Making a 60KW dynamometer

[johansen]

Induction motors do not have an intrinsic voltage. They have a soft saturation volts per hz limit.


A 400v 50hz motor is going to be producing 800 volts at 100hz, if you let it.. that is if you need it to. Torque follows the volts per hz squared


A european 380v 50hz motor at 50hp is an american 480v 60hp 60hz motor.

[NiHaoMike]

I'm still thinking that compressing air with a second modified engine would be far more practical. At the least, the problem of dissipating the energy is as simple as just blowing it outside.

[langwadt]

I'm still thinking that compressing air with a second modified engine would be far more practical. At the least, the problem of dissipating the energy is as simple as just blowing it outside.

or just get a telma retarder, something like this: https://www.telmausa.com/Downloads/OC442107a.pdf

it is also reasonably simple to control because you just need PWM, often just a dimmer and a rectifier

[shamooooot]

Induction motors do not have an intrinsic voltage. They have a soft saturation volts per hz limit.


A 400v 50hz motor is going to be producing 800 volts at 100hz, if you let it.. that is if you need it to. Torque follows the volts per hz squared


A european 380v 50hz motor at 50hp is an american 480v 60hp 60hz motor.

Thank you again for you reply. And for helping me better understand it.

400V/50Hz =8V/Hz , then 8V/Hz×100Hz=800V that's 6000rpm for a 2-poles motor. And the same ratio could be applied to switch calc. between European and US motors.

If we capped the motor/engine to 4000rpm top speed, it would mean 67 Hz 73 Hz with the safety margin, and 533 V 578 V. This would greatly limit the VFD options..

The torque for the Baldor motor is approximately 60.73 Nm, which means the breakdown torque will be around 120, while the engine torque could reach 185-210 Nm. So it might not be a good fit for this engine.

Going back to Berni replies, a synchronous generator could indeed save me a lot of troubles, especially that I need less than 100VDC and couple of amps to control it and I would still need the load bank..

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 expansive, but perhaps I am assuming wrong, here are two example motors:

75kw 6000rpm is 119Nm/75kw 5500rpm is 130Nm: $6K
132kw 5500rpm Torque is 229Nm: $9K

[shamooooot]

I'm still thinking that compressing air with a second modified engine would be far more practical. At the least, the problem of dissipating the energy is as simple as just blowing it outside.

or just get a telma retarder, something like this: https://www.telmausa.com/Downloads/OC442107a.pdf

it is also reasonably simple to control because you just need PWM, often just a dimmer and a rectifier

Interesting and indeed could be the safest option as they only might require cooling, I also noticed they come with their own ECU, and could be get used for around $4K https://www.ebay.com/itm/186265022860, I wonder how much the new ones are?

Also compared to motors how are their range with speed and load, and control precision etc..

[langwadt]

I'm still thinking that compressing air with a second modified engine would be far more practical. At the least, the problem of dissipating the energy is as simple as just blowing it outside.

or just get a telma retarder, something like this: https://www.telmausa.com/Downloads/OC442107a.pdf

it is also reasonably simple to control because you just need PWM, often just a dimmer and a rectifier

Interesting and indeed could be the safest option as they only might require cooling, I also noticed they come with their own ECU, and could be get used for around $4K https://www.ebay.com/itm/186265022860, I wonder how much the new ones are?

Also compared to motors how are their range with speed and load, and control precision etc..

doesn't look like that are that much more new https://www.store.telmausa.com/en/retarders/

[Berni]

Yep those electromagnetic retarders are pretty much what you want.

You likely won't find a synchronous generator for the exact specs you want since they are designed to run at a fixed RPM (usually driven by a large low rpm diesel engine) so you would need to 'qualify' it for your application. As said with external excitation you can control how they behave, so you can make it operate at RPM very different from the design RPM. The only limitation is how fast you can spin the rotor without it exploding from centripetal forces and what the bearings will handle. Overspining it will also make it less efficient due to the eddy losses going up from the higher frequency, but overspecing it should give it enough cooling to handle 200Hz no problem. But you could extend the RPM range by attaching a automotive manual gearbox to it and selecting the appropriate gear for the RPMs you want to run at.

