Microprocessor controlled motor controller with current limiting protection

A solid motor controller for controlling the current in an electrical motor. The motor controller includes a processing unit, and a solid state switch operatively connected to the electrical motor. The motor controller senses the current through the motor, compares the current with a reference current level, and limits the current in the motor by rapidly switching the switch between an ON and an OFF state when the current exceeds the reference current level. The controller de-energizes the motor by placing the switch in the OFF state when the current exceeds the reference current for a predetermined period of time. The controller also includes a second solid state switch which is controlled to rapidly stop the rotation of the motor. The motor controller also activates a high current warning when the current exceeds a high current limit. Further, the motor controller senses the battery voltage, provides a low voltage warning indication when the battery voltage falls below a predetermined value, and shuts down the motor when the battery voltage falls below a second predetermined value. Finally, the controller measures the temperature near a selected component and shuts down the motor when the temperature exceeds a predetermined value.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to motor controllers and in particular to
 solid state motor controllers for electric motors adapted for use with
 portable power equipment, such as lawn and garden equipment.
 2. Related Art
 Motor controllers are designed to interrupt power to a motor in the event
 of a fault in the motor. A fault in the motor can cause excessive current
 to flow through the motor windings and thereby overheat the windings and
 damage the motor.
 To provide overcurrent protection, motor controllers on small motors
 typically use a relay approach. In the relay approach, when an overcurrent
 condition is sensed, a circuit breaker connected in series with the motor
 trips open to interrupt current flow to the motor and halt the motor.
 However, circuit breakers do not limit current, they merely interrupt the
 current path when a particular current threshold has been exceeded for a
 certain period of time. A problem with using mechanical devices such as
 circuit breakers to interrupt the current path is that there can be a
 significant delay between the time the high current condition occurs and
 the time the circuit breaker trips open. Due to the time delay, high
 current can flow to the motor as well as to other components of the system
 before the circuit breaker opens, thereby causing damage to the system.
 This can be a particular problem with small motors which may be easily
 damaged by rapid power surges and for which the aprroach may be
 ineffective.
 Another problem with the relay approach is that the circuit breaker is
 either shut or tripped open. If the current is below a threshold, the
 circuit breaker remains shut, but if the current exceeds the threshold,
 the circuit breaker trips open to interrupt the current path. Such an
 approach is susceptible to spurious trips from transients. If the circuit
 breaker opens during momentary transients, the operator is forced to reset
 the circuit breaker before the motor can be restarted thereby causing
 unnecessary delay and inconvenience.
 Also, circuit breaker based protection systems do not readily lend
 themselves to the addition of auxiliary control features, for example,
 thermal protection and battery monitoring, in a compact, integrated
 package.
 Therefore, what is needed is a motor controller which can quickly and
 reliably limit motor current flow and thereby prevent excessively high
 current through the motor and the system, even for sharp power surges.
 What is also needed is a motor controller which can automatically shut down
 the motor if current limiting is required for a predetermined period of
 time.
 What is also needed is a motor controller which is capable of quickly
 stopping the motor after an automatic shutdown condition occurs.
 What is also needed is a motor controller which provides the flexibility to
 easily add additional control features for operator interface.
 What is also needed is a motor controller which includes the above-cited
 feature and is adapted for use with portable power equipment, such as lawn
 and garden equipment.
 SUMMARY OF THE INVENTION
 The motor controller of the present invention is a solid state based motor
 controller which includes a microprocessing unit to provide a number of
 advantages over motor controllers using a relay approach, particularly in
 portable equipment applications. The microprocessing unit comprises any IC
 device capable of accepting input signals comparing the input signals with
 predetermined thresholds and providing output signals based on the input
 signals, the comparison or any programmed manipulation of the input
 signals. A solid state design that uses a microprocessor is better from a
 reliability standpoint because the current can be easily limited to any
 chosen level. A solid state design is also advantageous because many other
 features can easily be added, for example, a battery level indicator or a
 load level gauge, to the overall control package.
 In the present motor controller, the microprocessor senses the current
 through the motor, compares the sensed motor current with a first current
 threshold level and provides a warning if the sensed motor current exceeds
 the first current threshold level. The microprocessor also compares the
 sensed motor current with a second current threshold level and actuates a
 current limiting device if the sensed motor current exceeds the second
 current threshold level. If the current limiting condition, namely the
 sensed current exceeding the second current threshold level, exists for a
 predetermined period of time, the microprocessor shuts down the motor.
