

The objective is to find Rlimit that satisfies the desired current rating i

For example, Using a power supply of 12 volts, what current limiting resistor should be chosen to deliver 1 amp of current to a motor with a coil resistance of 5 ohms? Assume 2 volt drop due to the transistors.

12v = 2v + (1 amp * 5 ohm) + (1 amp * Rlimit)

12v = 2v + (5v) + (1 amp * Rlimit)

5v = 1 amp * Rlimit

Rlimit = 5v / 1 amp = 5 ohm

What will the voltage be across this resistor? 5v
What current will flow through this resistor? 1a
What power rating should this resistor be? 5v * 1a = 5 watt, minimum

This calculation is for determining the current in a single coil. Most motors and stepper circuits are rated in terms of this coil rating - amps per phase. If the hi-torque, or half-step sequence is to be used, then more than 1 coil will be activated, thus more current will be required of the power supply.

Ideally, two matched current limiting resistors should be used, one in series with each of the 2 motor coils. The unipolar driver can get away with sharing a single power resistor, with the center taps tied together.

In summation, make your calculations carefully, and always apply caution when making any kind of modification to a circuit. Check the circuit often for over-heating.

[dertsap]

5v 1.5 amp running at 12v :
1 resistor use a 2.33ohm 20watt minimum
2 resistors use a 4.67 ohm 10.5 watt minimum

calculations are from software supplied from stepper world

[pcw]

Thank You for your help.