Why not using a Synchronous Machine as a Dynamometer? 2

[Shamooooot]

Hello guys

I am planning to make an AC dynamometer for research purposes to test PSA 4-cylinder diesel engines with the following specifications:
- Power max.: 60KW.
- Torque max.: 220 N.m.
- Speed max.: 4500rpm speed.
* Can't use the grid.
* Going to use dynamic torque sensor to log power, speed, and torque.

I thought of the induction motor option, but quickly changed my mind after seeing the added cost of the VFD with regenerative braking and the braking resistor.

I am now thinking of using a synchronous machine, which could indeed save me a lot of troubles, especially that I need less than 100VDC and couple of amps to control it and it would only need a load bank.. but after reading in the forum and in other places it doesn't seem to be a popular option in dyno applications. I wonder why is that and if I am missing something in such setup.

It seems to be hard to find synchronous motors that have such high speeds, and motors at that power and torque seems to be too expensive, I believe a 3600rpm rated motor can survive 4000-4500rpm. And as this won't be used as an actual generator or tied to the grid and the power would be dissipated to the load bank the job doesn't look complicated. Especially, that these motors are fairly common and could be found at a good price.

I understand that an eddy current may be the best and safest option for the job, but I am required to use an "AC dynamometer".

Thanks in advance..

 

[LionelHutz]

VFD and motor doesn't need a gearbox. That page has so little context I don't know why it's written that way.

 

[Shamooooot]

VFD and motor doesn't need a gearbox. That page has so little context I don't know why it's written that way.

So I need a bigger VFD than the motor to overvolt the motor to get enough torque.

A motor that can handle 55KW at 4000 rpm and can handle 29kW at 1500rpm.

For example a 380V, 50Hz, 1500rpm, 30kW, Y connected, and rewire it as delta for half the nominal voltage which doubles the current flowing, but this might be tough to find.

A 100hp VFD to drive a 50hp motor re-wired for half the nominal volts of the motor, which doubles the current.

But running this motor at 4000rpm, V/F = 3.8:

• Frequency = (4000 × 4) / 120
• Frequency = 133.33 Hz

• Required voltage = 133.33 Hz × 3.8
• Required voltage = 506.65V

Such VFD might be as well really hard to find!.

So I have two options:
1- Find a 208V motor that will output 453.33V at 4000rpm:
voltage = Frequency × V/F = 133.33Hz × 3.4 = 453.33V

2- Find 240V motor that is Y, and rewire it as delta to get: 266.66 V

Is that correct or ?

 

[BrianPetersen]

I still don't understand why you're not simply buying a commercially-available engine dynamometer and skipping all this fretting-about. Please explain.

 

[Shamooooot]

I still don't understand why you're not simply buying a commercially-available engine dynamometer and skipping all this fretting-about. Please explain.

These systems are quite costly and sadly I can't afford them..

 

[davidbeach]

But you can afford twice that much to kludge together something that does half as much with twice the difficulty?

 

[BrianPetersen]

These systems are quite costly and sadly I can't afford them..

What's your time worth? What's your schedule worth?

 

[LionelHutz]

You don't seem to get how power works with a motor and VFD, or through a gear reducer for that matter.

With gear reducers the power out = power in - losses.

This means your 30kW can't do 55kW unless you run it at approximately 2750rpm.

So, sure lets overspeed it to 2750rpm. This is a 1.84:1 gear reducer - 4000:2750.

Using this gear reducer means when the engine is running at 1500rpm, the electric motor is running at 815rpm.

Now the motor needs to be capable of 29kW @ 815rpm. Oops, this 30kW motor is only capable of 16.3kW @ 815rpm.

You need to put more thought into your selections.

I wouldn't attempt to run a 1500rpm motor at 4000rpm, but I would do it with a 3000rpm motor.

 

[Shamooooot]

But you can afford twice that much to kludge together something that does half as much with twice the difficulty?

The systems I was quoted are easily 20 times the cost of doing it by myself, so it is indeed worth the trouble.

 

[Shamooooot]

What's your time worth? What's your schedule worth?

I am free for a couple of months and I have much to do, so you can say my time doesn't worth much..

 

[Shamooooot]

You don't seem to get how power works with a motor and VFD, or through a gear reducer for that matter.

With gear reducers the power out = power in - losses.

This means your 30kW can't do 55kW unless you run it at approximately 2750rpm.

So, sure lets overspeed it to 2750rpm. This is a 1.84:1 gear reducer - 4000:2750.

Using this gear reducer means when the engine is running at 1500rpm, the electric motor is running at 815rpm.

Now the motor needs to be capable of 29kW @ 815rpm. Oops, this 30kW motor is only capable of 16.3kW @ 815rpm.

You need to put more thought into your selections.

I wouldn't attempt to run a 1500rpm motor at 4000rpm, but I would do it with a 3000rpm motor.

I really don't think I get it..

I don't want to use a gearbox/speed reducer as suggest in the paper.

If I understood you correctly you mean that I should get a 55kW/3000rpm that is rated for lower voltage 208V or 240V.

For example here are 2 motors I found locally, if I would run the first one that is rated for 400V/3000rpm at 4000rpm this will result in 533V.
For the 2nd one it would 506V.

Then I need a VFD that is capable of handling this voltage, which is rare and maybe impossible to find 2nd hand.

 

[waross]

Contrast the price of an induction motor and a four quadrant, "on the fly" capable VFD that will operate without a grid connection, with the price of a used alternator and a reduction gearbox.
Budget and availability may influence your choice.
Be aware, While the squirrel cage rotor of a 3600 RPM induction will probably withstand 4500 RPM, (about 156% mechanical forces),
It may be impossible to find a 60 kW, 3600 RPM alternator.
The rotor windings of an 1800 RPM alternator may not survive 4500 RPM. (Over 6 times the mechanical forces.)

 

[waross]

Over the last year, Richie Bros Auctions have sold in North America the following sizes of used generators:
120 KVA $5000
54 KVA $2500
125 KVA $17,500
120 KVA $3500
100 KVA $40,000
83 KVA $8000
125 KVA $9750
75 KVA $1100
135 KVA $4,000
140 KVA $12,500
169 KVA $6250
60 KVA $6750
750 KVA $12,500

The Down side:
These are single bearing machines. You will have to have fabricated a drive end end-bell, bearing and shaft extension.
Possible solutions.
You may be able to have a gear reduction system included in your machine shop work.
If the gen-set engine is in poor condition you may be able to cannibalize the back end of the engine block.
Up side; If the gen-set engine is in good condition you may be able to sell it to recover some of your costs.
The savings buying a used gen-set at auction may more than cover the cost of machining.