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Electric drive motors
From among the many electric motors available only a few have been found particularly suitable for electric power tools on account of their outstanding characteristics. The series motor or series-wound motor is the most common motor type found in electric appliances. This motor which is equally suitable for both direct current and alternating current is also known as an AC/DC or universal motor. Its advantages are the high rotational speed, independent of frequency, and its high torque which can reach breakdown torque if the load is steadily increased. Its potential for speed adjustment by advanced electronic control, the high power reserves, the high peak load allowance and low power-to-weight ratio are indisputable advantages if used in electric power tools.
The rotary current asynchronous motor operating at standard frequency has not become widely accepted. Heavy-duty construction tools and in particular power tools used in heavy industrial shift work often contain rotary-current asynchronic motors operated at increased system frequency. These tools are also called high-frequency or high-cycle tools. Rotary current or three-phase motors have the advantage that their rotational speed remains nearly constant between idling and full-load. Their rotational speed can only be changed by modifying their technical design. One refers to 2, 4, 6, or 8 pole models which only allow for pre-determined speeds at the usual system frequency of 50 Hz.
n = ? x 60 : number of pole pairs (1/min)
If other rotational speeds are required, this can only be achieved by changing the frequency through dynamic or static frequency converters. Dynamic converters consist of a rotary current drive motor with an attached generator; static converters contain semi-conductor components. The usual high frequencies today are 200 Hz and 300 Hz, other frequencies have not become accepted. Rotational speeds which can be obtained with the increased frequencies are listed below:
| Polepair | 50 Hz | 200 Hz | 300 Hz |
| 2 pole motor | 3000 1/min | 12000 1/min | 18000 1/min |
| 4 pole motor | 1500 1/min | 6000 1/min | 9000 1/min |
| 6 pole motor | 1000 1/min | 4000 1/min | 4500 1/min |
| 8 pole motor | 750 1/min | 3000 1/min | 4500 1/min |
The advantage is based on the law of increase which states that by increasing the frequency (from 50 to 200 Hz) and keeping the volume constant the output will also increase by the same ratio of 1:4. If only low power output is required, the tool volume can be reduced to one quarter while the output power stays the same.
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