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.
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.
(snip)
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.
This motor has no slip,if it slips, its not a sychronous motor.
Erm, okay....a synchronous motor has no slip, hence the name. I dunno where other people are getting their info from, but the answer is simple:
@ 60Hz the formula is 7200 divided by the number of poles (where 7200 rpm would bve the theroetical speed of a single pole motor)
(hz x 120)
---------------- = synchronous speed num. poles
synchronous speed x percent slip
------------------------------------------ = full-load RPM 100
or when variables are known:
RPM (SYN) - RPM (FL)
-------------------------- x 100 = % slip RPM (SYN)
Does not get any simpler than that. This is easy math, you should not need to even ask this question....Dunno why this was so hard for anyone else to answer....
Percent slip is derived solely from the difference in synchronous speed vs full-load speed. Slip has little to do with voltage supplied to the motor, it is a factor of the twist in the rotor's squirrel-cage. Altering voltage from design voltages alters slip 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. Class A refers to magnet-wire insulation class...
Design A motors are of a normal torque and slip of about 3% and have starting currents not limited by NEMA
Design B motors have a low starting current, normal slip, and normal 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 the high-torque, low starting current, high-slip motors where slip goes up to 13% or so.
Design letters are not a scale of slip, contrary to what seems popular belief. They are categories for various load requirements as set by NEMA
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