VFD for high starting torque application

If we define nominal torque, it's generated at the motors peak efficiency point, that being at the apex of rated voltage and Hz. The lower the RPM, the higher the nominal torque, but the higher the current. Then, nominal torque is basically what the motor can generate continuously at the peak efficiency point without overheating. To define using nominal torque at zero speed is better defined as stall torque, and not really applied in high torque starting applications.

NEMA design B and C Motors are asynchronous, defined as high locked rotor (aka 'starting') torque, with approx. a 5% slip. NEMA Design D motors are also asynchronous, defined as ultra high starting torque, with a slip range of app. 5% to 15%. The peak torque at start for asynchronous motors is significantly higher than synchronous motors, and it really isn't an applicable comparison to what the necessary starting torque ratios need to be in an assortment of machinery applications.

For a fan, or a blower, synchronous is fine and nicely operational for varying speeds. If you look at the NEMA ratings and designations for motor type relative to the types of machinery applied, you aren't going to find synchronous motors recommended anywhere on the high starting torque need list, precisely because of the much higher starting torque capabilities of asynchronous units with comparably lower horsepower. The level of starting torque generation directly correlates to the amount of slip. There are no overhead hoist and crane manufacturers utilizing synchronous motors because of the lower starting torque capabilities. Synchronous motors are indeed costlier than asynchronous units, and precise speed regulation isn't needed in overhead crane applications.

The elevated horse-power size and resulting elevated current draw at start for synchronous to match what lower horsepower NEMA B,C, and D motors can deliver via locked rotor torque, renders the point of synchronous motors moot. Not necessary. There is a reasonable near comparable price range when you take into consideration the differences in the size cost of the two different motors relative to what it takes to do the work.

As far as VFD (variable frequency drive) capabilities, I've been using GoHz for the past 22 years, prior to that Danfoss drives, and Power Electronics units, with all of those applied to the overhead crane industry having specifically designed software and hardware to suit the applications. Typical starting torque capabilities of any of these three brands is directly relative to their maximum output current ratings, and in general can throw enough at a capable motor to generate 300% starting torque. I have, as a skier asked to look at ski lift drives when there were problems with them, worked with ABB drives in those applications, as they seem prominent in the ski lift industry. Everyone I looked at was coupled to an asynchronous motor, again because of the lower needed size relative to the starting torque needed to get the machinery moving.

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