SG Transmission > Technical Information > Installation Information

Boost

When a product requires cycling at 50 times per minute or more, it is recommended that solid state switching by means of “boost” is used. A boost voltage of three times the nominal voltage should be applied to the clutch or brake for only as long as absolutely necessary to perform the required function, whilst preventing coil damage.

SG Transmission supply high-speed switches suitable for use with our products.

They are:
3-75-1-001 un-boxed Hi-speed Switch
3-75-1-002 boxed Hi-speed Switch

Back EMF

Electromagnetic Devices, particularly large ones, show the phenomenon of reverse polarity voltage spikes (or Back EMF) when the operating current is disconnected.
Energy must be dissipated by placing a voltage dependant resistor (VDR) or diode with a Zener diode across the coil.
This situation is aggravated with semiconductor switches as the switching times are very short and semiconductor devices are very sensitive to voltage spikes.

1.) Diode with Zener-diode (fig.1)
The diode is only effective as long as the value of the spike to be dissipated is higher than the nominal voltage of the zener-diode. The additional diode ensures that no current flows via the zener-diode in the conducting direction.

Advantages
• Very short release time
• Small transient voltage
• Defined transient voltage

Disadvantages
• For small electrical power only
Varistor method (fig.2) (VDR)

2.) The varistor (VDR) has a high resistance at nominal voltage. Only a very small current flows through the varistor while the coil is energised. However when the voltage spike occurs the resistance of the varistor drops considerably, thus damping the peak voltage.

Advantages
• Low priced
• Small delay in armature movement

Disadvantages
• Not suited to all electrical power
• Not qualified for high switching sequences

Fault Finding Guide

Excessive Slipping / Inconsistent Operation

Some degree of slip occurs naturally during the operation of electromagnetically operated clutches and brakes. If the unit has been selected correctly then this slip should not be excessive. New units may need bedding-in to obtain immediate optimum torque, (see Bedding-in Section). The following are additional points, which should be considered when investigating operational problems: -

1.) Large Air-gaps: the effective torque of a clutch or brake is affected by the size of the air-gap. A large air-gap reduces the clamping force between the interface of the clutch or brake surfaces thus reducing the torque. Excessive air-gaps can also result in inconsistent or intermittent operation. This occurs when the magnetic field no longer has the strength to pull the armature plate across the air-gap against the action of the return spring. The optimum air-gap value ‘g’ is given in the ‘Dimensional Information Section’ of this manual.

2.) Low Supply Voltage: for optimum performance we recommend that the supply voltage to any clutch or brake field is maintained at the device’s nominal voltage, within the range +5% to -10%. This voltage for d.c devices must also be full-wave rectified if derived from an a.c supply. Smoothing also improves performance. Any reduction in this supply voltage will affect the magnetic field strength and may cause performance related problems.

3.) Electrical Problems:
(a) Open Circuit: Unit fails to operate at all, return for repair/replacement
(b) Short Circuit: Unit continually ‘blows’ fuses, return for repair/replacement
(c) Partial Short Circuit: Unit has a lower than expected resistance compared to rated wattage & voltage, return for repair/replacement.

4.) Contamination: this product is designed to be used in DRY running conditions, therefore the presence of water or oil and grease can seriously reduce torque. In applications where such contaminants are present particular care must be taken to prevent these from reaching the friction surfaces. This may be achieved by providing adequate sealing and shielding.
Light contamination of the friction surfaces may be removed by lightly wiping the surfaces with a commercial degreasing fluid. Heavy contamination can never be removed successfully as any contaminant below the surface will inevitably be dissipated throughout the friction material, resulting in a further failure. These units should be replaced or returned for refurbishment.

5.) Excessive Wear: some degree of wear is unavoidable because of natural slip, but the presence of abrasive materials or foreign bodies in the atmosphere can accelerate this process. Accelerated wear results in need for frequent air-gap resetting operations and the premature failure of the unit.

High operating temperatures and excessive heat build-up can also result in rapid wear, therefore good natural or forced ventilation may be required in some applications, particularly where rapid cycling or large inertia loads are cycled.

Bedding-in (all Clutch/Brake Devices)

The torque of clutches and brakes of the type we manufacture is affected by the average coefficient of friction across the faces of the armature and the clutch\brake, and the clamping pressure at the interface of these parts. This effective pressure is increased if the surfaces are in good intimate contact when engaged.

The degree of intimate contact is controlled to a minimum level during production by grinding the opposing faces in contact. This increases during normal operation where some slip occurs naturally, the amount of slip depends upon the magnitude of the load, speed and cycle rate thus increasing the torque. We consider bedding-in to be essential and in most cases inevitable.
If maximum torque is required immediately after installation then this process can be accelerated by lowering the operating voltage to 40/50% of the nominal value. This induces artificial slip during operation and results in an increase in the degree of contact.

Care must be taken not to slip the device excessively as this will lead to an undesirable thermal overload causing electrical failure. Thermal overload may be avoided by cycling the unit on and off, allowing enough time for cooling between the periods of slip.