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Electric Motors

The various winding configurations of electric motors have lost much of their significance with the advent of electronic controls. The use of series fields over shunt, of DC power over AC have lost their advantage. Electronic Converters can give a shunt wound motor just as much starting torque as a series wound motor, and can convert AC to DC and back again to optimize voltage, current and frequency for maximum power and efficiency.

No matter how much the controls change, the basic fact remains that all electric motors use an alternating current in one or more windings. AC can be used direct, DC can be converted to AC either electronically or mechanically by the use of a commutator and carbon brushes. Traditional mechanical methods are simple and reliable, but cannot be optimal for all load-speed conditions. Electronic methods are often more efficient over a wider range of load-speed conditions, but are expensive and more easily damaged by transient voltages or currents.

Another basic fact is that motor torque is proportional to the number of armature amp-turns, armature size, and the strength of the magnetic field around the armature. The magnetic field increases with the amp-turns of the field. The increase is proportional until the magnetic path begins to saturate. The strength of the magnetic field is basically limited by the quality of the magnetic material. The amount of the magnetic field is limited by the area of the magnetic material and heat which limits field current. Armature current is also limited by heat. All small, high power motors require external cooling.

Motor power is proportional to torque and speed. One of the biggest advantages of electronic controls is the efficient conversion of power to high frequency AC. Motors with an excess of 400 Horsepower have been built with speeds greater than 20,000 RPM. The ability to put a lot of power is a small space is only available with electronic conversion.

Limitations of mechanical commutation limit maximum motor speed. Part of the job of commutation is to "discharge" windings as they leave one pole and head for another. In series wound motors, the location of the brushes is often shifted to optimize one direction of rotation. Large shunt wound motors often employ commutating field windings (aka interpole windings) between the main field windings. These are wired in series with the armature and induce a voltage in the armature to help neutralize the energy in the windings being commutated. Without commutating field windings, the brushes must dissipate this energy and can overheat at high speeds. High speed operation greatly increases brush wear.

Shunt Motors

The amount of voltage generated by a shunt motor is proportional the magnetic field and motor speed. The speed at which this generated voltage is equal to battery voltage with maximum field current is a critical design parameter in electric vehicle design. This speed is the maximum speed at which full torque is available, and is also the slowest speed at which simple regenerative braking can charge a battery. (Call this regen speed.) Dynamic braking can be used at slower speeds by connecting the armature to a resistive load. Above this speed, the motor is horsepower limited. Of course commutation limitations and mechanical strength limit maximum motor speed.

A typical driving cycle would start accelerating a vehicle from a stop with maximum field current, and armature current limited to a safe value. A constant motor torque would give a constant acceleration (neglecting increasing vehicle losses) until regen speed is reached. Above this speed, the armature is connected directly to the battery and armature current is maintained by a field current which decreases with increasing speed until maximum motor speed is reached. Torque decreases with decreasing field current so acceleration also decreases. At maximum speed, field current is increased to decrease armature current and torque as necessary to maintain a constant speed. To slow down, field current is further increased as necessary to cause the motor to reverse armature current and thus charge the battery. The vehicle slows down until maximum field current is reached at regen speed. Below this speed, dynamic breaking can be used to augment mechanical brakes. Note that mechanical brakes must be used in addition to regenerative braking if a fast stop is necessary. Maximum armature current must not be exceeded. As the motor temperature increases, maximum armature and fields currents must be reduced which significantly reduces maximum motor torque.

Note all the energy supplied to accelerate the vehicle is not recovered during regenerative breaking. One of the major losses is that a battery is recharged at a higher voltage then it is discharged. (i.e. a 144 volt battery might take 170 volts to charge, but only return 120 volts under load.) Another loss is the motors I2R losses, the heat created by current and resistance in the armature, and the power consumed by the field.

To correctly design an electric vehicle, the motor characteristics must be known. For a shunt motor the following questions must be asked:

          (Including maximum and max continuous)

As an example, the following shunt wound motor has been designed:

     DESIGN BATTERY VOLTAGE                 144 volts
     DESIGN BATTERY RESISTANCE              .06 ohm
     REGEN RPM                             3600 rpm
     MAXIMUM RPM                           5000 rpm
     MAXIMUM CONTINUOUS TORQUE               30 lb - ft
     MAXIMUM OUTPUT POWER                    40 hp
     MAXIMUM ARMATURE CURRENT               250 amps
     MAXIMUM FIELD CURRENT                  3.5 amps
     RESISTANCE OF ARMATURE                0.04 ohm
     RESISTANCE OF FIELD                     40 ohms


     field amps        volts/rpm  torque/armature-amp
       0.0               0.001            0.01
       0.5               0.009            0.07
       1.0               0.017            0.14
       1.5               0.024            0.19
       2.0               0.030            0.24
       2.5               0.035            0.28
       3.0               0.038            0.30
       3.5               0.040            0.32

