General Properties of Electric Motors

Home | Articles | Forum | Glossary | Books


All electric motors are governed by the laws of electromagnetism, and are subject to essentially the same constraints imposed by the materials (copper and iron) from which they are made. We should therefore not be surprised to find that at the fundamental level all motors -- regardless of type -- have a great deal in common.

These common properties, most of which have been touched on in this section, are not usually given prominence. Books tend to concentrate on the differences between types of motors, and manufacturers are usually interested in promoting the virtues of their particular motor at the expense of the competition. This divisive emphasis causes the under lying unity to be obscured, leaving users with little opportunity to absorb the sort of knowledge which will equip them to make informed judgments.

The most useful ideas worth bearing in mind are therefore given below, with brief notes accompanying each. Experience indicates that users who have these basic ideas firmly in mind will find themselves able to understand why one motor seems better than another, and will feel much more confident when faced with the difficult task of weighing the pros and cons of competing types.

Operating temperature and cooling


Above: Steel frame cage induction motor,150 kW (201 h.p.), 1485 rev/min. The active parts are totally enclosed, and cooling is provided by means of an internal fan which circulates cooling air round the interior of the motor through the hollow ribs, and an external fan which blows air over the case.

The cooling arrangement is the single most important factor in determining the output from any given motor.

Any motor will give out more power if its electric circuit's worked harder (i.e. if the current is allowed to increase).The limiting factor is normally the allowable temperature rise of the windings, which depends on the class of insulation.

For class F insulation (the most widely used) the permissible temperature rise is 100K, whereas for class H it's 125K. Thus if the cooling remains the same, more output can be obtained simply by using the higher-grade insulation. Alternatively, with a given insulation the output can be increased if the cooling system is improved. A through ventilated motor, for example, might give perhaps twice the output power of an otherwise identical but totally enclosed machine.


Wiki Series: Laboratory Manual for Electronics

Three-Phase Motors

Objectives

  • • Determine if a 9 lead dual voltage three-phase motor is connected wye or delta.
  • • Test continuity of motor windings.
  • • Test motor windings for poor insulation.
  • • Connect a dual voltage 9 lead three-phase motor.

Many 3-phase squirrel cage motors are designed in such a manner that they can be connected to 240 or 480 volts. The majority of these motors have 9 "T" leads at the terminal connection box. The manner in which these T leads are connected determines if the motor will operate on 240 or 480 volts.

Wye and Delta Connections There are two basic ways of connecting the stator windings for three-phase dual voltage motors: wye and delta. The standard numbering for stator windings of both wye and delta connected motors is shown in ill. 1. Stator windings are numbered by starting at an outer point with #1 and proceeding around in an ever-decreasing spiral. Notice that the opposite end of the winding that begins with #1 is #4. The opposite end of the winding that begins with #2 is #5. These numbers have been standardized and are used by most manufacturers.

ill. 1-- Standard numbering for three-phase motors. Standard numbering for a wye connected motor; Standard numbering for a wye connected motor ill. 2 High-voltage connections. High-voltage delta connection; High-voltage wye connection

High- and Low-Voltage Connections

When a motor is to be connected for high-voltage operation (generally 480 to 575 volts), the windings are connected in series, as shown in ill. 2. In a series circuit the sum of the voltage drops across the components must equal the applied voltage. If 480 volts were to be connected across the delta connected motor, each of the windings of that phase would have voltage drop of 240 volts.

When a motor is to be connected to operate on low voltage (generally 240 to 208 volts), the stator windings are connected in parallel, as shown in ill. 3. Components connected in parallel have the same voltage applied to them. Therefore, if 208 volts is connected to the motor, all stator windings would have an applied voltage of 208 volts.

Determining If the Windings are Connected Wye or Delta

An ohmmeter can be used to determine if a 9 lead motor is connected in wye or delta. In a wye connected motor, the opposite ends of leads 7, 8, and 9 are connected together, as shown in ill. 1. If an ohmmeter indicates continuity between these three leads, the motor is wye connected. In a delta connected motor, the ohmmeter should indicate continuity between 1, 4, and 9; 2, 5, and 7; and 3, 6, and 8.

ill. 3--Low-voltage connections. Low-voltage delta connection ; Low-voltage wye connection

ill. 4 --Hand-crank megger

Testing Stator Windings

To test the condition of the insulation of stator windings, it's necessary to use an ohmmeter that can supply a high voltage and measure high resistance. This device is a megohmmeter or megger, shown in ill. 4. The megger measures the resistance of the insulation.

