# Good article on brushless DC?



## McRat (Jul 10, 2012)

Seems my reading is giving conflicts.

Is it just an A/C 3ph motor? Or is there a difference? Wiki says it has an inverter built in, and the stator has windings. Not the one I have in my hand, which like many things today, comes with no instructions or a way to find out who actually makes it.

What would happen if I plugged it into 3ph 208vac through a step down transformer array?

I'm just curious as to what they really are. Mine has a 3 thick wires on it marked W-X-Y? Or something like that. I assume switching any two wires will reverse the motor?

Sidebar - This motor is rated at 100amps, but that wire doesn't look any bigger than 10 gauge. WTF? Is that a pyrotechnic device, or is there something I'm overloooking?


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## major (Apr 4, 2008)

McRat said:


> Is it just an A/C 3ph motor? Or is there a difference?


Hi Rat,

In my book, BLDC is an AC motor. It is a synchronous machine whereas induction motors are asynchronous meaning the rotation is at a frequency slower than the applied electric frequency. AC motors can be single or multi phase not just 3 phase although that is most common for larger machines (more than a few hp). 

The induction motor induces current into the rotor and establishes a rotor field from that. A synchronous machine has the rotor field from either windings (like the alternator) or from permanent magnets (like the BLDC). One characteristic of synchronous motors is they have no starting torque so if you hooked the motor you have to a 3 phase line it may just sit there and buzz. The BLDCs you see for EV propulsion have hall sensors and custom variable frequency inverters to overcome this problem and with that often boast high starting torque.

#10 wire will handle 100A for a short time 

major


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## McRat (Jul 10, 2012)

Thanks! The only one I have is 2000w @ 22vdc and after looking at it's construction, it's not going to do much to propel anything that weighs over 20 lbs. But the fact you can have a 3HP motor that is the size of a pack of cigarettes is amazing.


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## PStechPaul (May 1, 2012)

Much of this depends on how motors are rated. Industrial 3 phase motors are very conservatively rated and take into account ambient temperature and normal overloads and will run at their continuous ratings for months and even years with little attention and a high degree of reliability. Size and weight are of little cocern for something that is bolted to a factory floor and just expected to deliver it rated torque, speed, and power all day long and just occasionally perhaps need some minor maintenance like cleaning and lubrication and sometime snew bearings.

Vehicular traction motors, especially for non-rail applications, have much greater premium on size and weight, as well as efficiency, and that can be difficult to achieve. It seems like the motor of choice has become the BLDC, and there are many such motors with amazing specs, but often I think these are greatly exaggerated and you must look carefully at the duty cycle and the temperature rise and special requirements such as liquid cooling. The controllers are often rather exotic and expensive, and the motors are costly and fragile because of the rare earth magnets.

A properly designed AC induction motor can fairly easily achieve a 2x-6x boost of power by using high quality laminations and good controllers so they will come close to matching the specs of BLDCs. And my research and experimentation with switched reluctance motors show that they may well be even better than BLDCs in the long run. 

The electric motor industry seems to stay in certain "comfort levels" of what has been well proven in the past, and true innovation is considered risky and is often promoted by small companies who really don't have all the proper testing equipment and data, and are anxious to "jump on the bandwagon" with their "new" designs which they promote in the hopes of quick sales and immediate success.


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## McRat (Jul 10, 2012)

Yup. Especially with 3-ph ac motors, the MTBF is stunningly high. Way in excess of what is needed for EV's. I had a 7.5hp 208 motor running for 17 years with a monthly runtime average of 120hours/m at full load. Often at temps high enough to carmelize oil.

It was not retired due to failure, it was retired to upgrade. Even for urban commutes, that would be about 750,000 miles at a 30mph average.

But they are built like hammers. Not expected to ever fail.

In comparison, a residential 6.5HP 240v motor croaked twice in 3 years with under 30 hours/m. They look similar but one weighs twice what the other weighs.


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