EP0143091A1

EP0143091A1 - An induction heater for heating annular articles, particularly roller-bearings

Info

Publication number: EP0143091A1
Application number: EP19840850307
Authority: EP
Inventors: Carsten Aronsson
Original assignee: SPM Instrument AB
Current assignee: SPM Instrument AB
Priority date: 1983-10-21
Filing date: 1984-10-18
Publication date: 1985-05-29
Also published as: SE439718B; SE8305807L; DE3476314D1; EP0143091B1; SE8305807D0

Abstract

The invention relates to an induction heater for heating annular articles, and includes a U-shaped transformer core (1) and a magnetic yoke which can be lifted from the transformer and upon which the article to be heated can be fitted. When heating the article, mains current is supplied to a primary core (2) on the transformer core, via a first switch (4). In accordance with the invention, the switch (4) is connected in parallel with a PTC-resistor (5, or 5') which when the switch (4) is broken, upon completion of the heating period, first conducts the primary current, which gradually decreases to an extremely low value of about 10 mA, due to the heating of the PTC-resistor (5,5'), causing demagnetization of the heated article.

Description

The invention relates to an induction heater for heating annular articles, particularly roller-bearings, which comprises a U-shaped transformer core provided with a primary coil and a magnetic yoke which is designed to close a magnetic circuit through the transformer core and to have fitted thereonto an annular article which functions as a single-turn secondary coil for heating through an induced current, and further comprising means for coupling the primary coil to an alternating current mains network and means for gradually decreasing the current from said mains network through the primary coil, so as to demagnetize the annular article.
Such induction heaters are known to the art and are marketed by a number of manufacturers, for example Firma Grainer of Switzerland. They are primarily used when wishing to heat ball bearings or some other kind of ring-shaped article, in order to mount the same on a shaft and obtain a shrink-fit on said shaft as the article cools. When the annular article is made of a ferromagnetic material, the article must normally be demagnetized, which is effected by allowing the alternating current conducted through the primary coil to decrease gradually down towards zero.
In the known induction heaters this decrease in current is accomplished by gradually "choking" a controllable semi-conductor valve, eg. a triac, incorporated in the circuit.This takes place due to the fact that the valve is phase-angle controlled, wherewith in each voltage period the valve is triggered gradually at progressively later time periods, so that the initially sinus-shaped current wave-form is replaced with progressively shorter current pulses. Such a valve, which during the heating phase is intended to conduct all of the current through the primary coil and thereafter, during the demagnetizing phase, to conduct over progressively shorter time periods, must be provided with means for conducting away the heat generated. A further disadvantage is that phase-angle controlled, electric semi-conductor valves (triacs, thyristors) generally emit radio interference signals, which can often be particularly troublesome.
An object of the invention is to eliminate these disadvantages. Another object is to provide a simple and inexpensive construction in respective of those circuits which are intended to be active in the reduction of current for the demagnetizing function.
These and other objects, together with associated advantages, are achieved in accordance with the invention in that the coupling means and the means for gradually reducing the current include a switch which is connected in series to the primary coil and a PTC-resistor which is connected in parallel with said switch and which has a resistance which is low at room temperature, less than about 20 ohms, and a resistance which when heated in response to load, is at least 100 times greater.
PTC-resistors are a type of resistor whose resistance increases with temperature. They are available on the market and are normally specified by their resistance value at room temperature and the so-called switch temperature, at which temperature the resistance increases quite strongly at relatively small relative changes in temperature. The operating mode of the invention is as follows. The primary coil is initially connected, via the switch, which is closed, directly to the mains network (50 or 60 Hz; e.g. 220 or 110 v), wherewith the transformer induces in the annular article a current which produces heat, as a result of the inherent resistance of the article. The article is heated to the extent desired, by suitably determining the duration of the heating phase. Upon completion of the heating phase, the article is demagnetized, which is the phase in which the present invention becomes operative. When the switch is broken, current is conducted through the PTC-resistor, which gradually becomes hot, whereby the current therethrough, and therewith the current through *the primary coil, gradually decreases, e.g. from a primary current of 10-15A, down to about 10 mA.
The heated article is magnetized in alternate directions during successive half-periods of the mains frequency, with progressively reduced amplitudes, causing demagnetization in the same way as when the current is decreased in the known heating methods.
The switch is suitably arranged to co-act with a timing device which breaks the switch after a given period of time has lapsed, this length of time being determined in accordance with the size of the article to be heated, the shape of the article and other properties thereof, and in accordance with the desired final temperature. Preferably, the switch consists of a pair of contacts in a relay.
It will be apparent from the aforegoing that the PTC-resistor becomes heated. Consequently, before the PTC-resistor can be used again it must cool down. If desiring to decrease the time taken to complete a full cycle, it is often suitable, to this end, to include two or more PTC-resistors in parallel, of which only one is switched-in at a time, e.g. by means of individual switches, which may be triacs. Because these valves are not phase-angle controlled, but either switched on or off, only negligible electromagnet interference signals are generated.
Conveniently, the circuit may incorporate, for example, a signal lamp which lights up when the heating phase is completed. In many instances,this heating phase can take several minutes, during which time the operator may not have the whole of his attention concentrated on the heater.
The invention will now be described with reference to an embodiment thereof.