 In one form thereof, the motor controller includes first and second solid
 state switches, connected to the microprocessor through a driver unit, for
 limiting the current through the motor and for shutting down the motor.
 The current limiting feature is implemented by rapidly switching the first
 solid state switch, which is connected in series with the motor and the
 power source, between an ON and an OFF state. The rapid switching prevents
 the current through the motor from rising above a predetermined level. The
 motor is shut down by placing the first solid state switch in the OFF
 state and rapidly switching the second solid state switch, which is
 connected in parallel with the motor, between an ON and an OFF state to
 quickly dissipate the energy in the motor windings and stop the motor.
 In applications where the motor is driven by a battery, particularly
 portable lawn and garden equipment, the microprocessor also senses the
 battery voltage level and actuates a warning if the battery voltage falls
 below a first voltage threshold level, and shuts down the motor if the
 battery voltage falls below a second voltage threshold level. The
 microprocessor also senses the temperature around a selected component and
 shuts down the motor if the sensed temperature exceeds a predetermined
 temperature threshold level. The motor shutdown is rapidly achieved using
 the method described above.

Corresponding reference characters indicate corresponding parts throughout
 the several views. Although the drawings represent an embodiment of the
 present invention, the drawings are not necessarily to scale and certain
 features may be exaggerated in order to better illustrate and explain the
 present invention. The exemplification set out herein illustrates an
 embodiment of the invention, in one form, and such exemplification is not
 to be construed as limiting the scope of the invention in any manner.
 DETAILED DESCRIPTION OF THE INVENTION
 The embodiment disclosed below is not intended to be exhaustive or limit
 the invention to the precise form disclosed in the following detailed
 description. Rather, the embodiment is chosen and described so that others
 skilled in the art may utilize its teachings.
 The present invention is a microprocessor controlled motor controller which
 uses a current limiting device to prevent excessive current though an
 electric motor. The present invention is particularly suitable for use
 with common portable lawn and garden maintenance equipment. Such lawn and
 garden maintenance equipment usually comprise a rotating blade operatively
 coupled to a shaft of the electric motor. Such lawn and garden maintenance
 equipment include, but is not limited to, lawn mower, tiller, snowblowers,
 and the like.
 FIG. 5 illustrates a conventional lawn mower 50 having electric motor and
 power supply assembly 52 which comprises motor M1 operatively coupled to a
 rotating blade (not shown) disposed under deck 55 and controlled by motor
 controller 10 which is contained in housing 56. Lawn mower 50 includes
 operator-controlled bail switch assembly 57. For clarity, the various wire
 connections are not shown in FIG. 5. It is to be understood that a variety
 of equipment arrangements are possible and motor controller housing 56 may
 be placed at many different locations on the lawn and garden equipment.
 Referring to FIG. 1, motor controller 10 comprises processing unit U1 which
 is operatively connected to and controls the operation of motor M1. Motor
 M1 may comprise any suitably sized motor used in portable lawn and garden
 maintenance equipment, for example, fractional and integral horsepower
 motors. An operator controls the starting and stopping of motor M1 using
 bail switch assembly 57 coupled to motor controller 10 via wire harnesses
 connected to motor controller 10. When present motor controller 10 and
 motor M1 are used in lawn and garden maintenance equipment, such as
 lawnmowers, the operator-controlled bail switch assembly 57 may comprise
 any one of a number of conventionally known bail assemblies which require
 the user to actuate both a start button and a control lever. Such bail
 assemblies include, but are not limited to, model no. 602392 manufactured
 by Capro Inc. of Swainsboro, Ga.
 As shown in FIG. 5, operator-controlled bail switch assemblies may comprise
 start button 58 and lever 59 which must be actuated at the same time to
 start motor M1. To start motor M1, the user first depresses start button
 58 and keeps start button 58 depressed while drawing lever 59 toward the
 end of the lawnmower handle. When lever 59 has reached the run position,
 start button 58 is locked in and may be released. Thus, the start
 procedure requires the user to use both hands. When a trip condition
 occurs and motor M1 is shut down, start button 58 is released from the
 locked in position. To restart the motor, the user must release lever 59
 and repeat the start procedure.
 Processing unit U1 may comprise any IC data processing device capable of
 and programmed for accepting input signals, comparing the input signals
 with predetermined threshold levels and/or manipulating the input signals
 or comparison data as required, and outputting various control signals in
 response to the input signals and/or signal manipulations or comparisons.
