Controls for resistance welding machines, Mechanical Engineering

Assignment Help:

Controls for resistance welding machines

The main controls required for a resistance welding machine are:

i. Initiation and termination of the current.
ii. Controlling the magnitude of the current.
iii. Timing and controlling the mechanical operations of the welding machine.

Three main categories of controls are:

a. Welding contactors
b. Timing and sequence controllers and
c. Other controls and regulators.

Welding control is lifted in the primary circuit of the welding transformer. The contactors may be mechanical (foot pedal), magnetic (electromagnetic) or electronic of which electronic contacts are the most useful and reliable. The electronic contacts may be ignitrons, thyratrons or SCR's. We have to go in for ignitron tubes it the primary current exceeds 40 A.

Time and sequence controls can be pneumatic, R - C controlled, motorised or digital. Synchronous and non synchronous controls are two main possibilities. Non - synchronous controls provide variations of timing and current input to the machine from closing and opening of the welding contractor at random points on the wave form. On the other hand, synchronous controls provide switching at the same electrical angle each time. Non - synchronous controls are accurate for weld times of 20 cycles or longer.

Heat control: The major adjustment of the welding current is done by tap changing. For fine intermediate current control, electronic heat control is used. Ignitron tubes with SCR or thyratron firing is employed.

Upslope control: Upslope permits the welding current to be increased over several cycles from a low value to that needed for welding, instead of having the full welding current applied instantly. This eliminates the expulsion of metal (splashing). This is used at high current values and for welding scally stock and for almost all plate metals.

Downslope control: This permits the welding current to decay gradually to a low value, instead of ending suddenly, and it helps to produce good welds in some types of heat treatable metals by lengthening the cooling time of steels (less than 0.15 % carbon).

Current and voltage regulators: Electronic current regulators are used to maintain a constant welding current under severe conditions by compensating for welding current variations or load changes resulting  from insertion of magnetic materials into the throat of the welding machine.

Preheat and postheat control: These controls are important for flash butt welding. The joint is first heated up in one or more stages by applying pressure to the joint and then separating it before the weld temperature is reached. This may be repeated several times, depending on the type of work and the capacity of the machine in relation to the size of the joint. By adopting this interrupted preheating process, it is possible to increase the maximum capacity of the machine, as the preheating operations reduce the final kVA needed to raise the joint to weld temperature. In sophisticated flash butt welding machines, the preheating is digitally controlled and ignitrons in the circuit, for each preheat cycle, the time for which the jobs are in contact, the time taken for retraction of one job, the time for which the jobs are allowed to cool and the time for bringing the jobs together can be independently set with the thumb wheel switches. The number of preheat cycles can also be selected.


Related Discussions:- Controls for resistance welding machines

Heat pump - thermodynamics, Heat Pump - thermodynamics: Heat Pump: H...

Heat Pump - thermodynamics: Heat Pump: Heat pump is reversed heat engine that removes heat from a body at low temperature and transfer heat to the body at higher temperature

Robotic and manufacturing automation, Consider the robotic manipulator of F...

Consider the robotic manipulator of Fig. 1, with a Revolute and a Prismatic Joint. Figure 1, shows the side view of the manipulator while figure 2. shows the top view of the manipu

Function generation by relative pole method, how mechanism can be synthesiz...

how mechanism can be synthesized using relative pole method?

Evaluate principal stress and maximum shear stress, At a point in a beam, t...

At a point in a beam, the bending stress is 50 MPa shear stress and tensile is 20 MPa. Calculate :- (i) The principal stress and maximum shear stress. (ii) The tensile stress

Battery-electrical system , Battery: Battery is a device that produces dir...

Battery: Battery is a device that produces direct current (DC). The chemical energy of battery ingredients converts into electrical energy in the form of direct current. This is a

Maximum shear stress, An aircraft flap actuator housing made of cast magnes...

An aircraft flap actuator housing made of cast magnesium alloy ZAG3A-T4 with uniaxial yield strength σ Y = 14,000 psi.  At the suspected critical point the calculated state of str

Determine moment of this normal force - rectangular beam, Determine moment ...

Determine moment of this normal force - rectangular beam: A rectangular beam contains a width of 100 mm and a depth of 200 mm. This is utilized as a simply supported beam & th

Find out the centre of gravity of solid sphere, Find out the centre of grav...

Find out the centre of gravity of solid sphere: Find out the C. G. of a body formed by a solid sphere placed over a solid cylinder of similar radius a as illustrated in Figure

Evaluation of force exerted by jet on the plate, (a) What are the assumptio...

(a) What are the assumptions made during evaluation of force exerted by jet on the plate ? (b) A jet of water which has diameter of 8 cm. issues with a velocity of 40 m/sec. and

Explain interpoles or commutating poles, Interpoles or commutating poles ...

Interpoles or commutating poles Interpoles are ?xed to the frame in among main poles. They are used for sparkless commutation, i.e, to improve commutation.

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd