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#128668 - 09/24/03 03:01 PM motor amp draw
ds247 Offline
Member

Registered: 05/17/03
Posts: 18
Loc: California
I have was taking amperage measurements on some PSC motors and found that the common wire would have a higher reading than the run wire. In one or two cases the higher reading was above the full load amps of the motor while the other reading was below the fla. The motors were not heavily loaded. Which reading should be used for the comparison to the fla. Thanks.

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#128669 - 09/24/03 04:38 PM Re: motor amp draw
maintenanceguy Offline
Member

Registered: 12/02/01
Posts: 303
Loc: Southern NJ, USA
I always read fla on the common.

amp reading on the start plus amp reading on the run winding equals amp reading on the common. (except that there's a little phase shift so it's off a little)

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#128670 - 09/25/03 03:57 PM Re: motor amp draw
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
For the most basic single-speed PSC motor, the usual designations for connections are C, S & R. Full-load current is correctly measured on the common lead. [You should also see the same reading you clipped around the S & R leads simultaneously.] Rated FLC should be listed on the equipment label, and not be exceeded at all after starting. If it’s high, check for correct line voltage on the C & R leads. If it’s normal, the next thing to check is excess mechanical load. Often, appliance-grade fractional-horsepower “39-frame” PSC motors are furnished with sleeve bearings, so if any significant shaft sideplay is evident, plan on replacing the motor for the bearings are rarely economically replaceable.

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#128671 - 09/27/03 01:26 AM Re: motor amp draw
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
I was thinking that the higher than rated FLA reading was due to an overloaded Motor, but since the posting person mentioned Motors not fully loaded, then maybe the Capacitor is oversized and/or sticky bearings?

As already mentioned, ampere readings should be made on the Common line (for overall FLA), then across the Run winding lead, and finally the Auxiliary winding lead, to compare the figures.

Auxiliary winding should have high load current during Locked Rotor Start, eventually topping off at a percentage of the main "Run" winding's load current.

Excessive current in the Auxiliary winding is limited by the Capacitor (Xc).
Having too large value of a Capacitor will result in an increasing Auxiliary winding current, along with a corresponding shift backwards in time phase, as the Motor accelerates.

Just a little more info to add to the great info already posted!

Scott35

p.s. what's this/these Motor(s) driving? I hope only Fans if they are straight PSC (not Cap Start/Run).
S.E.T.
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#128672 - 09/29/03 07:34 PM Re: motor amp draw
ds247 Offline
Member

Registered: 05/17/03
Posts: 18
Loc: California
Thanks for the valuable replies. These motors are driving fans.
Since we are on the subject of capacitors and motors I am wondering about a few other things. If a motor has no information about the correct size capacitor is it reasonable to compare the fla or horsepower to another motor listed in a catalog or other similar piece of equipment and use the cap size of that motor? Any other way to size run caps when no spec is given on the motor? Any way to size start caps on CSR motors when no spec is given?
Also, can anyone state some general "rules" about what happens when run or start caps are oversized or undersized. My understanding is that an oversized run cap will speed up the motor and make it run hotter. I presume that an undersized run cap will slow down the motor but will it also overheat? What are the effects of over and undersized start caps? Thanks very much.

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#128673 - 09/29/03 08:10 PM Re: motor amp draw
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
There is no simple formula for sizing start or run capacitors—it is determined in motor design, and simply part of the OEM gear, or for MRO/replacement, specified on the motor packaging.

You may be able to approximate the value by experimentation, but appliance motors are built with so little headroom that shotgunning is risky.

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#128674 - 09/29/03 09:49 PM Re: motor amp draw
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
I was planning to pass this one on to Bjarney, since He can offer the most practical answers to Motor issues!

I agree also that component / Motor design is independent, making it a long shot to use a similar Motor's Capacitor
size (in µF) and achieve proper operation.
The total Reactance in the Auxiliary winding needs to be rather tightly set, in order to achieve optimum performance
and highest Motor efficiency (the equipment's total efficacy).

I guess you could try a "hit/miss" approach with a run Capacitor, but if the Capacitor is too large, it might fry the aux.
winding.

Too large of a Capacitor (start or run) will cause an increasing load current plus an increasing backwards shift of the
reducing magnetic flux, as the Rotor speed increases.
This will suck as far as proper operation goes! (non-uniform torque, increased heat/wattage to overcome the odd flux
behavior, so on...).

too low of Capacitor value (too small) will make starting difficult, and a lag in overall torque + Motor efficiency (once
again, Motor efficacy is not met).

For Start Caps, too large would result in an overloaded start winding - both creating too much heat in the winding and
causing a non-uniform starting torque.
The increased current would be applied for a longer period of time, and easily result in a tripped OCPD
(well, only if certain devices are not being used )

Too small of start Cap, and the Motor will struggle to start a heavy load, resulting in excessively long start circuit usage
- and a similar effect as the "too large" scenario!

Anyone object to this information? please feel free to chime in! We are all here to learn from each other, so please pass
on your knowledge.

Scott35
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#128675 - 09/29/03 10:14 PM Re: motor amp draw
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Scott, I buy your explanation of start-capacitor sizing 100%.

As for PSC/'run' capacitor size, one trial method may be to try various ratings under normal mechanical load and see which value produces lowest current in the 'C' lead.




[This message has been edited by Bjarney (edited 09-30-2003).]

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#128676 - 10/01/03 04:43 PM Re: motor amp draw
ds247 Offline
Member

Registered: 05/17/03
Posts: 18
Loc: California
Thanks for clarifying some things for me.

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