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Home > Archive > Electrical Engineering > March 2006 > Motor starter/relay sizing question
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Motor starter/relay sizing question
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| astjohn@gmail.com 2006-03-18, 11:21 pm |
| Hey all,
I am a mechanical engineer currently trying to size a motor
relay/starter for a laboratory apparatus in Canada. My electrical
experience is limited to small homebrew pcb circuits. I am not exactly
sure what I need for the following application. Any help is greatly
appreciated.
I have a vacuum pump connected to a Leeson Motor which needs to turn on
and off automatically.
The motor specs are:
Voltage = 220V
Freq. = 60 Hz.
Full Load = 5.4 A
Locked Rotor = 54.6 A
Phase = 1
The signal that I can send to turn the motor off or on is provided by a
National Instruments Digital Output Module:
http://sine.ni.com/nips/cds/view/p/lang/en/nid/12166
signal = 5-30 VDC, sourcing 2A
The first thing that came to mind was "relay". However, after reading
and researching about relays and starters, I have become thoroughly
confused. =-)
I do, however, have a copy of the National Electrical Code Handbook
(NFPA 70) - so I do know a little about required/recommended overload
protective devices etc...
Any help in the right direction such as the type of common components
that I need, where to get them, or even general info/webpages would
really be appreciated.
Adam
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| electrician@electrician.com 2006-03-19, 12:21 am |
| I just ran your motor parameters through my new free online motor
circuit calculator at http://www.electrician.com and the results came
out ok for 3/4 and 1 hp. However you did not give the horsepower, but
it looks like you have either a 3/4 hp or 1 hp motor based on locked
rotor current using Table 430.251(A).
The output page is quite large so I am not copying it here.
If the output from the module is a Class 2 power limited circuit
(which it probably is) then you will have to keep it separated from the
power wiring as covered in article 725. If it is Class 2 use a 16/2 or
18/2 shielded cable and ground the shield at only one end.
You can buy motor controllers (starters) with 24 volt DC coils but
this is not the normal method of controlling a motor starter or
controller. Instead use a 24 volt DC Potter Brumfield relay and wire
the motor controller with a 120 volt coil through the contacts.
Control the PB relay with your signal.
I am not an electrical engineer, but am a licensed electrical
administrator in Alaska and licensed electrician wireman in Alaska and
Washington that plugs his site here from time to time.
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| astjohn@gmail.com 2006-03-19, 11:21 am |
| Yes, sorry, I completely forgot the horsepower (3/4 hp).
I'll check out your website, and into the info you provided.
Thank you.
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| electrician@electrician.com 2006-03-19, 8:21 pm |
| If you have the money in the budget, an Allen Bradley Variable
Frequency Drive is really a great way to go. They are basically a
programmable motor controller that can be interfaced with your computer
network. They are also very reliable and are now common in industrial
locations. They can be used for on off control (or proportional
control) and will take you signal directly as an input. They come with
an instruction book that is sufficient for learning how to do the
programming after about 10 hours of study. The future use of these
VFD's is immense since they have about 350 programmable variables. And
I don't work for AB, but have installed and programmed their VFD's. I
was impressed. As I recall, they are a little spendy.
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| Bob Ferapples 2006-03-20, 4:21 am |
| On 19 Mar 2006 15:42:48 -0800, electrician@electrician.com wrote:
>If you have the money in the budget, an Allen Bradley Variable
>Frequency Drive is really a great way to go. They are basically a
>programmable motor controller that can be interfaced with your computer
>network. They are also very reliable and are now common in industrial
>locations. They can be used for on off control (or proportional
>control) and will take you signal directly as an input. They come with
>an instruction book that is sufficient for learning how to do the
>programming after about 10 hours of study. The future use of these
>VFD's is immense since they have about 350 programmable variables. And
>I don't work for AB, but have installed and programmed their VFD's. I
>was impressed. As I recall, they are a little spendy.
WRONG!
He has a 1 phase motor! VFDs are only for 3 phase motors.
Just buy a relay that has contacts rated for starting a 3/4HP motor.
it will be larger than the plug-in style plastic case relays typically
called "ice cube relays", because they are only rated for 1/2HP. 3/4HP
will mean a slightly larger one, but still very cheap. Most 1 phase
motors like that are already internally protected so you will not need
any other protective device other than a fuse or circuit breaker sized
for the wire going to the motor.
Also check out the relay coil "inrush current" when you find one, and
compare that to the amp rating of the output you have. If the relay
draws too much for that output, you may need to use another small
relay to drive the larger one, a systems called an "interposing
relay".
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| Bob Ferapples 2006-03-20, 5:21 am |
| On Mon, 20 Mar 2006 08:04:53 GMT, Bob Ferapples <licku45@yqahoo.com>
wrote:
>On 19 Mar 2006 15:42:48 -0800, electrician@electrician.com wrote:
>
>
>WRONG!
