I am re-posting this from the automotive forum to get a little more exposure, as I don't feel the underlying issue is really at all automotive related.
We are utilizing a brake installed on an electric motor to ensure the equipment doesn't move when throttle is not applied -- a pretty simple application using the controller and supplemental electronics that are paired with the system. The brake is more of a parking brake, however, and simply halts the vehicle too abruptly, despite the extremely low travel speeds. In the interest of not shearing anything within the drivetrain components, I want to install a capacitor to drop the voltage from nominal voltage (Vs) down to a known "pull-in" voltage which the brake manufacturer specifies as where the coil begins to push the contact plates away from the driveshaft -- where it all begins -- at about 19V. So:
Vs = 24V = Starting voltage (we are driving, brake is energized/pulled AWAY from shaft)
Vf = 19.2V = Ending voltage (we are stopping/almost stopped, brake is almost completely engaged to the shaft in it's normally STOPPED condition)
R = 7.68 ohms; Calculated as a 75W rated brake, presumably the continuous rating. This may be more complex during brief moments of pull-in. (75 / 24) gives 3.125. (24 / 3.125) = 7.68.
I've decided a 1s stop will be a good initial test, so will be sizing my cap accordingly.
For capacitor discharge functionality, I have:
Vc = Vo * e^(-t / RC)
19.2 = 24 * e^ (-1 / (7.68)(C)
C = ~0.584F
I think the more technical aspect of this revolves around the series resistor for the charging aspect of the circuit. Since max Q is observed at t=0 during charging, or "not braking," I will want to charge as quickly as possible so C does not delay acceleration when the operator opts to drive. I don't really want to use a massive power resistor, so I think the technical analysis may reside in heat dissipation through the series resistor when considering how often this charge cycle may be active, i.e., tapping the accelerator repeatedly when inching the equipment into position. This would not be even close to continuous, so would using a 10W resistor or something with the value I need to relatively quickly charge the cap be acceptable? Thoughts?
One of the primary concerns brought up already was that R1 should be sized relatively low to Rbrake so the voltage drop doesn't render the brake useless. This is becoming an issue because I need sub-ohm resistance there, which causes a massive current spike to charge the cap just after SW1 turns on. A 24V regulator is powering this system and I don't want to overload it. There is a metric in the spec sheet for the regulator that specifies "max capacitive load" as "unlimited," though I'm not sure this is the appropriate thing that addresses that initial surge.
See attached file. The "10" on R1 is not the value intended, just hadn't removed it.
Thanks,
We are utilizing a brake installed on an electric motor to ensure the equipment doesn't move when throttle is not applied -- a pretty simple application using the controller and supplemental electronics that are paired with the system. The brake is more of a parking brake, however, and simply halts the vehicle too abruptly, despite the extremely low travel speeds. In the interest of not shearing anything within the drivetrain components, I want to install a capacitor to drop the voltage from nominal voltage (Vs) down to a known "pull-in" voltage which the brake manufacturer specifies as where the coil begins to push the contact plates away from the driveshaft -- where it all begins -- at about 19V. So:
Vs = 24V = Starting voltage (we are driving, brake is energized/pulled AWAY from shaft)
Vf = 19.2V = Ending voltage (we are stopping/almost stopped, brake is almost completely engaged to the shaft in it's normally STOPPED condition)
R = 7.68 ohms; Calculated as a 75W rated brake, presumably the continuous rating. This may be more complex during brief moments of pull-in. (75 / 24) gives 3.125. (24 / 3.125) = 7.68.
I've decided a 1s stop will be a good initial test, so will be sizing my cap accordingly.
For capacitor discharge functionality, I have:
Vc = Vo * e^(-t / RC)
19.2 = 24 * e^ (-1 / (7.68)(C)
C = ~0.584F
I think the more technical aspect of this revolves around the series resistor for the charging aspect of the circuit. Since max Q is observed at t=0 during charging, or "not braking," I will want to charge as quickly as possible so C does not delay acceleration when the operator opts to drive. I don't really want to use a massive power resistor, so I think the technical analysis may reside in heat dissipation through the series resistor when considering how often this charge cycle may be active, i.e., tapping the accelerator repeatedly when inching the equipment into position. This would not be even close to continuous, so would using a 10W resistor or something with the value I need to relatively quickly charge the cap be acceptable? Thoughts?
One of the primary concerns brought up already was that R1 should be sized relatively low to Rbrake so the voltage drop doesn't render the brake useless. This is becoming an issue because I need sub-ohm resistance there, which causes a massive current spike to charge the cap just after SW1 turns on. A 24V regulator is powering this system and I don't want to overload it. There is a metric in the spec sheet for the regulator that specifies "max capacitive load" as "unlimited," though I'm not sure this is the appropriate thing that addresses that initial surge.
See attached file. The "10" on R1 is not the value intended, just hadn't removed it.
Thanks,
