Motors 1

[Chocolatecat]

on a graph of torque vs synchronous speed. Does 100% synchronous speed mean that the synchronous speed is equal to the motor speed. If not, how do you find the synchronous given that the poles and rotor speed is unknown

 

[LittleInch]

Well you need to use SOME data. Synchronous speed is determined by electrical frequency and number of poles. You can't find speed without those two things.

what graph? Post an example

 

[waross]

Motor rated speed for Squirrel Cage Induction Motors is generally 40 RPM to 60 RPM lower that synchronous speed. There are exceptions.

Does 100% synchronous speed mean that the synchronous speed is equal to the motor speed.

More accurate to say that when the motor speed is 100% of the synchronous speed then the motor speed is equal to the synchronous speed.
Motor speed is variable, synchronous speed is a value and is not variable.

 

[dvd]

This is a nice resource - Cowern Papers

 

[Gr8blu]

Chocolatecat:
Synchronous speed is found using the following relationship (for AC machines ONLY):
(synch speed in RPM) = 120 x (input frequency of current waveform, in hertz) / (number of poles)

Asynchronous machines - which means both squirrel cage and wound rotor induction designs - tend to operate a bit below synchronous speed when motoring, and a bit above when generating.
Synchronous machines operate right at synchronous speed.
Direct current machines operate wherever the heck they want (speed does not correspond to frequency AT ALL since DC has no frequency). Nor does it relate to number of poles, at least directly.

The speed at which an induction machine operates is called the "slip speed". The frequency that corresponds to that is called "slip frequency". To determine the slip frequency, plug the known variables (speed in rpm, number of poles, and 120) into the formula above and solve. "Per unit slip" is found using the following:
per unit slip = [absolute value of (synchronous speed - slip speed)] / (synchronous speed) ... with all speeds in the same units (usually rpm)

 

[waross]

The speed at which an induction machine operates is called the "slip speed".

Slip speed in my world is synchronous speed minus actual speed.
Typically 40 RPM to 60 RPM.
Slip frequency is the frequency that is seen by the rotor and is determined by the slip speed.
Typically 1.3 Hz to 2 Hz.
My version of slip speed is often used to apply speed torque curves to VFD driven motors to predict characteristics at different applied frequencies.
A 100% loaded motor will run at applied frequency/speed minus slip speed, from very low speeds up to rated speed.
That is, for a 60 Hz, 1760 RPM rated motor (40 RPM slip) if a VFD outputs 30 Hz, the motor will run at 900 RPM - 40 RPM or 860 RPM at full load/current..
And for a 60 Hz, 1760 RPM rated motor (40 RPM slip) if a VFD outputs 15 Hz, the motor will run at 450 RPM - 40 RPM or 410 RPM at full load/current..

 

[dvd]

Here is my take on that as an ME -

 

[waross]

Rotor speed equation at full-load.
Rotor Speed = synchronous speed minus slip speed.
The important value is the frequency seen by the rotor or the slip frequency.
Many of us refer to a speed corresponding to the slip frequency as the slip speed.
eg: 60 Hz-4 pole, rated speed 1760 RPM:- 1800 RPM minus 1760 RPM equals 40 RPM slip.
Synchronous speed minus actual speed = slip speed.
Some textbook agree with us.