But yeah those retarders are the same thing except that the 'load resistors' are integrated in the same device. You need a similar supply to run the coils as you would to run the excitation coils in a synchronous generator, just that the current it generates is eddy currents rather that current put on terminals to go into the load bank. This makes it a cheaper solution since you need less parts and they are designed to run at a range of RPMs. The downside of these particular ones is that they are air cooled and so can only provide max braking force for a short period of time before the rotor gets too hot (hot rotor = higher resistance = less eddy currents). So you likely want to overspec it a fair bit and maybe add some blower fans to help cool it after the run when it is not spinning anymore. As mentioned earlier in the thread real dynos solve this by instead magnetizing the rotor, making eddy currents in the stator and then cooling the stator with water.

But id say such a retarder has a reasonable chance of being found in a scrapyard in unknown condition for cheap. Not much to go wrong in one, either bad bearings or toast coils, both can be rebuilt yourself.

[langwadt]

Yep those electromagnetic retarders are pretty much what you want.

You likely won't find a synchronous generator for the exact specs you want since they are designed to run at a fixed RPM (usually driven by a large low rpm diesel engine) so you would need to 'qualify' it for your application. As said with external excitation you can control how they behave, so you can make it operate at RPM very different from the design RPM. The only limitation is how fast you can spin the rotor without it exploding from centripetal forces and what the bearings will handle. Overspining it will also make it less efficient due to the eddy losses going up from the higher frequency, but overspecing it should give it enough cooling to handle 200Hz no problem. But you could extend the RPM range by attaching a automotive manual gearbox to it and selecting the appropriate gear for the RPMs you want to run at.

But yeah those retarders are the same thing except that the 'load resistors' are integrated in the same device. You need a similar supply to run the coils as you would to run the excitation coils in a synchronous generator, just that the current it generates is eddy currents rather that current put on terminals to go into the load bank. This makes it a cheaper solution since you need less parts and they are designed to run at a range of RPMs. The downside of these particular ones is that they are air cooled and so can only provide max braking force for a short period of time before the rotor gets too hot (hot rotor = higher resistance = less eddy currents). So you likely want to overspec it a fair bit and maybe add some blower fans to help cool it after the run when it is not spinning anymore. As mentioned earlier in the thread real dynos solve this by instead magnetizing the rotor, making eddy currents in the stator and then cooling the stator with water.

But id say such a retarder has a reasonable chance of being found in a scrapyard in unknown condition for cheap. Not much to go wrong in one, either bad bearings or toast coils, both can be rebuilt yourself.

this one has specs for cold,3min,+60min https://www.telmausa.com/Downloads/OC442107a.pdf

and at some point cooling the engine your are testing also becomes an issue

[Jeroen3]

Belts are a thing often used to adapt the speeds for synchronous machines. Very common, and widely used.
Truck PTO have them a lot and direct drive diesel trains have them for on board power generation.

The only drawback is their torque limit and slipping. But otherwise easy to put together.

[max_torque]

Few notes from me, as some on with over 30 years engine and chassis dyno experience between 5bhp and 5,000bhp!


1) If the actual plan is to test engines, just buy a COTS engine dyno.  This way you can get on with actually testing engines (rather than waste thousands of $$ and hours developing a dyno, which is a completely pointless activity if the aim is to test and develop engines. Second hand dynos of all types pop up now and then. In reality the dyno itself if mearly one of the systems you will have to install and develop, especially if you want any measure of precision and repeatability


2) If you want a quick and dirty solution to mearly load and engine for a short period, then a hydraulic motor and restrictor (needle) valve provides a way to produce a huge reaction torque as a hyraulic motor the size of a ice cream container can often do 1,000 Nm or more at 100bar!.   Heat is your enemy, but because you are not really using dyno parts, those parts are cheap and generally available second hand. This system cannot be run continuously without cooling of course, but it can work for short periods



Personnally however, just buy a dyno and get on with it, otherwise you'll spend 99% of your effort being a dyno developer and hence only spending 1% of the remaining time on your actual engine...

[shamooooot]

Thank you guys, I appreciate it..

I am still looking into the synchronous option, perhaps as johansen mentioned a 3600rpm rated motor can survive 4000rpm. The engine specs mentions (Speed at maximum power 4000(rpm)).

