Pony Motor Design

[ccdubs]

Hi all,

I am trying to rate a pony motor (ind m/c) for a synchronous generator. I
have the following information:

genny losses at rated speed (1500 RPM) = 8kW
genny rotor inertia = 9.75kgm^2
various induction motor torque v speed data

I want the pony motor to drive the genny to slightly higher than synchronous
speed, at which point the auto-sync unit will connect the genny to the grid
(for sync condenser application). The pony motor is to drive the genny at
1505 RPM by using a belt drive with a gear ratio slightly higher than 1:1.

What I have done is to assume that the 8kW is roughly related to the square
of speed and have thus extracted loss values for the data points I have for
the induction motors TvS curves (every 150RPM). I have then subtracted the
loss power from the rated power of the ind m/c for each point, effectively
leaving me with the power/torque available to accelerate the rotor inertia.
Using torque=inertia*angular acceleration (T=J*alpha) I could then determine
alpha and then using speed difference between data points (150RPM) I can
calculate time for acceleration.

I then summed the total time to reach near rated speed and applied 20secs as
a suitable accel time. From here I found that an 11kW motor was within this
limit. Next I found the generator power loss for 1505 RPM and found the
rated ind m/c speed for this power. This speed relationship then gave me
the belt drive ratio.

Is this method of pony motor selection viable? Do you think that using a
belt drive with a gear ratio will work? My concern is that this is not
accurate enough and I may have to resort to using speed control of the pony
motor. Any inaccuracy in the power loss at rated speed will result in
either an increase or decrease in rotor speed and thus auto
synchronisation may not occur.

Anyone have any feedback with respect to this idea and feasibility, or
perhaps have a better way to avoid the expense of speed controller?

Cheers

 

[electricuwe]

I think you need some kind of controller for accellarating such a large inertia with a induction motor of that size. Check that first. Then you may decide to select a device which is suitable for starting as well as for speed control, e.g. a VFD.

 

[jbartos]

Suggestion: The induction motors that drive large inertia loads, e.g. large axial fans, may have an acceleration time about 17-19 seconds. These motors are built for the slow acceleration; therefore, it may be a good idea to select the motor for slow accelerations or variable speed drive inverter duty motor.