AU702818B3 - Electric motor control - Google Patents

Description

2 ELECTRIC MOTOR CONTROL This invention relates to an electric motor control, and in particular, but not exclusively, to an electric motor control for a motor used in hazardous environments such as in collieries and the like.

Electric motors are used to drive fans in underground coal mines in order to provide ventilation to the mines.

Auxiliary ventilation fans are used to provide additional ventilation to restricted areas. Electric motors are designed to efficiently convert electrical energy to rotational energy. However, such motors can also convert rotating energy to electrical energy (known as back EMF) when motors continue to rotate under their own inertia, or when driven by external means. In the case where the motors are used to drive fans in collieries or the like, the fans can be driven by the flow of air created by the auxiliary ventilation fans. We have determined that the open circuit terminal back EMF voltage of a motor used to power a fan with the rotor of the fan driven by an external **force can be comparable to the nominal supply voltage and in certain circumstances the back EMF voltage is capable of generating a spark across the starter terminals of the motor. If such fan is in a colliery or the like such sparking can be sufficient to cause methane gas ignition even at low motor speeds.

We have determined that the back EMF voltage generated by external powering of fans, and the resultant spark which may be created across starter terminals of the fan, may well be responsible for certain underground explosions in coal mines.

The rotor blades of the fan to which the motor is connected may be driven by, after power is shut off to the motor, by H:\Gale\Keep\speci\Vale.petty.do 9/07/98 3 the motor's own inertia by natural ventilation passing through the rotor of the fan or by ventilation caused by the colliery main ventilation system passing through the rotor. Tandem fans can represent a more series problem as an "off" fan can be driven at high speeds by an "on" fan.

The generation of a spark across starter terminals in a motor is normally not a problem to personnel as the motor and starter are under covers in a flame proof enclosure.

However, personnel performing maintenance on the motors or accessing the starter terminals (which may be some distance from the motor) are at risk either directly to electric shock or to possible gas ignition.

The present invention therefore provides a motor control 15 circuit for an electric motor, including: power supply means for supplying power to the motor to operate the motor; **circuit means for shorting the motor after shut off of power to the motor; and actuating means for selectively switching off power from the power supply means to the motor and when *said power supply is switched off to only then permit coupling of the circuit means to the motor to short any back EMF which may be generated.

Since the invention provides for the circuit means to be coupled to the motor after shut off of power to the motor to short back EMF, access can then be permitted to starter terminals or other parts of the power supply without fear of generation of sparks across the starter terminals which may ignite any methane gas in the environment of the starter terminals. Thus, maintenance or accessing of starter terminals etc can be performed safely.

Preferably the power supply includes an isolator switch or circuit breaker which is opened to shut off power to the motor.

H:\ale\Keep\ peci\Vale.Petty.doc 9/07/98 4 Preferably the circuit means includes shorting switches, contactors or circuit breakers which are closed after shut off of power to thereby couple the circuit means to the motor to short circuit the motor to thereby short back EMF voltage.

In some embodiments the circuit means can include resistive or reactive current limiters.

In other embodiments the circuit means may be earthed.

.In one preferred embodiment of the invention, a supply isolating circuit breaker includes a mechanical or electric *15 interlock which may include an operating handle which is held captive in an on position until the isolating circuit breaker is switched off to shut off power to the power *.supply, and wherein the operating handle is engageable with the shorting switch, contactor of circuit break of the circuit means to close the shorting switch, contactor or circuit breaker. Preferably engagement of the operating handle with the shorting switch, contactor or circuit breaker of the circuit means also enables opening of an enclosure door to permit internal access to the control.

.25 Thus, whenever the enclosure door is opened for maintenance etc the supply of power is turned off by operation of the operating handle and the shorting switch associated with the circuit means is automatically placed in the short circuit mode by operation of the operating handle to thereby ensure both power supply, voltage and possible motor back EMF voltage are shorted.

Preferred embodiments of the invention will be described, by way of example, with reference to the accompanying drawings in which: Figure 1 shows a first embodiment of a motor control; H:\Gale\Keep\speci\Vale.Petty.doc 9/07/98 5 Figure 2 shows a second embodiment of the motor control; Figure 3 shows a third embodiment of the motor control; Figure 4 shows a fourth embodiment of the motor control; Figure 5 shows a fifth embodiment of the motor control; Figure 6 shows a sixth embodiment of the motor control; and Figure 7 is a more detailed mechanical view of the preferred embodiment of the invention.

