DC Injection Braking

[albionj]

Is there any way of calculating the braking torque from the amount of DC current applied to the motor?

 

Non of the manufacturers seem to be able to quantify the braking ability in terms of torque or time for a given inertia.

[marke]

Hello albionj

 

Welcome to the forum.

 

The DC braking torque is not difficult to work out, but it is dependent on each motor. There are no standard figures.

I will step you through the operation and develpment of the braking torque on an induction motor with a DC field.

 

• Under normal operation, line voltage and frequency are applied to the stator of the induction motor. This results in a rotating magnetivc field in the stator with the speed of rotation dependant on the frequency of the voltage applied.

• If the rotor is not spinning at the same speed as the stator field, there is stator flux that cuts through the rotor winding.

• The stator flux cutting through the rotor winding induces a current in the rotor. The frequency of the rotor current is dependant on the difference between the stator field speed and the rotor speed.

• If the rotor is stationary, the rotor current is equal to the line current. If the rotor is at synchronous speed, the rotor frequency is zero.

• The rotor current creates a rotor field. The rotor field rotates around the rotor at a speed determnined by the rotor current frequency. The resultant rotor field speed is equal to the stator field relative to the stator.

• The torque generated by the motor under full voltage conditions is usually shown (graphed) relative to rotor speed. This can also be shown relative to rotor current frequency.

• Now take the case where we appliy DC to the stator winding. This results in a stationary stator field.

• A rotor spinning within the stationary stator field will cut through flux line and a rotor current will be induced. If we consider the frequency of the current in the rotor, we get the same torque curve as we would DOL except the magnitude is going to change with current.

• With a DC field, if the rotor is spinning at synchronous speed, the rotor frequency is equal to the line frequency. At zero speed, the rotor current is at zero Hz (and equal to zero)

Armed with this information above, you can take a speed torque curve for a given motor and translate the x axis from "speed" to rotor current frequency. You can then mirror this to get a curve with the rotor current frequency at zero speed on the left had side. This is the breaking curve. The magnitude of the curve depends on the square of the braking curent.

 

I hope that this makes sense. I will schedule to create a web page on this topic when I get a chance.

If you have further questions, please ask.

 

Note, there are many different forms of DC braking. The smoother the current, the better the result.

 

Best regards,

[jraef]

Excellent response from Marke.

I'll only add that his last comment, that there are different methods of DC injection braking, is the main reason why it is difficult to give an exact value of braking torque output. Component size relates to maximum currents and times, so economics plays a major role in the output capacity. It is possible to produce a torque value equal to the Breakdown Torque (peak torque) from the acceleration curve, so the brake could make the motor stop in the same time it takes to start it DOL. But that would mean very large compponents, and most people are not interested in paying that much. So since most users are simply looking for a stopping time better than coast-to-stop times, it becomes a cost / benefit puzzle and there are many ways to approach it.

 

It's often more productive to define your stopping requirements realistically so as not to overbuy, then take them to your reputable vendors for proposals. For instance, I am working right now on setting up a 900HP 460V brake for an 8 foot diameter wood chipper. The customer insisted that he wanted to stop that chipper head as fast as he could, so he purchased a 1200A brake. It was able to stop the chipper in 30 seconds, but the current was too much for the transformer and the building shook violently. As it turns out, he was perfectly happy with stopping his chipper in 2 minutes, because the alternative coast time was 45 minutes. He could have purchased a much smaller brake had he been more forthcoming in stating his requirements.

[marke]

A further comment ot the above postings.

 

When you start a machine, you dissipate energy in the rotor that is equal to the full speed kinetic energy of the load. You must be careful the the rotor of the motor is capable of dissipating this energy.

 

When you apply a DC brake to a motor, you onec again dissipate the full speed kinetic energy of the driven load in the rotor of the motor. If the rotor is already very hot, this can result in damage!!

 

Best regards,