Parking brake driver [tour 22]

Parking Brake Driver

Regenerative braking relies on the voltage generated by the rotating armature to cause braking so it is speed dependent. If the speed is slow enough there is no effective braking, so the machine will creep down hill.

A parking brake is a mechanical brake which can be fitted to some motors. It is activated by a spring and is released by a voltage supplied by the controller when it requires the motor to rotate. ‘Parking brake’ feature is the electronic switch to activate this parking brake.

The circuit applies power to the brake as the motor starts up and removes the power about a second after the motor stops.

The driver can also be used to switch the field of a shunt wound motor, or to switch an air solenoid or relay to release brakes or for some other purpose.

The parking brake driver fitted to all our controllers is able to drive a nominal 1A maximum.

Current Limit

If you miswire your brake circuit you can short-circuit the brake. Power transistors (such as are used to switch the brake coil) don’t like switching into a short circuit – it makes them try and short the battery out. So they promptly expire in a puff of smoke. This doesn’t happen to a controller that has been properly wired, so the only person likely to cause a problem is the electrician wiring the machine up in the customer’s works.

But it’s awkward for a controller manufacturer to tell the customer he’s blown it and, although it is clearly misuse, it’s awkward trying to make a customer pay for repairs to a new unit. It’s much easier to fit extra circuitry to all controllers and to make all customers pay the price! You can then sell the feature as part of the specification, even though it doesn’t actually help the average customer any!

So ‘Brake current limit’ may be fitted by the manufacturer to protect himself against the incompetent customer! This limits the available current to something which won’t immediately destroy the transistor.

We do this only on the 4QD series (where the brake driver transistor is a pain to replace) and the Pro. Moreover as the brake transistor gets hot under this fault, we cause the fault also to switch off the controller for complete safety.

Anti rollback

When a controller senses the armature back emf, the logic to apply power to the coil is ‘Demand speed not zero OR motor going’. So as soon as you apply the throttle (apply some demand speed) the brake releases. The brake won’t come on again until the throttle is at zero and the motor has stopped.

Unfortunately if the machine is now parked on a hill with the ignition on – which is fine: this is what the parking brake is for – and someone bumps it with another machine, you can get a problem with some controllers. If the bump is hard enough the parking brake won’t hold the machine and it will start to move. The motor is now going – so the parking brake is released and the vehicle rolls away.

The ‘Anti rollback’ feature stops this and is fitted to 4QD series. It is not applicable to the DNO and Pro-120- which don’t sense armature voltage and where (since relays are used for reversing) the armature is shorted out at stand-still.

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