regenerative or dynamic braking

16 years 1 week ago #1578317 by SAwheeler
regenerative or dynamic braking was created by SAwheeler
can the controller handle the high current from the Etek motor when braking?

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16 years 1 week ago #1609650 by cosma
Replied by cosma on topic Re:regenerative or dynamic braking
It is possible that our controllers have been used with this motor but we do not have direct knowledge of this.

Here is the theory of what happens during braking with such a motor.

When the motors is operating at full speed and then slowed down rapidly, the motor's inertia cause energy to flow back through the controller to the battery.

The issue is that at some point during the deceleration, the power output will reach 0 (i.e. stop). When that happens, the MOSFETs are shorting the motor.

The problem with a motor like the Etek, is that it has a very low low impedance and a high inertia. So if the motor is still spinning while it is shorted through the FET, much of its energy will disipate inside the MOSFETs, rather than in the winding as it would with higher impedance motors.

Consider the following case: motor was running at full speed for a while at 24V. It is then bruttaly decelerated. When the motor is eventually shorted in the output MOSFETs, we'll assume that the motor is still rotating at half speed (because of inertia), that would mean a back emf of 12V.

Assuming that the Etek's armature resistiance is 0.039 ohm (to be verified), and the controller's bridge resistance is 0.006 ohm, we will have a total current of 12V/0.045ohm = 266A.

The power that is disipated in the controller MOSFET vs in the motors widing is computed as follows:

Power (W) = I^2 * R. So we see that of the 3120W that are burned off at that time, 424W are burned inside the controller vs. 2760W in the motor windings.

This is actually within the expected abilities - but near the limits - of our standard controller. If the Etek's armature resistance is indeed 0.039ohm, we would recommend that the High Efficiency version of our controller be used. This controller has twice the number of MOSFETs resulting in half as much power being dissipated inside the controller during braking.

Cosma


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