Selecting nonfriction stopping methods

Nov. 1, 2000
Selecting the best method for controlling how a motor stops maximizes machine productivity. This article will help you evaluate the numerious nonfriction methods.

For dc motors, there are two electrical (nonfriction) methods to bring a machine to a controlled stop. These include dynamic braking (DB) and regenerative braking. For ac motors, the choices are more numerous and confusing. They include motor-loss braking, dynamic braking, dc injection braking, and regenerative braking. As a starting point, Table 1 gives common stopping techniques for typical loads.

Braking dc motors

Assuming a field supply is maintained to a dc motor, it operates as a motor if the armature voltage is high enough so power flows to the motor. However, should the applied voltage be lower than that, the machine turns into a generator and supplies energy to an electrical load if one is connected to the motor. In the absence of a load, the motor coasts to a lower speed. The lower the armature voltage, the lower the final speed.

Dynamic braking. By connecting a power resistor across the dc motor armature, the motor- turned-generator has a load to absorb and dissipate the rotating energy. Thus, the motor stops much quicker than if it coasts to rest. To make this connection, a normally closed contact (M, NC) is installed on the motor contactor, Figure 1. When a Stop button is pushed, the contactor (M) drops out opening the NO contact; the NC contact closes and connects the resistor across the dc motor armature.

The stopping time depends on load inertia, motor speed when the button was pushed, motor internal resistance, and ohmic value of the DB resistor. This resistor usually has a fixed resistance value. Therefore, the stopping characteristics are fixed. However, some DB resistors do have an adjustment that is usually set during start up. Once set, the stopping characteristics are then established.

Regeneration. If a second power module, with its polarity reversed, is connected across (in parallel with) the motoring power module, Figure 2, the drive can put (regenerate) power back to the ac power supply. Of course, this assumes the regulators of the motoring and regenerating power modules are interocked to avoid both conducting at the same time and turning into a super smoker.

Unlike dynamic braking, this regenerative method offers controlled braking, because the regulator controls the rate at which the power flows back into the ac line. This controlling function is the same as when the forward power unit controls the applied power to the motor during motoring operations. Moreover, this rate of power return can be changed during the regenerating phase.

The increased capabilities of regeneration naturally cost more than does the DB method, but for many applications — such as unwinders and some continuous web handling machines — the added control is well worth the investment.

Braking ac motors

Although ac motors are simplier in construction than dc motors, ac motors are much more complex electrically. Therefore, they offer additional braking methods. In order of increasing cost, the following are the major methods for electrically stopping ac motors that are powered by typical adjustable-frequency drives. Generally, these braking methods will not replace mechanical brakes for holding functions, as required in many cranes, hoists, and people movers.

Motor-loss braking. Also called “inherent- loss braking,” this system uses the tendency of ac-motor speed to follow the frequency of the applied power. If the motor internal losses — rotor resistance, plus windage and friction losses — are sufficient in relation to the rotating kinetic energy, the motor will slow down as the applied frequency decreases. However, if the frequency is reduced faster than the motor and other parts of the electrical system can absorb the energy, the motor, acting as an alternator, raises the dc bus voltage. If the bus voltage exceeds a set value, the drive shuts down and the motor coasts to rest.

Dynamic braking. Continuing the above situation, many drives have a capacitor and a resistor connected across the dc bus. These components can absord and dissipate a certain amount of energy produced by a rotating motor shaft, thus dynamically braking the ac motor. More advanced drives have a dc-bus-voltage regulator, Figure 3, that controls the current flowing through the DB resistor, thence the deceleration rate. If an application has a high-inertia load or requires a short stopping time, the drive manufacturer freqently installs the resistor externally to the drive enclosure. Such an arrangement offers increased heat dissipation capacity.

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DC injection braking. One characteristic of dynamic braking is the typical nonlinear stopping characteristic, Figure 4. To increase the deceleration rate (decrease the stopping time), dc can be applied to the ac motor windings during the last 10% to 20% of the motor’s speed. For example, a motor is turning at 1,500 rpm when the Stop command is issued. DB reduces the speed to 150 rpm, then injecting dc reduces the stopping time by say 4 seconds than if it were stopped by DB alone.

DC injection braking, which can apply 10% to 30% of the rated motor torque, is often used to stop a motor before setting a mechnaical brake.

There is a special caveat for those considering dc injection braking — assure the ac motor has sufficient thermal capacity to handle the dc. Although, the dc is applied to the stator windings, most of the motor heating is in the rotor, and this can be the weak link in this braking system. Even though this feature is offered in standard ac drives, its use must be approached carefully. It has been used with much success, but the faster the motor speed when the dc is applied, the more care must be the taken in motor selection.

Full regeneration. To distinguish this method from the dynamic braking scheme, which dissipates electrical energy in a resistor across the dc bus, full regeneration puts power back to the ac line. To do this, requires adding a full power unit with reverse polarity connected across the first stage of the drive, Figure 5. This regenerative unit puts the electrical energy from the dc bus to the ac power line in the same manner as a regenerative unit does in a dc drive.

With regeneration, you can control the rate of decelaration and the specific characteristics. If you desire, it can have Scurve deceleration, Figure 6, for reduced jerk during the stopping process.

DC bus voltage during deceleration

One of the determinates in selecting a braking method for ac drives is how the bus voltage changes during deceleration. The following graphs show the inter-relationships.

To show how the solid-state control turns the DB resistor on and off during braking (regeneration), the amount of the variation of the saw-tooth bus voltage is exaggerated. This switching limits the bus voltage to 110% of rated voltage. Should the bus voltage exceed this value, the drive shuts down.

Graph A shows the values for a drive with a 25% friction load. The rating of the drive was selected to accelerate the load in the specified time.

A centrifugal fan load produces a different set of values, graph B. Because the torque requirements change in relation to the square of the speed, the bus current and motor torque follow the characteristic square curve.

The following engineers contributed valuable information to this article: Cleveland Machine Controls — Vice President of Engineering Robert Klimo, plus Bing Cheng, and David Felty; General Electric — Brian Ricci; Mitsubishi Electronics America — Cliff Cole.

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