Current limit [tour 9]

Current limit – drive

Protection is fitted to all controllers so that the initial starting surge (or the current that would flow if the armature were locked) cannot exceed that which the MOSFETs can handle. The standard current limit is fitted only to protect the MOSFETs and you should not rely on it to protect other components in the controller, or to protect the wiring or motor. These are all application dependant so – if you want these also to be protected (and your expected quantity warrants the effort) – contact the office for advice.

All controllers made by 4QD use extremely fast current detection which even protects the MOSFETs against short circuits in the wiring (though this is not guaranteed) so there is no minimum motor resistance or inductance.

The current limit automatically compensates for the internal temperature of the MOSFETs – since this affects the current they can handle – so the available current decreases as the MOSFETs get hot. See also ‘Thermal shutdown’.

However MOSFETs can get very hot without failing: on test we can easily make them unsolder themselves! Clearly if they do this other components will be likely to suffer. The automatic compensation in the MOSFET cannot change its behaviour to stop solder melting so, if you overload any controller long enough, you may cook it to death. This is why some controllers also have thermal shutdown (or overheat protection).


Current limit – regeneration

If the vehicle is driven down too steep a hill (or the demand speed is suddenly reduced so that very hard braking results) the current generated by the motor could exceed that which the MOSFETs can safely handle.

Since this would blow the MOSFETs it must be protected against so all controllers that give regenerative braking are also fitted with a current limit to stop such failure. In practise failure from excessive regen current is quite rare and we could sell controllers without it. However 4QD have a very simple yet very sophisticated hiside current sensing system which is unique to 4QD – until some other manufacturers copy it! We did consider patenting this circuit but decided not to. If any other manufacturer is reading this, we’ll be happy to share our system with you for a small fee!

Again: this limit is set to protect the MOSFETs only. It is not intended to control the vehicle’s handling or to protect the mechanisms or motor. Contact 4QD for advice if you require this.


Current limit adjustment

We are sometimes asked whether the current limit can be altered. Yes it can, but since its prime purpose is to protect the MOSFETs we always need to ask why the user wishes to alter the current and is such alteration sensible. The 4QD series do actually have a current limit adjustment preset built in. Other controllers do not.

Drive current can be reduced from the standard value by altering a resistor and can be altered over quite a wide range.

Regen current is not so adjustable and cannot easily be altered, nor is there normally any point in doing so.

Power limit

One reason you might want to alter the current limit is to restrict system power. Sorry: altering motor curremt limit wil not do this. The motor voltage os speed dependant and the motor power is current times amps, so a limited current wil give no power at slow speeds.

Motor protection

You might want to protect the motor by limiting the current. Yes: this is theoretically sensible. However motors have a large thermal mass. The current they can take is therefore very dependant on the time the current is drawn (i2T rating. Motor manufacturers do not quote time values for current so matching the controller to what is safe for the motor turns out to be extremely difficult. To do it properly also calls for a software-based controller with some very clever programming and it would require lots of parameters to be user-definable, so not something we can help with.

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