Geoff Whaling
09-22-2015, 6:13 PM
In a thread on the G0766 there is confusion over motor current draw which comes about because we don't understand the technology, the technical terms and because the motor ID plate, manual & specifications only give part of the story.
The main things to take away from the "power / motor current" discussion are,
Follow the designer’s recommendation to connect the lathe (motor / VFD combination) to a specified supply i.e. 220v 16A minimum rated circuit.
Don’t run the lathe’s motor at near stall or under continuous heavy loads, i.e. heavy roughing cuts or bowl saving, for long periods of time.
It is preferable that the lathe/motor is on a dedicated circuit. The lathe should be the only machine running on the circuit when the lathe is in operation. i.e. use another circuit, not another power point on the same circuit, for the dusty.
The reasons a 220v 16A circuit is recommended are,
(Single & 3 phase) Electric motors draw high current on start up – “motor start current.” The temporary high load on the electricity supply is called “inrush current” and can create other issues such as temporary voltage drop - “brown out.”
Once the motor reaches its running speed under normal operating load its “speed with rated load” the current draw from start up reduces to its typical “motor running current.” A motor running under no load reaches a theoretical “synchronous speed.” Electric motors are not 100% efficient and perform differently under changing loads, motor slip etc.
A 4 pole motor typically runs at 1725 rpm (60hz) 1425 (50hz) “speed with rated load” and at 1800 rpm (60hz) 1500 (50hz) “synchronous speed”. See a trend here - this is why variable frequency drives work.
Current draw increases dramatically when the motor is overloaded “Motor locked-rotor current” i.e. stalled or operated under near stall conditions. Motors are specified with a “service factor” that represents the motor’s ability to handle temporary demand / overload increases and sustained operating loads.
Constant operation at near stall conditions reduces the motor service life, increases power consumption, overheats components including power circuits in the shop and in worst case scenarios can overcome the insulating & thermal properties of the motor and supply wiring, potentially creating a fire.
Motors typically should be run at the designed voltage. “The actual current drawn by the motor depends upon the driven load and on the operating voltage at the motor terminals. If the load increases, the current also increases. If the motor operates at a voltage below its nameplate rating, the operating current will increase.”
Lower operating voltages mean increased current. Operating a lathe or any electric motor on longer than necessary power cords potentially can create a significant supply voltage drop from the increased resistance of the longer wires (V=IR). Avoiding “voltage drop” and preventing unsafe thermal overloads are the main reasons for regulations which limit the length of extension cords, prohibit coupling leads and recommend increased wire gauges (less resistance).
The only means we had to vary motor speed on a machine such as a wood lathe was through mechanical devices such as pulley sets, reeves drives etc.
Then Variable frequency drives (VFD’s) were invented and gave us the ability to modify an electric motors running speed by varying / modifying the frequency and voltage supplied to the electric motor.
Essentially the VFD’s wood turners use convert a single phase AC power supply to DC then creates a pseudo 3ph AC supply which the controller can modify to suit our particular application. This permits us to control motor speed, increase efficiency, save power (& money), boost torque at certain speeds, and include other nice little features like controlled acceleration and deceleration, improved motor overload protection etc.
The VFD current from the supply is no longer a simple relationship to the electric motors horsepower / voltage / current and typically is much lower in overall operation than that from an electric motor with out VFD control, which leads to significant power savings.
http://www.vfds.com/blog/how-to-read-a-motor-nameplate
Good basic intro to VFD’s https://www.youtube.com/watch?v=wDZANW2HeJ8
In depth discussion video available http://www.vfds.com/blog/what-is-a-vfd
Nice ready guide for non-VFD controlled motors http://www.rm-electrical.com/wp-content/uploads/2014/11/MOTOR-CURRENT-RM-Technical.pdf
Note a simple electric motor without VFD control will typically draw the figures in the PDF. So your 1 hp dusty on a 220v supply could be drawing 7.3A on its own, 2 hp/13A; 3 hp/19A!
The main things to take away from the "power / motor current" discussion are,
Follow the designer’s recommendation to connect the lathe (motor / VFD combination) to a specified supply i.e. 220v 16A minimum rated circuit.
Don’t run the lathe’s motor at near stall or under continuous heavy loads, i.e. heavy roughing cuts or bowl saving, for long periods of time.
It is preferable that the lathe/motor is on a dedicated circuit. The lathe should be the only machine running on the circuit when the lathe is in operation. i.e. use another circuit, not another power point on the same circuit, for the dusty.
The reasons a 220v 16A circuit is recommended are,
(Single & 3 phase) Electric motors draw high current on start up – “motor start current.” The temporary high load on the electricity supply is called “inrush current” and can create other issues such as temporary voltage drop - “brown out.”
Once the motor reaches its running speed under normal operating load its “speed with rated load” the current draw from start up reduces to its typical “motor running current.” A motor running under no load reaches a theoretical “synchronous speed.” Electric motors are not 100% efficient and perform differently under changing loads, motor slip etc.
A 4 pole motor typically runs at 1725 rpm (60hz) 1425 (50hz) “speed with rated load” and at 1800 rpm (60hz) 1500 (50hz) “synchronous speed”. See a trend here - this is why variable frequency drives work.
Current draw increases dramatically when the motor is overloaded “Motor locked-rotor current” i.e. stalled or operated under near stall conditions. Motors are specified with a “service factor” that represents the motor’s ability to handle temporary demand / overload increases and sustained operating loads.
Constant operation at near stall conditions reduces the motor service life, increases power consumption, overheats components including power circuits in the shop and in worst case scenarios can overcome the insulating & thermal properties of the motor and supply wiring, potentially creating a fire.
Motors typically should be run at the designed voltage. “The actual current drawn by the motor depends upon the driven load and on the operating voltage at the motor terminals. If the load increases, the current also increases. If the motor operates at a voltage below its nameplate rating, the operating current will increase.”
Lower operating voltages mean increased current. Operating a lathe or any electric motor on longer than necessary power cords potentially can create a significant supply voltage drop from the increased resistance of the longer wires (V=IR). Avoiding “voltage drop” and preventing unsafe thermal overloads are the main reasons for regulations which limit the length of extension cords, prohibit coupling leads and recommend increased wire gauges (less resistance).
The only means we had to vary motor speed on a machine such as a wood lathe was through mechanical devices such as pulley sets, reeves drives etc.
Then Variable frequency drives (VFD’s) were invented and gave us the ability to modify an electric motors running speed by varying / modifying the frequency and voltage supplied to the electric motor.
Essentially the VFD’s wood turners use convert a single phase AC power supply to DC then creates a pseudo 3ph AC supply which the controller can modify to suit our particular application. This permits us to control motor speed, increase efficiency, save power (& money), boost torque at certain speeds, and include other nice little features like controlled acceleration and deceleration, improved motor overload protection etc.
The VFD current from the supply is no longer a simple relationship to the electric motors horsepower / voltage / current and typically is much lower in overall operation than that from an electric motor with out VFD control, which leads to significant power savings.
http://www.vfds.com/blog/how-to-read-a-motor-nameplate
Good basic intro to VFD’s https://www.youtube.com/watch?v=wDZANW2HeJ8
In depth discussion video available http://www.vfds.com/blog/what-is-a-vfd
Nice ready guide for non-VFD controlled motors http://www.rm-electrical.com/wp-content/uploads/2014/11/MOTOR-CURRENT-RM-Technical.pdf
Note a simple electric motor without VFD control will typically draw the figures in the PDF. So your 1 hp dusty on a 220v supply could be drawing 7.3A on its own, 2 hp/13A; 3 hp/19A!