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Selecting drives for electric vehicles

Aug. 1, 2000
Industrial, electric-vehicle drives and controls vary according to vehicle design and duty cycle. Here is a primer on drive-system design of fork-lift and automatically guided vehicles.

Battery powered electric vehicles, such as electrically powered forklift trucks, automatically guided vehicles (AGVs), sweeping machines, personnel-transport vehicles, and delivery vans, offer clean, reliable, and economical transportation. Emissions from electric vehicles are low to nonexistent, their range is adequate, and their cost of operation is low.

These vehicles generally have similar components: a battery, a motor (or motors), a motor controller (or controllers), a speed or torque reference (accelerator pedal), and safety equipment such as fuses and contactors. Their differing drive requirements result from their duty cycles, which range from the heavily loaded fork-lift trucks to the lightly loaded AGVs.

Fork-lift trucks

Electric fork-lift trucks are available in many configurations, such as the familiar rubber-tired, four-wheel truck with rear drive wheels under a heavy frame that carries the battery and a counterweight to balance the load on the front forks. They are rated by how much load the forks can safely lift, usually in thousands of pounds with 3,000 and 5,000-lb ratings common.

Selecting and applying the appropriate drive system can be an engineering challenge. The masses of the load, battery, frame, and counterweight make acceleration and braking difficult. High torque is needed to get a 10,000 lb, or heavier, vehicle moving so the usual practice is to power the truck with a serieswound dc traction motor.

Motor characteristics. Series-wound traction motors have fields, connected in series with the armature, capable of carrying the motor armature current. These motors offer high torque at low speed or stall, and increasing speed as the torque load diminishes. The armature voltage in the circuit, Figure 1, is equal to the battery voltage less the voltage drop in the series field. The speed of the motor is equal to the armature voltage less the resistance voltage drop divided by a constant times the field flux:

Ffld = Field flux
Ia = Armature current, A
kn = Speed-field flux constant
N = Motor speed, rpm
R = Resistance, ohm
Va = Armature voltage, V

Armature current flowing through the field coils produces field flux. Reducing armature current reduces the field flux, which increases the speed, assuming the motor produces enough torque to move the load.

The torque produced by this motor is proportional to the product of armature current and field flux:

T = ktIaFfld (2)

kt = Torque constant
T = Motor torque

If the armature current and the field flux are low, the developed motor torque will also be low. Under these conditions, the motor is running at high speed and light load. Applying a torque load to the motor causes the armature to slow down and draw more current from the battery; the additional current increases the field flux and, along with increased armature current, produces more torque. The multiplying action of the torque caused by the product of field flux and armature current creates a large value of torque at stall. This is just what a heavy truck needs to accelerate from a standstill or climb a ramp.

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Using a series-wound motor to power a fork-lift truck offers another advantage: the ability to get high speed at a light load. To increase motor speed, a fieldshunting contactor and resistor let some of the armature current bypass the series field, Figure 2, reducing the field flux. Thus, empty trucks can move long distances in the shortest possible time, while retaining the low-speed, hightorque capability.

Motor control. To properly control a series-wound motor-powered truck, it is necessary to control the current — not the voltage — delivered to the motor. By controlling the current, Figure 3, the operator controls the torque applied to the wheels of the vehicle in much the same way as when driving an automobile. Most vehicles with series-wound motors use thyristor or transistor choppers to control motor current.

The current rating of the chopper is important when selecting equipment with series-wound motors. The rated motor and chopper current for a 5,000- lb rated truck is about 400 A. The battery must supply this current level and the chopper must be properly mounted and cooled (by heat sink or fans).

One disadvantage of using series motors is the inability to reverse the motor by simply reversing the applied voltage to both the field and armature. Doing so maintains a positive value of torque, not a negative value needed for reversing , equation 2.

Most controls reverse torque with controlled slow-down from high to zero speed followed by smooth acceleration to the desired reverse speed. To reverse the motor, chopper controls reverse the series field connections, and thus the field flux direction, with a reversing contactor, Figure 4.

Reversing the motor while the truck is in motion requires a controller that can prevent excessive plugging torque and currents, which can cause abrupt direction reversals and loss of vehicle control. Plugging cycles may also damage commutators, which reduces motor life.

Automatically guided vehicles

AGVs use clean, efficient electric controls and motors to move objects on precisely controlled routes or automatically guided trajectories. They do not have the heavy lifting capacity of fork-lift trucks, and usually carry finished or partially finished goods, tooling fixtures, dies and punches, or other fragile and valuable objects. Often, these loads demand smooth, controlled accelerations and stops, and precise navigation, location, and positioning.

These unmanned vehicles receive velocity and path directions from on-board computers that respond to commands over a variety of communication media. Several vendors offer reliable guidance with buried wires or fluorescent paint traces. More sophisticated techniques use laser auto-location systems to help AGVs accurately locate their positions from laser-illuminated coded targets. With this information, and the stored floor-plan, AGV controls compute safe and efficient routes from their present position to a commanded destination.

