motor calcs!!!

[Part P]

Hi, I wonder if anyone might know the formula to derive the Rotor speed of a
synchronous motor - having only the: Percentage slip, Amount of poles and
the c.p.m (hertz) of the supply to work with? Perhaps you might point me to
a site that carries this type of info???

( Exams later this week you see )!

Thanks in advance.

 

[John Popelish]

Hi, I wonder if anyone might know the formula to derive the Rotor speed of a
synchronous motor - having only the: Percentage slip, Amount of poles and
the c.p.m (hertz) of the supply to work with? Perhaps you might point me to
a site that carries this type of info???

( Exams later this week you see )!

Thanks in advance.

The words, "synchronous" and "slip" do not get along well, together.

The torque of a synchronous motor is proportional to the phase lag of
the rotor with respect to the stator magnetic phasor. Once you load
the rotor enough to get into a continuous slip situation (lose
synchronization), the torque becomes cyclic as the phase relationship
between stator and rotor continue to shift.

This sounds like a trick question to see if you understand the
difference between synchronous and induction motors.

 

[Part P]

The words, "synchronous" and "slip" do not get along well, together.

The torque of a synchronous motor is proportional to the phase lag of the
rotor with respect to the stator magnetic phasor. Once you load the rotor
enough to get into a continuous slip situation (lose synchronization), the
torque becomes cyclic as the phase relationship between stator and rotor
continue to shift.

This sounds like a trick question to see if you understand the difference
between synchronous and induction motors.

John.

Hmm... so the rotor speed of an "induction" motor can be calculated from the
aforementioned variables then?

I just re- checked my notes and it does say "Synchronous" motor!! Like you
say, it's maybe a mistake/trick.

What is the appropriate equation for an induction motor please?

Thanks for your help. N.

 

[John Popelish]

Hmm... so the rotor speed of an "induction" motor can be calculated from the
aforementioned variables then?

No, but you might estimate it. There is a typical slip at full rated
load for a class of induction motors, with class A having the lowest
slip. But to calculate anything about torque, you not only need to
know the frequency and number of poles, you need to know the power
supplied to the motor and the full rated power. Voltage and frequency
are not enough, since these would give the same answer for a 1/2 HP
motor and a 3000 HP motor. Slip is a fairly linear function of
torque, but you need to know what torque produces the full rated slip
at what full rated torque. Did you attend classes on AC motors?

I just re- checked my notes and it does say "Synchronous" motor!! Like you
say, it's maybe a mistake/trick.

What is the appropriate equation for an induction motor please?

Zero torque at zero slip. Full rated slip at full rated torque. The
slope between these two points depends on the class of the motor, with
higher letters producing more slip at full rated torque.

 

[Part P]

No, but you might estimate it. There is a typical slip at full rated load
for a class of induction motors, with class A having the lowest slip. But
to calculate anything about torque, you not only need to know the
frequency and number of poles, you need to know the power supplied to the
motor and the full rated power. Voltage and frequency are not enough,
since these would give the same answer for a 1/2 HP motor and a 3000 HP
motor. Slip is a fairly linear function of torque, but you need to know
what torque produces the full rated slip at what full rated torque. Did
you attend classes on AC motors?


Zero torque at zero slip. Full rated slip at full rated torque. The
slope between these two points depends on the class of the motor, with
higher letters producing more slip at full rated torque.

Much obliged John.

 

[Sjouke Burry]

Hi, I wonder if anyone might know the formula to derive the Rotor speed of a
synchronous motor - having only the: Percentage slip, Amount of poles and
the c.p.m (hertz) of the supply to work with? Perhaps you might point me to
a site that carries this type of info???

( Exams later this week you see )!

Thanks in advance.

This motor has no slip,if it slips,
its not a sychronous motor.

 

[Electromotive Guru]

Erm, okay....a synchronous motor has no slip, hence the name. I dunn
where other people are getting their info from, but the answer i
simple

@ 60Hz the formula is 7200 divided by the number of poles (where 720
rpm would bve the theroetical speed of a single pole motor

(hz x 120
---------------- = synchronous spee
num. pole

synchronous speed x percent sli
------------------------------------------ = full-load RP
10

or when variables are known

RPM (SYN) - RPM (FL
-------------------------- x 100 = % sli
RPM (SYN

Does not get any simpler than that. This is easy math, you should no
need to even ask this question....Dunno why this was so hard fo
anyone else to answer...

Percent slip is derived solely from the difference in synchronou
speed vs full-load speed. Slip has little to do with voltage supplie
to the motor, it is a factor of the twist in the rotor'
squirrel-cage. Altering voltage from design voltages alters sli
slightly, but base slip is determined by measuring rotor skew
Frequency has absolutely no effect on slip

Class A motor? I think you might be mistaking your terminology. Clas
A refers to magnet-wire insulation class..

Design A motors are of a normal torque and slip of about 3% and hav
starting currents not limited by NEM

Design B motors have a low starting current, normal slip, and norma
torque. The typical general-purpose motor

Design C has high starting-torque, low starting current, and low slip
This design is usually for sticky loads that are hard to start

Slip does not change much until you get to design D, which are th
high-torque, low starting current, high-slip motors where slip goe
up to 13% or so

Design letters are not a scale of slip, contrary to what seems popula
belief. They are categories for various load requirements as set b
NEM