Obviously, the armature current is the difference between the applied EMF and the back EMF divided by the armature resistance. For example, if the applied voltage is 105 volts, and the back EMF is 100 volts, then 5/.05 = 100 amps. If either the motor speed or field current is increased, the back EMF would increase, the armature current would decrease, and torque would decrease. At zero armature current there would be no torque. With further increase in motor speed or field current, the armature current would flow in the opposite direction which would make the motor a generator. i.e. it would absorb mechanical power and supply electrical power.

The back EMF times the armature current represents real work, the power that is output after frictional losses are deducted. I2R losses include:

     1.  (Armature current)2 x (Armature Resistance) 
     2.  (Field current)2  x  (Field Resistance)

For example, with the motor at full continuous power:

     Speed  =  3600 rpm
     Torque =    30 lb - ft
     Power  =  3600 x 30 x 2 x pi / 550    =  20.5 hp
               20.5 x .746                 =  15.3 KW
     Motor Voltage  =  144 - .06 x 125     = 136.5 volts
     Armature Current=(136.5 - 131.5)/0.04 = 125 amps
     Field Current                         = 2.7 amps
     Back EMF         = .0365 x 3600       = 131.5 volts
     Power Input  = 136.5x125 + 2.7x2.7x40 = 17.35 KW   
     Efficiency   =  15.3/17.35            = 88%

Note that the (17.35 - 15.3) 2.05 KW loss is dissipated as heat.

Now if the field current is increased to 3.5 amps, then the back EMF will equal the battery voltage (i.e. .04 * 3600 = 144 volts) and then the armature current and therefore torque would become 0. Or if the speed was increased to 3798 rpm then the back EMF would again equal battery voltage and the armature current and torque would be 0. With the field current at 2.7 amps and a speed of 3996 rpm, then the armature current would be minus 125 amps (i.e. the motor would be a generator charging the battery) and the torque would be minus 30 lb - ft.

If this seems complicated, it really isn't. Just remember the following:

1. The field current determines the strength of the motor's magnetic field.

2. The strength of the magnetic field times speed determines the back EMF. (motor voltage without armature current)

3. The difference between the applied motor voltage and back EMF, divided by the armature resistance, determines the armature current.

4. The strength of the magnetic field times the armature current determines the output torque.

5. The output torque times motor speed determines the output power.

6. The sum of (Armature current)2 x (Armature Resistance) and (Field current)2 x (Field Resistance) determines input power.

7. Output power divided by input power determines efficiency.

Series Wound Motor

To give a feel for a typical Series Wound Motor, a typical Advanced DC Motor is discussed below. Note that the picture below shows the basic construction of a series would motor.

Typical Advanced DC Motor

Advanced DC Motors Model 203-06-4001

  amps = (5 * torque) + 85

  RPM = (286 * volts) / (10 + torque)0.5

The Advanced DC Motor Model 203-06-4001 is a typical, series wound motor that is often used in Electric Vehicles. It can be reasonably characterized with the above two equations. Note that torque is measured in ft-lbs. The motor is about 8 inches in diameter, almost 15 inches long (not including shaft), and weighs about 107 pounds. It is designed for a maximum voltage of 120 volts. With an input of 91 volts and 178 amps, it can continuously deliver 19 horsepower at 5000 rpm. With an input of 86 volts and 322 amps, it can deliver 31.5 horsepower at 3600 rpm for about 5 minutes. Both of these ratings assume standard ambient temperature. Power generation is limited by heat. Maximum speed is about 8000 rpm, but high speed operation results in greatly increased brush wear.

Series wound motors are also known as traction motors since they can generate great torque at low speed. But only for a short time. Great torque requires great current which quickly heats the motor.

It should be noted, that comparing electric motors with gasoline engines is like comparing apples and oranges. Gasoline engines are rated at their peak power, and electric motors are rated at their continuous power. It is for this reason that a 30 hp electric motor will generally perform like a 50 hp gasoline engine in a same weight vehicle. Of course the addition of a large number of batteries will diminish the vehicle's ability to accelerate or climb hills.