Re: [CAD_CAM_EDM_DRO] Re: servo voltage question

Pete,
I have a stepper here that has 36 VDC, 72VDC and 170VDC(!) curves on
it's data sheet.

erie


Peter Reilley wrote:

>A question on voltage. There must be a maximum voltage that
>can be applied a motor because of insulation breakdown.
>For a 4 volt stepper motor what voltage would be too high?
>What is the voltage rating of the insulation on the wire used
>in the average stepper motor? Are 30 year old stepper motors
>going to be significantly worse than new motors? Is there
>a voltage limit for the brush assembly used in servo motors?
>
>Do companies give these ratings? If not what are some good
>guesses?
>
>Pete.
>
>
>----- Original Message -----
>From: "Mariss Freimanis" <mariss92705@...>
>To: <CAD_CAM_EDM_DRO@yahoogroups.com>
>Sent: Saturday, January 29, 2005 4:06 AM
>Subject: [CAD_CAM_EDM_DRO] Re: servo voltage question
>
>
>
>
>>Jon,
>>
>>Even if it is a "minor war" I never received the invitation to
>>participate; I feel let down.
>>
>>Switching type drives apply a voltage to a motor. This voltage has
>>little to do with the power supply voltage. All the motor sees is
>>some percentage of that.
>>
>>Voltage by itself doesn't really matter and a user doesn't control it
>>or measure it anyway. What matters is a motor's maximum rated RPM and
>>torque.
>>
>>It is perfectly OK to use a 40VDC rated motor with a 70VDC power
>>supply so long as you don't ask for any speed over what the motor is
>>rated for at 40VDC. The motor won't know any better because all it
>>will see is 40V. The PWM will go to a 60% duty cycle (0.6 times 70V =
>>40V more or less). Everything is cool then.
>>
>>The drive is equipped with a current limit trimpot. Current is
>>limited on a pulse-by-pulse method. If you have a 15A rated peak
>>motor, set the limit on the trimpot to that then.
>>
>>Overvoltage under your control by not asking for a speed above what
>>the motor is rated for. Over-torque can be prevented by setting the
>>torque limit trimpot correctly. That's what it's there for.
>>
>>Otherwise go with your 72VDC supplies for 40VDC or even 24VDC motors.
>>They won't know the difference if you behave.
>>
>>Mariss
>>
>>
>>
>>
>>--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>>
>>
>>>cnc_4_me wrote:
>>>
>>>
>>>
>>>>What power supply voltage do you use for servos with a pwm
>>>>drive...Example, if your motor is rated for 40VDC do you use a
>>>>
>>>>
>>40VDC
>>
>>
>>>>power supply or do you use something higher like 70VDC...
>>>>
>>>>
>>>>
>>>>
>>>I've been having a minor war with Mariss Freimanis at Gecko over
>>>
>>>
>>this.
>>
>>
>>>You have to know how the voltage rating of the motor was written.
>>>If it is an absolute voltage limit, never to be exceeded, then you
>>>
>>>
>>don't
>>
>>
>>>want to go over it. This will be true for "real" servo motors made
>>>
>>>
>>by
>>
>>
>>>such makers as Electro-Craft (now Reliance Electric), Yaskawa,
>>>
>>>
>>Fanuc,
>>
>>
>>>etc. Those motors also have a peak current rating, which is also
>>>
>>>
>>never to
>>
>>
>>>be exceeded.
>>>
>>>But many motors sold in the servo market have a very different
>>>
>>>
>>voltage
>>
>>
>>>rating.
>>>Makers such as Ametek use this. It is a "bus voltage" rating. You
>>>
>>>
>>can
>>
>>
>>>connect
>>>the motor to a DC bus of that voltage with the motor stationary,
>>>
>>>
>>and it will
>>
>>
>>>not be harmed. To illustrate, the typical Ametek motor, sold by
>>>Camtronics and
>>>many others, has about a 4 Ohm armature resistance, a 12 A peak
>>>
>>>
>>current
>>
>>
