Motor Theory Education

To piggyback on many of the above comments, I would say your students would need to be aware of what TYPE of mechanical applications would benefit from using an AC vs DC motor, servo, stepper, VFD controlled, etc.
 
To piggyback off Dogleg43, who is piggybacking on many of the above comments, it would be even better to take your students right into the industry for a day if possible and show them real world applications as well.

This comment is not available for piggybacking.
 
[Not piggybacking, but going on a tangent, from @alive15]


Could a sampling of the many VFD- and motor-related threads on this forum be worthwhile? Many of the concepts taught might become more solidly understood in the students' minds when applied to working through real problems.


Another worthwhile activity might be to build a simple motor: start with the basic parts (wire, metal); make the windings; examine electromagnetism in both directions (current in windings => magnetic field; moving magnetic field => current in windings); feel the magnetic force; put it together and watch it spin. Some students may have trouble understanding "magic" (a la Arthur C. Clarke), but feeling a force change when the current is turned on may help it click. There might not be time to build every kind of motor, but they all use the same fundamentals.
 
tim_callinan said:
Im looking for some opinions and perspectives - I'm a mechatronic instructor at a community college. I'm not a motor expert but I'm starting to learn more about it.

Can I ask the group a question - is power factor, corrective capacitors to minimize reactive power, apparent power - is this stuff critical for a mechatronic technician? Its kind of blowing the heads of my students unfortunately.

Also - do they need to understand how every single type of AC motor works in detail?
I could be very wrong but it seems like capacitor start induction motor and 3 phase motors are the main ones out there?
Thank you for any advice. Hope everyone is keeping safe....
Tim
Click to expand...

"SLIP" would be something that deserves at least a mention, somewhere.
 
jraef said:
The utility must supply kVA to make sure everyone gets enough. But users are billed for kWh, and the difference between kVA and kW is power factor (W = V X A x PF). So the lower your PF is, the less KWh you pay for. But the UTILITY still has to deliver you the full kVA, so if your PF is low, they penalize you with a multiplier on your power bill. You can correct PF at the loads by adding PFC capacitors. But also understand that VFDs also correct the PF as seen by the distribution system, so PF of the motor becomes irrelevant if it is being run by a VFD.
Click to expand...


How will they know you are using PF correction ?
 
Power factor is like the weather. Everybody talks about it but nobody does anything about it.

That isn't quite true, of course. Power factor correction capacitors are part of some engineers' standard design on constant speed motors. And one advantage of VFDs is the high power factor.

If you PM me with your email I can send you the Powerpoint I use for the section on motors for the controls course I teach at UW Madison.
 
Originally posted by daba:

How will they know you are using PF correction ?
Click to expand...

That's the beauty of it. They don't care. You are paying for the effects of the actual displacement power factor, not whether or not you are trying to do something about it. Whether the money you spent to TRY and better your power factor is actually bearing fruit is immaterial as far as the power company is concerned.

On a tangentially related note, I once had a customer who specified active front end drive systems because his power company was beating him up over harmonic power factor. While AC drives will generally not create an issue with displacement power factor they can be a beast in terms of harmonic power factor. An AFE will usually do pretty well in terms of harmonic power factor, maintaining the fidelity of the AC current waveform entering the system. I'm still not 100% convinced these things paid for themselves but the customer said they did.

Keith
 
Last edited:
daba said:
How will they know you are using PF correction ?
Click to expand...

By "they" I assume you mean the utility?

They don't know, or care, how you correct the PF, they measure the VARs you require and use that to penalize you. If you use PFC capacitors to provide the VARs, or your own Synchronous Condenser, or a Static VAR Compensator, it all has the same effect; reducing the VARs that the utility supplies, hence the penalty they would assess based on that is reduced or eliminated.
 
kamenges said:
That's the beauty of it. They don't care. You are paying for the effects of the actual displacement power factor, not whether or not you are trying to do something about it. Whether the money you spent to TRY and better your power factor is actually bearing fruit is immaterial as far as the power company is concerned.

On a tangentially related note, I once had a customer who specified active front end drive systems because his power company was beating him up over harmonic power factor. While AC drives will generally not create an issue with displacement power factor they can be a beast in terms of harmonic power factor. An AFE will usually do pretty well in terms of harmonic power factor, maintaining the fidelity of the AC current waveform entering the system. I'm still not 100% convinced these things paid for themselves but the customer said they did.

Keith
Click to expand...
Saw your response too late...

Several of the AFE drive suppliers are now allowing you to set up, in programming, the ability of allowing the VFD to EXPORT VARs to help compensate for other inductive loads. The general rule-of-thumb is that you can export about 10% of the VFD's capacity as VARs, although that depends on a lot of what-ifs.

The same is true of some of the Active Harmonic Filters on the market now as well.
 
Just think of the Power Factor like the Vacuum on an old car engine... Depending on the engine load, the vacuum drops or increases, same (but opposite) for Power Factor.

Remember the distributor vacuum advance? This was timing compensation for the phase relationship of spark to fuel on an engine when under loaded :) Current and voltage have similar relationships in an AC motor.

It's like the efficiency curve, motors are mostly designed to run at full load, and when they are partially loaded the efficiency/power factor fall off... That should be enough for them to get a basic understanding.
 

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