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- Comparison between a universal motor and a DC
series motor: A universal motor is one which operates
both on AC and DC supplies. It develops more horsepower
per Kg. weight than any other AC motor, mainly due to its
high speed. The principle of operation is the same as that
of a DC motor. Though a universal motor resembles a DC
series motor, it requires suitable modification in the
construction, winding and brush grade to achieve sparkless
commutation and reduced heating when operated on AC
supply, due to increased inductance and armature reaction. A universal motor could, therefore, be defined as a series
or a compensated series motor designed to operate at
approximately the same speed and output on either direct
current or single phase alternating current of a frequency
not greater than 50 Hz, and of approximately the same
RMS voltage. Universal motor is also named as AC single
phase series motor, and Fig 1 shows the multi-line
representation according to B.I.S. 2032, Part IV. The main parts of a universal motor are an armature, field winding, stator stampings, frame, end plates and brushes as shown in Fig 2. The increased sparking at the brush position in AC operation is reduced by the following means
- Providing compensating winding to neutralize the armature M.M.F. These compensating windings are either short-circuited windings or windings connected in series with the armature.
- Providing commutating inter-poles in the stator and connecting the inter-pole winding in series with the armature winding.
- Providing high contact resistance brushes to reduce sparking at brush positions.
Comparison: Universal Motor vs. DC Series Motor
Feature | Universal Motor | DC Series Motor |
Power Supply | Can run on AC and DC supplies. | Can run smoothly on DC supply. However, when connected to AC supply, it produces heavy sparks at brush positions and becomes hot due to armature reaction and rough commutation. |
Compensating Winding | Is a must for large machines. | Does not require compensating winding. |
Inter-poles | Provided in larger machines. | Does not require inter-poles normally. |
Brushes | High resistance grade brushes are necessary. | Normal grade brushes will suffice. |
Air Gap | Air gap is kept to the minimum. | Normal air gap is maintained. |
- Operation: A universal motor works on the same principle
as a DC motor, i.e. force is created on the armature
conductors due to the interaction between the main field
flux and the flux created by the current-carrying armature
conductors. A universal motor develops unidirectional
torque regardless of whether it operates on AC or DC
supply. Fig 3 shows the operation of a universal motor on
AC supply. In AC operation, both field and armature
currents change their, polarities, at the same time resulting
in unidirectional torque.
- Characteristic and application: The speed of a universal
motor is inversely proportional to the load, i.e. speed is low at full load and high on no load. The speed reaches a
dangerously high value due to low field flux at no loads. In
fact the no-load speed is limited only by its own friction and
windage losses. As such these motors are connected
with permanent loads or gear trains to avoid running at
no-load, thereby avoiding high speeds.
- Change of rotation: Direction of rotation of a universal motor can be reversed by reversing the flow of current through either the armature or the field windings. It is easy to interchange the leads at the brush holders as shown in Fig 5. However, when the armature terminals are interchanged in a universal motor having compensating winding, care should be taken to interchange the compensating winding also to avoid heavy sparking while running.
- Speed control of universal motor: The following methods are adopted to control the speed of a universal motor.
- Series resistance or applied voltage control method: The motor speed is controlled by connecting a variable resistance in series with the motor. Foot-pedal operated sewing machines incorporate such a control. Fig shows the connections.
- Tapped field method: In this method,the field winding is tapped at 2 or 3 points and the speed is controlled by the varying field MMF. Fig shows such a connection. Most of the domestic food mixers employ this method of speed control.
- Centrifugal switch method: A centrifugal mechanism adjusted by an external lever is connected in series with the motor as shown by Fig 8. If the speed reaches beyond a certain limit, according to the lever setting, the centrifugal device opens the contacts and inserts the resistance R in the circuit, which causes the motor speed to decrease. When the motor speed falls and reaches a predetermined value, the centrifugal switch contact closes, the motor gets reconnected to the supply and the speed rises. Some advanced type of food mixers employ this sort of speed control. A capacitor is used across the centrifugal switch to reduce the switching spark and to suppress the radio interference. Apart from the above methods of speed control, a thyristor is used in certain food mixers to control the speed electronically.
- Troubleshooting of universal motor As the name suggest universal motors can operate on either AC or DC supply. By a compromise of design fractional horse power motors may be built to operate satisfactorily on either 240 V 50 Hz AC or direct current at 240 volts. Such motors are known as universal motors.
Advantages of universal motors
These motors develop high starting torque and have the ability to adjust the torque and speed proportionally when loaded.
Universal motors can operate on direct current or AC supply.
Tapped fields provide an easy method of controlling speed.
Disadvantages of universal motors
Since these motors operate at very high speed upto 40,000 rpm considerable air noise is present.
Because of the large increase in the power input under stalled conditions and the loss of motor cooling, they can burn out within a short time when overloaded too much.
Useful for intermittent duty application only.
They produce radio and television interference.
Troubleshooting Chart for Universal Motor
1. Trouble: Motor fails to start
Causes Mode of Testing Rectification a) No voltage due to blown fuse. Test by test lamp or voltmeter. Replace the blown fuse. b) Open overload relay of starter. Test by test lamp or voltmeter. Reset or rectify the overload relay contact. c) Low voltage due to improper supply voltage. Test by voltmeter. Rectify the loose connections in the switch & fuse. d) Open circuited field or armature. Test by ohmmeter or Megger. If possible, join properly or replace the winding. e) Improper contact of carbon brushes with commutator. Visual inspection and test by test lamp. Adjust proper contact of brushes. f) Dirty commutator. Visual inspection and test by test lamp. Clean by buffing the commutator using smooth sandpaper. 2. Trouble: Shock to the operator
Causes Mode of Testing Rectification a) Grounded field or armature circuit due to weak insulation. Test by Megger or test lamp. Rectify the defect and apply shellac varnish to the winding. 3. Trouble: Overheating of motor
Causes Mode of Testing Rectification a) Insufficient earth. Test by Megger or test lamp. Provide proper earth to the motor. b) Shorted coil of field or armature. Visual inspection and resistance measurement. Rewind field or armature coil which is shorted. c) Tight bearing due to worn out or lubricated bearing. Test the shaft free for rotation. Check for shoulder or rear bushing. Clean the bearings and check for damage or replace them. d) Heavy sparking at commutator due to shorted coils, dirt, or other damage. Visual inspection. Clean the commutator and remove shorted field or armature. 4. Trouble: Humming sound / Lack of torque due to overheat
Causes Mode of Testing Rectification a) Grounded field or armature. Test by Megger. Rewind the winding. b) Short circuited field. Test by Ohmmeter. Rewind shorted field winding. c) Shorted armature coil. Test by Growler. Rewind shorted armature winding.