> He has a 1 phase motor! VFDs are only for 3 phase motors.
>
>Just buy a relay that has contacts rated for starting a 3/4HP motor.
>it will be larger than the plug-in style plastic case relays typically
>called "ice cube relays", because they are only rated for 1/2HP. 3/4HP
>will mean a slightly larger one, but still very cheap. Most 1 phase
>motors like that are already internally protected so you will not need
>any other protective device other than a fuse or circuit breaker sized
>for the wire going to the motor.
>
>Also check out the relay coil "inrush current" when you find one, and
>compare that to the amp rating of the output you have. If the relay
>draws too much for that output, you may need to use another small
>relay to drive the larger one, a systems called an "interposing
>relay".
Sorry, didn't have your output info available when responding to the
other posting, but at 2A capacity, you will be able to drive a 3/4HP
rated relay no problem, so forget about the interposing relay
business.
Here's a relay like what you need. A bit of overkill, but for $7.24
it's worth it.
http://www.newark.com/NewarkWebComm...SKU=09B8542&N=0
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| electrician@electrician.com 2006-03-20, 3:21 pm |
| <WRONG!
He has a 1 phase motor! VFDs are only for 3 phase motors. >
Where have you been? There are a ton of Single phase VFD's available.
Just search single phase VFD at Google.
Also, I seriously doubt the ice cube relay design or ice cube relay
with "an interposing relay" that you refer to can handle the locked
rotor current, inrush current and fault current without a detailed
analysis. The original question was posted by a Mechanical Engineer
that admits he has limited experience and who probably is unfamiliar
with fault current. The safe and simple approach, in this case,
appears to be a motor starter combination with a ground-fault and
short circuit protector and controller with overload relays with an
auxiliary relay driven by a Class 2 DC signal located in a separate box
that we often install in industrial locations. A LISTED motor starter
for 3/4 hp motors usually can handle the fault current, inrush current,
and locked rotor current without further considerations. It makes
things simple and provides a guaranteed high degree of reliability
which I think is of practical importance for the mechanical engineer.
Like I said before, I am not an engineer, but do have considerable
experience installing motor circuits that have been designed by very
experienced PE's.
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| electrician@electrician.com 2006-03-20, 4:21 pm |
| I will give you credit on one item. The VFD's outputs are three phase,
but the input can be single phase. So we were both half right on that
one.
The literature online for the AB Powerflex 40 is misleading in that it
states that the VFD can be used for 120, 230, 460 volts but it does not
state the phases so I called Allen Bradley at 440-646-5800 and asked
them. Apparently, there is a harmonic problem with single phase
outputs that has not been solved yet. I have installed only three
phase VFD's and relied on the online material for the single phase use
- my error.
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| electrician@electrician.com wrote:
>
> Also, I seriously doubt the ice cube relay design or ice cube relay
> with "an interposing relay" that you refer to can handle the locked
> rotor current, inrush current and fault current without a detailed
> analysis. The original question was posted by a Mechanical Engineer
> that admits he has limited experience and who probably is unfamiliar
> with fault current. The safe and simple approach, in this case,
> appears to be a motor starter combination with a ground-fault and
> short circuit protector and controller with overload relays with an
> auxiliary relay driven by a Class 2 DC signal located in a separate box
> that we often install in industrial locations. A LISTED motor starter
> for 3/4 hp motors usually can handle the fault current, inrush current,
> and locked rotor current without further considerations. It makes
> things simple and provides a guaranteed high degree of reliability
> which I think is of practical importance for the mechanical engineer.
> Like I said before, I am not an engineer, but do have considerable
> experience installing motor circuits that have been designed by very
> experienced PE's.
>
The Magnecraft relay has a rating of 3HP at 240V. It should not have a
problem with locked rotor (inrush?) current. I have not seen ground
fault protection on a 3/4HP motor. The Magnecraft lit does not indicate
a value for short circuit protection. A 5.4A motor could use a 15 or 20A
branch circuit breaker. The relay does not provide overload protection
for the motor. If the motor is marked "Thermally protected" that is
adequate, else something else would be needed. The relay is a UL
Recognized Component, indicating it is intended to be used in a
manufactured device. That may or may not indicate a limitation for this
application. If this is hardwired, an inspector would want a UL listed
relay with a HP rating (if the motor is thrermally protected) or a motor
starter (which has overload protection). "NEMA" sized motor starters
would last longer if this is a long term application. For a single
application use for a plug in device for noncritical application the
relay is should be adequate.
A disconnect is also required. That could be a 2 pole switch rated 3/4
HP @240V. Wall switches rated 15 or 20A would work.
bud--
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