 In the disclosed embodiment, processing unit U1 comprises processor
 PIC16C620 manufactured by Microchip Technology of Chandler, Ariz.
 As shown in FIG. 1, processing unit U1 is connected to various sensors and
 threshold level circuits in order to sense various parameters, compare the
 sensed parameters with various threshold levels and provide outputs to
 control motor M1 and actuate warning indicators. Processing unit U1 senses
 the current in motor M1 via sensing line 30 of motor current sensor 18,
 the temperature around a selected component via sensing line 32 of
 temperature sensor 22, and the voltage of supply battery (not shown) via
 sensing line 31 of battery voltage sensor 20. Processing unit U1 is also
 connected to voltage threshold circuit 24 and current threshold circuit 26
 via input lines 34 and 35, respectively. Threshold circuits 24 and 26 each
 provide an upper or lower threshold level on input lines 34 and 35
 depending on the output on output line 33.
 Processing unit U1 compares the sensed motor current with the upper and
 lower current threshold levels provided on input line 35, to control the
 operation of motor M1. If the sensed motor current exceeds the lower
 current threshold level, processing unit U1 actuates a high current
 warning indicator disposed on daughterboard 12. If the sensed motor
 current exceeds the upper current threshold level, processing unit U1
 actuates current limiting protection using current limiting/cutoff switch
 14 as described further below. If the current limiting condition continues
 for more than a predetermined period of time, six seconds in the present
 case, processing unit U1 shuts down motor M1 using a combination of
 current limiting/cutoff switch 14 and braking switch 16, as also described
 further below. Driver unit U2 provides sufficient power to drive switches
 14 and 16 between the ON and OFF states.
 Processing unit U1 also compares the sensed battery voltage with the upper
 and lower battery threshold levels provided on input line 34. If the
 battery voltage falls below the upper voltage threshold level, processing
 unit U1 actuates a low battery voltage warning indicator on daughterboard
 12. If the battery voltage falls below the lower voltage threshold level
 for a predetermined period of time, indicating the battery is almost
 completely discharged, processing unit U1 shuts down motor M1 and keeps
 the low battery voltage warning indicator in the ON condition after motor
 M1 has been shut down.
 Finally, if the temperature sensed by temperature sensor 22 exceeds a
 predetermined temperature threshold level, processing unit U1 shuts down
 motor M1. The details of motor controller 10 are now described below.
 The power supply for motor controller 10 is shown in FIG. 2. Filtered +24 V
 is initially provided by a supply battery (not shown) through battery
 positive 4, battery negative J5, D1, R17, and C9. The +5 V power supply
 comprises voltage regulator U3, D9 and C7. The +12 V power supply to
 driver unit U2 comprises R18, D10, Q6, and C1.
 As shown in FIGS. 3-4, motor M1 is connected to the supply battery through
 connections J6 and J7. Connection J6 is connected to the positive terminal
 of the battery and J7 is connected to ground via current limiting/cutoff
 switch 14 which comprises power MOSFET Q3. Power MOSFET Q3 is connected in
 series between connection J7 and ground and controls the current flow
 through motor M1. When power MOSFET Q3 is turned ON, current can flow
 through motor M1. When power MOSFET Q3 is turned OFF, current flow through
 motor M1 is interrupted. The state of power MOSFET Q3 is controlled by
 processing unit U1 through output pin 5 of driver unit U2.
 Processing unit U1 senses current flow through motor M1 via motor current
 sensor 18, which comprises R16 and D6, by sensing the current flow through
 power MOSFET Q3. Since the voltage across power MOSFET Q3 varies linearly
 with the current flowing through power MOSFET Q3, the voltage across power
 MOSFET Q3 is proportional to the current through motor M1. Current sensing
 line 30 is connected to pin 2 of processing unit U1. The other end of
 sensing line 30 is connected between R16 and D6, which are connected in
 parallel with power MOSFET Q3. Zener diode D6 clamps the voltage on
 sensing line 30 to prevent excessive input voltage on pin 2.
 During normal operation, processing unit U1 compares the sensed motor
 current with an upper and a lower current threshold level provided by
 current threshold circuit 26, which comprises a voltage divider circuit
 having R5, R7 and R10. The current threshold level is provided at pin 17.
