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Delicacy of VFD's and three phase motors
VFD's and motors have been occasional topics here.
It was asked elsewhere how sensitive VFD's and induction motors were to frequent starts and reversals. The short answer is not at all. In fact they're practically bullet proof in the absense of deliberate and malicious abuse.
Once apon a time when motor manufacturers offered real engineering data in their catalogs they included among other things curves representing the number of times their motors could be plug reversed under different conditions of connected inertia. I recall that a 5 HP 1750 RPM three phase induction motor with no inertia but that of its own armature could be plug reversed across the line 30 times a minute and still stay within its thermal ratings.
"Plug reversing" means applying reverse power to a motor already running at full forward RPM to full RPM in reverse. A "plug stop" means reversing the motor only until it comes to a stop at which time the power is automatically (or manually) shut off.
A standard item of industrial electrical apparatus is a gadget called a "zero speed switch". EE's in their lab sessions learn to wire reversing starters to zero speed switches as a routine part of their training. The switch has a rotating part that attaches to the back end of the motor shaft. A pair of contacts opens when the motor crosses zero RPM. A zero speed switch wired in an across the line starter circuit will automatically plug the motor to a stop when the stop button is depressed. Great big induction motors are routinely plug stopped and reversed.
While plugging a motor results in a heat gain to the windings, they are designed for it. There's very little thermal break between a third of the winding's volume and the mass of the iron stator which acts as a big heat sink and radiator. The two thirds that form the exposed portion of the windings is, after all, made of copper, a metal that's a superb conductor of heat as well as electricity. Between the internal fan and the heat sink of the stator, an induction motor is thermally as well as electrically robust. There's not a thing delicate about a three phase motor.
My discussion is intended to inform and assure people concerned about their VFD/motor spindle drives. So long as the heat input from momentary overloads can escape by convection, radiation, or ventillation the motor will not be harmed provided it doesn't exceed the thermal characteristics of its insulation due to excessive mechanical and, hence, electrical overload - and that is the job of the VFD.
A VFD is reversed either by the pushbuttons on the digital input keypad on the face of the drive or by wiring it to a start/stop/reverse control station according to the diagram in the manual. No intervening switches or contacts should be present between the VFD and the motor. Starting and stopping and reversing is all the taks of the VFD's control algorithms.
When you set the accell and decell you're establishing times to achieve rated RPM. You can set the accell to 1 second and the decel to 10 seconds or vice versa or any other figure that works. The time to a given setpoint RPM is proportional to the setpoint divided by the rated RPM. Accel time to 875 RPM is half the time to 1750.
The VFD dynamic brake extracts energy from the rotating armature by induction and dumps it back into the DC buss via the output transistors. If you set the decel too short, the energy goes into over-charging the capacitor. The dynamic brake circuit is designed to monitor the DC buss voltage and if it exceeds limits dumps excess charge into the dynamic brake resistor. If no resistor is present, the buss overcharges, the safety circuitry trips, and the VFD goes into over-voltage shutdown.
There's another form of dynamic brake in the VFD called DC injection. This can be used alone or in combination with the electronic braking. DC injection is not as sophisticated or flexible and its possible to fry the armature (not the stator) if very large over-running loads are present.
VFD's are designed with elaborate thermal protection algorithms and sophisticated current limiting. If the parameters are correctly set to reflect the motor's nameplate ratings it's utterly impossible to fry the connected motor or the VFD by manipulating the control station or physically overloading the motor - ahem, provided the motor is properly ventillated at low RPM.
If you overload the motor, the VFD automatically reduces the frequency and slows the motor down - to zero if need be. If the overload continues too long the VFD will shut down.
Furthermore, VFD's do not plug reverse a motor if the accel and decel parameter is set to zero. They accel and decel well within that part of the motor's slip where the frequency leads or lags the armature by an amount that produces the maxiumum torque. A stalled induction motor develops about half the max overload torque.
Max overload torque occurs at about 150% - 200% of nameplate full load Amps instead of the 7 times inrush Amps you get when starting a motor across the line. A VFD 150% FLA start has more intial torque and draws less start current than an across the line start.
If you want crisp starts and stops for manual machine tool operation, set the accel parameter for 0.7 seconds and the decel to 3.0. If you don't have a dynamic brake resistor installed you might not be able to stop that quick without going into over-voltage trip. Also remember to consider conected inertia. A big chuck spinning at 1000 RPM stores a lot of energy.
I've mentioned "dynamic brake resistor" several times. For the 2 to 3 HP machinery most of us work with it's just a 50 Watt resistor. The factory part might cost a $100 - $200. It's a sucker part - like buying ball bearings and standard fasteners under Sears part numbers. A common wire wound resistor of the same resistance and wattage will do exactly the same job for maybe $10 - $15. You choose.
Most all VFD are made by a single manufacturer, Yaskawa, made to suit the many resellers' concepts of their markets. There's a very good chance that whatever VFD you buy it has close similarities to comparable drives offered by Yaskawa. Your VFD manual is sure to offer a factory part number for the dynamic brake resistor but maybe no resistance or wattage value. I have a book that covers a wide range of VFD accessory data and among the tables are resistance and wattage values for DB resistors.
Gimme an email with your drive's nameplate electrical specifications and I'll look up a close cousin to the factory dynamic brake resistor; one you can buy at the local major electronics supply house.
You guys with VFD's with threaded spindle lathes consider the decel issue carefully. VFD's can stop a motor with a helluva lurch. You don't want to spin off the chuck. That makes nasty marks on the ways and gives the operator sore toes.
My suggestion to you VFD owners is to run the sox of the drive and motor. If you set it up right, you can't possibly hurt it. If you run into problems of nuisance shut downs, tweak the appropiate parameter.
