Why not using a Synchronous Machine as a Dynamometer? 1

[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..

 

[TE163]

If you mean using a sync motor as a load; yes, that can be done. I used to do that all the time in my test lab. I used servomotors for this since the company I worked made servomotors we had lots of extras.

Connect the motor leads to a 3 phase variac, take the output of the variac to the resistive load connected in a wye. You can now dial in the load you want.

You can get a 2 pole motor to go to 4000RPMs like this. The issue is heat. You may have to put a fan on the motor.

 

[BrianPetersen]

So if you need an engine dynamometer, why are you not using something that someone else has already built, tested, and debugged which is expressly for that very purpose?

https://superflow.com/products/engine-dynamometers/

 

[cranky108]

I think sync motors were made for 400Hz systems, as well as 60Hz, so you might be able to find one with a higher speed rating. My recollection is 400Hz is used in aircraft to make things smaller.

 

[Gr8blu]

Hmm. General statement is that the "off-the shelf" machine is mechanically designed to handle either 20 or 25% above top nameplate speed without coming apart and injuring someone. The limit is dependent on the governing design standards (IEC = 20%, NEMA = 25%).

What this means is that there is some room for an overspeed trip setting (typically in the 105-110% speed range) so that it will SAFELY trigger the shutdown process and bring the machine to a stop WITHOUT exceeding the 120 (or 125) percent limitation. Remember that a machine continues to speed up because of the built in time delays (i.e. detect the overspeed condition, send the trip signal to the control, have the control initiate a shutdown sequence, and have the sequence run its course to bring the machine to a stop).

If you're already operating at 4000 rpm, you're at 111% of rated speed - which means you time available to trip and shut down safely is already reduced significantly. At 4500 rpm, you've used up all the mechanical safety in the rotor construction (3600 * 1.25 - 4500).

Converting energy to motion for more than half a century

 

[waross]

400 Hz sounds appealing, but higher frequency may mean more poles, not more RPM.
Start looking for a suitable reduction gear.
At 2:1 reduction you have a wide range of new and used/surplus machines available.
A 60 kW Variac to control the loading will be very expensive and may not be available.
Consider a direct connection to the load bank and control the output by varying the excitation.
A surplus automotive transmission may be an economical solution to speed reduction.

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

 

[Shamooooot]

Thank you guys, it is relieving to know that it can be done.

As waross said I thought I would just connect the sync machine to the load bank directly. I can control the load bank with about 1KW steps, and I can control the excitation current.

I am more worried about the control unit now, I read that this setup might be dangerous (changing load and/or excitation current) as it might make the generator lose synchronism which might be detrimental and might destroy completely destroy the generator.

Or am I wrong as I hope and it's simpler than that?

 

[waross]

Losing sync is an issue when systems are running in parallel.
With a single machine connected to s load bank there is no synchronism to lose.

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

 

[TugboatEng]

Induction motors in that power range are relatively tolerant of overspeed. I think you'd have a hard time finding one with a 3600 rpm rating that wouldn't tolerate 4500 rpm. Synchronous machines, on the other hand, have wound rotor construction and are sensitive to throwing the copper windings off the rotor when oversped.

An automatic voltage regulator may have some issues with the higher speed. Maybe consider using your load bank's highest setting only and using a variable output constant current DC power supply to vary the load. A generator that size will likely have a 50 ohm field and need 3-10 amps to achieve rated voltage/power so you'll need 20-50 volts.

 

[Shamooooot]

Thank you again.

TugboatEng
I see now that I need either an AVR or a fixed 60KW load bank with a DC power supply. Did you mean a constant current or a constant voltage DC power supply?.

This is needed because the "Voltage drops significantly with increasing load due to armature reaction and internal voltage drops".

If I used an AVR I would lose the control over excitation current and this won't have a significant effect on the dyno.
If I used fixed load bank, I would lose the load changing option and won't be able to control real power and torque.

This AVR costs about $500 Link

 

[waross]

You are overthinking this.
1. Use gear reduction to get the speed down to whatever your generator will withstand.
2. Control the exciter with a DC supply capable of 0 to about 100 Volts.

That will be your basic dynamometer for fixed speed..
As you increase the voltage to the exciter field, the voltage will increase and the load current and the kW load will increase.
If you want automatic fixed loading as the test motor speed changes, that is another issue.

 

[TugboatEng]

I believe a constant current power supply will provide better control but either type of supply will work.