With reference to Figure 1, a motor control 10 is shown for controlling the power supply to a motor 4. The controller includes a power supply 12 which includes a single or three phase supply for supplying power to the motor 4. The power supply 12 includes an isolator switch or circuit breaker 2 which is operated to selectively allow power to be supplied by the power supply 12 to the motor 4 or opened to shut off power to the motor 4 when it is opened.

A circuit 16 for shorting back EMF voltage after shut off of power to the motor 4 is coupled to the motor and power supply 12. The circuit 16 includes a shorting switch, contactor or circuit breaker 3 which is closed after shut off of power to the motor 4 so that the circuit 16 effectively short circuits the motor 4 and to thereby short back EMF voltage. Thus, the circuit 16, after closing of the shorting switch 3, effectively short circuits the motor 4 to thereby short back EMF voltage which may otherwise cause sparks at starter terminals (not shown) which may ignite methane gas after opening of covers or a flame proof enclosure in which the motor control 10 and starter terminals are included.

A mechanical and/or electric interlock 6 is coupled between H:\Gale\Keep\speci\Vale.Petty.doc 9/07/98 6 the isolator switch or circuit breaker 2 and the shorting switch 3 so that when the circuit breaker 2 is opened, the shorting switch 3 is closed.

Figure 2 is similar to the embodiment of Figure 1 except that the circuit 16 is earthed at 18.

Figure 3 is also similar to Figure 1 except that the circuit 16 includes resistive or reactive current limiters 5 in the circuit 16.

Figure 4 is also similar to Figure 3 except that the :circuit 16 includes the resistive or reactive current limiters 5 as per the circuit 3 and the circuit 16 is also earthed at 18.

Figure 5 shows an embodiment which instead of having the shorting switch 3 in parallel as in the other embodiments the shorting switch 3 is arranged in series.

Figure 6 shows an embodiment similar to Figure 5 in which the circuit 16 includes the resistive or reactive current limiters 5. In the embodiment of Figure 6, part of the circuit 16 shown in dotted lines could be eliminated so S 25 that only two of the phases are short circuited which would be sufficient to demagnetise the motor 4 and ensure that no back EMF is generated.

The shorting switch 3 may couple the circuit 16 to the power supply 12 and therefore short circuit the motor and short back EMF voltage for only a short period of time after switching off of the motor or continually and, as noted above, across one or more phases of the power supply with or without reference to ground. Coupling of the circuit 16 to the power supply 12 by closure of the shorting switch 3 for only a short period of time will be sufficient to short circuit the motor so that even if the H:\Gale\Keep\speci\Vae.Petty.doc 9/07/98 7 rotor blades of a fan which the motor powers do rotate, it will short circuit back EMF voltage.

Since modifications within the spirit and scope of the invention may readily be effected by persons skilled within the art, it is to be understood that this invention is not limited to the particular embodiment described by way of example hereinabove.

e H:\Gale\Keep\speci\Vale .Petty.doc 9/07/98 8 THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A motor control circuit for an electric motor, including: power supply means for supplying power to the motor to operate the motor; circuit means for shorting the motor after shut off of power to the motor; and actuating means for selectively switching off power from the power supply means to the motor and when said power supply is switched off to only then permit coupling of the circuit means to the motor to short any back EMF which may be generated.

15 2. A motor control circuit as claimed in claim 1, S wherein the circuit means includes shorting switches, contactors or circuit breakers which are closed after shut off of power to thereby couple the circuit means to the motor to short circuit the motor to thereby short back EMF 00 20 voltage.

*0 3. A motor control circuit as claimed in any one of toet the preceding claims, wherein said actuating means is *associated with a door of an enclosure of said circuit 25 means so that if said door is opened, said actuating means couples the circuit means to the motor to shut off the power to the motor and to short any back EMF which may be generated.

Dated this 9th day of July 1998 VALE STATUTORY AND MINING SERVICES PTY LTD By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent Attorneys of Australia H:\Gae\Keep\speci\Vale.Petty.doc 9/07/98