Peak torque of Cap Start vs Cap Start/Cap Run single phase induction motors

[John_Vreede]

My query is around the relative torque output of Capacitor Start (CS) and Cap Start/Cap Run (CSCR) induction motors, especially when they are loaded up to the point where they will stall.

My investigation to date turns up the graph at the top of the attachment to illustrate torque vs motor rpm for various types of induction motors (everyone who uses this graph seems to have copied&pasted from each other and I don’t know where the original came from)
However my query is that I think that the shape of the CSCR graph is wrong, or at least doesn’t match my observation of the one I have (see 'Background' in the attached document) nor the many other published speed/torque curves.

Will some knowledgeable electrical eng. person pls answer:
• Which single phase induction motor type has the greater peak (breakdown) torque, CS or CSCR, if they were both the same rated HP?
and
• Which type stalls at lower rpm?

This relates to which motor type is better to run a bandsaw (high friction load) that regularly is running near its breakdown torque (again see attachment for more info)

Thanks - jv

 

[waross]

The published generic curves are valid for the model of motor that was tested.
The characteristics depend quite a bit on the design of the rotor, and on the value of the capacitor.
Integral HP, three phase, industrial motors are available as designs A, B, C, and D, all with different speed/torque characteristics.

While rotors may look identical they may be very different internally.
There is a hidden "Squirrel cage" winding cast into the rotor.
The shape of this winding is a major factor in determining the motor characteristics.
Some factors affecting motor performance.
The resistance of the squirrel cage winding.
The depth of the of the squirrel cage winding below the surface of the rotor.
Some designs may use a double squirrel cage winding, One winding near the surface for good starting torque and a deeper winding for better running characteristics.
Similar performance to a double squirrel cage winding may be achieved with figure 8 shaped squirrel cage bars.
Your observed torque values,if done accurately, may be more accurate than a generic curve.
Don't expect all single phase motors to have identical torque curves unless they are the same make and model.
It is what it is and you go what you got.
And continually running into the overload region will shorten the motor life.
There is a correlation between insulation temperature and insulation expected life.
One cut with the motor running into overload is not good.
Ten similar cuts in succession without an intervening cool-down time may lead to motor failure.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dArsonval]

This thread interests me not so much for the subject, but for the issue related to discrepancies between plotted graphs found on the Internet.

The graph I have is similar to Bill's and based on NEMA Standards MG 10, Table 1.
It was re-drawn by someone sitting at a computer, somewhere.
It has basically the same data as Bill's... but it's not denoting visually the same exact "thing".
Both graphs show different curve paths. They are close, but not identical.

I also agree that if one is measuring things accurately, the OP's observed torque values are likely more authentic than any generic representations found on the Internet.

Here are two more graphs lifted from Cyril G. Veinott's 1939 book. I have his 1948 second edition also, but the graphs are the same.
In his text, he notes that the capacitor-start motor has substantially the same torques as the two value motor, and that they are practically identical.

I would also add as Bill somewhat eludes... one could take two identical brand new motors out-of-a-box, test them... and get
different results.

The Op is on the right path in doing one's own homework on the subject.

John

 

[waross]

I would expect that two identical motors would test similar, but if the two motors were the same rating but from different manufacturers, all bets are off.
And please note that the Design letters and characteristics apply to three phase industrial motors, not single phase motors.
BTW My graph was lifted from the Cowern Papers.
Thanks for the single phase graphs.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[John_Vreede]

Thanks guys, both posts have something new for me.
I had thought there would be some archetypal differences between the two, but the two graphs and Veinott's description would say not. Yet the fact that the 2 types, by design, reach rated power at different revs (Cap start's at 95% of synchronous and Cap Start/Cap run's at 90-92%) says the curves should be different.
Got to accept that the differences may not be significant, esp if power delivery from 2 similar motors form different manfr's will probably be different.
Guess I'll find some CS motors and stall them.
Thanks again guys - jv

 

[waross]

One of the design choices between a CS motor and a CSCR motor is a balance between copper cost and capacitor cost. A capacitor start motor may have more copper.
Capacitor Start:
Once the motor is up to speed, the start winding is idle and does not produce any torque. All of the torque must be developed by the run winding.
Capacitor Start Capacitor Run:
Both windings contribute to the torque and the designer may choose to use less copper in the run winding.
The failure of the run capacitor may go undetected and result in motor overheating and possible burnout.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!