As this won't be used as an actual generator or tied to the grid and the power would be dissipated to a load the job doesn't look complicated. Especially, that these motors are fairly common and could be found at a good price.

Also, I don't really understand how long a test can be carried using the eddy current option before it loses its efficiency. Or how big of an eddy current brake should be get to stay safe for the specified engine..

[shamooooot]

Hello Again

I did some search in my local area and I actually found surplus induction motors and VFD's, the prices are really good and could be cheaper than any other option.

I came across a paper that describes almost exactly what I need https://repositorio.comillas.edu/jspui/handle/11531/17559 but I am not sure about some few things..

From the paper:

The only possible method to start the asynchronous machine when the rotor is already turning is the flying start. Thus, the availability of flying start is a requirement for the frequency converter.

The starting point is the maximum torque and the rotational speed that the diesel engine can provide.
- The maximum torque that the diesel engine can provide is 190 N∙m.
- The maximum torque can be generated in a speed interval between 1800 rpm and 2800 rpm.

Parameter that has to be taken into account
-    If the slip is positive the asynchronous machine acts as a motor.
-    If the slip is negative the asynchronous machine acts as a generator.
In the steady state of this application the asynchronous machine has to act as a generator. Thus, the rotor speed, which is the rotational speed of the diesel engine shaft, has to be bigger than the synchronous speed to achieve a negative slip. (page 30)

The normalized rotational speed of the asynchronous machine has to be included in the interval of the diesel engine operation speed, as observed. (page 31)

1000 rpm: this speed is not in the interval between 1800 rpm and 2800 rpm, so in theory it is not possible to use these asynchronous machines. But if a speed reducer with a 2:1 ratio is used the diesel engine can work around 2000 rpm speed. This rotational speed is included in the interval and there is a margin for achieve positive and negative slip values.
The option with the less pair of poles (1000 rpm) is chosen in order to have a better power factor. Choosing the 1000 rpm option the product torque∙rotational speed (τ∙ω) is going to be smaller and thus the power dissipated. (page 32)

The nominal power of an asynchronous machine that appears in the nameplate is the result of multiplying the nominal torque by the nominal angular velocity.
However, in the dimensioning of a drive application the parameters that have to be considered  are  the  maximum  torque  and  the  normalized  synchronism  velocity. Multiplying these parameters, the minimum power of the asynchronous machine is obtained. (page 33)

When sizing the motor and hence the VFD the author uses the the range of speed at peak torque, so if my engine peak torque is at 1500rpm, then I should aim for 1500rpm motor for minimizing slip and maximizing stable power output and no reducer should be required. The rotor speed need to exceed the synchronous speed to enter negative slip so the motor absorbs power.

Power P=T×ω will be 29.06kW, and current will be 56.7A (Voltage: 400 V, Power Factor: 0.85, Efficiency: 0.90)

So a 4-pole, 30kW, 1500rpm, 190nm asynchronous motor should be sufficient.

The braking resistor calculated on (page 47) is 8Ohm and 47kW  , assuming duty cycle is going to be 100%!.

Are these assumptions valid, or am I mistaken?!

[Jeroen3]

The braking resistor is going to need to be able to take all input shaft power, maybe even more to keep the dc link voltage of the converter within limits.
I've seen a setup like this only once and they were using 4-quadrant capable siemens drives capable of injecting back to grid.

[johansen]

When sizing the motor and hence the VFD the author uses the the range of speed at peak torque, so if my engine peak torque is at 1500rpm, then I should aim for 1500rpm motor for minimizing slip and maximizing stable power output and no reducer should be required. The rotor speed need to exceed the synchronous speed to enter negative slip so the motor absorbs power.

Power P=T×ω will be 29.06kW, and current will be 56.7A (Voltage: 400 V, Power Factor: 0.85, Efficiency: 0.90)

So a 4-pole, 30kW, 1500rpm, 190nm asynchronous motor should be sufficient.

The braking resistor calculated on (page 47) is 8Ohm and 47kW  , assuming duty cycle is going to be 100%!.

