Stepper motor question

I have a 6-wire motor which I plan to operate as a 5-wire motor. As such
I've found the two center tap wires and will treat them as a single wire
going to a supply voltage of 40.5 VDC. Measuring the resistance of the
coils gives me 2.2 ohms to each of the remaining wires.

The motor itself is rated for 2 A. Using i = V - (Rated A * motor R) /Rated
R yields a result of 18 ohms for a current limiting resistor.

The closest I can find (would like a power resistor) is 15 ohms which would
give 2.35 A to the motor. (17% over)

Will this overheat the motor?

The motor is on a desktop CNC machine run by a unipolar driver (???) and is
for the Z axis. Best guess is running short periods of time (30 seconds
max, but usually only a couple of seconds).

Or, can someone suggest a source for a 18 ohm resistor? (To replace an
Ohmite TCH35P7R0J resistor).

On Fri, 26 Feb 2010 14:44:17 -0700, "thermo102" <thermo102_at_comcast.net
wrote:

I have a 6-wire motor which I plan to operate as a 5-wire motor. As such
I've found the two center tap wires and will treat them as a single wire
going to a supply voltage of 40.5 VDC. Measuring the resistance of the
coils gives me 2.2 ohms to each of the remaining wires.

The motor itself is rated for 2 A. Using i = V - (Rated A * motor R)
/Rated
R yields a result of 18 ohms for a current limiting resistor.

The closest I can find (would like a power resistor) is 15 ohms which
would
give 2.35 A to the motor. (17% over)

Will this overheat the motor?

The motor is on a desktop CNC machine run by a unipolar driver (???) and
is
for the Z axis. Best guess is running short periods of time (30 seconds
max, but usually only a couple of seconds).

Or, can someone suggest a source for a 18 ohm resistor? (To replace an
Ohmite TCH35P7R0J resistor).

---
That's a 7 ohm 35 watt resistor, so if it was part of the old circuit
there might be more going on than meets the eye and an 18 ohm resistor
might not be what you want.

What more can you tell us about how you'll be driving the stepper?

JF

"John Fields" <jfields_at_austininstruments.com> wrote in message
news:f0tgo5ttla4mai2fbebu9ip1utn5i3fftu_at_4ax.com...
On Fri, 26 Feb 2010 14:44:17 -0700, "thermo102" <thermo102_at_comcast.net
wrote:

I have a 6-wire motor which I plan to operate as a 5-wire motor. As such
I've found the two center tap wires and will treat them as a single wire
going to a supply voltage of 40.5 VDC. Measuring the resistance of the
coils gives me 2.2 ohms to each of the remaining wires.

The motor itself is rated for 2 A. Using i = V - (Rated A * motor R)
/Rated
R yields a result of 18 ohms for a current limiting resistor.

The closest I can find (would like a power resistor) is 15 ohms which
would
give 2.35 A to the motor. (17% over)

Will this overheat the motor?

The motor is on a desktop CNC machine run by a unipolar driver (???) and
is
for the Z axis. Best guess is running short periods of time (30 seconds
max, but usually only a couple of seconds).

Or, can someone suggest a source for a 18 ohm resistor? (To replace an
Ohmite TCH35P7R0J resistor).

---
That's a 7 ohm 35 watt resistor, so if it was part of the old circuit
there might be more going on than meets the eye and an 18 ohm resistor
might not be what you want.

What more can you tell us about how you'll be driving the stepper?

JF

Guess I left out most important part. This motor is to replace another
motor which has 133 ohms arcross the coils. I'm pretty sure the 18 ohm value
is what I need for the new motor, but can't seem to find one of that value.

About 30 years ago, I was told by an engineer that most electrical stuff had
a fudge factor of about 10% or more built into them. Is this more or less
true today also? If so, then I can probably use the 15 ohm resistor without
burning up the motor.

On Sat, 27 Feb 2010 09:55:03 -0700, "thermo102" <thermo102_at_comcast.net
wrote:


"John Fields" <jfields_at_austininstruments.com> wrote in message
news:f0tgo5ttla4mai2fbebu9ip1utn5i3fftu_at_4ax.com...
On Fri, 26 Feb 2010 14:44:17 -0700, "thermo102" <thermo102_at_comcast.net
wrote:

I have a 6-wire motor which I plan to operate as a 5-wire motor. As
such
I've found the two center tap wires and will treat them as a single wire
going to a supply voltage of 40.5 VDC. Measuring the resistance of the
coils gives me 2.2 ohms to each of the remaining wires.

The motor itself is rated for 2 A. Using i = V - (Rated A * motor R)
/Rated
R yields a result of 18 ohms for a current limiting resistor.

The closest I can find (would like a power resistor) is 15 ohms which
would
give 2.35 A to the motor. (17% over)

Will this overheat the motor?

The motor is on a desktop CNC machine run by a unipolar driver (???) and
is
for the Z axis. Best guess is running short periods of time (30 seconds
max, but usually only a couple of seconds).

