Motor current status sensor

A method and circuit design for establishing the operating status of a direct drive, multiple speed motor. More specifically, the invention furnishes a method for providing feedback to an air distribution system indicating first whether the system motor is operating and second which speed winding has been selected. The circuit design senses the voltage differences between the speed windings of the motor and also between the first speed motor winding and the power supply. The voltage difference sensed by the circuit will determine whether a light emitting diode (LED) illuminates. The illumination of the LED indicates to the system controller whether the motor is operating and the speed winding that has been selected, thus providing the necessary feedback to the air distribution system controller.

BACKGROUND OF THE INVENTION 
The present invention relates to the field of integrated function control 
and more particularly to the control of motors in air distribution 
systems, such as an HVAC or dual-firing rate furnace systems. 
Most modern furnace designs involve direct drive fans for air distribution. 
These fans are typically designed for use at multiple speeds, allowing the 
user to operate the fans at varying speeds. In fact, many designs for 
integrated function controls directly incorporate the speed select 
function. One example of this type of system is the air distribution motor 
in an HVAC system. Another example is the two-speed inducer motor in a 
dual-firing rate furnace system. 
There are times when proof of motor operation within these systems is 
necessary for a particular function to be performed. One example is the 
twinning module concept, where two furnaces are connected to one duct 
system. Because the twinning module concept operates with one duct system, 
it is necessary to activate the fans of both furnaces at the appropriate 
speed so that backflow will not occur in one of the furnaces. This does 
not mean that both systems must be supplying conditioned air; if only one 
system is needed in full operation, it is only necessary to operate the 
fan of the other system. 
Current air distribution systems are capable of commanding both fans to 
operate, but the systems provide no feedback to the system controller 
verifying that both fans are indeed operating at the appropriate speed. 
The feedback to the system is necessary to prevent inefficiency in the 
system and also damage to the furnace caused by backflow. Backflow 
produces an inefficient air distribution system by redirecting the 
conditioned air away from the residence or building into the second 
furnace system. Additionally, the backflow into the second furnace system 
may cause the second fan to rotate backwards, causing damage to the 
furnace when the motor does start. 
Current methods are available to provide feedback to the system controller. 
The first of these is through the use of direct sensing, which essentially 
involves the use of either photo or magnetic sensing devices to detect the 
movement of the blades of the fan. This method requires significant 
additional hardware and is therefore a very expensive addition to an air 
distribution system. Furthermore, the system must be designed particularly 
for this method; it may not be updated to include such a sensing device. 
This causes the direct sensing method to be both an expensive and 
inefficient alternative. 
Another available method is current sensing, which involves the use of a 
current transformer and resistor to detect a certain current threshold. 
Like direct sensing, current sensing requires additional equipment, making 
it a very expensive feedback option. Additionally, because different 
furnaces use different motors that operate on different currents, a 
different circuit must be used for each current level. Like direct 
sensing, this method is both inefficient and expensive. 
Therefore, a need exists for a method of providing feedback to the air 
distribution system controller of a motor. Such a method must be able to 
indicate that the motor is indeed operating and that it is operating at 
the correct speed. The method should be compatible with varying furnace 
designs, efficient, and inexpensive. 
SUMMARY OF THE INVENTION 
The broad object of this invention is to furnish a device for providing 
feedback to an air distribution system controller indicating first that 
the system motor is indeed operating, and second that the proper speed 
winding has been selected. 
It is a further object of the invention to provide such a device that is 
compatible with varying furnace systems, efficient, and inexpensive. 
These and other objects of the invention, which will be apparent to one 
skilled in the art, are accomplished by the present invention through the 
addition of two voltage sensing circuits. These voltage sensing circuits 
represent a method for establishing the operating status of a direct 
drive, multiple speed motor, that is, the circuits can establish whether 
the motor is operating and can indicate which speed winding has been 
selected. The method for doing so is comprised of first, sensing the 
voltage difference across the speed windings of the motor and also the 
voltage difference between the first speed winding and the power supply 
and second, indicating whether such voltage differences are above a 
predetermined threshold. It is assumed that the direct drive, multiple 
speed motor has at least first and second speed motor windings and a power 
supply. 
A first voltage sensing circuit verifies whether the motor is operating by 
sensing the voltage difference across the speed windings of the motor and 
indicating whether the voltage difference is above a predetermined voltage 
level. This is accomplished by a resistor, a diode, and a photocoupler 
utilizing a light emitting diode (LED), all connected in series between 
the speed windings of the motor. The LED will indicate whether the motor 
is operating by illuminating. 
A second voltage sensing circuit verifies which winding has been selected 
by sensing the voltage difference between the high speed motor winding and 
the power line and indicating whether the voltage difference is above a 
predetermined voltage level. This sensing and indicating is accomplished 
by a resistor, a diode, and a photocoupler utilizing a light emitting 
diode (LED), all connected in series between the first speed motor winding 
and the power supply. The illumination of the LED indicates that the first 
speed motor winding has a voltage difference from the power line and 
therefore is not the winding selected. 
