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Simple solution for the P85D contactor problem ?

joer00

Member
Jan 26, 2013
346
65
Tampa
Obviously the power problem of the P85D are the contactors and not anything else. Thinking about this I came to a simple solution, but since I am not an engineer I must miss something. So experts, please chime in.

The "old" contactors worked in the P85 and as of Tesla are good for up to 1300 amps. The rear drive inverter does only 1200 amps, so all good here.

So why not just putting and extra lower rated (non expensive , no special metal and microprocessor etc) contactor which feeds the front motor ?

Theoretically they can do 2600 amps together , good for close to 1000 HP.

Problem solved :)
 

stopcrazypp

Well-Known Member
Dec 8, 2007
11,359
6,519
Obviously the power problem of the P85D are the contactors and not anything else. Thinking about this I came to a simple solution, but since I am not an engineer I must miss something. So experts, please chime in.

The "old" contactors worked in the P85 and as of Tesla are good for up to 1300 amps. The rear drive inverter does only 1200 amps, so all good here.

So why not just putting and extra lower rated (non expensive , no special metal and microprocessor etc) contactor which feeds the front motor ?

Theoretically they can do 2600 amps together , good for close to 1000 HP.

Problem solved :)
The contactor is at the battery and its job is to disconnect the battery completely from everything else. I don't believe putting contactors in parallel (as you suggest) works because you can't guarantee that both of them open or close at the exact same time. If they are not in sync, you pretty much guarantee you will overload one of them (as it won't be rated to handle the full load).
 

Ingineer

Electrical Engineer
Aug 8, 2012
1,507
3,712
Yes, there are only 2 blades coming out of the pack into the car. There aren't separate cables for front/rear. They would have to change the physical layout of the pack and the car body, and it would make the packs incompatible. The design has quick-connect type connectors that automatically mate when the pack is installed, both for the electrical and glycol cooling loop.
 

scottm

Legacy account
Jun 13, 2014
3,070
2,349
Canada
The contactor is at the battery and its job is to disconnect the battery completely from everything else. I don't believe putting contactors in parallel (as you suggest) works because you can't guarantee that both of them open or close at the exact same time. If they are not in sync, you pretty much guarantee you will overload one of them (as it won't be rated to handle the full load).

Parallel would be a good idea, they both don't have to close at the exact same time. The car doesn't open/close the contactor under load, only when unloaded. Otherwise they'd fry in ONE cycle.
 

stopcrazypp

Well-Known Member
Dec 8, 2007
11,359
6,519
Parallel would be a good idea, they both don't have to close at the exact same time. The car doesn't open/close the contactor under load, only when unloaded. Otherwise they'd fry in ONE cycle.
That's only true if nothing is going wrong. But I believe the contactors have to the duty to be able disconnect the battery if something goes wrong, even under full load. If you have two in parallel, won't you run into risk of one welding if you don't open them at exactly the same time?
 
That's only true if nothing is going wrong. But I believe the contactors have to the duty to be able disconnect the battery if something goes wrong, even under full load. If you have two in parallel, won't you run into risk of one welding if you don't open them at exactly the same time?
Don't know if the front and rear motors have separate VFDs? If there are two drives, two contactors make sense to me because they don't have to be paralleled.
 

tom66

Member
Dec 17, 2013
625
28
United Kingdom
We've seen batteries marked DUAL MOTOR so I wonder what the difference is if it's not an extra connector at the front.

Currently HVbat power for the DC-DC/AC/heater/etc comes down the right(?) hand side of the bottom of the door frame. The wires aren't huge, and would certainly need upgrading for the additional front motor power. So how does that work on the newer cars? Maybe thick wiring down both sides, or a different wiring setup altogether?
 

Ingineer

Electrical Engineer
Aug 8, 2012
1,507
3,712
The Dual Motor packs have a larger fuse (and maybe other changes). There is only 1 electrical exit for ALL Model S packs; In the rear. There is no front connector except the glycol quick connects. The power then enters the rear HVJB under the rear seat, and from there it exits to the rear motor on RWD cars, or on AWD; to 2 separate fuse modules, one for front, one for rear. The power cables for the drive unit travel down the driver side rocker. There is also a smaller cable set on all models fused with a 100A fuse in the rear HVJB that travels down the passenger side rocker (assuming LHD cars in these cases).
 
The Dual Motor packs have a larger fuse (and maybe other changes). There is only 1 electrical exit for ALL Model S packs; In the rear. There is no front connector except the glycol quick connects. The power then enters the rear HVJB under the rear seat, and from there it exits to the rear motor on RWD cars, or on AWD; to 2 separate fuse modules, one for front, one for rear. The power cables for the drive unit travel down the driver side rocker. There is also a smaller cable set on all models fused with a 100A fuse in the rear HVJB that travels down the passenger side rocker (assuming LHD cars in these cases).

That's interesting information and useful to know. I had assumed that Tesla would have put a second connector on the front because of the high draw from the front motor (in the P85D, up to 221hp / 165kW) which would require substantially thick cabling to make practical. It's interesting that they make two separate runs for the accessory stuff (DC-DC, battery heater, cabin heat, HVAC, etc.) and motor. I wonder if the motor power now operates under a second set of contactors. Can't think why they would make two separate runs when the accessory/HVAC power is so much lower than the motor power.
 
The only reason i can see for the separate runs is due to the 100 A fuse on the accessory line--i would think that they want a direct connection with no fuse for the front motor as is done in the rear to avoid nuisance blows of a fuse during normal operation.

What is the purpose of the fuse in the pack? My guess is that it is not to protect any wiring per se since everything except the inverter(s) has a fuse for the wires, but to open the pack in the event of an emergency/short circuit situation.
 

Ingineer

Electrical Engineer
Aug 8, 2012
1,507
3,712
That's interesting information and useful to know. I had assumed that Tesla would have put a second connector on the front because of the high draw from the front motor (in the P85D, up to 221hp / 165kW) which would require substantially thick cabling to make practical. It's interesting that they make two separate runs for the accessory stuff (DC-DC, battery heater, cabin heat, HVAC, etc.) and motor. I wonder if the motor power now operates under a second set of contactors. Can't think why they would make two separate runs when the accessory/HVAC power is so much lower than the motor power.
To save a few feet of #8 copper, they'd have to add yet another HVJB in the front. This would also make the RWD/AWD cars even more different complicating the assembly line.

The only reason i can see for the separate runs is due to the 100 A fuse on the accessory line--i would think that they want a direct connection with no fuse for the front motor as is done in the rear to avoid nuisance blows of a fuse during normal operation.

What is the purpose of the fuse in the pack? My guess is that it is not to protect any wiring per se since everything except the inverter(s) has a fuse for the wires, but to open the pack in the event of an emergency/short circuit situation.
Yes, let's say there is an accident that shorts the pack connector, you'd want a fuse upstream of that.
 

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