We would like to introduce some fundamental characteristics and selecting points of DC and AC motors.
Characteristics of DC Motor
About Torque Curve
Torque speed curve of DC motors is a straight line as shown to the left.? As load torque is increased, motor speed slows down and motor will be run until almost closed to the starting torque.? When driving at the efficient point, DC motors can be operated for a long period of time.? However when over-loading is continued which cause malfunction or damage, protection measures is required in advance.
With Consideration for Voltage Variation
As shown to the left, speed and torque is changed in proportion to the applied voltage.? Thus, when voltage is changed within the usable range, the same motor can be adjustable to the speed characteristics according to specific applications.? Please avoid using dc motors under supplied over the rating value since it may cause the temperature rise and damages.
Countermeasure against Electrical Noise
Motors with brush usually generate electrical noise.? There are various types of noise available such as radiating out from motor, running through the lead-wires and others.? In order to prevent adverse effects on motors’ peripheral circuitry, careful countermeasure is important.? We will propose the best countermeasure against noise.
Brush life is equal to the life of dc motors using with brush.? We can extend some degree of motor life by changing brush material and other countermeasures,
For further details, please feel free to contact with us.
Characteristics of AC Motor
About Torque Curve
Synchronous speed is determined according to the qty of motor pole and power frequency.
In case of induction motors, no-load speed is inevitably lower than synchronous speed.
Starting torque is an instantaneous starting torque and if loaded torque is smaller than this, motor will not rotate.
Stall torque is the maxmam torque motor generates.? When motor is applied more than stall torque, motor will stop.
⑤Rated Speed, ⑥Rated Torque
They are the most suitable speed and torque when AC motors are used.? As shown to the left, the most recommended motor design is to match the intersection point
between motor’s torque curve and rated torque.
About Temperature Rise
Motors are limited to operate according to the temperature.? Generally, coil winding in a motor is increased in temperature at most.? Esco’s standard design of motors with class E insulation (allowable maximum temperature: 120℃), thus arrange to design under 115℃.
Motors’ temperature rise(deg) ＝ 115℃ － ambient temperature（℃）
About overheat protective device
When a motor is either over-loaded, ambient temperature is increased more than a designed value, or input is increased by any other causes, motor temperature will be drastically increased.? If this is left in that state, insulation inside the motor is deteriorated.? In case of serious condition, coil winding will be burned out that may cause fires.? In order to protect motors from those abnormal phenomena, following overheat protective devices will be implemented.
In case the coil winding temperature goes up to the designed value or above,
thermosensitive device inside the thermal fuse will be operated, and circuit will be cut off and then cannot be recovered.? We usually set up temperature of thermal fuse just above the allowable maximum temperature.
In case the coil winding temperature goes up to designed abnormal value, bimetal contact point inside the thermal protector will be operated and circuit will be cut off.? Then, temperature of inside motor goes down, the thermal protector will automatically restart.? We usually set up temperature of protector within the allowable maximum temperature.
Even if a motor is locked, this type of motor temperature will not be increased more than allowable maximum temperature because impedance of motor winding is designed large.