[This message has been edited by Forrest Addy (edited 02-09-2003).]
It was asked elsewhere how sensitive VFD's and induction motors were to frequent starts and reversals. The short answer is not at all. In fact they're practically bullet proof in the absense of deliberate and malicious abuse.
Once apon a time when motor manufacturers offered real engineering data in their catalogs they included among other things curves representing the number of times their motors could be plug reversed under different conditions of connected inertia. I recall that a 5 HP 1750 RPM three phase induction motor with no inertia but that of its own armature could be plug reversed across the line 30 times a minute and still stay within its thermal ratings.
"Plug reversing" means applying reverse power to a motor already running at full forward RPM to full RPM in reverse. A "plug stop" means reversing the motor only until it comes to a stop at which time the power is automatically (or manually) shut off.
A standard item of industrial electrical apparatus is a gadget called a "zero speed switch". EE's in their lab sessions learn to wire reversing starters to zero speed switches as a routine part of their training. The switch has a rotating part that attaches to the back end of the motor shaft. A pair of contacts opens when the motor crosses zero RPM. A zero speed switch wired in an across the line starter circuit will automatically plug the motor to a stop when the stop button is depressed. Great big induction motors are routinely plug stopped and reversed.
While plugging a motor results in a heat gain to the windings, they are designed for it. There's very little thermal break between a third of the winding's volume and the mass of the iron stator which acts as a big heat sink and radiator. The two thirds that form the exposed portion of the windings is, after all, made of copper, a metal that's a superb conductor of heat as well as electricity. Between the internal fan and the heat sink of the stator, an induction motor is thermally as well as electrically robust. There's not a thing delicate about a three phase motor.
My discussion is intended to inform and assure people concerned about their VFD/motor spindle drives. So long as the heat input from momentary overloads can escape by convection, radiation, or ventillation the motor will not be harmed provided it doesn't exceed the thermal characteristics of its insulation due to excessive mechanical and, hence, electrical overload - and that is the job of the VFD.
A VFD is reversed either by the pushbuttons on the digital input keypad on the face of the drive or by wiring it to a start/stop/reverse control station according to the diagram in the manual. No intervening switches or contacts should be present between the VFD and the motor. Starting and stopping and reversing is all the taks of the VFD's control algorithms.
When you set the accell and decell you're establishing times to achieve rated RPM. You can set the accell to 1 second and the decel to 10 seconds or vice versa or any other figure that works. The time to a given setpoint RPM is proportional to the setpoint divided by the rated RPM. Accel time to 875 RPM is half the time to 1750.
The VFD dynamic brake extracts energy from the rotating armature by induction and dumps it back into the DC buss via the output transistors. If you set the decel too short, the energy goes into over-charging the capacitor. The dynamic brake circuit is designed to monitor the DC buss voltage and if it exceeds limits dumps excess charge into the dynamic brake resistor. If no resistor is present, the buss overcharges, the safety circuitry trips, and the VFD goes into over-voltage shutdown.
There's another form of dynamic brake in the VFD called DC injection. This can be used alone or in combination with the electronic braking. DC injection is not as sophisticated or flexible and its possible to fry the armature (not the stator) if very large over-running loads are present.
VFD's are designed with elaborate thermal protection algorithms and sophisticated current limiting. If the parameters are correctly set to reflect the motor's nameplate ratings it's utterly impossible to fry the connected motor or the VFD by manipulating the control station or physically overloading the motor - ahem, provided the motor is properly ventillated at low RPM.
If you overload the motor, the VFD automatically reduces the frequency and slows the motor down - to zero if need be. If the overload continues too long the VFD will shut down.
Furthermore, VFD's do not plug reverse a motor if the accel and decel parameter is set to zero. They accel and decel well within that part of the motor's slip where the frequency leads or lags the armature by an amount that produces the maxiumum torque. A stalled induction motor develops about half the max overload torque.
Max overload torque occurs at about 150% - 200% of nameplate full load Amps instead of the 7 times inrush Amps you get when starting a motor across the line. A VFD 150% FLA start has more intial torque and draws less start current than an across the line start.
If you want crisp starts and stops for manual machine tool operation, set the accel parameter for 0.7 seconds and the decel to 3.0. If you don't have a dynamic brake resistor installed you might not be able to stop that quick without going into over-voltage trip. Also remember to consider conected inertia. A big chuck spinning at 1000 RPM stores a lot of energy.
I've mentioned "dynamic brake resistor" several times. For the 2 to 3 HP machinery most of us work with it's just a 50 Watt resistor. The factory part might cost a $100 - $200. It's a sucker part - like buying ball bearings and standard fasteners under Sears part numbers. A common wire wound resistor of the same resistance and wattage will do exactly the same job for maybe $10 - $15. You choose.
Most all VFD are made by a single manufacturer, Yaskawa, made to suit the many resellers' concepts of their markets. There's a very good chance that whatever VFD you buy it has close similarities to comparable drives offered by Yaskawa. Your VFD manual is sure to offer a factory part number for the dynamic brake resistor but maybe no resistance or wattage value. I have a book that covers a wide range of VFD accessory data and among the tables are resistance and wattage values for DB resistors.
Gimme an email with your drive's nameplate electrical specifications and I'll look up a close cousin to the factory dynamic brake resistor; one you can buy at the local major electronics supply house.
You guys with VFD's with threaded spindle lathes consider the decel issue carefully. VFD's can stop a motor with a helluva lurch. You don't want to spin off the chuck. That makes nasty marks on the ways and gives the operator sore toes.
My suggestion to you VFD owners is to run the sox of the drive and motor. If you set it up right, you can't possibly hurt it. If you run into problems of nuisance shut downs, tweak the appropiate parameter.
[This message has been edited by Forrest Addy (edited 02-09-2003).]
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