 The current threshold level is alternated between the upper and lower
 current threshold levels by alternating the output of pin 10 of processing
 unit U1. When the output of pin 10 is low, at ground level in this case,
 R7 and R5 are in parallel and sensing line 35 provides the lower current
 threshold level. When the output of pin 10 is high, 5 V in this case, R7
 and R10 are in parallel and sensing line 35 provides the upper current
 threshold level. Processing unit U1 alternately receives these current
 threshold levels at pin 17 and compares these levels with the sensed
 current level received at pin 2.
 The current threshold levels may be adjusted as desired by adjusting the
 values of resistors R5, R7 and R10. In this case, current threshold
 circuit 26 provides a lower current threshold level of about 28 amps and
 an upper current threshold level of about 50 amps. The lower current
 threshold level is used in conjunction with a high current warning
 indicator. The upper current threshold level is used to activate the
 current limiting function and the motor shutdown function.
 When the current level sensed at pin 2 exceeds the lower current threshold
 level provided at pin 17, processing unit U1 activates high current
 warning indication LED D5 on daughterboard 12. The warning indicators are
 disposed on daughterboard 12 which comprises wiring harness J11, DC jack
 J3, and LED's D4 and D5. Daughterboard 12 is connected to motor controller
 10 through wire harness J8 and powered through R1 and D11. To activate the
 high current warning indicator, processing unit U1 provides a high output
 at output pin 7, which turns ON transistor Q1, which in turn allows
 current flow through LED D5. Capacitor C4 is connected across transistor
 Q1 to reduce the voltage fluctuation across transistor Q1.
 In lawn and garden power equipment application, the warning notifies the
 user of a possible obstruction in the blade or rotating member coupled to
 the motor. For example, in the case of a lawnmower, the warning indication
 notifies the user of a reduced quality of cut, possibly due to obstruction
 or build-up of grass around the blade, and that continued use may reduce
 the performance of the unit. The user may then attempt to improve the unit
 performance by, for example, raising the deck, cutting at a slower pace
 and/or reducing the width of the cut.
 As noted above, when the current level sensed at pin 2 exceeds the upper
 current threshold level provided at pin 17, processing unit U1 activates
 the current limiting feature, and when the current limiting condition
 continues for more than about six seconds, processing unit U1 shuts down
 motor M1. Processing unit U1 limits the current through motor M1 by
 controlling the state of power MOSFET Q3 and shuts down motor M1 by
 controlling the state of power MOSFET Q3 and the state of braking switch
 16 which comprises MOSFET Q4. The states of power MOSFETs Q3 and Q4 are
 controlled via output pins 5 and 7 of driver unit U2.
 Power MOSFET Q3 is maintained in the ON state when motor M1 is energized
 and in the OFF state when motor M1 is deenergized. However, during the
 current limiting condition, power MOSFET Q3 is rapidly switched between
 the ON and OFF states to limit the current through motor M1. When power
 MOSFET Q3 is turned OFF, the current through motor M1 begins to decay. By
 rapidly switching power MOSFET Q3 ON and OFF, the motor current can be
 switched between a rising and decaying state and thus maintained below a
 predetermined value. Therefore, processing unit U1 controls the current
 limiting feature by controlling the switching action of power MOSFET Q3.
 In the present invention, current through motor M1 is limited to less than
 about 50 amps, corresponding to about 0.6 V-0.7 V on sensing line 30.
 When the current limiting condition continues for about six seconds,
 processing unit U1 shuts down motor M1 by interrupting the current and
 braking motor M1. Placing power MOSFET Q3 in the OFF state interrupts the
 current through motor M1 and rapidly switching MOSFET Q4 between the ON
 and OFF states brakes motor M1. As shown in FIG. 3B, the source and drain
 of MOSFET Q4 are connected directly across motor M1 to provide a short
 circuit across motor M1 when MOSFET Q4 is turned ON. As described below,
 rapidly switching MOSFET Q4 ON and OFF provides a periodic short circuit
 to rapidly stop motor M1 within a predetermined time period. It is
 important to note that MOSFETs Q3 and Q4 are not ON at the same time.
 As is known, a motor acts as an inductive load which stores energy. To
 rapidly stop the motor, the stored energy must be rapidly dissipated. With
 the short circuit, the combination of wires, motor windings and MOSFET Q4
 quickly dissipates the energy stored in motor M1. The energy is quickly
 dissipated by allowing short circuit current to flow through power MOSFET
 Q4 and associated wiring as MOSFET Q4 is rapidly switched ON and OFF.
