DC Motors

Nov. 15, 2002
Industrial applications use direct current motors because the speed-torque relationship can be varied to almost any useful form -- for both motor and regeneration applications in either direction of rotation.

Industrial applications use direct current motors because the speed-torque relationship can be varied to almost any useful form -- for both motor and regeneration applications in either direction of rotation. Continuous operation of dc motors is commonly available over a speed range of 8:1. Infinite range (smooth control down to zero speed) for short durations or reduced load is also common.

Dc motors are often applied where they momentarily deliver three or more times their rated torque. In emergency situations, dc motors can supply over five times rated torque without stalling (power supply permitting).

Dynamic braking (motor-generated energy is fed to a resistor grid) or regenerative braking (motor-generated energy is fed back into the dc supply) can be obtained with dc motors on applications requiring quick stops, thus eliminating the need for, or reducing the size of, a mechanical brake.

Dc motor speed can be controlled smoothly down to zero, immediately followed by acceleration in the opposite direction -- without power circuit switching. And dc motors respond quickly to changes in control signals due to their high ratio of torque to inertia.

Motor types: Wound-field dc motors are usually classified by shunt-wound, series-wound, and compound-wound. In addition to these, permanent-magnet and brushless types are also available, normally as fractional-horsepower motors. Motors may be further classified for intermittent or continuous duty. Continuous-duty motors can run without an off period.

Speed control: There are two ways to adjust the speed of a wound-field dc motor. Combinations of the two are sometimes used.

Shunt-field control: Reel drives require this kind of control. Material is wound on a reel at constant linear speed and constant strip tension, regardless of diameter.

Control is obtained by weakening the shunt-field current of the motor to increase speed and to reduce output torque for a given armature current. Since the rating of a dc motor is determined by heating, the maximum permissible armature current is approximately constant over the speed range. This means that at rated current, output torque varies inversely with speed, and the motor has constant-horsepower capability over its speed range.

This system is good for only obtaining speeds greater than the base speed. A momentary speed reduction below base speed can be obtained by overexciting the field, but prolonged overexcitation overheats the motor. Also, magnetic saturation in the motor permits only a small reduction in speed for a substantial increase in field voltage.

Maximum standard speed range by field control is 3:1, and this occurs only at low base speeds. Special motors have greater speed ranges, but if the speed range is much greater than 3:1, some other control method is used for at least part of the range.

Armature-voltage control: In this method, shunt-field current is maintained constant from a separate source while the voltage applied to the armature is varied. The speed is proportional to the counter emf, which is equal to the applied voltage minus the armature circuit IR drop. At rated current, the torque remains constant regardless of the speed (since the magnetic flux is constant) and, therefore, the motor has constant torque capability over its speed range.

Horsepower varies directly with speed. Actually, as the speed of a self-ventilated motor is lowered, it loses ventilation and cannot be loaded with quite as much armature current without exceeding the rated temperature rise.

Selection: Choosing a dc motor and associated equipment for a given application requires consideration of several factors.

Speed range: If field control is to be used, and a large speed range is required, the base speed must be proportionately lower and the motor size must be larger. If speed range is much over 3:1, armature voltage control should be considered for at least part of the range. Very wide dynamic speed range can be obtained with armature voltage control. However, below about 60% of base speed, the motor should be derated or used for only short periods.

Speed variation with torque: Applications requiring constant speed at all torque demands should use a shunt-wound motor. If speed change with load must be minimized, a regulator, such as one employing feedback from a tachometer, must be used.

When speed must decrease as the load increases, compound or series-wound motors may be used. Or, a power supply with a drooping volt-ampere curve could be used with a shunt-wound motor.

Reversing: This operation affects power supply and control, and may affect brush adjustment, if the motor cannot be stopped for switching before reverse operation. In this case, compound and stabilizing windings should not be used, and a suitable armature-voltage control system should supply power.

Duty cycle: Direct current motors are seldom used on drives that run continuously at one speed and load. Motor size needed may be determined by either the peak torque requirement or heating.

Peak torque: The peak torque that a dc motor delivers is limited by that load at which damaging commutation begins. Brush and commutator damage depends on sparking severity and duration. Therefore, peak torque depends on the duration and frequency of occurrence of the overload. Peak torque is often limited by the maximum current that the power supply can deliver.

Motors can commutate greater loads at low speed without damage. NEMA standards specify that dc machines must deliver at least 150% rated current for 1 min at any speed within rated range, but most motors do much better.

Heating: Temperature is a function of ventilation and electrical/mechanical losses in the machine. Some losses, such as core, shunt-field, and brush-friction losses are independent of load, but vary with speed and excitation.

The best method to predict operating temperature is to use thermal capability curves available from the manufacturer. If curves are not available, temperature can be estimated by the power-loss method. This method requires a total losses versus load curve or an efficiency curve.

For each portion of the duty cycle, power loss is obtained and multiplied by the duration of that portion of the cycle. The summation of these products divided by the total cycle time gives the average power loss. The ratio of this value to the power loss at the motor rating is multiplied by the rated temperature rise to give the approximate temperature rise of the motor when operated on that duty cycle.

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