The utilization of the two voltage sensing circuits yields a device that 
provides feedback to an air distribution system controller indicating not 
only that the motor is operating, but also that the proper winding has 
been selected. This invention represents a device that is universal in 
use, efficient, and inexpensive.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to FIG. 1, there is schematically illustrated a circuit 
arrangement 10 for establishing the basic operating status of a direct 
drive, multiple speed (DDMS) motor according to the preferred exemplary 
embodiment of the invention. The circuit arrangement in FIG. 1 is a part 
of a larger air distribution system, such as a furnace used for heating a 
residence or business. This air distribution system includes control 
circuitry, permitting the system to command the motor to turn on and off, 
and circuit arrangement 10 provides feedback to the controller indicating 
whether the motor is operating. The embodiment of the invention 
illustrated in FIG. 1 represents a two speed motor, but it should be 
understood that the invention can be adapted to any direct drive, multiple 
speed motor. 
The circuit design consists of a resistor, diode, and photocoupler 
utilizing a light emitting diode (LED), all in series. The first of these 
circuits is connected between the speed windings of the motor and senses 
the voltage difference across the speed windings of the motor. This 
circuit provides feedback to the system as to whether the motor is 
operating. The second circuit is connected between the high speed motor 
winding and the power supply and senses the voltage between the high speed 
motor winding and the power line. This circuit provides feedback to the 
system as to which winding has been selected. 
The circuit is designed such that when one of the circuits senses a voltage 
that is above a certain level, the LED will illuminate. This process will 
thus indicate to the system controller whether the motor is operating and 
whether the proper winding has been selected. 
The voltage differences that result across the speed windings during the 
operation of a direct drive, multiple speed motor are fairly constant. 
Thus, the voltage sensing circuit design, unlike current sensing designs, 
may be used with furnaces that use different motors. Furthermore, the 
voltage sensing design is one which may be added while updating the 
original air distribution system. Unlike direct sensing designs, the air 
distribution system need not be designed particularly for voltage sensing. 
These qualities of the voltage sensing design make voltage sensing a 
highly compatible, efficient, and inexpensive method of providing feedback 
to the air distribution system controller. 
The DDMS motor is designated generally by motor windings 13. The capability 
to operate at multiple speeds requires a motor to have multiple windings, 
which is generally accomplished by incorporating taps on the motor 
windings. On a two speed motor, there are necessarily three taps--low 
speed tap 14, high speed tap 16, and neutral tap 18--connected to the 
motor windings. High speed tap 16 divides motor winding 13 into two 
windings: motor winding 20, located between taps 14 and 16, and motor 
winding 22, located between taps 16 and 18. Motor winding 22 represents 
the high speed motor winding; motor winding 20 and motor winding 22 
together represent the low speed motor winding. The selection of speed is 
accomplished by applying the supply voltage to the appropriate tap. 
Inherent in this approach is that the motor acts like a transformer, i.e., 
if one winding is energized with the supply voltage, typically 110 volts 
applied across nodes 24 and 26, a scaled voltage will be induced across 
the other windings. This transformer action will take place only if the 
magnetic field configuration is normal, and this will be true only if the 
electric currents are normal, that is, within an expected range for the 
given circuit configuration. For example, a voltage can be applied to the 
high speed motor winding and if a sufficient current flows in that 
winding, an increased voltage, relative to neutral, can be detected across 
the low speed motor winding. Conversely, if the low speed motor winding is 
energized, a lower voltage, relative to neutral, will appear across the 
high speed motor winding. In any event the voltage relative to neutral is 
always higher on the low speed motor winding than it is on the high speed 
motor winding, effecting a voltage difference between the two motor 
windings when the motor is operating. Circuit arrangement 10 utilizes this 
induced voltage differential to verify the presence of sufficient currents 
to run the motor by sensing the voltage differential and providing 
feedback to the system controller confirming that the motor has started. 
Blower enable switch 28 is the connection between the circuit arrangement 
and the air distribution system. The system controller commands the motor 
by opening and closing the blower enable switch. The system is able to 
control the speed of the motor by changing the position of speed select 
switch 30. When the switch is connected to low speed contact 15, the motor 
should run on low speed. When the switch is connected to high speed 
contact 17, the motor should run on high speed. 
Light emitting diode (LED) and photocoupler pair 32, resistor 34, and diode 
36 together provide the means for sensing the voltage difference across 
the low and high speed motor windings and indicating whether such voltage 
difference is above a predetermined threshold. In the embodiment 
illustrated in FIG. 1, resistor 34 is a 3900 ohm, 1 watt resistor. The 
supply signal 25 energizing circuit arrangement 10 is an alternating 
current (ac) signal. Since only the positive cycle of the signal is 
needed, Diode 36 is provided to restrict the voltage signal to its 
positive half cycle. The LED and photocoupler pair 32, in series with 
Diode 36, detects any voltage difference greater than 10 Volts, and when 
such a difference is detected, the LED illuminates. 
When the motor is operating at low speed, the blower enable switch 28 is 
closed and the speed select switch 30 is connected to the low speed 
contact 15. The low speed tap will therefore be energized with the supply 
voltage, 110 Volts, and the transformer action will induce a voltage 
across the high speed motor winding of approximately 85 Volts. This 
difference in voltage across the low and high speed motor windings is 
detected by the LED and photocoupler, and the LED subsequently 
illuminates, indicating to the system controller that the motor is indeed 
operating. 
