Subject: Re: Motor Question

From: [email-masked] (gary preckshot)
Date: 10 Feb 1994 20:58:59 GMT

In article <64=[email-masked]>, [email-masked] (John De Armond) wrote:
> 
> [email-masked] (Vincent Iorio) writes:
[...]
> > "What does servce factor mean?"
> 
> I can't imagine why anyone would argue over this term.

There's no significant argument, and John has quoted the (noncommital) GE
description.  This doesn't answer the questions "why is there a service
factor?" or "what does it mean to me?"

The real significance of service factor to most shop equipment is that it
allows some leeway in selecting overload heaters for motor starters.  The
standard service factor, as John notes, is 1.15 for 1800 rpm single phase
motors 1 hp to about 10 hp.  This is quite sufficient to absorb variations
in overload heaters without endangering the motor.  For most shop
equipment, service challenges are small, and service factor is not a
significant factor.  In fact, the machinery designer may not have even
known what it was for, and merely selected the standard available motor for
low cost.

For industrial applications, the crux of service factor is that the motor
maker warrants that the motor will be undamaged by continuous operation
under the service factor, but *not* efficiency or power factor.  An
industrial designer, constrained by power factor and efficiency demands,
selects motors for warranted power factor and efficiency at rated load. 
This still leaves the designer with overload considerations, typically
because of multiple starts, reversals, or overloads placed on the machinery
by customers because of ignorance or intentionally pushing the machinery to
its limits.  So the designer selects a service factor to accommodate
service beyond equipment ratings.  This isn't so stupid as it may seem,
because increasing rated horsepower may be more expensive than buying a
larger service factor.  It's an economic decision, mostly, although not for
starting or reversing service.  Starting and reversing currents tend to be
related to motor rated horsepower, so that increasing horsepower may not
compensate for lots of starts, hard starts, or reverses.

Why don't you see service factors on compressor or special duty motors? 
The reason is that service factor assumes a standard duty profile, which
neither of these duty ratings has.  Moreover, compressor rated motors
typically exceed continuous duty current rating over some or all of the
pumpup cycle.  Consequently, the profile rating already includes whatever
overload capability the manufacturer built into the motor, and service
factor is irrelevant.

Knowing the definition of a specification is good, but knowing the why of
it helps you to apply it more precisely.