This article is to introduce the function of a PTC thermistor used in the electric moto of a vacuum pump.
The electric induction motor is an important part for a vacuum pump. In some cases, the motor is accidentally clogged, generate a lot of heat and result in motor burning when a certain temperature is exceeded. A PTC thermistor can be used to protect the motor from such overheating. The protection is realized by thermally bonding one sensor (for single-phase) or three sensors (for three-phase) to a motor winding at stator side. When the PTC protector heats up to a given temperature, the automatic circuit breaker is activated to cut off the power supply.
Built-in thermal protector in a motor winding
Overheating protection for three-phase induction motors
PTC stands for “Positive Temperature Coefficient”. The resistance rises significantly with increasing temperature in a PTC sensor, or thermistor, or thermal protector. This property is perfectly used to protect circuits from overheating in electric motors.
Such thermistors are made of special semiconductor ceramics with a high positive temperature coefficient (PTC). The resistance is low at room temperature and the value increases when the temperature is above a certain limit. The protectors for electric motors are specially designed so that they can be inserted in the motor windings, as a built-in part of the motor.
The PTC’s resistance values for motor protection are specified in DIN 44081/44082. It is ideally between 40 – 200 ohms but can be anywhere between 35 – 250 ohms.
Nowadays, a built-in thermal protector is commonly assembled as default within a single-phase motor. For a three-phase motor however, the PTC thermistor is still considered as an extra request. The low demand of 3-phase motors with PTC sensor causes higher production cost and much longer lead time although it’s not difficult to achieve technically. When the customer insists a special production for a 3-phase motor, take both points into consideration.