 Also, the short circuit provided by MOSFET Q4 allows the energy to be
 safely dissipated through a solid state device disposed in an enclosure
 rather than through the outer enclosure of the power equipment thereby
 providing additional protection for the operator. For example, in the case
 of many lawnmower designs, the stored energy is dissipated by running a
 high current pulse through exterior portions of the lawnmower. Releasing
 such energy so close to the user can be a hazard. The present invention
 obviates this problem by dissipating the energy through the MOSFET and
 other internal components.
 The combination of D12, R21, R22, D13 and D14, protects MOSFET Q4 against
 voltage spikes and sets the gate voltage as necessary. Processing unit U1
 places MOSFET Q4 in the ON or OFF state via output line 29 connected to
 driver unit U2. The combination of D13 and D14 pulls up MOSFET Q4 slightly
 to protect MOSFET Q4 when MOSFET Q4 is in the OFF state during large
 voltage spikes. MOSFET Q4 in the ON state is capable of dissipating large
 amounts of energy, but is susceptible to damage in the OFF state.
 Therefore, during voltage spikes, D13 and D14 momentarily provide
 sufficient voltage to the gate of MOSFET Q4 to marginally turn ON MOSFET
 Q4.
 Line 40 is connected to pin 6 of driver unit U2 to provide a stable ground
 at line 40 during motor shutdown. Since power MOSFET Q3 is OFF at
 shutdown, line 40 provides a stable ground connection for connection J7
 via driver chip U2. The stable ground in combination with the input to the
 gate of MOSFET Q4 ensures a sufficient voltage difference to assure that
 MOSFET Q4 fully switches to the ON state.
 Motor controller 10 also monitors the condition of the supply battery (not
 shown) and provides a warning indication when battery voltage drops below
 a predetermined voltage threshold level. Processing unit U1 compares the
 battery output voltage sensed via input line 31 to a voltage threshold
 level sensed via input line 34 to provide a warning if the battery voltage
 drops below a predetermined level.
 The battery voltage is sensed via input line 31 through a network
 comprising R8, R13, C2 and D2. The voltage divider comprising R8 and R13
 provides a reduced voltage level to processing unit U1. Capacitor C2 is
 connected across resistor R8 to reduce any fluctuation to the signal to
 pin 1. Diode D2 is connected to node 23 to clamp the voltage at node 23
 and prevent excessive input voltage to pin 1.
 The voltage threshold level signals are provided via input line 34 through
 a voltage divider network comprising R6, R9 and R12. Similar to the
 current threshold levels, upper and lower voltage threshold levels are
 provided to pin 18. Again, the output of pin 10 alternates from +5 V and
 ground, to alternately place R6 in parallel with R9 and R12, to
 alternately provide the upper and lower voltage threshold levels.
 Low battery voltage warning LED D4 is activated when the battery voltage
 drops below the upper voltage threshold level, in this case 21.5 V. To
 activate the warning, processing unit U1 provides a high output at output
 pin 6, which turns on transistor Q2 and allows current flow through LED
 D4. This alerts the user that the battery is nearly discharged. The
 remaining time varies according to conditions and can range from 5 to 10
 minutes. Capacitor C5 reduces the fluctuation across transistor Q2.
 If the battery voltage drops below the lower voltage threshold level, 19.25
 V in this case, and remains below that level for about six seconds,
 processing unit U1 shuts down motor M1 using the procedure described
 above. The low battery voltage warning remains ON after shutdown in the
 event of a low voltage shut down to indicate to the user that the
 batteries are about 100% discharged and should be placed on recharge.
 The battery is recharged by connecting the battery to a charging unit (not
 shown) through DC jack J3. As shown in FIG. 4, DC jack J3 includes an
 interlock which prevents motor M1 from being energized when the charging
 unit is connected to DC jack J3. The interlock is implemented using lines
 38 and 39 which are in electrical contact with each other when the charger
 is disconnected, but are electrically disconnected when a charger is
 inserted into DC jack J3. Line 39 is connected to run/stop line 36, which
 is connected to pin 3 of processing unit U1, via wire harnesses J8 and
 J11. Line 36 informs processing unit U1 whether to run or stop motor M1.