To test the motor, connect one megger lead to the case of the motor and the other to one of the motor line leads (it's assumed that the motor is connected for high or low voltage). The meter should indicate several million ohms of resistance. A low resistance reading indicates a motor that may fail in the near future. Low readings can also be caused by moisture in the stator winding. If a motor is left standing in a high humidity climate for long periods without being operated, moisture can form in the stator. If this is the case, it may be necessary to place the motor in a warm, dry environment until the moisture evaporates.

LAB EXERCISE

Materials Required

1 - 9 lead dual voltage three-phase motor

1 - 208-volt three-phase power supply

1 – ohmmeter

1 - megohmmeter

1. Using the 9 lead three-phase motor provided, separate all nine leads.

2. Use an ohmmeter to test for continuity between T7, T8, and T9. Does the meter indicate continuity between these leads? Yes/no

3. If the answer to step 2 is yes, proceed to step 4. If the answer is no, skip step 4.

4. Use the ohmmeter to test for continuity between the following T leads. Write yes beside the leads that indicate continuity and no beside the ones that don't.

T1-T2 _ T1-T3 _ T1-T4 _ T1-T5 _ T1-T6 _ T1-T7 _ T1-T8 _ T1-T9 _ T2-T3 _ T2-T4 _ T2-T5 _ T2-T6 _ T2-T7 _ T2-T8 _ T2-T9 _ T3-T4 _ T3-T5 _ T3-T6 _ T3-T7 _ T3-T8 _ T3-T9 _ T4-T5 _ T4-T6 _ T4-T7 _ T4-T8 _ T4-T9 _ T5-T6 _ T5-T7 _ T5-T8 _ T5-T9 _ T6-T7 _ T6-T8 _ T6-T9 _ T7-T8 _ T7-T9 _ T8-T9 _

5. Compare the continuity readings with the schematic drawing of a 9 lead wye connected motor shown in ill. 1. Do the readings confirm the connection diagram? Yes/no

6. Use a megohmmeter to test the insulation of the stator windings. Set the megger output voltage for a value close to the rated voltage of the motor. Connect one lead of the megger to the case of the motor. Measure the resistance between the motor case and each of the following:

T1-case _ ? T2-case _ ? T3-case _ ? T7-case _ ?

7. Connect the motor for low-voltage operation. Refer to the connection diagram on the motor nameplate or the diagram shown in ill. 3 for a low-voltage wye connection.

8. Connect the motor to a 208 volt, three-phase power source.

9. Turn on the power and notice the direction of rotation. Viewing the motor from the rear, does the motor turn in a clockwise or counterclockwise direction?

10. Turn off the power supply.

11. Reverse two of the line leads connected to the motor.

12. Turn on the power and notice the direction of rotation. Did the motor reverse its direction of rotation? Yes/no

13. Turn off the power supply and disconnect the motor. Return the components to their proper place.

QUIZ

1. What are the two ways that the stator windings of three-phase motors can be connected?

2. An ohmmeter reveals that a 9 lead dual voltage motor has continuity between T7, T8, and T9. Is the motor wye or delta connected?

3. An ohmmeter reveals that there is continuity between T1 and T4. Does this indicate a wye connected motor?

4. An ohmmeter reveals continuity between T5 and T7. Does this indicate a delta connected motor?

5. A 9 lead dual voltage motor is to be connected for high-voltage operation. An ohmmeter test indicates that the stator windings are delta connected. To which T lead(s) should T5 be connected?

6. A 9 lead dual voltage motor is to be connected for low-voltage operation. An ohmmeter test indicates that the motor is delta connected. To which T lead(s) should T6 be connected?

7. A 9 lead dual voltage motor is to be connected for low-voltage operation. An ohmmeter test reveals that the motor is delta connected. To which T lead(s) should T2 be connected?

8. A 9 lead dual voltage motor is to be connected for low-voltage operation. An ohmmeter test reveals that the motor is wye connected. To which T lead(s) should T4 be connected?

9. What piece of test equipment should be used to test the insulation of the stator winding?

10. How is the direction of rotation of a three-phase motor reversed?

<< Prev. | Next >>


Torque per unit volume

For motors with similar cooling systems, the rated torque is approximately proportional to the rotor volume, which in turn is roughly proportional to the overall motor volume .

This stems from the fact that for a given cooling arrangement, the specific and magnetic loadings of machines of different types will be more or less the same. The torque per unit length therefore depends first and foremost on the square of the diameter, so motors of roughly the same diameter and length can be expected to produce roughly the same torque.

Power per unit volume--importance of speed

Output power per unit volume is directly proportional to speed.

Low-speed motors are unattractive because they are large, and there fore expensive. It is usually much better to use a high-speed motor with a mechanical speed reduction. E.g., a direct drive motor for a portable electric screwdriver would be an absurd proposition.

Size effects --specific torque and efficiency

Large motors have a higher specific torque (torque per unit volume) and are more efficient than small ones.