Or, can someone suggest a source for a 18 ohm resistor? (To replace an
Ohmite TCH35P7R0J resistor).

---
That's a 7 ohm 35 watt resistor, so if it was part of the old circuit
there might be more going on than meets the eye and an 18 ohm resistor
might not be what you want.

What more can you tell us about how you'll be driving the stepper?

JF

Guess I left out most important part. This motor is to replace another
motor which has 133 ohms arcross the coils. I'm pretty sure the 18 ohm
value
is what I need for the new motor, but can't seem to find one of that
value.

About 30 years ago, I was told by an engineer that most electrical stuff
had
a fudge factor of about 10% or more built into them. Is this more or less
true today also? If so, then I can probably use the 15 ohm resistor
without
burning up the motor.

---
Dunno.

Do you have a schematic and, again, what more can you tell us about how
you'll be driving the stepper?

http://www.mouser.com/Passive-Components/Resistors/Power-Resistors-Wire-Wound/_/N-2grd3?P=1z0yllmZ1z0x6z1Z1z0vl7s

JF

Since my last post, I downloaded mouser's catalogue. Took my time to find
the page with the ohmite resistors and printed it. Then I found an on-line
calculator for resisters in parallel. Trial and error gave me the
following:

A 24 ohm and a 75 ohm in parallel will give me 18.18 effective ohms. That's
close enough for me. I was hoping for a simple change out, but this will
work. I plan on sandwiching the two resistors back to back on a homemade
heatsink and letting the leads of each one share the spot where the single
resistor was previously. (Which is making them parallel, right?)

To answer your question, I don't have a schematic, and the stepper(s) are
being driven via a PC parallel port.

I'll order the resistors tomorrow and see what happens. Wish me luck!

A 24 ohm and a 75 ohm in parallel will give me 18.18 effective ohms.

18 ohms is a standard value.

"John Fields" <jfields_at_austininstruments.com> wrote in message
On Sat, 27 Feb 2010 09:55:03 -0700, "thermo102" <thermo102_at_comcast.net
"John Fields" <jfields_at_austininstruments.com> wrote in message
On Fri, 26 Feb 2010 14:44:17 -0700, "thermo102" <thermo102_at_comcast.net
I have a 6-wire motor which I plan to operate as a 5-wire motor. As

What more can you tell us about how you'll be driving the stepper?
Guess I left out most important part. This motor is to replace another
Do you have a schematic and, again, what more can you tell us about how
you'll be driving the stepper?
To answer your question, I don't have a schematic, and the stepper(s) are
being driven via a PC parallel port.

"John Fields" <jfields_at_austininstruments.com> wrote in message
news:uodlo5ddejh1fsr6kugsctdchjcgb4odnv_at_4ax.com...
On Sat, 27 Feb 2010 09:55:03 -0700, "thermo102" <thermo102_at_comcast.net
wrote:

The closest I can find (would like a power resistor) is 15 ohms which
would
give 2.35 A to the motor. (17% over)

Will this overheat the motor?

Best guess is running short periods of time (30 seconds
max, but usually only a couple of seconds).

have you considered PWM,

On 6 Mar 2010 02:44:56 GMT, Jasen Betts <jasen_at_xnet.co.nz> wrote:


have you considered PWM,

---
Have you read the subject line? ;)

On 2010-03-06, John Fields <jfields_at_austininstruments.com> wrote:
On 6 Mar 2010 02:44:56 GMT, Jasen Betts <jasen_at_xnet.co.nz> wrote:


have you considered PWM,

---
Have you read the subject line? ;)


are you saying that PWM can't be used to limit the current in stepper
motors?

Well, I tried! The power resistors I installed are acting as bleed off's I
guess. A waste of money, but they don't seem to be harming anything, so I
plan to leave them there.

The new motor did run, but with exaxtly the same current draw as the old
one.

Hopefully the following is meaningful, and will help answer my question:
What do I have to do to supply the new motor with approximately 2.0 amps
operating current?

Pins 2,3,4,&5 of the parallel port go to a DM7 400N Logic NAND gate.

A trace goes from the NAND gate to another transistor which I cannot
identify, and then on to a large condensor. There is a trace branching off
(through a 1K resistor) to the 'base' of TIP 120. 5 volts from the parallel
port here?

Am I correct that this 1K resistor is the actual current limiting resistor?

I 'think' there is a 42 Volt DC applied to the 'emitter' of TIP 120. Then
the 'collector' goes to the motor.

Of course I have four TIP 120's with the same configuation. (one for each
'coil' of the motor).

The OLD motor had 133 ohms resistance across the coils, and the above gave
approximately 0.29 operationg amps.

The NEW motor I'm working with has 2.2 ohms resistance across the coils.
What value resistor should be used to replace the existing 1K's to achieve
approximately 2.0 operating amps?

Else you'll need to replace the DM7400N with someething stronger;
the DM7400N chip can only produce 0.4 ma so it's not going to work,
A SN74AHC00N chip can procude 8mA on the outputs which is probably
close enough to 20mA for it to work, replace the 1K resistors with
wire links.