题目内容
（请给出正确答案）
If a DC motor runs hot, the cause may be _____.
A.high mica condition
B.low ambient temperature
C.clogged ventilation ducts
D.an open in the shunt field
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B、slip ratio
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A.differ
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An acrobatics performs motor cycling on the inner surface of a cylindrical wall. Assume the total masses of the acrobatics and the motor are m and radius of the cylinder is R. The actor rides the motor to make spin circular motion with speed of
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Three kinds of motors are used in electric shavers. Some contain a tiny universal motor, others contain a vibrator type motor. Cordless razors contain NiCad cells, miniature DC motor, and charging components all within the shaver unit. A scraper or series of scrapers attached to the motor moves in reciprocating fashion—that is, in a back-and-forth, straight line way.
Reciprocating motion is established by means of a small pinion. Essentially, the pinion and scraper are connected by a stud set in an offset position in the pinion. Thus, the scraper is prevented from making a full-circle turn as the motor revolves; its travel is limited to the size of the razor head.
There are variations in the way different makes of electric razors are built, but the principle just described applies to all, with the exception of those that employ rotary heads.
Electric razors generally use very thin perforated shields to enclose the whiskers as skin is pressed against the shields. The rapidly moving scraper shears off the hairs extending through the surface of the shield.
The power source to operate an electric shaver's motor may come from a wall outlet by means of a line cord or through a tiny nickel-cadmium battery in cordless electric razors. Batteries of cordless electric razors have to be recharged periodically by being connected to a wall outlet through use of a line cord.
PROBLEMS, CAUSES, REPAIRS
The electric razor is one appliance that owners—even those with technical ability--cannot repair. Replacement parts are not usually available, not even to professional appliance technicians. The shaver must be returned to the factory when a problem occurs.
The key word to keep in mind regarding caring for your electric razor is "clean." The razor should be brushed clean of hair after every use to maintain its effectiveness and extend its life.
How many types of motors are generally used in the shaver？
A．Two.
B．Three.
C．Four.
D．Not known.
A．heating is least essential
B．electricity rates are lowest
C．its compressor runs the fastest
D．outdoor temperatures hold steady
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