Brake technology has remained largely unchanged in recent years. Manufacturers choose brakes generally on reliability, easy installation and maintenance, and particularly low cost. In Europe more than 250,000 electromagnetic brakes are sold each year for hoist and lift duties. The majority of these have DC coils. At first this seems an odd choice with an AC mains supply, but DC versions offer better wear capacity than brakes with AC coils, as well as greater reliability, wider voltage tolerance and lower manufacturing costs, particularly for larger sizes.
Brakes are constructed with a single rotating disc with asbestos-free friction material bonded to each side. Hence there are two braking surfaces. The brake disc rotates on the shaft geared to the load, and floats axially on splines. One side of the disc rubs onto a fixed surface, for example the rear face of a motor or part of the crane. The other side rubs onto a floating plate called an armature. The armature is pressed onto the brake disc by springs and withdrawn by the electromagnetic force of the coil when it is energised. When the power is off, or if there is an interruption to the supply, the brake engages. Brakes used to be termed ‘failsafe’, but for liability reasons this term has generally been dropped and ‘spring applied’ is used, although failures are very rare and even then are often due to a lack of maintenance. Brakes are supplied with options such as a manual release, sealing and electrical switchgear, in particular rectifiers.
Although brake technology has not changed significantly, the industrial environment has. In the past the lifting equipment manufacturer would often have a high level of brake selection expertise, and would make his own choice. For reasons of downsizing, such engineering specialists are now not so common. Also increasing safety standards, plus concerns over liability, means that the crane manufacturer needs to select the brake in partnership with the brake supplier. The need for a brake supplier with knowledge of both the product and its application in lifting equipment has never been greater.
As mentioned above, there is a need to get from the AC hoist supply voltage to the DC brake coil voltage. As the crane manufacturer may have different control voltages, there are several methods of doing this.
One simple solution is to use a half wave rectifier connected to the 415V AC mains. This gives a reduction factor of 2.2 so the output is 188V DC. The brake generally has 180V coils, the discrepancy not being a problem as DC coils have a reasonably wide voltage tolerance.
Such a simple solution is not ideal as brake coils are inductive and therefore the current rise time, equating to the brake release time, is relatively long. This can be a particular problem with larger brakes. Slow brake release can mean overheating and wear from fighting between brake and motor torques, and also a jerky drive. This problem can be solved with a device called a forced voltage rectifier. The forced voltage rectifier has circuitry that initially acts as a full wave rectifier. From 415V AC this energises the brake coil at 380V DC for a short period, typically half a second. This 2:1 over-voltaging is generally satisfactory to get fast brake release. Care is needed to ensure that the integrity and reliability of the forced voltage rectifier is proven in hoist and lift applications.
Crane manufacturers with 110V AC control circuits might use full wave rectification to achieve 96V DC. They can then get over-voltage of the brake and fast brake release by selecting a 48V DC brake coil. The drop from 96V to 48V can be achieved by series resistances, which are low cost and effective. They allow the full voltage through initially until the inductive effect of the coil takes over.
It is not unknown for an inexperienced manufacturer to use a standard brake motor to raise a load. He then finds that the brake fails to stop the falling load, which hits the ground sometimes with no braking at all. This is because standard brake motors are set up for AC switching of the brake. The current to the brake is disconnected on the AC side of the rectifier. Although a simple and low cost solution, this leaves intact a circuit where the motor driven by the falling load acts as a generator and powers the brake off. For all braked loads it is necessary to isolate the brake with DC switching, i.e. interrupting the DC current after the rectifier. Unfortunately this results in high induced voltages at the switch contacts from the electrical energy in the brake coil, often above 1,000V. Spark suppression is essential. Experienced suppliers can offer rectifiers with built-in suppression rated for these high energy levels.
Handling the vertical movement of cranes and hoists is without doubt the most difficult application for electromagnetic brakes. The worst case is where the brake is being asked to stop a falling load, as happens routinely on small hoists, and in an emergency on larger inverter driven hoists.
Consider what happens when the motor supply is disconnected. Up until this point the brake has been energised with the full coil voltage. The load being lowered begins to accelerate under the influence of gravity, restrained only by the efficiency losses in the hoist drive. It is necessary to get the electrical energy out of the coil before the brake springs can cause engagement. Although the brake engagement times are shorter than the brake release, the actual values are significant, typically between 30ms and 200ms. Thus for that period the load accelerates. Speeds at the motor can rise from (4 pole) 1,500min-1 up to 1,800min-1 before the brake balances the load and then starts to reduce the speed.
Clearly brakes must be sized with capacity more than the load torque to be able to stop the load, normally in less than 200ms. Other factors come into play, however. The energy in the system relates to the square of the speed. So if the lowering speed has increased from 1,500min-1 to 1,800min-1, energy rises by 44%. The energy must be calculated in order to estimate temperature rise and loss of torque due to brake fade. The calculation is also important in the longer term to determine the lifetime of the brake. Hence it is essential that the brake supplier has accurate knowledge of brake response times, energy capacity and how the coefficient of friction varies with speed and temperature, in order to select a brake which will stop the load safely under widely different operating conditions.
In the future we may see the introduction of so-called mechatronic systems. These use electronics to change the spring applied brake from an on-off device to one that can give variable torque, for example from a fieldbus signal. Mechatronics then gives the advantage of braking times being independent of load, or direction of travel. Furthermore, over-voltaging for fast brake release and reduced holding voltages for faster brake engagement, can be better controlled. Mechatronic technology exists now, and its introduction for cranes and hoists depends on proven reliability and performance justifying the additional cost.
