While conventional brush-commutated DC motors may have advantageous characteristics, including convenience of changing operational speeds, there may be disadvantages such as brush wear, electrical loss, noise and radio frequency interference caused by sparking between the brushes and the segmented commutator, which may limit the applicability of such brush-commutated DC motors in some fields such as the furnace blower control field. Electronically commutated motors, such as brushless DC motors and permanent magnet motors with electronic commutation, have now been developed and generally are believed to have the above-discussed advantageous characteristics of the brush-commutated DC motors without many of the disadvantages thereof while also having other important advantages. Such electronically commutated motors are disclosed in the David M. Erdman U.S. Pat. Nos. 4,015,182 and 4,459,519, for instance. These electronically commutated motors are advantageously employed, for instance, in various air handling applications such as air conditioning for cooling and warming.
Controls for electronically commutated motors include speed controls which generally monitor motor voltage and torque control which generally monitor effective motor current. Such torque controls generally employ a shunt resistor connected to the motor windings which taps a small portion of the motor current to generate a voltage signal across the shunt which has a magnitude directly proportional to the effective motor current. The voltage signal across the shunt is not isolated. Furthermore, high voltage motors (on the order of 400 volts) which carry substantial motor current (approximately 50 amperes) require large shunt resistors having precise resistance values which are expensive and may require cooling which adds to the cost of the controls. In such high voltage motors, the use of a shunt resistor results in the unnecessary dissipation of some power and the voltage signal generated thereby tends to have a very low signal-to-noise ratio.