Crane motor controls generally come in two varieties:
Across the line controls are generally limited to one and two speeds (dependent on motors used), and result in large inrush currents caused by the motor having to instantly run up to full operating speed. This is most noticeable with the hoisting motion. This also causes the movement of the crane to be erratic as the crane tries to switch from stationary to full speed in as short a time as possible. This also causes excessive mechanical and electrical wear and tear to the equipment.
For smaller capacity and light duty cranes the above may not be as important an issue, but for any crane used on a regular basis, VFD controls are always recommended. Since the speed of the crane can be very precisely controlled, most of the mechanical and electrical wear and tear is significantly reduced. The crane motions are electrically brought to a standstill by generator actions rather than mechanical force, and the motor brakes are used ONLY to prevent rotation and to hold the crane in place. As the VFD's are controlled by programming, the speed of any motion can be adjusted to any value at any time. This feature can be used to easily adjust a new crane's speed to an existing crane. As well, acceleration and deceleration can be adjusted to match operational requirements as necessary. Over speeding of the hoist is possible under varying load conditions to allow decreased cycle times. and built in safety features greatly increase the expected life span of the motor being controlled.
An overhead crane pendant provides detailed control over the individual speed and motion of the equipment. The operator of the crane can utilize the pushbuttons on the pendant to control each specific function of the crane from bridge travel, trolley direction, lift speed, etc. The pendant is specifically designed and wired to suit the functions and configuration of the crane that it controls.
The pendant is typically mounted in one of two ways. It can either be mounted to a sliding track system which allows the user of the crane to stay safely back from the load being lifted. The second typical arrangement is mounting the pendant to the electric trolley hoist itself. When the pendant is mounted to the trolley, the user will have to move and operate the equipment while being positioned beside the load.
VFD is a common short form for Variable Frequency Drive and they are used to control the motors on the crane by varying the frequency. By varying the frequency of motors you are able to set custom speeds and allow for the equipment to accelerate or decelerate smoothly. The pendant pushbuttons can be used with a VFD in a very similar way that two speed controls are used.
The buttons first step can be engaged to maintain the current speed and the second step can be used to accelerate. If the operator releases the button completely, the crane will begin to decelerate. If the operator intends to maintain the current slower speed then they simply press the button again into its first step position. It is important to be aware that when the crane begins to decelerate it is not coasting uncontrollably; rather it is being controlled by a dynamic braking system that is programmable within the VFD unit.
The wireless radio remote functions exactly like a pendant does however it uses a radio frequency rather than being wired directly into the crane. Radio controls come with many essential safety features and they allow the users to operate the crane from a safe working distance. Mobility is greatly increased for the operator when using a radio control and they allow for multiple cranes to be controlled from the same remote as required.
The power supply represents the electrical service that is available within the building in which the crane or material handling equipment is being installed. An example of a common power supply is 460 volts / 3 phase / 60 hertz.
All enclosure are rated for different applications and environments. The enclosures contain all of the electrical components for the overhead cranes or other types of material handling equipment. They protect the electrical components from the surrounding conditions within the working environment and they are rated by either NEMA (The National Electrical Manufacturers Association or by CEMA (The Canadian Electrical Manufactures Association).
The following ratings identify the three main levels of protection.
NEMA 1 rating protects operators or service crews from making accidental contact with the enclosed electrical components and preventing them from electric shock inury.
NEMA 12 covers the same safety features as NEMA 1 and also protects the internal electrical components from airborne contaminates such as dust, and water. NEMA 12 is rated for indoor environments.
NEMA 4 is a higher level of protection than NEMA 12 / CEMA 12, it is designed to protect the internal electrical components from contaminants (dirt, dust, oil, water, etc.). NEMA 4 is rated for either indoor or outdoor applications and can also protect the enclosure from leaks (for example if the unit gets hosed down or becomes extremely wet for other reasons).
NEMA 4X is a higher level of protection than NEMA 4 with the additional quality of being resistant to corrosive environments Panels with this rating are typically made from either Fibreglass or Stainless Steel. Highly recommended for any outdoor or marine environment.