Basic principle
Single-phase asynchronous motors require auxiliary means to generate a rotating field. This is because a magnetic field generated with pure alternating current would only oscillate back and forth between two poles without providing a direction. For this reason, an auxiliary winding is installed in a 230V~ motor, which is supplied with current via a capacitor. This generates a calculable phase shift to the mains frequency. This creates a time-shifted magnetic field in the auxiliary winding, which determines the direction of rotation and provides torque.
Variants
The auxiliary winding and its power supply are realized using two different designs:
- Single-phase motor with running capacitor (also known as PSC motor)
This design is simple, robust and suitable for applications with a uniform load curve and low starting torque. The run capacitor remains permanently in the circuit. - Single-phase motor with starting and running capacitor (also known as “capacitor starting and running motor”)
A powerful starting capacitor is also used here, which is used during start-up. When the desired speed is reached, it is switched off via a mechanical centrifugal switch or an electronic relay, and a normal operating capacitor takes over its role. This design significantly increases the starting torque.
When is the run capacitor sufficient?
Single-phase motors with only a run capacitor are ideal for applications with constant low to medium loads, smooth starting and low starting torque. Typical applications are
- Fans, ventilators
- Conveyor belts with light loads such as packaged textiles
- Small pumps with slow acceleration such as pool pumps
- Simple workshop machines such as stationary drills
These motors have a simple design, low power consumption and are often cheaper to buy. Their energy efficiency is good in continuous operation – however, the starting torque (Ma) is limited to around 30-50% of a comparable three-phase motor, depending on the size. This means that a conveyor belt with a 0.37 kW Y2MD071M2B motor with Ma/Mn 0.6 can only start if it is loaded with less than 60% of the maximum permissible load.
When is a starting capacitor required?
Single-phase motors with starting and running capacitors are used wherever a high torque is required at startup. Typical cases are
- Compressors
- Pump applications with high starting resistance (e.g. piston pumps)
- Mixers for viscous materials such as dough, cement or paint
Thanks to the additional starting capacitor, these motors achieve a starting torque that can be between 100% and 200% of the nominal torque. In many commercial and semi-industrial applications, this is a prerequisite for a reliable start without starting problems. Even if a three-phase connection is available, the increased starting torque of a single-phase motor with two capacitors can be utilized in a targeted manner. A mixer with a YLHE80A4 motor with Ma/Mn 1.7 can easily require 60% more torque during start-up than during operation.
What to do if the torque requirement is unknown?
Single-phase motors with two capacitors are permissible for most applications, even if the increased starting torque from the second capacitor is not required. Although they are usually more expensive, especially if switching is carried out by means of relays, these additional costs are generally manageable.
However, there are applications for which a motor with two capacitors, or a single-phase motor in general, is not advisable. If the starting torque is too high, other machine parts may be destroyed in the event of a blockage. Applications with a large number of switching cycles or prolonged operation at less than 75% of the rated power can lead to premature wear of the capacitors.
MOLL-MOTOR has both variants in stock. Sizes 63 – 112, outputs from 0.12 kW to 3.7 kW are available for immediate delivery from Stockerau near Vienna. MOLL-MOTOR will be happy to assist you in selecting the right motor, offering individual shaft adaptations and special designs. With over 75 years of experience and a stock of over 50,000 motors, we help OEM customers, maintenance engineers and designers to find the optimum solution quickly and reliably.