Are these assumptions valid, or am I mistaken?!

so peak torque is usually 2/3rds peak rpm, so you can get by with a motor sized for the torque and just run the motor out past its design volts per hz limit (keep in mind torque and slip follow the volts per hz squared)


So I still think you can run a 30hp 1800 rpm 240/480v motor re-wired for 138 volts delta, out to 4500 rpm and run it off a 100hp 480v vfd, and i believe the motor can handle this just fine. the vfd will be able to provide 480v at 100hp and this corresponds to 6200 rpm at the base volts per hz curve of the motor. you should be able to get 100 hp out of a 30hp motor, at 6200 rpm. this is 76KW.

sending the rotor out to be dynamically balanced is not an option at 6200 rpm, but you can probably get by with a stock, good quality 1800 rpm motor, running it out to 4500 rpm.


your dc brake resistor needs to be sized to handle the 60KW you expect to get out of the motor. at 750 volts that's 80 amps or about 9 ohms.

[shamooooot]

When sizing the motor and hence the VFD the author uses the the range of speed at peak torque, so if my engine peak torque is at 1500rpm, then I should aim for 1500rpm motor for minimizing slip and maximizing stable power output and no reducer should be required. The rotor speed need to exceed the synchronous speed to enter negative slip so the motor absorbs power.

Power P=T×ω will be 29.06kW, and current will be 56.7A (Voltage: 400 V, Power Factor: 0.85, Efficiency: 0.90)

So a 4-pole, 30kW, 1500rpm, 190nm asynchronous motor should be sufficient.

The braking resistor calculated on (page 47) is 8Ohm and 47kW  , assuming duty cycle is going to be 100%!.

Are these assumptions valid, or am I mistaken?!

so peak torque is usually 2/3rds peak rpm, so you can get by with a motor sized for the torque and just run the motor out past its design volts per hz limit (keep in mind torque and slip follow the volts per hz squared)

So I still think you can run a 30hp 1800 rpm 240/480v motor re-wired for 138 volts delta, out to 4500 rpm and run it off a 100hp 480v vfd, and i believe the motor can handle this just fine. the vfd will be able to provide 480v at 100hp and this corresponds to 6200 rpm at the base volts per hz curve of the motor. you should be able to get 100 hp out of a 30hp motor, at 6200 rpm. this is 76KW.

sending the rotor out to be dynamically balanced is not an option at 6200 rpm, but you can probably get by with a stock, good quality 1800 rpm motor, running it out to 4500 rpm.

your dc brake resistor needs to be sized to handle the 60KW you expect to get out of the motor. at 750 volts that's 80 amps or about 9 ohms.

Thank you johansen, I was suspicious about the mentioned details in the paper and I couldn't find if they actually did make the system and if it worked as intended or not..

So I need to find 208/240V, 50/60Hz, 1500/1800rpm, Y connected, 30kW induction motor, and connect it as delta, but this might be tough to find.
Or I get a 3000rpm/380V motor and run it above 3000rpm without exceeding the rated voltage of the VFD.

I found these motors locally (but it was when I was looking for higher power):
https://ibb.co/NNsJhWG
https://ibb.co/zVwcTz4
https://ibb.co/0JNF85X
https://ibb.co/6rP4d85
https://ibb.co/m5bMS1N

Also I would like to ask do I actually need such a beefy VFD (100hp)?

Engine:
Max. Power: 55kW
Top Speed: 4000rpm
Max. Torque: 185Nm
Speed at max. Torque: 1500rpm

THANK YOU

[johansen]

Also I would like to ask do I actually need such a beefy VFD (100hp)?

Engine:
Max. Power: 55kW
Top Speed: 4000rpm
Max. Torque: 185Nm
Speed at max. Torque: 1500rpm

THANK YOU

you need a bigger vfd than the motor so you can overvolt the motor to get enough torque, when you exceed the base rpm.

so you need two things, you need a motor that can handle 55KW at 4000 rpm and you need a motor that can handle 39hp at 1500 rpm. this corresponds  to a 50hp motor at 1800 rpm.

a 100hp vfd will be able to drive a 50hp motor re-wired for half the nominal volts of the motor, which doubles the current flowing. you have to do this so that the motor can be driven with enough voltage at the high end of over 2 times the base speed.

induction motors produce or consume torque proportional to volts per hz squared.