Common 60hz voltages for three-phase motors are 230 volt, 460 volt and 230/460 volt. Two hundred volt and 575 volt motors are sometimes encountered. In prior NEMA standards these voltages were listed as 208 or 220/440 or 550 volts. Motors with these voltages on the nameplate can safely be replaced by motors having the current standard marking of 200 or 208-230/460 or 575 volts, respectively.

Motors rated 115/208-230 volt and 208-230/460 volt, in most cases will operate satisfactorily at 208 volts, but the torque will be 20% - 25% lower. Operating below 208 volts may require a 208 volt (or 200 volt) motor or the use of the next higher horsepower, standard voltage motor.

Starting Torque: Force produced by a motor as it begins to turn from standstill and accelerate (sometimes called locked rotor torque).

Full Load Torque: The force produced by a motor running at rated full-load speed at rated horsepower.

Breakdown Torque: The maximum torque a motor will develop under increasing load conditions without an abrupt drop in speed and power (sometimes called pull-out torque).

Pull-Up Torque: The minimum torque delivered by a motor between zero and the rated RPM, equal to the maximum load a motor can accelerate to rated RPM.

NEMA Locked Rotor: For three phase motors, 60Hz & 50Hz at rated voltage. (Design B torques in black; Design C torques in blue)

Automatic Reset : After the motor cools, this line-interrupting protector automatically restores power. It should not be used where unexpected restarting would be hazardous.

Manual Reset : This line-interrupting protector has an external button that must be pushed to restore power to the motor. Use where unexpected restarting would be hazardous, as on saws, conveyors, compressors and other machinery.

Resistance Temperature Detectors : Precision-calibrated resistors are mounted in the motor and are used in conjunction with an instrument supplied by the customer to detect high temperatures.

Reliable, easy-to-use units are available today for controlling the speed of AC and DC industrial motors. Both types use solid state devices for power control. DC drives are the more straightforward, commonly using silicon controlled rectifiers (SCR's) to convert AC line voltage to controlled DC voltage, which is then applied to the armature of a direct current motor. The more voltage applied to the armature, the faster it will turn. DC drives of this type represent an excellent value for motors up to approximately 3 HP, allowing 60:1 speed regulation and full torque even at reduced speeds. The most common type of AC drive today begins much the same way as a DC drive does - by rectifying "pulsing" AC line voltage to pulse-free DC voltage. However, instead of outputting the DC voltage, the AC drive must re-introduce pulses into the output in order to meet the needs of an AC motor.

This is done using solid-state switches, such as insulated gate bipolar transistors (IGBT's) or gate turn off SCR's (GTO's). The result is a control technique known as pulse width modulation (PWM), perhaps the most highly regarded type of AC drive for many industrial applications. Motor speed varies with the frequency of the pulses introduced into the output voltage.

Pulse width modulated AC drives offer an extremely wide speed range, a host of control functions including programmable acceleration and deceleration ramps and several preset speeds, excellent energy efficiency and, in many cases, speed and torque precision equal to or closely approaching that of a DC system. Perhaps the major reason for their growing popularity, however, is their ability to work with the wide range of AC induction motors available for industry, usually at a price competitive with that of a DC drive package.