When the motor is operating at high speed, the blower enable switch 28 is 
closed and the speed select switch 30 is connected to the high speed 
contact 17. The high speed tap will therefore be energized with the supply 
voltage, 110 Volts, and the transformer action will induce a voltage 
across the low speed motor winding. Since the voltage on the low speed 
motor winding is always greater than that on the high speed motor winding, 
the induced voltage across the low speed motor winding will be 
approximately 135 Volts. Again, the difference in voltage between the low 
and high speed motor windings is detected by the LED and photocoupler, and 
the LED will illuminate, indicating to the system controller that the 
motor is operating. 
If the motor is not operating, the blower enable switch 28 is effectively 
open, and the speed select switch may be connected to either the low or 
high speed contact. Neither tap, however, will be energized with the 
supply voltage since the blower enable switch is open. Therefore, no 
voltage difference will appear across the low and high speed motor 
windings, and the LED and photocoupler will detect no voltage difference. 
In this situation, the LED will not illuminate, indicating to the system 
controller that the motor is not operating. 
If there is a mechanical impediment to the motor operating, such as a 
locked rotor condition, current will initially flow through the circuit as 
if the motor were operating. Because the motor does not start, the 
electrical energy from supply 25 is not converted to mechanical energy and 
the motor windings will begin to overheat. When this occurs, thermal 
cutout 37 opens the circuit, eliminating the current flow and thus 
eliminating the voltages across the motor windings. For the short time 
until this occurs, circuit 10 will indicate that the motor is operating, 
but once thermal cutout 37 opens the circuit, the LED and photocoupler 
will sense that there is no voltage, and the LED will not illuminate, 
indicating that the motor is not operating. 
While FIG. 1 illustrates only the circuit that indicates whether the motor 
is operating, FIG. 2 illustrates a circuit arrangement 12 that indicates 
not only whether the motor is operating but also that the proper speed 
winding has been selected. The second task is useful, for example, in a 
dual-firing rate furnace system, which delivers two levels of gas flow 
rate to the burner. If the same mixture of gas and air is to be 
maintained, a dual speed inducer would be required, i.e., low fire 
requires a low speed inducer, and high fire requires a high speed inducer. 
Circuit arrangement 12 can be used to indicate whether the proper speed 
winding has been selected and whether the motor currents are approximately 
normal, that is, within the expected range, ensuring that the proper 
air-flow is obtained. 
Circuit arrangement 12 is marked with the taps, motor windings, switches, 
nodes, and power supply identical to those in FIG. 1. The additional 
elements in this arrangement compare the line voltage to the high speed 
motor winding voltage in a manner similar to that with which circuit 
arrangement 10 compares the low speed motor winding voltage to the high 
speed motor winding voltage. Circuit arrangement 12 employs a photocoupler 
utilizing a light emitting diode (LED) 38, resistor 40, and diode 42 to 
provide the means for sensing the voltage difference between the supply 
and the high speed motor winding and indicating whether such voltage 
difference is above a predetermined voltage level. In the embodiment of 
the invention illustrated in FIG. 2, resistor 40 is a 3900 ohm, 1 watt 
resistor, and resistor 46 is a 15000 ohm, 2 watt resistor. Diodes 44 and 
42, like diode 36, are used to restrict the voltage signal to its positive 
half cycle. 
When the motor is operating at low speed, the blower enable switch 28 is 
closed and the speed select switch 30 is connected to the low speed 
contact 15. The low speed tap will therefore be energized with the supply 
voltage, 110 Volts, and the transformer action will induce a voltage 
across the high speed motor winding of approximately 85 Volts. In this 
situation, there is a voltage difference between the supply voltage, 110 
Volts, and the voltage across the high speed motor winding, approximately 
85 Volts. The LED and photocoupler pair 38 will detect this difference in 
voltage, and the LED will illuminate, indicating to the system controller 
that the low speed motor winding has been selected. 
When the motor is operating at high speed, the blower enable switch 28 is 
closed, and the speed select switch 30 is connected to the high speed 
contact 17. The high speed tap 16 will therefore be energized with the 
supply voltage, 110 Volts, and the low speed winding will be energized 
with an induced voltage of approximately 135 Volts. In this situation, 
there is no voltage difference between the supply voltage, 110 Volts, and 
the voltage across the high speed motor winding, also 110 Volts. 
Therefore, the LED and photocoupler pair 38 will not detect a voltage 
difference, and the LED will not illuminate, indicating to the system 
controller that the high speed motor winding has been selected. 
Circuit arrangement 12, therefore, performs both functions described above: 
the circuit indicates to the system controller first whether the motor is 
operating and second whether the correct speed winding has been selected. 
It will be understood that the foregoing description is one of preferred 
embodiment of the present invention and is not limited to the specific 
forms shown. Modifications may be made in the design and arrangement of 
elements within the scope of the present invention as expressed in the 
appended claims.