Can you make a dynamometer from a partciular kind of motor/g

[Richard]

I suppose if you couple a motor to a generator and load the generator, that
will load the motor so the generator could be used to make a dynamometer?

 

[Nobody]

On Sun, 11 Jan 2009 09:37:30 +0000, Richard wrote:

I suppose if you couple a motor to a generator and load the generator,
that will load the motor so the generator could be used to make a
dynamometer?

You can make an adequate dynamometer with a loaded generator, estimating
the torque from the current, although "real" ones use a torque sensor.

 

[Jasen Betts]

On 2009-01-11, Richard <notty@emailo.com> wrote:

I suppose if you couple a motor to a generator and load the generator, that
will load the motor so the generator could be used to make a dynamometer?

yeah, that could be done.

you won't get a direct engine power reading from the generator's output though as
efficiency will likely change dependant on RPM and load,

so you still have to measure torque and RPM then calculate to get the engine's power.

 

[Richard]

"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:gkcia4$8vc$1@reversiblemaps.ath.cx...

On 2009-01-11, Richard <notty@emailo.com> wrote:
I suppose if you couple a motor to a generator and load the generator,
that
will load the motor so the generator could be used to make a
dynamometer?

yeah, that could be done.

you won't get a direct engine power reading from the generator's output
though as
efficiency will likely change dependant on RPM and load,

so you still have to measure torque and RPM then calculate to get the
engine's power.

Can't you measure watts from the generator, make necessary corrections for
losses, and get the power of the motor?

 

[Richard]

"Richard" <notty@emailo.com> wrote in message
news:6su7kpF860aoU1@mid.individual.net...

"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:gkcia4$8vc$1@reversiblemaps.ath.cx...
On 2009-01-11, Richard <notty@emailo.com> wrote:
I suppose if you couple a motor to a generator and load the generator,
that
will load the motor so the generator could be used to make a
dynamometer?

yeah, that could be done.

you won't get a direct engine power reading from the generator's output
though as
efficiency will likely change dependant on RPM and load,

so you still have to measure torque and RPM then calculate to get the
engine's power.

Can't you measure watts from the generator, make necessary corrections for
losses, and get the power of the motor?

Well, I'm thinking that you can measure watts consumed in a load from the
generator. But you would need to know a lot about the generator
characteristics and be able to measure the rotation in rpm. Do you absolutly
have to measure torque?

 

[Jasen Betts]

On 2009-01-11, Richard <notty@emailo.com> wrote:

"Jasen Betts" <jasen@xnet.co.nz> wrote in message
news:gkcia4$8vc$1@reversiblemaps.ath.cx...
On 2009-01-11, Richard <notty@emailo.com> wrote:
I suppose if you couple a motor to a generator and load the generator,
that
will load the motor so the generator could be used to make a
dynamometer?

yeah, that could be done.

you won't get a direct engine power reading from the generator's output
though as
efficiency will likely change dependant on RPM and load,

so you still have to measure torque and RPM then calculate to get the
engine's power.

Can't you measure watts from the generator, make necessary corrections for
losses, and get the power of the motor?

No, efficiency will vary dependant upon operating parameters,
for an extreme example consider the case when the generator's output is
short-circuited.

 

[langwadt@fonz.dk]

On 12 Jan., 10:46, Jasen Betts <ja...@xnet.co.nz> wrote:

On 2009-01-11, Richard <no...@emailo.com> wrote:





"Jasen Betts" <ja...@xnet.co.nz> wrote in message
news:gkcia4$8vc$1@reversiblemaps.ath.cx...
On 2009-01-11, Richard <no...@emailo.com> wrote:
I suppose if you couple a motor to a generator and load the generator,
that
will load the motor so the generator could  be used to make a
dynamometer?

yeah, that could be done.

you won't get a direct engine power reading from the generator's output
though as
efficiency will likely change dependant on RPM and load,

so you still have to measure torque and RPM then calculate to get the
engine's power.

Can't you measure watts from the generator, make necessary corrections for
losses, and get the power of the motor?

No, efficiency will vary dependant upon operating parameters,
for an extreme example consider the case when the generator's output is
short-circuited.

for a DC motor/generator:
voltage = speed/speed_constant ( in rpm/V),
current = current * torque_constant ( in Nm/A)

why shouldn't it still work if shorted?,
torque will be determined by the current through the DC resistance of
the motor.
the voltage across the DC resistance will be determined by the speed.

lots of dynamometers are a simple drum or disc with a know moment of
inertia, with
speed being sampled and acceleration thus torque calculated from that

-Lasse

 

[Nobody]

On Mon, 12 Jan 2009 14:24:02 -0800, langwadt@fonz.dk wrote:

so you still have to measure torque and RPM then calculate to get the
engine's power.

Can't you measure watts from the generator, make necessary corrections
for losses, and get the power of the motor?

No, efficiency will vary dependant upon operating parameters, for an
extreme example consider the case when the generator's output is
short-circuited.


for a DC motor/generator:
voltage = speed/speed_constant ( in rpm/V), current = current *
torque_constant ( in Nm/A)

Typo: torque = current * torque_constant

For speed, I'd be inclined to measure frequency rather than voltage, with
a tacho sensor if necessary (e.g. if the load is a DC generator and
the AC component's fundamental can't be reliably separated from its
harmonics).

why shouldn't it still work if shorted?, torque will be determined by the
current through the DC resistance of the motor.

The main issue is that the constants only provide an approximation.
Depending upon the precision, that may be enough. Or beyond that, it may
suffice to perform a one-time calibration step to measure torque as a
function of current and speed (and temperature if that can be measured
easily) and use the results to interpret subsequent electrical
measurements.

the voltage across the DC resistance will be determined by the speed.

There are two components to the voltage drop: back-EMF and current through
the winding resistance. The former increases with speed, the latter with
current (and thus torque). The speed constant only determines the former.

When free-running (high speed, low torque), back-EMF dominates;
near stall (low speed, high torque), winding resistance dominates.

If voltage = speed/speed_constant was an absolute, any given voltage would
produce a fixed speed regardless of load (up to and including welding the
axle to the body).

lots of dynamometers are a simple drum or disc with a know moment of
inertia, with speed being sampled and acceleration thus torque
calculated from that

That only works if it's desired (or acceptable) for the speed to vary as
the integral of the applied torque (e.g. rolling road scenario). It won't
work if you want to measure behaviour under varying loads, or stall torque
(although for the latter, a passive load won't work either).