In large motors the specific electric loading is normally much higher than in small ones, and the specific magnetic loading is somewhat higher. These two factors combine to give the higher specific torque.

Very small motors are inherently very inefficient (e.g. 1% in a wrist watch), whereas motors of over say 100 kW have efficiencies above 95%.

The reasons for this scale effect are complex, but stem from the fact that the resistance volt-drop term can be made relatively small in large electromagnetic devices, whereas in small ones the resistance becomes the dominant term.

Efficiency and speed

The efficiency of a motor improves with speed.

For a given torque, power output rises in direct proportion to speed, while electrical losses are - broadly speaking - constant. Under these conditions, efficiency rises with speed.

Rated voltage

A motor can be provided to suit any voltage.

Within limits it's always possible to rewind a motor for a different voltage without affecting its performance. A 200 V, 10 A motor could be rewound for 100 V, 20A simply by using half as many turns per coil of wire having twice the cross-sectional area. The total amounts of active material, and hence the performance, would be the same.

Short-term overload

Most motors can be overloaded for short periods without damage.

The continuous electric loading (i.e. the current)cannot be exceeded without damaging the insulation, but if the motor has been running with reduced current for some time, it's permissible for the current (and hence the torque)to be much greater than normal for a short period of time. The principal factors which influence the magnitude and duration of the permissible overload are the thermal time-constant (which governs the rate of rise of temperature) and the previous pattern of operation. Thermal time constants range from a few seconds for small motors to many minutes or even hours for large ones. Operating pat terns are obviously very variable, so rather than rely on a particular pattern being followed, it's usual for motors to be provided with over temperature protective devices (e.g. thermistors)which trigger an alarm and /or trip the supply if the safe temperature is exceeded.

QUIZ

1) The current in a coil with 250 turns is 8A. Calculate the MMF.

2) The coil in (1) is used in a magnetic circuit with a uniform cross section made of good-quality magnetic steel and with a 2 mm air gap. Estimate the flux density in the air-gap, and in the iron.

(u_0 = 4 pi x 10^-7 H/m.)

How would the answers change if the cross-sectional area of the magnetic circuit was doubled, with all other parameters remaining the same?

3) Calculate the flux in a magnetic circuit that has a cross-sectional area of 18 cm^2 when the flux density is 1.4T.

4) A magnetic circuit of uniform cross-sectional area has two air-gaps of 0.5 and 1 mm respectively in series. The exciting winding provides an MMF of 1200 Amp-turns. Estimate the MMF across each of the air-gaps, and the flux density.

5) The field winding in a motor consumes 25W when it produces a flux density of 0.4T at the pole-face. Estimate the power when the pole-face flux density is 0.8T.

6) The rotor of a DC motor had an original diameter of 30 cm and an air-gap under the poles of 2 mm. During refurbishment the rotor diameter was accidentally reground and was then undersized by 0.5 mm. Estimate by how much the field MMF would have to be increased to restore normal performance. How might the extra MMF be provided?

7) Estimate the minimum cross-sectional area of a magnetic circuit that has to carry a flux of 5 mWb. (Don't worry if you think that this question cannot be answered without more information you are right.)

8) Calculate the electromagnetic force on:

a) a single conductor of length 25 cm, carrying a current of 4A, exposed to a magnetic flux density of 0.8 T perpendicular to its length.

b) a coil-side consisting of twenty wires of length 25cm, each carrying a current of 2A, exposed to a magnetic flux density of 0.8 T perpendicular to its length.

9) Estimate the torque produced by one of the early machines illustrated in ill. 11 given the following:-Mean air-gap flux density under pole-face = 0.4 T; pole-arc as a percentage of total circumference =75%; active length of rotor = 50 cm; rotor diameter = 30 cm; inner diameter of stator pole = 32 cm; total number of rotor conductors = 120; current in each rotor conductor = 50 A.

10) Motor designers often refer to the 'average flux density over the rotor surface'. What do they really mean? If we want to be really pedantic, what would the average flux density over the (whole) rotor surface be?

11) If the field coils of a motor are rewound to operate from 220V instead of 110V, how will the new winding compare with the old in terms of number of turns, wire diameter, power consumption and physical size?

12) A catalogue of DIY power tools indicates that most of them are available in 240 V or 110 V versions. What differences would you expect in terms of appearance, size, weight and performance?

13) Given that the field windings of a motor don't contribute to the mechanical output power, why do they consume power continuously?

14) For a given power, which will be larger, a motor or a generator?

15) Explain brie fly why low-speed electrical drives often employ a high-speed motor and some form of mechanical speed reduction, rather than a direct-drive motor.

Top of Page

PREV: Equivalent Circuit NEXT: POWER ELECTRONIC CONVERTERS FOR MOTOR DRIVES HOME