[wjdtpghks29]

Why do you use contactors? When we wiring, we can drive without using a connector. We are thinking of contactors as automatic switches.Is the only role of the contactor a switch?We are using Kelly controller and sayoo zj400d contactor

 

[Functional Artist]

The contactor is a BIG BEEFY switch that is capable of "breaking" the flow of current. (sometimes 100's of Amps)

It can be used by the driver to turn the system "off", in an emergency
...&/or the controller to turn the system "off" if any of the pre-set limits are reached (like low voltage cut-off, controller overheating etc.)

 

[wjdtpghks29]

Can't we use a kill switch as the switch used by the driver?
Then does it disconnect if 600a flows through the connector of 400a? Isn't that the role of a fuse? I'm asking because I don't know

 

[Functional Artist]

Heavy Duty kill switches are used, by the driver, in many applications
...but, usually as a "secondary" option.

You should have properly rated components "matched" to your system

A fuse is a "changeable" "weak spot" in a circuit
...& if there isn't a "fuse" in a power circuit & a dead short situation arises
...then, the "entire" wire or cable "becomes" the fuse

I'd say you have lots of looking & reading to do

Most of these questions have been answered "in detail" in many different threads, here on the forum

There's lots of kool builds
...& really smart folks on here

You just gotta look around

 

[kennybobby]

Yes it is a switch.

Notice the key switch in the diagram--it is used to start the car to READY mode for driving.

But the key switch contacts are tiny and cannot handle the high currents needed to drive, so a HV contactor is used to carry the high currents. The contactor is commanded (switched) by the position of the key switch and the Kelly controller logic. It is somewhat like a transistor amplifier in that a small current into the base can control a large current thru the collector.

The use or not of connectors has no real influence on this circuit.

In an ICE car the key switch starts the car and turns it OFF; the same is true in an EV, the key switch can turn the car ON/OFF, AND it can also be turned OFF by the controller if safety limits are exceeded. There is no need for a big red button kill switch.

A fuse is used in a circuit to protect the wire from an overcurrent situation which could overheat and melt the wire and start a fire.

On the inside, contactors have special metal contact points and a magnetic circuit to quench the plasma arc of an interruption of current. The contacts can be damaged if the circuit opens with current flowing and the contactor would likely need to be replaced in such an event.

 

[remy_martian]

A fuse is in a circuit to prevent a fire. Period. It rarely is fast enough to prevent damage to solid state devices.

A contactor is in a circuit to disconnect the high voltage battery for safety. It/they should ideally be in the battery box and kill voltage on exiting power cables.

A contactor is ALSO used to CONNECT the controller/inverter to the battery in two REQUIRED steps to keep from blowing its innards up from current inrush. It normally switches zero current, but carries 100's of amps.

Contactors are expensive, but they are NOT optional.

 

[Electric Forklift Guy]

Why do you use contactors? When we wiring, we can drive without using a connector. We are thinking of contactors as automatic switches.Is the only role of the contactor a switch?We are using Kelly controller and sayoo zj400d contactor View attachment 131011
View attachment 131010

It's so the controller can still turn off the motor when the transistor shorts out.

I kind of sucks when the motor goes to full torque and you have no way to turn it off .

So we have a line contactor that won't close until the controller tests itself and then will pop open at the first sign of trouble.

 

[remy_martian]

That's an awfully expensive way to shut down a runaway motor in modern 3 phase traction applications, though. Pretty much guaranteed to blow up the inverter with the voltage spike from the motor inductance.

For EVs there's a safety requirement (I forget the standard number) to be able to shut off power to the CONTROL (driver circuits) side of the inverter. No driver, MOSFETs/IGBTs are off, no power to the motor.

I think its pretty rare for the output device phases to fail short and even if they did, the short would need to be sinusoidal and correctly phased to turn a brushless motor 😛

The contactor might be a last resort motor disconnect in the event of a software or pedal fault, and it would effectively shut down a rogue motor, but its primary purpose is to isolate the HV battery from the car and initialize the DC link in the inverter. Again, cutting power to the drivers is low current stuff and will shut down a rogue.

All bets are off if you have a brushed DC system, which I think your post was intended to cover (it's important for people to understand you cannot disconnect a three phase inverter from the battery under load without serious damage) but the contactor again is meant to isolate the battery as its main purpose.

 

[DJBecker]

It's much easier to appreciate the role of a contactor in a series-wound DC motor controller. If the power devices short (which is a common way for MOSFETs to fail), the motor will operate at full power. Full power is unlikely to blow a fuse, but is very likely to cause major damage.

An AC controller is less likely to fail with a short, let alone two shorts that would allow current to flow through the motor. But if it happened, you wouldn't be able to do anything about the motor windings heating until they failed, probably with a catastrophic fire in the motor and battery pack.

Even with a simple failure that didn't cause a disaster, you would want a contactor as a safety disconnect for service.

 

[Ken S]

The reader is encouraged to draw a schematic of an H-Bridge (or it's three-phase cousin) and ponder what happens 180 degrees after one of gate devices shorts its output a DC rail.

 

[Ken S]

Back to the original topic, "Why do you use contactors?"

you would want a contactor as a safety disconnect for service.

Very true. I would still want a manually operated service disconnect in the HV circuit. Even if you are familiar with what's safe to touch, and what's not, it's easy to make a mistake when you're in a hurry. The chances are even greater when you have a helper. Consider what kind of isolation a "lock-out / tag-out" procedure would require. (ie: where would you put an isolation device to prevent a clown from accidentally electrocuting another?)

Most of us have felt residential AC voltage. Higher voltage DC in a circuit designed to deliver 500A is a different animal. Proper isolation and personal protective gear are essential to safe operating procedures.

But enough of my soapbox rant....
"Why do you use contactors?"

1. Provide a means for the controller to cut power to the Hi-power components
2. Facilitate emergency shutdown (required by NHRA, btw)
3. Improve the safety of standard maintenance work

 

[remy_martian]

The reader is encouraged to draw a schematic of an H-Bridge (or it's three-phase cousin) and ponder what happens 180 degrees after one of gate devices shorts its output a DC rail.

Bait - the mouse goes for the cheese.

Properly designed driver circuits have a circuit that detects & prevents "shoot-through" in an h-bridge or "its three-phase cousin", so possibly nothing at the HV supply in that case, since the DC link caps would supply the initial transient current - if the isolated driver power supply survives. The system would shut down and generate a fault code.

An HV short to the gate, though, and, then its driver would also likely take out the gate driver power supplies (note: plural) if the gate short back drives its isolation transformer. Again a properly designed traction inverter will shut down its output switching devices when the gate driver supplies are shut off -- it's a safety requirement in production inverters.

The failure modes between IGBTs and MOSFETs can be different. The former usually fails short, the latter can fail open. In extreme current conditions, the bond wires/ribbons in the transistor should act as fuses.

So, an IGBT-based traction motor controller/inverter with a blind, non-shoot-through-protected gate driver, could short the HV supply until the traction inverter's fuse, which is ahead of the DC link caps, pops. However, it's the DC link caps that supply the hellacious currents and make the battery and fuse oblivious to them during the transition, causing massive arc-fault damage.

Did I get it right?

 

[Ken S]

However, it's the DC link caps that supply the hellacious currents and make the battery and fuse oblivious to them during the transition, causing massive arc-fault damage.

Did I get it right?

That's about what I had in mind - shoot-through is bad. (so are over-temperature, and over-current, both of which are possible if you are lucky enough to have your failed transistor remove itself from the circuit by burning its wires)

The remainder of my long-winded reply regarding cascade failures is hereby deleted.