The universal motor is a special kind of ac series motor that performs almost as well as a dc motor. It delivers a high starting torque, its speed can be adjusted, it is small in size, and it is inexpensive.
There are two main differences between the dc series motors and universal motors.
- The universal motor has different winding ratios and thinner iron laminations.
- The universal motor operates on AC power with dc performance.
One of the most outstanding features of a universal motor is its high-power output per pound of weight.
- A typical 0.5 hp, 1750 rpm induction motor weighs about 30 lb.
- A typical 0.5 hp, 19,000 rpm universal motor weighs only about 2.5 lb.
This high ratio of power to weight is a result of the motor’s ability to run at high speed.
Most universal motors drive a tool through a speed reducer. The speed reducer may consist of gears. In other equipment, the speed reducer may be a belt drive or a chain drive.
In some applications, increasing the speed of the motor permits reducing its size, weight, and cost. An example of these savings occurs in vacuum cleaner motors. The size of the fan can be reduced be reduce because the same volume of air can be moved by a smaller fan running at a higher speed. For example, a 1750 rpm motor would require a 53″(inches) fan to move air at the same rate as a 19,000 rpm motor with a 5″(inches) fan.
A universal motor can stand occasional overloads. The stall torque maybe 4 to 10 times the continuous rated torque. For short periods of time, it is possible to operate the motor at a torque up to the stall value.
Performance Characteristics of Universal Motors
Universal motors rated as “small” generally deliver 0.01 to 0.035 hp. Other universal motors deliver 0.05 to 2.0 hp at a rated speed of 5000 rpm or greater.
Although a universal motor performs nearly as well as on ac power as on dc, there are some important differences.
At a given power output and speed, the universal motor can run on a lower effective potential difference if the input is dc or low-frequency ac.
Universal motors are designed to have speed and torque characteristics on ac power as on dc, but other features must be sacrificed. Because most universal motors run on 50 or 60 Hz, they are designed to give the longest brush life at this frequency. The dc characteristics are of secondary importance. For any given power output, universal motors can be built in a range of speeds. The minimum speed of the motor is determined by the limits of efficiency and by the speed at which the motor ceases to be universal. The maximum speed is determined by the bearings and by the strength of the rotating parts.
The Efficiency of Universal Motors
The efficiency of universal motors ranges from 30% for small motors to 75% for large motors. For example, the efficiency of a 9/16 horsepower universal motor at 17,500 rpm is about 70%. A 0.1 horsepower motor, running at 12,000 rpm, has an efficiency of about 50%.
Under a no-load condition, a universal motor “runs away”. Its speed is limited only by air resistance and by friction in the bearings. For this reason, large universal motors are permanently connected to the load. On portable power tools, the bearings, gears, and cooling fans provide enough load to keep the speed down to a safe, manageable value.
Speed Control of Universal Motor
Controlling the speed of a universal motor is quite simple. The speed is sensitive to the current in the armature and to the strength of the magnetic field around the armature. If varying the potential difference across one winding or the other, then, both values we control.
A rheostat or an adjustable transformer can vary the speed from zero to maximum. However, they generally reduce the motor’s torque at low speeds.
Diagrams of two such control circuits see below. The major use of variable-speed universal motors is in portable power tools and in sewing machines
Some universal motors need to operate at two or three different speeds, but not at the speeds in between. These motors are called multi-speed motors rather than variable-speed motors. A multi-speed motor is adjusted by varying the magnetic field by means of a tapped field winding. The diagram above shows a multi-speed universal motor with such a tapped field winding.
A fixed resistor in the circuit serves as a speed governor. The resistor is short-circuited unless the motor runs too fast. Then the shorting contacts open and add the resistor to the motor circuit.
Speed control can also be accomplished by an electronic controller. Most controllers use a silicon-controlled rectifier (SCR). The controller is designed to maintain nearly full-speed torque at low speeds.
Commutation in universal series motors can interfere with the reception of radio and television signals. This interference can be kept to a minimum by placing a radio-noise filter in the power line. Generally, no filter is used as an appliance motor.
Universal Motor Life
Universal motors generally, do not have long operating lives. A typical appliance motor requires brush replacement after 200 to 1200 hours of service.
The time it takes to accumulate this much service depends on the kind of appliance and on how it is used.
- An appliance used infrequently and for short periods of time may take 15 to 25 years to accumulate 200 to 1200 hours of service.
- An appliance operated constantly will take only 8 to 48 days to accumulate the same number of hours of service.
- A larger and more expensive universal motor may be able to run for 3000 to 5000 service hours before, the brushes must be replaced.
For long motor life, the commutator should be resurfaced and reconditioned when the brushes replaced. For short-life motors, there is one commutator bar for each armature slot. To increase the brush life, two commutator bars are provided for each armature slot on some motors.
Electronic speed controllers can cause a major reduction in brush life. Low cost, half-wave circuits have the worst effect. Full-wave rectifiers have the least effect. So, harsh environmental conditions will also shorten the life of the universal motor.
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