The electric drives used in these vehicles are not the conventional choppers used in fork-lift trucks. Usually, the controls and motors are battery-powered servo amplifiers and heavy-duty, permanent- magnet-field motors. The voltage rating of the amplifiers and motors is similar to those in a fork-lift truck since the battery technology is common to both vehicles. However, the continuous current rating is generally lower for AGVs, 30 and 60-A current ratings with 333% overload ratings for clearing low barriers and turning. The four-quadrant operation of servo-type controllers gives the traction motor precise velocity and direction control without reversing contactors. Regeneration of the vehicle’s kinetic energy back to the battery provides smooth braking, which slightly increases the effective capacity of the battery.

Some AGV designs use two sets of permanent- magnet motors and controllers. Two independently controlled motors provide turning control without requiring a special steering motor and wheel arrangement. A common design provides two driving wheels and two castered wheels. Independent speed control of each wheel guides the vehicle on the desired path. Other designs use three powered wheels, one of which is the steered wheel.

AGV motor-controller design. To minimize height of the AGV, compact motors are built into the wheel assembly and send power to the tire through planetary gear trains. An additional motor, for direction control, turns the entire wheel assembly, motor and all. This design makes heavy demands on the motor and controller, because small turning radii make the tires scrub. In sharp turns, up to 333% of rated torque is necessary to successfully make the turn.

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Rating a motor and controller for continuous currents of 333% is uneconomical, so the usual practice is to time the overload. For example, a controller and motor may be rated to deliver 333% current for 2 sec, then automatically reduce the current to 250% for 5 sec, and then drop to 100% of maximum current for a selected time to allow the controller and motor to cool. Such time selections enable the AGV to make turns and climb barriers before the current reduction interferes with proper operation.

Proper operation also demands proper installation that takes into account the thermal rating of the controller and motor. Motors for AGVs and all other applications have a maximum ambient temperature rating and temperature rise. The manufacturer of the motor may also specify mounting configurations to meet these ratings. A thermostatic switch embedded in the motor windings can protect against over temperature. Frequent shutdown due to an open thermostatic switch is a warning of overload, degraded installation conditions, or accumulated soil and debris.

As with the motor, heat is the biggest enemy of controller reliability. Most controllers use high-power transistors — either bipolar, MOS-FET, or IGBT devices — to control motor current.

Also keep in mind that a transistor controller may have an optimum operating frequency that may also be a multiple of an interrupt frequency in the on-board computer. The controller is a high-power pulse generator and may interfere with proper computer operation; therefore, it should have an adjustable frequency to eliminate unwanted side effects.

Controller venders specify maximum power dissipation and maximum mounting temperature for their devices. The AGV builder uses this information to design the controller’s mounting location. Controllers are available as base-plate cooled or fan-cooled devices.

Base-plate cooled devices, Figure 5, are for fork-lift truck designs with large iron counterweights. The counterweights have flat, machined surfaces and serve as large heat sinks. Controllers are often mounted to these surfaces.

AGVs usually do not have these large counterweights so a base-plate cooled controller may mount to a flat, structural section of the machine. The thermal impedance must be low enough to keep the controller’s temperature rise within ratings.

Fan-cooled controllers are self-contained and do not require a cooling analysis. However, there are caveats. The fans are generally brushless dc types, and rated at 24 or 48 V. However, battery voltages are often 32, 36, and 72 V. This possible mismatch adds complexity to the fan circuits, because the fans should be operated at their rated voltage. Also, it is a good idea to stock spare fans. As with all fancooled equipment, regular maintenance is important to clean, proper operation. Locating fans in equipment close to the floor of warehouses and factories is risky because of the presence of dust and dirt.

Battery charging

Battery maintenance and proper charging are primary concerns of any business using electric vehicles. A recent innovation for companies with many battery powered vehicles is a distributed charging facility. Such a system consists of several charging bays that attach the charging terminals to the vehicle, automatically or manually, when it is parked in the bay. The advantage of the system is that a smaller and lighter battery may be used if the vehicle can be taken out of service for short periods to be recharged. This system also eliminates some travel time and distance to move the vehicle to a central charging location.

The method of sequencing the charge is important. Be careful not to connect the vehicle battery terminals to live charging terminals as this can cause arcs. To eliminate this possibility, activate the charger after the connection is made.

Consider too, whether it is necessary to have the motor controller and onboard computer operational during the charging cycle. If this is a requirement, the controller and computer power supply must be rated to operate safely and reliably on battery voltages that are as high as the battery maximum charging voltage. The voltage on a nominal 72-V battery can rise to 83 V, or more, during charging.

For more information on selecting drives for electric vehicles from Cleveland Machine Controls, circle 411 on the reader service card.

Robert G. Klimo is vice president of technology, Cleveland Machine Controls, Cleveland.

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