Electric Motor Choice Help Please!

[fadingfastsd]

Hey guys, I'm new to the forum here, although I've browsed through it many times. A quick intro and then I'll get down to my question.

I'm a 28 year old Electrical Engineer by education (San Diego State University). I specialize in motorcycle electronic components, specifically charging system parts (stator/flywheel/CDI ignition/coils). I also do quite a bit of wiring harness design and other general automotive electrical consulting.

So, on to my question.
I am building some custom test equipment to bench test and profile power output of motorcycle stators, as well as stress test motorcycle voltage regulator/rectifier units.

The equipment is already assembled, however I am having trouble finding a suitable power plant. Motorcycle alternator systems consist of a 4-16 pole stator unit (depending on bike size and ignition configuration) & a permanent magnet flywheel. I need to drive the flywheel with accurate RPM control, ideally from 500 to 7000 RPM. If I use an AC motor, I know I need a 3 phase unit to give me adjustable RPM. My original design used a 1 horsepower 3 phase AC motor, driving roughly an 6" diameter sheave, with a v-belt connecting the driven shaft with roughly a 2" belt pitch sheave. The other end of this shaft held the flywheel. I used an Automation Direct GS2 series motor drive, which worked very well.

The motor controller (drive) took single phase 110VAC input, and provided 220VAC 3 phase output to drive the motor. Just spinning the flywheel on it's own, this worked very well, and provided accurate RPM control. However, to stress test these stators, I need to put a serious load on them to generate heat (usual failure mode - heat buildup in stator windings, causing short to ground or another winding at a spot of poor insulation). I do this by running through a large load box I have constructed. The load box contains a 3-phase input rectifier, followed by 2 large capacitors (to take the place of charging a battery on motorcycle). The stable DC output from the caps is then switched through various load resistors to allow up to 500Watts of power dissipation (calculated assuming 13VDC).

The issue I am having, is that a 1 horsepower motor cannot keep the flywheel spinning without bogging down severely as I put any sort of electrical load on the stator. Even with no electrical load, and the stator inside of the flywheel, the increased resistance of the magnetic field will limit RPM, and any load switched on and the motor bogs severely.
I would like to be able to generate a peak of about 35amps (@ 13VDC = 455Watts) from the stator, to really well stress the largest models I will need to test.

I believe I now need about a 6 horsepower motor to be able to drive the flywheel with a fully loaded stator engaged. My issue is...is this possible with the limitations I have available?
I have 110VAC & 220VAC Single Phase service available where I run the test equipment. From my research so far, it looks like most motor controllers with single phase input cap out at a 3 horsepower motor drive capability.

Are DC motors an option? To get down to the short of it, is it possible for me to drive a 6 horsepower electric motor with single phase 110V/220V service. I suppose I could swap out the breaker for this particular circuit for higher amperage (currently 15amp breakers).

Also, cost is somewhat of an issue. I could spend up to another $1000 if I absolutely have to for a new motor/drive setup.

Thanks so much for your help and input!

http://www.redlineelectrics.com

Motorcycle Electronics Consulting

 

[waross]

Your motor must provide enough power to supply not only the 455 Watt load bank but also the losses in the stator. Still, if your motor is bogging to the point that you feel that you need 6 HP to supply 455 Watts plus losses, I suspect that you are using the wrong drive ratio. For maximum power don't run your motor above rated speed and use belts to increase the speed.
I will be surprised if you need more than 1.5 or 2 HP with the proper ratio drive.
Reality check: When the motor starts to bog what power output does the VFD indicate? Probably a lot less than 1 HP.
You may find that actual HP over rated HP approximates rated speed over actual speed for speeds above rated speed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[fadingfastsd]

Waross, thanks for the quick reply.
I may be wrong about 6hp being necessary to do this. Honestly at this point I kind of forget how I came up with that number.
I was doing some more research tonight and did find a motor controller capable of driving up to a 7.5hp motor from 220VAC single phase input. Although that requires up to 25Amps, which will be pushing it I think with my existing wiring.

Since most 3 phase AC motors spin at a ~1725 RPM (off the top of my head, I think thats close)I was using a belt drive ratio to get about *3 multiplication at the driven end (about 6000 RPM which is sufficient).

