VFD with Synchronous motors 1

[NickParker]

When do you use VFD with synchronous motors as against VFD with induction motors?

 

[FacEngrPE]

VFD + Synchronous motor is the same as a VFD + Permanent Magnet AC Motor. Some additional energy consumption from the electricity used to energize the field, likely lower first cost.

For most use cases the selection of motor type is economic first cost vs operating cost.

 

[davidbeach]

A conventional rule of thumb for non-VFD applications is that synchronous motors should be considered when the motor rating in HP exceeds the motor speed in RPM. I don't know much the addition of the VFD impacts that, but there are undoubted a variety of factors. I would expect that expected/necessary rate of change of speed would matter considerably. An induction motor can slip and the amount of slip tells the drive what's going on with the motor. A synchronous motor won't slip at all until that's all it's doing.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

Synchronous motors may be selected for their ability to correct displacement power factor.
A VFD would negate that benefit.

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

 

[iop95]

Synchronous motors on VFD are needed mainly in precise positioning systems. Most of them are in closed loop.
Highly dynamic system requirments (so very low rotor inertia) may be another point to choose special design synchronous motors.
Such motors on VFDs are in fact brushless PM AC motors or switched reluctance motors.
Such motors have better efficiency compared with asynchronous motors (at least rotor losses are much lower) but at much higer motor price...

 

[jpts]

Synchronous machines are usually preferred in high-power, low-speed applications such as big conveyor motors, shaft generators (marine application) and so on. This is because induction machines don't work very well electromagnetically when they have to be designed for low-speeds. High pole-count means very low power factor and low efficiency due to big slip (excessive rotor loss). With synchronous machines you don't have these issues

 

[Gr8blu]

Choose the correct tool for the job. The basic decision boils down to cost (to manufacture) vs torque. Up to roughly 150 000 lb-ft of torque, a squirrel cage (or wound rotor) induction is more cost effective because the rotor lamination remains a single sheet. Once the rotor diameter is large enough to require segmental construction, the cost of the machine increases noticeably and quickly approaches (and surpasses) the synchronous cost. The next step is around 300 000 lb-ft of torque, where the change should be to DC.

VFD operation is for processes where there as multiple speed points. Or where the limit is on absolute starting current (amplitude). Large inertia rotors (or processes) are more likely to need drives to limit starting current, so the other system problems don't arise.

Last thing - a synchronous motor will require an excitation system that can operate from zero speed if the stator is operating on a drive. What this really means is either a brush-type synchronous machine, or one with an AC-AC brushless exciter. If the exciter is not capable of producing current at very low speed (frequency) then it becomes much harder to limit the line current to the stator and be able to accelerate the load.

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[electricpete]

Up to roughly 150 000 lb-ft of torque, a squirrel cage (or wound rotor) induction is more cost effective because the rotor lamination remains a single sheet. Once the rotor diameter is large enough to require segmental construction, the cost of the machine increases noticeably and quickly approaches (and surpasses) the synchronous cost.

Interesting - I never knew that the cutoff was related to segmented laminations, but it makes perfect sense. I know large motors tend to need insulated bearings (even on fixed frequency supplies) when they are large enough to require segmented stator laminations (the segmented lamination core is more vulnerable to magnetic asymmetries which can cause circulating currents). And those segmented lams take a heckuva lot more labor to assemble the core (as I learned only after experiencing sticker shock the first time we had to restack one of those due to core condition). And I never thought about it before, but of course the HP / RPM thumbrule is predicting torque and physical size.