12-17-2014, 11:10 AM
Greg,
Forgot to address your other question about how the VFD goes about ramping up the motor (AKA "soft start").
You probably already know that there is a direct relation between frequency and motor speed. The motor reaches its rated speed at rated frequency (60 HZ here in North America). If you run the motor at 30 HZ, it will run at about 1/2 rated speed, but you also have to reduce the voltage to the motor so as not to over-current. The drive knows how to figure this out. With older VFDs, when you ran at less than rated frequency, the available torque fell off quickly. The newer drives have "torque boost" capabilities that are able to give the right "kick" at the right instance with cycle to preserve more torque at lower speeds.
On a "soft start" the drive starts at a low frequency (like 5 hz) and ramps the frequency up to the setpoint (along with the voltage). You get to adjust how long this ramp is. I have my mill and lathe both set at a 3 second ramp. I don't do any machining in the first 3 seconds anyways. This makes for a quiet, gentle start. Pulls less current than "across the line." Puts less strain on belts and gears.
You also get to select the type of stop you want. You can have the VFD "ramp down" the speed on stop or "coast to a stop." I use "coast to a stop" on both of my machines. This allows me to use the mechanical brake that each machine has. If you were to try to use the mechanical brake while the drive was ramping down, you would be fighting the drive.
I don't make use of the "E-Stop" feature. E-Stop requires some additional electrical components for proper use.
Terry S.
Forgot to address your other question about how the VFD goes about ramping up the motor (AKA "soft start").
You probably already know that there is a direct relation between frequency and motor speed. The motor reaches its rated speed at rated frequency (60 HZ here in North America). If you run the motor at 30 HZ, it will run at about 1/2 rated speed, but you also have to reduce the voltage to the motor so as not to over-current. The drive knows how to figure this out. With older VFDs, when you ran at less than rated frequency, the available torque fell off quickly. The newer drives have "torque boost" capabilities that are able to give the right "kick" at the right instance with cycle to preserve more torque at lower speeds.
On a "soft start" the drive starts at a low frequency (like 5 hz) and ramps the frequency up to the setpoint (along with the voltage). You get to adjust how long this ramp is. I have my mill and lathe both set at a 3 second ramp. I don't do any machining in the first 3 seconds anyways. This makes for a quiet, gentle start. Pulls less current than "across the line." Puts less strain on belts and gears.
You also get to select the type of stop you want. You can have the VFD "ramp down" the speed on stop or "coast to a stop." I use "coast to a stop" on both of my machines. This allows me to use the mechanical brake that each machine has. If you were to try to use the mechanical brake while the drive was ramping down, you would be fighting the drive.
I don't make use of the "E-Stop" feature. E-Stop requires some additional electrical components for proper use.
Terry S.