Question:

What is the fastest, quickest or shortest amount of time that a PFD Soft Start Smart Motor Brake model can stop a motor? How does the PFD Smart Motor Braking work?

Answer:

The PFD soft starter is generally able to stop the motor in about 1.5 times the maximum period it takes to successfully start the motor without faulting. Therefore, if it takes 10 seconds to start the motor, then a stop time of about 15 seconds is a realistic expectation.

In most applications the stop time will take longer than the 15 seconds because of

• PFD sizing
• branch circuit protection limitations
• or constraints of the mechanically driven components

There will be additional noise and vibration in the motor and load caused by the braking action. Therefore, you should initially try to set the brake current to the minimum value, and increase to the lowest level, which provides satisfactory performance in the application. This will normally result in a stop time somewhat longer than the minimum possible. In addition, repeated starting and stopping of the motor may necessitate increasing the size of the PFD controller.

For general application purposes you may approximate that 1 brake maneuver shortly after a start is equal to two consecutive starts. PFD Smart Motor Braking puts a great amount of thermal burden on the SCR junctions and can cause the SMC controller to trip on an over temperature fault or damage the power poles if the temperature rise is faster than the PC (thermistor) can react to, since the PTC is located near the heat sink. The temperature at the SCR junction will rise faster than the temperature at the PTC location.

Lastly, a PFD Smart Motor Braking model uses one or more phases to cut the flux lines of the rotating motor. This causes the motor to decelerate. The higher the brake current setting the more brake current is applied. This braking energy, which can be in excess of 400% the FLA, will go into the motor as additional heat. High levels of brake current may trip your overload protection, branch fusing, or stress the motor insulation. Pay special attention if the brake current is still applied after the motor has come to rest - this can cause damage to the motor insulation winding and reduce the life of the motor.

How Does The PFD Smart Motor Braking Work?

The PFD Smart Motor Braking option is ideal for use in applications that require reduced stopping times such as machinery band saws, hammer mills, ball mills, and centrifuges. Each PFD controller, which has this installed option, incorporates a specially designed algorithm which is controlled by the microprocessor. Through advanced control of the current to the motor, braking can be completed without any additional equipment.

While the exact algorithm for braking is proprietary, the sequence is such that once the brake command is initiated the PFD begins to monitor the motors rotation. Using advanced control, the PFD will fire the SCR's at an optimal time to create negative torque. The sequence is such that the SCR’s fire randomly at first both in the positive and negative directions. As the motor speed is reduced below 50% the SCR’s can be fired consistently in one direction to bring the motor to rest. The PFD controller uses all 3-phases to brake.

Each time the SCR turns on there is a resulting current requirement from the motor. This is commonly referred to as phase fired DC and is not considered DC injection. If you were to use a scope or digital volt meter to observe the output, the data would appear to show short duration pulses of voltage (<6mS) and the corresponding current. In actuality these pulses are a direct result of the SCR turning on at specific times.

A few common application considerations to keep in mind while using PFD Smart Motor braking is the heating effect on the motor, the effect of additional current requirements on the up-stream system components such as circuit breakers, and SCR fusing.

Note: SCR fusing is optional and is not typically recommended when the combination of starting time and braking time exceeds 75 seconds or the duty cycle is aggressive.

For additional information please contact your nearest authorized distributor, sales representative, or call our customer service or technical support lines.