 When bail switch is operated to pull node 36A to ground, the unit will
 run. Therefore, when the insertion of a charger in DC jack J3 is detected
 by the break in electrical contact between lines 38 and 39, processing
 unit U1 prevents or stops the operation of motor M1 using the procedure
 described above. A suitable DC jack model for this purpose is switchcraft.
 Protection against overheating is provided by temperature sensing line 32
 connected to pin 9 of processing unit U1. Temperature sensing line 32
 includes thermistor RT1 having a resistance characteristic which varies
 with temperature. Thermistor RT1 is preferably disposed near power MOSFET
 Q3 which is one of the most temperature sensitive components of the
 present motor controller. Placing thermistor RT1 near power MOSFET Q3
 allows control circuit 10 to be responsive to a combination of ambient
 temperature and heating caused by the current flowing through motor M1.
 Processing unit U1 automatically shuts down motor M1 using the process
 described above when the temperature sensed by thermistor RT1 exceeds a
 predetermined temperature threshold level. In this case, processing unit
 U1 is set to shut down motor M1 when the detected temperature reaches
 about 150.degree. C.
 Battery drain due to current through battery voltage sensor 20 and
 threshold level circuits 24 and 26 is minimized by the switched ground
 connection to MOSFET Q5. MOSFET Q5 has a source and drain connected to the
 switched ground and ground, respectively, and a gate connected to pin 12
 of processing unit U1. Therefore, current flows through the respective
 sensing and threshold level circuits only when MOSFET Q5 is in the ON
 state. When motor M1 is operating, processing unit U1 turns MOSFET Q5 ON
 to allow current flow, but when motor M1 is shut down, processing unit U1
 maintains MOSFET Q5 in the OFF state to prevent current flow thereby
 reducing the battery drain. Essentially, MOSFET Q5 is turned ON to enable
 the various voltage dividers whenever MOSFET Q3 is turned ON.
 Operator-controlled bail switch 57 is connected to the present motor
 controller via wire harness connectors J8 and J11. As shown in FIG. 4,
 bail switch 57 is connected to input pins 4 and 8 of connector J11 and
 includes series connected contacts 60 and 61 associated with start button
 58 and lever 59, respectively. As pin 4 is connected to run/stop line 36,
 it can be seen that the actuation of start button 58 and lever 59, thus
 the closing of contacts 60 and 61, will pull line 36 to ground, thereby
 allowing motor M1 to run. Although the present invention uses two serially
 connected contacts which close to allow motor M1 to run, it is to be
 understood that any arrangement of contacts connected to the run/stop line
 36 which requires the operator to actuate start button 58 and lever 59 may
 be used.
 Resonator Y1 is connected to pins 15 and 16 and provides the timing for
 processing unit U1.
 The motor controller of the present invention as used with portable power
 equipment, such as a lawn and garden maintenance equipment which uses the
 modified bail switch, operates as follows. The user starts motor M1 by
 using a modified bail switch which requires the user to depress a start
 button and move a lever to the run position. The start button remains
 depressed as long as the operator presence control lever is in the run
 position. When motor M1 is started, processing unit U1 continuously
 monitors the current through motor M1.
 If the motor current exceeds the lower current threshold level, processing
 unit U1 will actuate a high current warning indicator to notify the
 operator of a possible problem. If the motor current exceeds the upper
 current threshold level, processing unit U1 switches MOSFET Q3 rapidly to
 limit the current through motor M1. If the current limiting condition
 exists for about six continuous seconds, processing unit U1 shuts down
 motor M1 using power MOSFETs Q3 and Q4. Here, the high current warning
 indicator remains lit after shutdown to indicate that the motor was shut
 down due to high current.
 If the battery voltage falls below the upper voltage threshold level,
 processing unit U1 activates a low battery voltage indicator. If the
 battery voltage drops below the lower voltage threshold level for more
 than about six continuous seconds, processing unit U1 shuts down motor M1
 using power MOSFETSs Q3 and Q4. Here, the low voltage warning indicator
 remains lit after shutdown to indicate that the motor was shut down due to
 low battery voltage.
 If the temperature sensed by the thermistor rises above a predetermined
 temperature threshold, processing unit U1 shuts down motor M1 using the
 procedure described above.
 In each of the shutdown cases described, the operator may restart motor M1
 by resetting the starting mechanism and repeating the starting sequence.
 In the case of the modified bail switch described above, the operator must
 release the operator presence control lever, depress the start button and
 move the lever to the run position.
 The present invention may be practiced by using the following values for
 the circuit elements described above: