Re: Bridgeport servo motors

>
> I have a 45 volt 800 VA toroid transformer. This should supply
> 60+ VDC. The motors max volt rating is 60v and I am not sure
> I want to go over it. There seems to be conflicting info as to
> whether it is ok to go over voltage on servo motors.
>
> So it is your opinion that I will be OK with these motors and a
> 3:1 reduction as long as I can handle 80 IPM rapids? I will have
> plenty of torque for taking nice size cuts in steel?
> This makes me feel better because I was second guessing my motors.
> I am only a hobbiest and the speed is not all that inportant. Heck,
> at least I won't have to stand there and crank the handwheels!
>
> Thanks for your opinion,
> Scott

Let's start with torque, at 3-1 you will have plenty of power for
cutting steel, probably enough torque to stall the spindle if using a
large cutter.

Now as for power supply.

First, when using a PWM power supply you can safely have a power
supply of greater voltage rating than the motor can safely handle.
The reason you can do this is that current burns motors up. And the
PWM drive has a settable current limit. So the higher power supply
voltage will let you get you max RPM and the max current setting will
stop the motor from burning up or demagnetizing. You would set your
current limit to 20 amps max.

Second, your 800 VA transformer is woefully inadequate. It is so far
undersized you could not even run 1 motor on it up to power. A week
or 2 ago I posted a complete example of how to design a power
supply. Search for this.

From: "cnc_4_me" <cnc4me@g...>
Date: Thu Jan 27, 2005 1:47 pm
Subject: Re: Toroidal transformer ratings?? What after being
rectified?


Wally




--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Polaraligned"
<polaraligned@o...> wrote:

>
>
> >
> > There is a difference between a real servo and a DC motor.

Servos

> > have lower rotor inertia, commutators optimized for smother
> > operation, and more peak torque reserve.
> >
>
>
> Which is going to equate to overall better, more predictable
> performance from the machine.
>
>
>
>
>
>
> > Well, this is what my research shows on required torque, speed,

and

> > reduction ratio.
> >
> > Most people on this forum agree the servo size used for

commercial

> > Bridgeport retrofits is around 30 lb-in or 480 oz-in.
>
>
> So this is the Peak torque rating for these motors?? I have been
> assuming that this is a continuous rating and my motors are a little
> small as opposed to being a little big.
>
>
>
> > Also I seem to
> > be getting the consensus that the pulley reduction is around 2-1

to

> > 2.5-1.
> >
> > Now let's look at a practical example. A friend has been using a
> > Bridgeport for 15 years. He is estimating that on average with a
> > heavy cut he is pushing 5 lb on the hand wheel. The hand wheel

has a

> > 3" lever length. So 3" x 5 lb = 15 in-lb continuous torque

needed.

> >
> > You motor has a peak rating of 37.5 lb-in. Estimating constant
> > torque we would divide peak by 5 = 7.5 lb-in. So you need 2x

this

> > number for the estimated 15lb-in force required. This tells me

you

> > need a minimum of 2-1 belt reduction.
> >
> > As you noted earlier more reduction gives you more power but

less

> > speed. A 3-1 reduction would give you a 22.5 lb-in at 80 IPM. A
> > nice continuous torque rating but a little slow on rapids.
> >
> > Lets talk about 80 IPM, I can tell you this. A typical power

feed on

> > a Bridgeport will give you a maximum speed of around 90IPM. If

you

> > have access to one of these you can see for yourself if you like

this

> > speed.
> >
> > From a personal standpoint, and this is coming from someone who

has

> > no patience….I get irritated using the power feed to go from one

side

> > of the table to the other. One time I actually disengaged it and
> > turned the crank by hand because I thought I could go faster (I

was

> > wrong). The reason for going from one side of the table to other

was

> > I had 2 setups on the table, one on each end…
> >
> > With that in mind I am shooting for a rapid of 120 IPM minimum on

my

> > machine.
> >
> > With all that said, if I were you I would probably go with 2.5

or 3-

> > 1 ratio, a fast rapid is nice. But if you can't take the cut you
> > want it doesn't do you any good.
> >
> > I also think you should use a 70 volt supply, with your current

limit

> > set to 20 amps. This way you will get maximum RPM without

running

> > over current.
> >
>
> I have a 45 volt 800 VA toroid transformer. This should supply
> 60+ VDC. The motors max volt rating is 60v and I am not sure
> I want to go over it. There seems to be conflicting info as to
> whether it is ok to go over voltage on servo motors.
>
> So it is your opinion that I will be OK with these motors and a
> 3:1 reduction as long as I can handle 80 IPM rapids? I will have
> plenty of torque for taking nice size cuts in steel?
> This makes me feel better because I was second guessing my motors.
> I am only a hobbiest and the speed is not all that inportant. Heck,
> at least I won't have to stand there and crank the handwheels!
>
> Thanks for your opinion,
> Scott