I was not running the motor over rated speed when I see the bog. Just spinning the flywheel alone works fine. Engaging the stator inside the flywheel and I would no longer be able to reach maximum RPM on the motor. Engage even a 10Watt resistive load on the stator, and the motor would begin to bog at any drive speed over 600RPM indicated on the controller unit.

How exactly can I calculate necessary horsepower to size the motor given my application needs? I need to be spinning up to an 8-9" diameter flywheel, and needing to source up to 35Amps from the stator. Although the 35 amps is calculated DC after rectifier and cap's on the load box.

Thanks again!

http://www.redlineelectrics.com

Motorcycle Electronics Consulting

 

[waross]

Recheck your motor connections and the VFD setup parameters.
You may have the motor wired for 460 Volts instead of 230 volts.
A wrongly configured VFD may also show this symptom.
I just reread your initial post and came across this
"the motor controller (drive) took single phase 110VAC input, and provided 220VAC 3 phase output to drive the motor."
Check the specs. I'm almost sure that the drive takes a 240 Volt input. Drives don't normally increase the voltage. 120 Volts to a 240 Volt drive would account for your problems.
Actually your voltage is probably 120 Volts rather than 110 Volts, or it should be.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[lectricwes]

What's the size of your drive? If you are using a drive to convert single phase to three phase, you will need to oversize your drive.

 

[jraef]

Issue 1) Up to about 1HP, there are indeed small VFDs that will take 120VAC 1 phase and put out 240V 3 phase, they have a voltage doubler circuit ahead of the input. You typically do NOT need to de-rate those, the rating is based on the current at 120V 1 phase since that is what they do. But beyond 1HP it becomes difficult to deal with the high current on the voltage doubler. I know people who have "rolled their own" successfully, I would not recommend it for a novice though.

Issue 2) using a VFD to increase speed above the motor base speed is fraught with issues. The chief one is, you cannot do it indefinitely. For the torque to remain constant, you must maintain the V/Hz ratio constant. But VFD is capable of putting out only as much voltage as is fed in (the above notwithstanding). So if you have a 240V VFD, it can only put out 240V max. A motor designed for 240V 60Hz has a V/Hz ratio of 4:1 and if you turn the speed up to 120Hz, the voltage is still 240V, so the V/Hz ratio drops to 2:1. Effectively that means your torque drops proportionately and worse, peak torque capability will drop by the square of the delta, so you will now have 1/2 normal torque and 1/4 the peak torque. If you are taking a 4 pole motor and trying to drive it at 7000RPM, you must be at roughly 400Hz and you have very little torque by the time it gets there. If you are trying to do that with a 16 pole motor, I'd be surprised if it creates enough torque to spin its own rotor.

Issue 3) Any VFD is theoretically capable of 1 phase input if over sized, but there are pitfalls as some brands have phase loss protection that cannot be turned off in programming, making them non-candidates. But plenty are capable of this. you need to double the size of the VFD of the motor being used in order to make sure the rectifier and bus capacitance is suitable for the task. So if you need 5HP, buy a 10HP VFD.



"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[ScottyUK]

2) cont.

You can get away with reconnecting a star-connected motor in delta to get up to 1.73x base frequency while maintaining rated torque, if the motor is star-connected at base frequency.

3) cont.

When calculating bus capacitance consider that with a three-phase input the unsmoothed bus voltage drops below 90% twice a cycle for a period of roughly 10°, and never drops below 86%. By comparision an unsmoothed single phase input drops below 90% twice a cycle for a period of roughly 125°, and drops to zero twice per cycle. The DC bus capacitor on a 3-phase drive is primarily there to provide a source of HF ripple current to the output stage of the drive; the capacitor on a single-phase drive additonally has to provide bulk energy storage for the periods when the rectified voltage is too low to be useful. If the on-board rectifier is capable of handling the high current peaks then it's normally possible to add additional capacitance to a three-phase drive used in single-phase-input applications. It is normally possible to use an external rectifier too.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

Hi Scotty
An IEC/NEMA issue. The OP mentioned the University of San Diego so he is probably in California. While IEC motor connections are often switched between star and delta, this conversion is seldom an option with a NEMA 9 lead motor. If the motor is wye or star connected in NEMA land there will be a buried wye point that is not accessible for reconnection in delta. Likewise a delta connected NEMA motor will typically have three buried delta corner connections that are not accessible for reconnection in star.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Thanks Bill. That winding configuration isn't that common over here but what you say makes sense.


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If we learn from our mistakes I'm getting a great education!