Figure 1 illustrates an embodiment schematically.
Figures 2 and 3 show a circuit diagram in greater detail, the diagram being largely self-explanatory.

Figure 1 illustrates schematically a transformer construction which includes a U-shaped transformer core 1 and a magnetic yoke 3, which can be removed to allow an annular metal article to be heated, such as a ball bearing (not shown), to be fitted thereonto. A primary coil 2 comprises a winding of 260 turns and is dimensioned for a current of 10-15 A. The transformer itself includes about 12 kg of transformer plate (for 110 V the number of turns is half this number and the current twice as strong). Mains current is passed from a master switch (not shown), through the coil 2 via a switch 4, which is a relay, whose coil current is controlled by a timing circuit T with adjustable switch-in time. There can be provided a mechanical clock or an electronic clock, e.g. powered with mains frequency, which is preferred.
Connected in parallel with the switch 4 in a respective branch line is a PTC-resistor 5, 5', each in series with a respective switch 7 and 8 effective in triggering said resistors alternately, these switches being in the form of triacs.
When using the transformer, the switch 4 is first closed for a given length of time of from 0.5 to 10 mins. and is then broken. One of the triacs 7, 8 is conductive. When the switch is broken, the current will flow via the one PTC-resistor, which soon becomes so hot that the current therethrough becomes progressively negligible.
In the illustrated embodiment there are used PTC-resistors of the make ITT (from the company Multi- komponent AB), each in the form of two parallel-coupled resistors (not shown) which when cold each have a resistance of 10 ohm and thus together a resistance of 5 ohm. A small drop in voltage soon occurs and the resistors become hot and the resistances thereof quickly increase to several kilo-ohms. According to the data sheet, the PTC-resistors had a switch temperature of 115°C.
The Figure 1 embodiment also includes a lamp 9, which r lights-up when the heating phase is terminated.
The detailed circuit diagram shown in Figures 2 and 3 is largely self-explanatory and incorporates conventional components. It will be observed that a voltage divider is placed over the primary coil and its output C is coupled to a circuit which via firing output terminals D and E selects either of the triacs 7 and 8 to be conductive next and to extinguish the conductive triac when the voltage across the primary coil 2 has fallen, and which further causes firing of a light diode 9 upon termination of the demagnetizing phase. An electronic clock circuit is supplied at B with an alternating current of 50-periods and, at a point in time determined by the setting of a potentiometer 10, opens the switching contact of the relay 4 via a signal A. Since such circuit diagrams are fully obvious to the electronics engineer, the information given above is deemed quite sufficient to render the switching circuit readily understandable.

Claims

1. An induction heater for heating annular articles, particularly ball bearings, in which heater there is included a U-shaped transformer core (1) provided with a primary coil (2) and a magnetic yoke (3) adapted to close a magnetic circuit through the transformer core and to receive thereon an annular article which functions as a single-turn secondary coil for heating with an induced current, and further comprising means for coupling the primary coil to an alternating current mains supply, and means for gradually reducing the current from the alternating current mains supply through the primary coil, to cause demagnetization of said article, characterized in that said means includes a first switch (4) connected in series with the primary coil (2), and a PTC-resistor (5) which is connected in parallel with said first switch and which has a low resistance at room temperature beneath about 20 ohms and a resistance when heated after applying a load which is at least 100 times greater, and also includes a further PTC-resistor (5'), each of said two PTC-resistors (5,5') being connected in parallel with said first switch (4) and in series with a respective second switch (7,8), said second switches being arranged to be switched-in alternatively.

2. An induction heater according to Claim 2, characterized in that said first switch is a relay (4) and in that an adjustable timer (6) is coupled to the coil of said relay.

3. An induction heater according to Claim 2, characterized in that said second switches (7,8) are triacs.