Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
2000-02-29
2001-12-11
Masih, Karen (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C378S169000, C378S169000, C378S169000, C378S169000, C378S169000, C378S169000
Reexamination Certificate
active
06329785
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to controllers for AC driven loads. It relates specifically to pulse width modulated (PWM) controllers for variable speed induction motors.
2. Discussion of the Related Art
Reduction of AC power to an inductive load is often problematic due to lack of a path for the necessary discharge current flow from an inductor when the current is interrupted. Also, power reduction, or interruption, may introduce undesired noise into the line. For example, a traditional dimmer switch utilizing triacs may introduce unwanted noise into the line. Variacs are good for reduction of AC power but are too expensive and bulky for many applications.
As another example, control of induction motors to achieve variable speeds is problematic due to increased noise and inefficient operation. Further, when line power is switched off to the load back emf from inductance may damage sensitive components in the power controller if not properly channeled. Ideally, an “electronic variac” being reasonable in size and cost would solve many practical problems for adjustable power delivery to a load.
PWM controllers have been proposed in the past for use with induction motors to resolve many of the shortcomings inherent in trying to use induction motors in variable speed applications. See, for example,
An Approach to Realize Higher Power PWM AC Controller
, Enjeti and Choi, Applied Power Electronics Conference and Exposition, 1993. APEC '93 Conference Proceedings, 1993, Eighth Annual, pages 323-327 (IEEE: 0-7803-0982-0/93). Enjeti and Choi teach that while the PWM controlled AC controller for regulating power to the motor will decrease unwanted harmonics, commutation problems for controlling switching of inductive load current can be difficult. They propose a four step switching strategy for control of two bidirectional semiconductor switches routing the load current. An experimental controller is detailed as a proof of concept vehicle for steady speed, generalized, inductive load applications.
Four-step switching is a method of controlling bidirectional switches in an alternating current (AC) application so that back electromotive force (emf) from an inductive load is never presented with an open circuit thus allowing it to increase to a large value that destroys the circuitry.
Referring to
FIG. 1
, the bidirectional switches S
1
, S
2
are configured from a pair of inverse serial connected MOSFET transistors S
1
A, S
1
B and S
2
A, S
2
B, respectively. The inverse parallel diodes D
1
A, D
1
B, D
2
A, D
2
B inherent to each MOSFET are shown because they are essential in providing a circulating current path when the inductive load is switched. The complete switch for an inductive load L is comprised of a series switch S
1
and a shunt switch S
2
. In general, the series switch S
2
provides current to the inductive load L and the shunt switch S
1
provides a circulating or freewheel path for the current in the inductive load L when the series switch S
2
is turned off.
It can be seen that conventional switching methods have problems in the configuration of FIG.
1
. If a dead band in switching time is provided between, for instance, the turn off of S
1
and the turn on of S
2
, then the back electromotive force would increase during the time that both switches were off and potentially destroy the circuit. If some overlap in switching time is allowed then the shoot through current as S
1
and S
2
are connected across the AC supply is potentially destructive.
To illustrate the operation of the four-step switching method, one complete switching transition will be described. With reference to
FIGS. 1 and 2
consider the transition between S
1
on and S
2
off to S
1
off and S
2
on, when the polarity of the AC supply is as shown.
Initially S
1
A and S
1
B are on and S
2
A and S
2
B are off.
(1) S
2
B is turned on, nothing happens because S
2
A is off.
(2) S
1
A is turned off, the potential on S
1
A source rises until the on S
2
B forward biases the inverse parallel diode of S
2
A. This provides a circulatory path for the inductive load current and the back electromotive force is trapped at slightly more than line potential.
(3) S
2
A is turned on, nothing significant happens because S
2
A's diode is already conducting.
(4) S
1
B is turned off, nothing happens because S
1
A is already off.
The transition is now complete. It will be appreciated that the switching sequence must be different if the line polarity is opposite and for that reason detection of line polarity must be provided. For a complete description of all transitions and polarities the reader is referred to the Enjeti and Choi article.
However, certain improvements to the Enjeti and Choi four step commutation controller were deemed necessary to make their controller scheme a practical reality for commercial applications of AC power control such as dimmer switches or blower motors of heating, ventilation and air conditioning (HVAC) systems where a variable speed, low noise, long life, fractional horsepower motor could greatly improve the efficiency of HVAC systems. It is these improvements which are the subject of the present invention.
SUMMARY OF THE INVENTION
The present invention comprises, in one aspect, to any single phase AC line power controller for controlling power to an AC operated device, or load. In an exemplary embodiment a PWM controller is coupled to the stator windings of a permanent split capacitor induction motor. The controller may be a retrofit package or integrated into the original motor unit. In a preferred embodiment the controller is used to control power to the main windings only. A separate triac switch is used for controlling power to the auxiliary windings. In other aspects of the present invention the controller comprises an economical power supply providing isolation and power to the controller and gate drivers. A digital circuit is implemented to provide PWM control signals as well as timing for the commutation, or switching, of the semiconductor switches which control inductive load current between the series circuit supplying power to the load and the freewheeling, or shunt, circuit which routes the inductive load current when the series circuit is not conducting. Enhanced control precision and control signal input is thereby attained as well as economy of parts supply and heat reduction in the controller. Also, the programmable logic is constructed to be intolerant of incorrect switching states to prevent motor damage to the controller or the load. Further refinements include isolating only the one set of switch drivers, such as the shunt switch drivers, while leaving the series circuit drivers nonisolated. The series switches provided are also rated at a higher power than the shunt switches. The controller further provides for a one hundred percent duty cycle of the series switches and turning off the shunt switches when the load draw is at a maximum for a given application. Over-current or shorting protection, as well as under-voltage protection is provided through fault sensing circuitry to prevent conditions injurious to the controller or load. Finally, series inductance and shunt capacitance values for a line filter are chosen to absorb energy stored in the inductance of the power line and mitigate resonant effects due to this inductance thus avoiding voltage spikes during commutation and consequent disruption or damage of the circuit.
By utilizing any, or all, of these aspects of the present invention singly or in combination, an economical, efficient, low noise, long life, commercially viable controller system may be obtained.
REFERENCES:
patent: 4641234 (1987-02-01), Bonal
patent: 4788485 (1988-11-01), Kawagishi et al.
patent: 5057760 (1991-10-01), Dadpey et al.
patent: 5268628 (1993-12-01), Dongll
patent: 5883490 (1999-03-01), Moreira
Sewan Choi: Analysis and design of a direct AC to AC matrix converter topology,A Thesis by Sewan Choi, pp. i-x and 1-77, Dec. 1992.
Enjeti, P.N. et al.: An approach to reali
Starkie Alan
Vrionis Nickolas G.
Gas Research Institute
Masih Karen
Pauley Petersen Kinne & Fejer
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