Gate motor types

Why are there so many motor types used in gate automation? Confused? We try to explain the differences and advantages of each gate motor type. 

Author  ;    Huw Jones

Controlling the beast   

There wasn't always this many options. In simpler times, 230V motors were the norm. Now we have DC motors, single and three phase AC motors, and the new kid on the block, the brushless motor.

The changes have been lead more by demands for better control than new technology.

We live in a world where news of gate related accidents has lead to greater awareness of the dangers. So the industry has had to improve its safety record, and that has lead to better control of the gate motor.

"Guns don't kill people, people kill people". A convenient piece of logic trotted out by the american gun lobby. But electric gates are required by their owners to operate at any time without supervision. An automatic gate is more akin to a heat seeking drone withhout the sophistication.

In simple terms, the motor is there to pull the gate open and closed. The control system on the other hand, is there to keep the beast under control, and stop us  

Reading the motor plate

Every motor has a plate that gives details on the motor power. The wattage (W) tells you the power of the motor, but that is only half the story.

A low power motor could move a gate very slowly given sufficient gearing. That is how the Egyptian slave built the pyramids. 

The 320Nm is the torque figure. It means that at the output shaft, this motor can exert a force of 320N (Newtons) through a 1 metre long lever arm, or 32N through a 10 metre long lever. 

The arm on this motor is actually about 10cm long, so it could lift a third of a tonne on the end of its lever. 

The rest of the plate tells us it is a single phase 230V motor suitable for Europe (50Hz) or north america (60Hz). 

Every motor has a data plate 

1 - ACIM motors

• FOR - low current mains power
• FOR - simple control panels
• AGAINST - low startup torque
• AGAINST - poor speed control
• AGAINST - extra high voltage regulation 

230V single phase mains powered motors were at one time the only popular option. They are still popular for low basic systems. 

What we describe as mains voltage is 230V in europe, 115V in North America. The polarity flips 50 times a second in Europe (50Hz) so it is called alternating current.

ACIM means alternating current induction motor. The alternating voltage is converted to an alternating magnetic field which induces a magnetic circuit is a series of magnetic paths inside the rotating shaft known as the rotor. 

A disadvantage of AC motors is their speed is proportional to the frequency of the alternating current, which is fixed by the national grid.

Another problem is poor efficiency, which leads to overheating. ACIM's need a thermal trip to stop them overheating when over used. A typical duty cycle is 30%, and the higher the power, the quicker it heats up. The motor may need 20 minutes to cool down.

ACIM's are characterised by the need for a starter capacitor. In truth, ACIM's have two sets of windings; forward and reverse. By passing current through the capacitor, the second winding gets power whose alternations are a quarter cycle (90°) behind the first. That delay keeps the motor spinning in the right direction.

2 - Three phase AC motors

Power stations generate power on three phases. Power is distributed by the national grid in three phases at very high voltage, then transformed down to 230V to supply a whole street.

While each phase is 230V, the voltage from one phase to another is 400V, so phases are kept a street's distance from each other for safety.

The national grid's prime concern is to share the load equally on the three phases. Industrial users are allowed 3 phase motors and heaters which are naturally balanced across phases.

Three phase motors have 3 sets of windings. There is no need for the starter capacitor because there is a natural 120° interphase delay. The direction of the motor is acheived by changing the order of the phases on the three windings.

Control panels tend to be expensive, electromechanical and agricultural. There is no control of speed or power regulation, so the gates need to run slowly to remain safe. 

3∅ motor advantages

• FOR - better startup torque and power
• AGAINST - 3∅ power is rarely available
• AGAINST - no torque or speed control
• AGAINST - extra high voltage regulation 

3 - DC motors

• FOR - safe low voltage wiring
• FOR - good speed and torque control
• FOR - battery backup
• FOR - efficient & high duty cycle
• FOR - wide torque band
• AGAINST - high current stress
• AGAINST - expensive controllers 

DC means direct current, the opposite of alternating current. The qualities of DC motors is rapidly making them the technology of choice.

The majority of DC motors have their windings on the rotating shaft, and a fixed magnet static in the casing. The problem then is getting relatively high currents from the fixed terminals to the rotor.

The carbon brushes that perform this transfer are the DC motor's achilles heel. Fortunately, a domestic gate motor runs for less than 10 minutes a day, so is likely to outlast the gate.

DC motors are efficient, meaning more input power is transfered to the gate, and less expended in noise  and heat. The current is near proportional to the torque output. 

Unlike AC motors, DC motors are not prone to over heating, so do not need a thermal trip. The duty cycle of DC gate openers is rated as intensive use.

For some applications, the ability to provide battery operation is the deal maker. We are fortunate that the UK national grid has fewer outages than in more challenging climates. Batteries are essential for off grid solar powered systems.

For the same power, a lower voltage means higher current. Every cable joint, connector, and cable is a source of resistance, and every resistance becomes a hotspot when passing high currents, which in time may be the source of a failure.

Most automation motors use 24Vdc. To half current, it is likely the manufacturers will increase motor voltages up to 48Vdc when semiconductors allow. 50Vdc is the practical ELV limit.

The demand for safety reflects the need for controlability. Although DC motors are much easier to control and more responsive, the electronics required becomes more complex. Fortunately, electronics becomes cheaper and more reliable, so DC motor systems are likely to increase in popularity.

4 - EC brushless motors

Brushless motors have been around for decades, but are new to gate automation. EC stands for 'electronic commutating' referring to the switching of the current from one winding to another. The experts say the adoption of brushless technology is inevitable in this field, but I am not so sure.

A brushless motor is a low voltage AC motor with a variable frequency controller. Think of combining a small 3 phase AC motor with a 3 phase invertor control panel.  (FYI; an invertor creates an AC output from a DC supply).

Some great claims are made for efficiency and sensitivity, but it is not clear where the improvements are gained. Claims of higher reliability for the motor are more plausible.

In automation, brushless motors have been used where there is constant use and the need for profiled speed control. Toll road barriers and automatic swing doors. The lack of brushes is a great advantage for high speed motors. Other aptmospheric applications include vacuums, oil baths, explosive gases. 

Brushless motors are not expensive to make, but it will be a few years before dedicated semiconductors are available to drive down the price of the control panels. Until then, brushless motors are unlikely to gain traction in competitive markets.

The automation industry has an insatiable thirst for safety brownie points. A case could be made for a controller that measures force through the motor connections.

Brushless motor advantages

• FOR - digital speed control
• FOR - battery backup options
• FOR - low wear & higher efficiency
• AGAINST - extra wiring