TechArchive

Original article date: November 1998

Choosing an electric motor and gearbox would appear simple until you appreciate there are more than 100 million possible combinations of motor and frame coupled with a variety of gearboxes.

The best starting point is probably to look at the chosen application and to examine three different basic motor types – induction commutator and permanent magnet. If the available power supply is DC the straightforward choice is a permanent magnet motor. In the Parvalux range power outputs span 7 to 350W with full load speed of 1500 to 5000rpm with reversible rotation as a standard feature.

If the supply is 220/240V DC the recommendation may be commutator motor either shunt would or series wound the latter also available for 220/240V AC. Output powers span a range from 10 to 190W with full load speeds of 1500 to 6500rpm.

The province of the induction motor is as a workhorse for 100/120 or 220/240V single phase AC or 380/440V three-phase. Outputs range from 3 to 250W at full load constant speeds of 900 1400 or 2800rpm.

Over the recommended operating range the speed of an induction motor is reasonably constant declining only slightly with increasing torque up to full rated load. This is largely true of DC shunt-wound commutator motors.

Is reversibility important? If so choose anything except a shaded pole induction motor. If instantaneous reversal of rotation while the motor is running a three-phase induction motor is necessary.

In applications involving a lot of stopping and starting a capacitor start induction run may be favoured where loads are not of high inertia and stop/starts are fewer than say 20 an hour. For more frequent switching the permanent capacitor induction motor should be considered. If the rate exceeds twice a minute a commutator motor may be best.

Can continuous duty motors/gearmotors be used intermittently?

Continuous duty products can be used intermittently. Operation at loads higher than nameplate ratings is possible for short periods as long as there are rest periods for cooling. Loads must not exceed the mechanical rating for the gears. The temperature of the motor should be monitored during testing to make sure it is not overheating.

Can single speed motors and gearmotors ever be operated as variable speed?

Three phase and some permanent split capacitor designs may be operated as variable speed with a frequency inverter. Commutator motors can be operated as variable speed using a thyristor controller.

What is meant by the term self-locking?

Self-locking refers to the tendency of some gearing to resist movement when the gearmotor is at rest and the load is attempting to move An example is a load on a conveyor belt trying to drive the system backwards. Parallel shaft gearheads and right-angle gearheads with ratios greater than 25:1 are often considered to be self-locking – they will resist movement up to their torque rating. As the gearing nears the end of its useful life or if subjected to overload it may wear to the point where it is no longer self-locking. Self-locking gearing is not recommended for preventing movement in applications where the movement may cause injury or damage when an external brake is recommended.

Can more than one motor be operated from a single control?

Operation of more than one motor from a single control is not normally recommended. However when cost is a primary concern a multiple motor/single control system with either permanent magnet motors and control or three-phase motors and control may be implemented.

More than one permanent magnet motor may be operated with a single permanent magnet control as long as the sum of the motor currents does not exceed the rated output current of the controller. Comprehensive testing is recommended because this type of system may produce fluctuation or drifting speed output.

More than one three-phase motor may be operated with a single three-phase control. Motor speeds should not fluctuate as long as the motors are not overloaded and the sum of the motor currents does not exceed the rated output current of the control.

• Parvalux
• 01202 512575

November 1998