>>>rating, and a stated 40 V maximum voltage. Simple Ohm's law
>>>
>>>
>>calculation
>>
>>
>>>states you can NEVER even obtain 12 A through this motor with a 40 V
>>>power supply. I = E/R or 40/4 = 10 A! Now, with the motor running
>>>
>>>
>>at
>>
>>
>>>1/2 its rated speed, the back EMF will be about 20 V. That
>>>
>>>
>>subtracts
>>
>>
>>>from the
>>>applied voltage, so you'd only have about 20 V left to drive
>>>
>>>
>>current
>>
>>
>>>through the motor.
>>>With a 40 V power source, you can only get (40-20)/4 = 5 Amps! If
>>>
>>>
>>you
>>
>>
>>>want to
>>>spin the motor faster, there's even less current possible. So, by
>>>limiting the supply
>>>to the 40 V rating, you lose at least HALF the possible power
>>>
>>>
>>output of
>>
>>
>>>this motor.
>>>But, you can't "erase" the magnets in it with only 40 V. If you
>>>
>>>
>>turn
>>
>>
>>>the voltage up
>>>a bit, the only thing standing between the the motor and
>>>
>>>
>>destruction of
>>
>>
>>>the magnets
>>>is the servo amp's current limit. If you trust that current limit
>>>
>>>
>>to do
>>
>>
>>>its job, then
>>>you should be able to safely turn up the supply voltage to get
>>>
>>>
>>rated
>>
>>
>>>performance
>>>from the motor.
>>>
>>>Now, that gets you into the safe operating area discussion. The
>>>
>>>
>>motor
>>
>>
>>>CAN be
>>>made to safely deliver maximum rated speed and maximum rated torque
>>>simultaneously
>>>for only some time interval. If you need both at the same time,
>>>
>>>
>>you'd
>>
>>
>>>better not
>>>demand it for more than a fraction of a second a couple of times a
>>>
>>>
>>minute.
>>
>>
>>>Really massive industrial servos are designed to survive more
>>>
>>>
>>severe
>>
>>
>>>service than
>>>lighter commercial-grade motors, and those often don't come with
>>>
>>>
>>safe
>>
>>
>>>operating
>>>area curves. So, you have to use some common sense in how far you
>>>
>>>
>>are
>>
>>
>>>going to
>>>push things. And, of course, a Gecko drive will happily deliver 10
>>>
>>>
>>A to
>>
>>
>>>a locked
>>>Ametek motor until the smoke comes out. I've seen it happen!
>>>
>>>Anyway, Mariss warns to strictly adhere to the manufacturer's
>>>
>>>
>>ratings.
>>
>>
>>>But, I hope
>>>I have shown that at least with some motors, you absolutely cannot
>>>
>>>
>>even
>>
>>
>>>achieve rated
>>>performance if you do that.
>>>
>>>Jon
>>>
>>>
>>
>>
>>
>>Addresses:
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>>FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
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>Addresses:
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>FILES: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO/files/
>Post Messages: CAD_CAM_EDM_DRO@yahoogroups.com
>
>Subscribe: CAD_CAM_EDM_DRO-subscribe@yahoogroups.com
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>List owner: CAD_CAM_EDM_DRO-owner@yahoogroups.com, wanliker@..., timg@...
>Moderator: pentam@... indigo_red@... davemucha@... [Moderators]
>URL to this group: http://groups.yahoo.com/group/CAD_CAM_EDM_DRO
>
>OFF Topic POSTS: General Machining
>If you wish to post on unlimited OT subjects goto: aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if you have trouble.
>http://www.metalworking.com/news_servers.html
>
>http://groups.yahoo.com/group/jobshophomeshop I consider this to be a sister site to the CCED group, as many of the same members are there, for OT subjects, that are not allowed on the CCED list.
>
>NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
>bill
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